JP2017066546A - Pitch-based extra fine carbon fiber, electrode mixture layer for nonaqueous electrolyte secondary battery, electrode for nonaqueous electrolyte secondary battery and nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a pitch-based extra fine carbon fiber having enhanced compatibility to an electrolyte and high conductivity.SOLUTION: A pitch-based extra fine carbon fiber high in compatibility to an electrolyte and high in conductivity by controlling a surface state of a carbon fiber having variation of a certain degree of fine diameter and high crystallinity and the pitch-based extra fine carbon fiber having surface oxygen concentration ratio (O/C) measured by an X-ray photoelectron spectroscopy of 0.02 to 0.15, CV value of the fiber diameter of 10 to 50% and crystallite size (d002) measured by an X-ray diffraction method of 0.340 nm or less is manufactured.SELECTED DRAWING: None

Description

  The present invention relates to a pitch-based ultrafine carbon fiber, an electrode mixture layer for a non-aqueous electrolyte secondary battery using the carbon fiber, a non-aqueous electrolyte secondary battery electrode on which the electrode mixture layer is formed, and the electrode. The present invention relates to a non-aqueous electrolyte secondary battery including the same.

  Carbon nanomaterials, especially ultrafine carbon fibers with an average fiber diameter of 1 μm or less, have excellent properties such as high crystallinity, high conductivity, high strength, high elastic modulus, and light weight. Used as a nanofiller for composite materials. Its use is not limited to reinforcing nano fillers for the purpose of improving mechanical strength. Utilizing the high conductivity of carbon materials, it can be used as an additive material for various batteries and capacitor electrodes, electromagnetic shielding materials, and antistatic materials. As a conductive nanofiller for use in an electric field, or as a nanofiller to be blended in an electrostatic coating for resin, studies are being made. In addition, it is expected to be used as a field electron emission material such as a flat display by taking advantage of the chemical stability, thermal stability and microstructure of the carbon material.

  For example, Patent Document 1 includes (1) 100 parts by mass of a thermoplastic resin and at least one thermoplastic carbon precursor selected from the group consisting of pitch, polyacrylonitrile, polycarbodiimide, polyimide, polybenzoazole, and aramid. A step of forming a precursor molded body from a mixture of 150 parts by mass; (2) stabilizing the thermoplastic carbon precursor in the precursor molded body by subjecting the precursor molded body to a stabilization treatment; (3) removing the thermoplastic resin from the stabilized resin composition under reduced pressure to form a fibrous carbon precursor, (4) carbonizing or graphitizing the fibrous carbon precursor The manufacturing method of the carbon fiber which passes through the process to perform is disclosed. Although the carbon fiber produced by this method has good characteristics, it is insufficient as battery characteristics when used as an electrode material, and further improvement in charge and discharge efficiency is required.

  In order to further improve the charge / discharge efficiency, it is required to improve the affinity between the carbon fiber constituting the electrode material and the electrolytic solution. As a method for improving the affinity between the carbon fiber and the electrolyte solution, a method of fixing metal particles on the surface of the carbon fiber, a method of heating in the presence of an oxidizing gas, an additive such as a surfactant is added. There are known methods for reducing the interfacial resistance with the electrolytic solution.

  In Patent Document 2, the surface of the carbon fiber is modified to improve the dispersibility between the fiber and the matrix and to enhance the interfacial strength by subjecting the carbon fiber produced by the vapor phase method to surface oxidation treatment. A method that can be performed is disclosed. However, the surface state and oxygen content of a suitable carbon fiber are not disclosed at all, and the conductivity and the affinity with the electrolytic solution are not satisfied at the same time.

  Patent Document 3 discloses a pitch-based ultrafine carbon fiber having a surface oxygen concentration ratio (O / C) of 0.02 or more, a uniform fiber diameter, excellent conductivity, and good dispersibility in an aqueous solvent. Is disclosed. However, there is no specific description of ultrafine carbon fibers having extremely high crystallinity and a fiber diameter of the order of microns or less. Further, it is described that carbon fibers are formed in a film shape on a base material, and the conductivity in the in-plane direction of the film is good.

International Publication No. 2009/125857 JP 2006-140142 A JP-A-2015-48561

  An object of the present invention is to provide a pitch-based ultrafine carbon fiber having improved affinity for an electrolytic solution and high conductivity. Furthermore, an electrode mixture layer for a nonaqueous electrolyte battery formed using this pitch-based ultrafine carbon fiber, an electrode on which the electrode mixture layer is formed, and a nonaqueous electrolyte secondary formed using the electrode To provide a battery.

  As a result of intensive studies in view of the above prior art, the present inventors have found that the fiber diameter has a certain degree of variation and the surface state of the carbon fiber having high crystallinity is controlled, thereby making it compatible with the electrolyte. It has been found that pitch-based ultrafine carbon fibers having high properties and high conductivity can be obtained, and the present invention has been completed.

  That is, the present invention for solving the above problems is described below.

[1] The surface oxygen concentration ratio (O / C) measured by X-ray photoelectron spectroscopy is 0.02 or more and 0.15 or less,
CV value of fiber diameter is 10-50%,
A pitch-based ultrafine carbon fiber having a crystallite size (d002) measured by an X-ray diffraction method of 0.340 nm or less.

[2] The intensity ratio (O = C / O—C) between O—C bonds derived from O1s orbitals and O═C bonds (O═C / O—C) measured by X-ray photoelectron spectroscopy is 1.4 or more. Pitch-based ultra-fine carbon fiber.

[3] The pitch-based ultrafine carbon fiber according to [1] or [2], wherein the average fiber diameter is 100 nm or more and less than 700 nm.

[4] The pitch-based ultrafine carbon fiber according to any one of [1] to [3], having an average aspect ratio of 30 to 1000.

[5] The pitch-based ultrafine carbon fiber according to any one of [1] to [4],
An electrode active material;
A binder,
An electrode mixture layer for a nonaqueous electrolyte secondary battery.

[6] current collector;
The electrode mixture layer for a nonaqueous electrolyte secondary battery according to [5] laminated on the current collector,
An electrode for a nonaqueous electrolyte secondary battery.

[7] A nonaqueous electrolyte secondary battery comprising the electrode for a nonaqueous electrolyte secondary battery according to [6].

  According to the present invention, it is possible to obtain a pitch-based ultrafine carbon fiber that is superior in affinity with an electrolytic solution and has high conductivity in the film thickness direction in the electrode mixture layer as compared with the carbon fiber obtained in the prior art. it can. This pitch-based ultrafine carbon fiber can be suitably used as a battery material.

  In addition to the battery material, the pitch-based ultrafine carbon fiber of the present invention includes a reinforcing nanofiller, a material added to a capacitor electrode, an electromagnetic shielding material, a conductive nanofiller for an antistatic material, and an electrostatic for resin. It is promising for applications as field electron emission materials such as nanofillers and flat displays to be blended in paints.

FIG. 1 shows XPS measurement results of CNF produced in Example 1. FIG. 2 shows the XPS measurement result of CNF produced in Example 2. FIG. 3 is a result of XPS measurement of CNF manufactured in Comparative Example 1. FIG. 4 shows XPS measurement results of CNF produced in Comparative Example 2.

  Hereinafter, the present invention will be described in detail.

1. 1. Pitch ultrafine carbon fiber for non-aqueous electrolyte secondary battery 1-1. Properties of pitch-based ultrafine carbon fiber The first aspect of the present invention has a surface oxygen concentration ratio (O / C) measured by X-ray photoelectron spectroscopy of 0.02 or more and 0.15 or less,
CV value of fiber diameter is 10-50%,
The pitch-based ultrafine carbon fiber has a crystallite size (d002) measured by an X-ray diffraction method of 0.340 nm or less.

  The pitch-based ultrafine carbon fiber of the present invention (hereinafter sometimes simply referred to as carbon fiber) has a surface oxygen concentration that indicates the ratio of oxygen atoms to carbon atoms on the surface of the carbon fiber when measured by X-ray photoelectron spectroscopy. The ratio (surface O / C) is 0.02 or more and 0.15 or less, more preferably 0.02 or more and 0.10 or less, and further preferably 0.02 or more and 0.06 or less. When the surface O / C is within this range, the affinity for the electrolytic solution, particularly propylene carbonate, is high, and the carbon fibers can be uniformly dispersed in the electrode mixture layer.

  The carbon fiber of the present invention has a strength ratio (O = C / O—C) of O—C bonds derived from O1s orbitals and O═C bonds (O═C / O—C) of 1.4 or more measured on the surface by X-ray photoelectron spectroscopy. Is preferred. When the strength ratio is 1.4 times or more, the presence of more hydrophobic O═C groups on the surface of the carbon fiber than the highly hydrophilic C—OH groups makes it possible to have an affinity for the electrolyte, particularly propylene carbonate. The dispersibility of the carbon fiber in the electrode mixture layer is high. The intensity ratio (O = C / O—C) between the O—C bond and the O═C bond is more preferably 1.5 or more.

The average fiber diameter of the carbon fiber of the present invention is preferably 100 nm or more and less than 700 nm. The upper limit is preferably 600 nm or less, more preferably 500 nm or less, further preferably 400 nm or less, and further preferably 300 nm or less. The lower limit is preferably 110 nm or more, more preferably 120 nm or more, and further preferably 150 nm or more.
If it is less than 100 nm, the bulk density is very small and the handling property is poor. Moreover, when an electrode mixture layer is constructed, the electrode strength tends to decrease. When it is 700 nm or more, a gap is likely to be generated in the electrode mixture layer, and it may be difficult to increase the electrode density.
Here, the fiber diameter in the present invention means a value measured from a photograph taken at a magnification of 2000 times with a field emission scanning electron microscope.

  In the carbon fiber of the present invention, the CV value of the fiber diameter is in the range of 10 to 50%. By being in this range, the fiber diameter varies moderately. Therefore, the contact property with the electrode active material having various particle sizes can be increased, and the performance as a conductive additive is improved. As a result, the electrical conductivity in the film thickness direction of the electrode mixture layer can be increased. The CV value of the fiber diameter is more preferably in the range of 20 to 50%, and further preferably in the range of 30 to 45%. Here, the CV value is a value obtained by dividing the standard deviation value by the average value and expressed as a percentage (%).

  The carbon fiber of the present invention has a distance (d002) between adjacent graphite sheets measured by wide-angle X-ray measurement of 0.340 nm or less, preferably 0.335 to 0.340 nm, and preferably 0.335 to 0. More preferably, it is 339 nm. When d002 is in the range of 0.335 to 0.340 nm, the electrode mixture layer exhibits high conductivity in the film thickness direction.

  The average fiber length of the carbon fiber of the present invention is preferably in the range of 1 to 100 μm, more preferably 1 to 50 μm, and even more preferably 5 to 20 μm. When the thickness is less than 1 μm, the conductivity in the electrode mixture layer, the strength of the electrode, and the electrolyte solution retention are lowered, which is not preferable. If it exceeds 100 μm, the dispersibility of the pitch-based ultrafine carbon fiber is impaired, which is not preferable. That is, when the pitch-based ultrafine carbon fiber is too long, the pitch-based ultrafine carbon fiber is easily oriented in the in-plane direction of the electrode mixture layer. As a result, it becomes difficult to form a conductive path in the film thickness direction.

  The ratio (L / D) of the average fiber length (L) and the average fiber diameter (D) of the carbon fiber of the present invention is preferably 30 or more, more preferably 50 or more, and 100 or more. Is more preferable. By setting the ratio (L / D) to 30 or more, a conductive path is efficiently formed in the electrode mixture layer, and the electrical conductivity in the film thickness direction can be increased. If it is less than 30, the formation of the conductive path is likely to be insufficient in the electrode mixture layer, and the resistance value in the film thickness direction of the electrode mixture layer may not be sufficiently reduced. The upper limit value of the ratio (L / D) is not particularly limited, but is preferably 1000 or less, and more preferably 800 or less.

The carbon fiber of the present invention preferably has a linear structure. Here, the linear structure means that the degree of branching is 0.01 piece / μm or less. Branching means a granular part in which carbon fibers are bonded to other carbon fibers at a place other than the terminal part. The main axis of the carbon fiber is branched in the middle, and the main axis of the carbon fiber is a branch-like minor axis. It means having. Note that vapor grown ultrafine carbon fiber (VGCF) is known as a branched carbon fiber.
In addition, this carbon fiber should just have a fibrous form as a whole, for example, the carbon material of less than 1 micrometer contacts and couple | bonds and has a fiber shape integrally (for example, , Spherical carbon is connected in a beaded manner by fusion or the like, and extremely short fibers are connected by fusion or the like.

The carbon fiber of the present invention preferably has high conductivity in a state where the packing density is low. This is because carbon fibers having high conductivity in a state where the packing density is low can impart conductivity at a lower additive concentration. Specifically, the powder volume resistance when filled at a packing density of 0.4 g / cm ³ is preferably 1 Ω · cm or less, and more preferably 0.5 Ω · cm or less. If it exceeds 1 Ω · cm, the amount of carbon fiber added to improve conductivity is increased, which is not preferable. The lower limit is not particularly limited, but is generally about 0.0001 Ω · cm. Further, the powder volume resistance when filled at a packing density of 0.8 g / cm ³ is preferably 0.1 Ω · cm or less, more preferably 0.050 Ω · cm or less, still more preferably 0.030 Ω · cm or less, Even more preferably, it is 0.028 Ω · cm or less. When it exceeds 0.1 Ω · cm, the amount of carbon fiber added to improve conductivity is undesirably increased. The lower limit is not particularly limited, but is generally about 0.0001 Ω · cm.

1-2. Production method of carbon fiber The production method of the carbon fiber of the present invention is not particularly limited. For example, it can be produced through the steps (1) to (4) described below.
(1) A step of forming a resin composition comprising 100 parts by mass of a thermoplastic resin and 1 to 150 parts by mass of a carbon precursor in a molten state, thereby obtaining a resin composite fiber by fiberizing the carbon precursor;
(2) a stabilization step of stabilizing the resin composite fiber to obtain a resin composite stabilized fiber;
(3) a thermoplastic resin removing step of removing only the stabilized fiber by removing the thermoplastic resin from the resin composite stabilized fiber;
(4) A carbonization firing step in which the stabilized fiber is heated in an inert atmosphere to be carbonized or graphitized to obtain carbon fiber.

<Thermoplastic resin>
The thermoplastic resin used in the method for producing carbon fiber of the present invention needs to be easily removed after producing the resin composite fiber. Examples of such thermoplastic resins include polyolefins, polymethacrylates, polymethylmethacrylates and other polyacrylate polymers, polystyrene, polycarbonate, polyarylate, polyester, polyamide, polyester carbonate, polysulfone, polyimide, polyetherimide, polyketone, and polylactic acid. Is exemplified. Among these, polyolefin is preferably used.

  Specific examples of the polyolefin include polyethylene, polypropylene, poly-4-methylpentene-1, and a copolymer containing these. From the viewpoint of easy removal in the thermoplastic resin removal step, it is preferable to use polyethylene. Polyethylene includes high pressure method low density polyethylene, gas phase method, solution method, high pressure method low density polyethylene such as linear low density polyethylene, homopolymer such as medium density polyethylene, high density polyethylene or ethylene and α-olefin. A copolymer of ethylene and another vinyl monomer such as an ethylene / vinyl acetate copolymer.

  The thermoplastic resin used in the present invention preferably has a melt mass flow rate (MFR) measured in accordance with JIS K 7210 (1999) of 0.1 to 10 g / 10 min, preferably 0.1 to 5 g / min. 10 min is more preferable, and 0.1 to 3 g / 10 min is particularly preferable. When the MFR is in the above range, the carbon precursor can be well microdispersed in the thermoplastic resin. Further, when the resin composite fiber is molded, the fiber diameter of the obtained carbon fiber can be further reduced by stretching the fiber. The thermoplastic resin used in the present invention has a glass transition temperature of 250 ° C. or lower in the case of amorphous and a melting point of 300 ° C. or lower in the case of crystallinity because it can be easily melt kneaded with the carbon precursor. Is preferred.

<Carbon precursor>
As the carbon precursor, mesophase pitch is preferably used as a raw material. Hereinafter, the case where mesophase pitch is used as the carbon precursor will be described. The mesophase pitch is a pitch that can form an optically anisotropic phase (liquid crystal phase) in a molten state. Examples of the mesophase pitch used in the present invention include those using a distillation residue of coal or petroleum as a raw material and those using an aromatic hydrocarbon such as naphthalene as a raw material. For example, a coal-derived mesophase pitch can be obtained by a process mainly including hydrogenation / heat treatment of coal tar pitch, a process mainly including hydrogenation / heat treatment / solvent extraction, and the like.

More specifically, it can be obtained by the following method.
First, a coal tar pitch having a softening point of 80 ° C. from which quinoline-insoluble components have been removed is hydrogenated at a pressure of 13 MPa and a temperature of 340 ° C. in the presence of a Ni—Mo catalyst to obtain a hydrogenated coal tar pitch. This hydrogenated coal tar pitch is heat-treated at 480 ° C. under normal pressure and then reduced in pressure to remove low boiling point components to obtain a crude mesophase pitch. By purifying the crude mesophase pitch using a filter at a temperature of 340 ° C. to remove foreign matters, a purified mesophase pitch can be obtained.

  The optically anisotropic content (mesophase ratio) of the mesophase pitch is preferably 80% or more, and more preferably 90% or more.

  Further, the mesophase pitch preferably has a softening point of 100 to 400 ° C, more preferably 150 to 350 ° C.

<Resin composition>
The resin composition comprising a thermoplastic resin and mesophase pitch (hereinafter also referred to as mesophase pitch composition) used in the carbon fiber production method of the present invention comprises 100 parts by mass of thermoplastic resin and 1 to 150 parts by mass of mesophase pitch. And comprising. The mesophase pitch content is preferably 5 to 100 parts by mass. If the mesophase pitch content exceeds 150 parts by mass, a resin composite fiber having a desired dispersion diameter cannot be obtained, and if it is less than 1 part by mass, the target carbon fiber cannot be produced at low cost. This is not preferable because it occurs.

  In order to produce carbon fibers having a fiber diameter of less than 700 nm, the mesophase pitch dispersion diameter in the thermoplastic resin is preferably 0.01 to 50 μm, and more preferably 0.01 to 30 μm. If the dispersion diameter of the mesophase pitch in the thermoplastic resin is out of the range of 0.01 to 50 μm, it may be difficult to produce a desired carbon fiber. In the mesophase pitch composition, the mesophase pitch forms a spherical or elliptical island phase, but the dispersion diameter in the present invention means the diameter when the island component is spherical, The major axis diameter is meant.

  The dispersion diameter of 0.01 to 50 μm is preferably maintained after the mesophase pitch composition is held at 300 ° C. for 3 minutes, more preferably maintained after being held at 300 ° C. for 5 minutes. It is particularly preferable that the temperature is maintained after being held at 300 ° C. for 10 minutes. Generally, when the mesophase pitch composition is held in a molten state, the mesophase pitch aggregates with time in the thermoplastic resin. When the mesophase pitch is aggregated and the dispersion diameter exceeds 50 μm, it may be difficult to produce a desired carbon fiber. The aggregation speed of the mesophase pitch in the thermoplastic resin varies depending on the type of the thermoplastic resin and mesophase pitch used.

  The mesophase pitch composition can be produced by kneading a thermoplastic resin and mesophase pitch in a molten state. Melting and kneading of the thermoplastic resin and the mesophase pitch can be performed using a known apparatus. For example, at least one selected from the group consisting of a single-screw kneader, a twin-screw kneader, a mixing roll, and a Banbury mixer can be used. Among these, a biaxial kneader is preferably used for the purpose of satisfactorily microdispersing the mesophase pitch in the thermoplastic resin, and in particular, a biaxial kneader in which each axis rotates in the same direction is used. preferable.

  The kneading temperature is not particularly limited as long as the thermoplastic resin and the mesophase pitch are in a molten state, but is preferably 100 to 400 ° C, and more preferably 150 to 350 ° C. When the kneading temperature is less than 100 ° C., the mesophase pitch is not in a molten state, and it is difficult to micro-disperse in the thermoplastic resin, which is not preferable. On the other hand, when the temperature exceeds 400 ° C., decomposition of the thermoplastic resin and mesophase pitch proceeds, which is not preferable. Further, the melt kneading time is preferably 0.5 to 20 minutes, and more preferably 1 to 15 minutes. When the melt kneading time is less than 0.5 minutes, it is not preferable because micro dispersion of mesophase pitch is difficult. On the other hand, if it exceeds 20 minutes, the productivity of the carbon fiber is remarkably lowered, which is not preferable.

  The melt-kneading is preferably performed in an inert atmosphere having an oxygen gas content of less than 10% by volume, more preferably performed in an inert atmosphere having an oxygen gas content of less than 5% by volume, and an oxygen gas content of 1% by volume. It is particularly preferred to carry out under an inert atmosphere of less than The mesophase pitch used in the present invention may be modified by reacting with oxygen during melt-kneading and may inhibit micro-dispersion in the thermoplastic resin. For this reason, it is preferable to perform melt kneading in an inert atmosphere to suppress the reaction between oxygen and mesophase pitch.

<Resin composite fiber>
The method for producing a resin composite fiber from the above mesophase pitch composition is not limited as long as a desired carbon fiber can be produced, but the mesophase pitch composition is melt-spun from a spinneret, and the mesophase pitch composition is melted from a rectangular die. A method for forming a film can be exemplified.

  In order to obtain the carbon fiber of the present invention, it is necessary to perform an operation for increasing the initial orientation of the mesophase pitch contained in the resin composite fiber at the stage of obtaining the resin composite fiber. As an operation for increasing the initial orientation of the mesophase pitch contained in the resin composite fiber, it is necessary to add a deformation for increasing the orientation of the mesophase pitch in the molten state. A method of applying strain due to shear and a method of applying strain due to elongation can be exemplified.

As a method of applying strain due to shearing, in a state where the mesophase pitch is melted, the linear velocity of the molten mesophase pitch composition is increased by using the die, so that the molten mesophase pitch composition is moved in the die channel. When passing, strain due to shear can be applied.
Further, as a method for applying strain due to elongation, there is a method in which the linear velocity of the melted mesophase pitch composition is increased toward the discharge side using a die in a state where the mesophase pitch is melted. Specific examples include a method of gradually reducing the cross-sectional area in the flow path toward the discharge side, and a method of drawing the mesophase pitch composition discharged from the die at a linear velocity higher than the discharge linear velocity.

  The temperature during the operation for increasing the initial orientation of the mesophase pitch needs to be higher than the melting temperature of the mesophase pitch, preferably 150 to 400 ° C, and more preferably 180 to 350 ° C. . When it exceeds 400 ° C., the deformation relaxation rate of the mesophase pitch is increased, and it is difficult to maintain the fiber form.

  Moreover, the manufacturing process of resin composite fiber may have a cooling process. Examples of the cooling step include a method of cooling the atmosphere downstream of the spinneret in the case of melt spinning. In the case of melt film formation, a method of providing a cooling drum downstream of the rectangular die can be used. By providing the cooling step, it is possible to adjust the region in which the mesophase pitch is deformed by extension, and to adjust the strain rate. Further, by providing a cooling step, the resin composite fiber after spinning or film formation is immediately cooled and solidified to enable stable molding.

<Resin composite stabilized fiber>
The resin composite fibers obtained as described above are stabilized (also referred to as infusible) mesophase pitch fibers contained in the resin composite fibers to produce resin composite stabilized fibers. Stabilization can be performed by a known method such as gas flow treatment using air, oxygen, ozone, nitrogen dioxide, halogen, etc., solution treatment using an acidic aqueous solution, etc., but infusibility by gas flow treatment from the viewpoint of productivity. Is preferred.

  The gas component to be used is preferably air, oxygen, or a mixed gas containing this from the viewpoint of ease of handling, and it is particularly preferable to use air from the viewpoint of cost. The oxygen gas concentration used is preferably in the range of 10 to 100% by volume of the total gas composition. If the oxygen gas concentration is less than 10% by volume of the total gas composition, it takes a long time to stabilize the mesophase pitch contained in the resin composite fiber, which is not preferable.

  The stabilization reaction temperature is preferably 25 to 350 ° C., and the stabilization treatment time is preferably 10 to 300 minutes.

  Although the softening point of the mesophase pitch is remarkably increased by the stabilization treatment, the softening point of the mesophase pitch is preferably 400 ° C. or higher, and more preferably 500 ° C. or higher, for the purpose of obtaining a desired carbon fiber. .

<Thermoplastic resin removal step>
Next, in the resin composite stabilized fiber obtained as described above, the thermoplastic resin contained therein is removed and the stabilized fiber is separated. In this step, the thermoplastic resin is decomposed and removed while suppressing thermal decomposition of the stabilizing fiber. Examples of the method for decomposing / removing the thermoplastic resin include a method for removing the thermoplastic resin using a solvent and a method for thermally decomposing and removing the thermoplastic resin.

  The thermal decomposition of the thermoplastic resin is preferably performed in an inert gas atmosphere. The inert gas atmosphere here refers to a gas atmosphere such as carbon dioxide, nitrogen, and argon, and the oxygen concentration is preferably 30 ppm by volume or less, and more preferably 20 ppm by volume or less. As the inert gas used in this step, it is preferable to use carbon dioxide and nitrogen from the viewpoint of cost, and it is particularly preferable to use nitrogen.

  When removing a thermoplastic resin by thermal decomposition, it can also carry out under reduced pressure. By thermally decomposing under reduced pressure, the thermoplastic resin can be sufficiently removed. As a result, fusion between carbon fibers or graphitized fibers obtained by carbonizing or graphitizing the stabilizing fibers can be reduced. The atmospheric pressure is preferably as low as possible, but is preferably 50 kPa or less, more preferably 30 kPa or less, still more preferably 10 kPa or less, and particularly preferably 5 kPa or less. On the other hand, since a complete vacuum is difficult to achieve, the lower limit of the pressure is generally 0.01 kPa or more.

  When the thermoplastic resin is removed by thermal decomposition, a trace amount of oxygen or inert gas may be present as long as the above atmospheric pressure is maintained. In particular, the presence of a trace amount of inert gas is preferable because there is an advantage that fusion between fibers due to thermal deterioration of the thermoplastic resin is suppressed. In addition, the trace amount oxygen atmosphere here means that oxygen concentration is 30 volume ppm or less, and the trace amount inert gas atmosphere means that inert gas concentration is 20 volume ppm or less. . The kind of the inert gas used is as described above.

  The thermal decomposition temperature is preferably 350 to 600 ° C, and more preferably 380 to 550 ° C. When the thermal decomposition temperature is less than 350 ° C., the thermal decomposition of the stabilized fiber may be suppressed, but the thermoplastic resin may not be sufficiently decomposed. On the other hand, when the temperature exceeds 600 ° C., the thermoplastic resin can be sufficiently thermally decomposed, but even the stabilized fiber may be thermally decomposed, and as a result, the yield at the time of carbonization tends to decrease. The time for thermal decomposition is preferably 0.1 to 10 hours, and more preferably 0.5 to 10 hours.

In the production method of the present invention, the stabilization step and the thermoplastic resin removal step are preferably performed by holding the resin composite fiber or the resin composite stabilized fiber on the support substrate at a weight per unit area of 2000 g / m ² or less. By holding on the support substrate, aggregation of the resin composite fiber or the resin composite stabilized fiber due to the heat treatment at the time of the stabilization treatment or removal of the thermoplastic resin can be suppressed, and air permeability can be maintained. .

  As a material of the supporting base material, it is necessary that deformation or corrosion does not occur due to a solvent or heating. Moreover, as the heat resistant temperature of the support base material, it is preferable that the supporting base material has a heat resistance of 600 ° C. or higher because it does not need to be deformed at the thermal decomposition temperature in the thermoplastic resin removing step. Examples of such materials include metal materials such as stainless steel and ceramic materials such as alumina and silica.

  Moreover, as a shape of a support base material, it is preferable that it is a shape which has air permeability to a surface perpendicular direction. As such a shape, a mesh structure is preferable. The mesh opening is preferably 0.1 to 5 mm. When the mesh opening is larger than 5 mm, the fibers are likely to aggregate on the mesh line by the heat treatment, and the stabilization of the mesophase pitch and the removal of the thermoplastic resin may be insufficient. On the other hand, when the mesh opening is less than 0.1 mm, the air permeability in the direction perpendicular to the surface of the support base material may decrease due to the decrease in the hole area ratio of the support base material, which is not preferable.

<Carbonization firing process>
The carbon fiber of the present invention can be obtained by carbonizing and / or graphitizing the stabilized fiber in an inert gas atmosphere. As a container used in that case, a crucible made of graphite is preferable. Here, carbonization refers to heating at a relatively low temperature (preferably about 1000 ° C.), and graphitization refers to growing graphite crystals by heating at a higher temperature (preferably about 3000 ° C.). Say.

  Nitrogen, argon etc. are mentioned as an inert gas used at the time of carbonization and / or graphitization of the said stabilization fiber. The oxygen concentration in the inert gas is preferably 20 ppm by volume or less, and more preferably 10 ppm by volume or less. The firing temperature during carbonization and / or graphitization is preferably from 500 to 3500 ° C, more preferably from 800 to 3200 ° C. In particular, the firing temperature during graphitization is preferably 2000 ° C. or more and less than 3000 ° C., and more preferably 2100 to 2900 ° C. When the temperature during graphitization is less than 2000 ° C., crystal growth is hindered, the crystallite length becomes insufficient, and the conductivity may be significantly lowered. Further, when the graphitization temperature is 3000 ° C. or higher, it is preferable in terms of crystal growth, but the oxygen content present on the surface of the carbon fiber is decreased, and O / C tends to be smaller than 0.02. The firing time is preferably 0.1 to 24 hours, and more preferably 0.2 to 10 hours.

<Crushing process>
The carbon fiber production method of the present invention may have a pulverization treatment step. The pulverization treatment is preferably performed in the thermoplastic resin removal step and / or the carbonization firing step. As a pulverization method, it is preferable to apply a fine pulverizer such as a jet mill, a ball mill, a bead mill, an impeller mill, or a cutter mill, and classification may be performed as necessary after pulverization. In the case of wet pulverization, the dispersion medium is removed after pulverization, but if secondary aggregation occurs significantly at this time, subsequent handling becomes very difficult. In such a case, it is preferable to perform a crushing operation using a ball mill or a jet mill after drying.

  The carbon fiber of the present invention does not need to be subjected to a generally known surface oxidation treatment in order to improve the dispersibility with the matrix resin and the dispersibility with water. For example, in order to improve the dispersibility of carbon fiber in water, it is known to use a mixed acid or hydrogen peroxide as the surface oxidation treatment (for example, WO2014 / 115852, paragraphs 0391 and 0398). Carbon fiber treated with mixed acid has a very large surface O / C value, and in the case of hydrogen peroxide, the value is small. (See Reference Examples 1 and 2 below).

2. Nonaqueous Secondary Battery Electrode Mixture Layer The second aspect of the present invention is a nonaqueous electrolyte secondary battery electrode mixture layer (hereinafter, also simply referred to as “electrode mixture layer”) using the carbon fiber. This electrode mixture layer contains at least an electrode active material, the carbon fiber of the present invention, and a binder. The electrode mixture layer of the present invention may further contain other carbon-based conductive assistant.

  The thickness (film thickness) of the electrode mixture layer of the present invention is not particularly limited, but is preferably 50 μm or more, more preferably 70 μm or more, further preferably 90 μm or more, and 100 μm or more. It is particularly preferred. The upper limit of the film thickness is not particularly limited, but is generally less than 1000 μm and particularly preferably less than 800 μm. When the film thickness is less than 50 μm, when an arbitrary capacity cell is to be manufactured, a large amount of separator and current collector are used, and the volume occupancy of the active material layer in the cell is reduced. This is not preferable from the viewpoint of energy density, and its application is considerably limited. When the film thickness is 1000 μm or more, cracks are likely to occur in the electrode mixture layer, and production is relatively difficult. Further, when the film thickness is 1000 μm or more, the transport of Li ions is likely to be hindered, and the resistance is likely to increase. Although it does not specifically limit as a measuring method of the film thickness of an electrode mixture layer, For example, it can measure using a micrometer.

  As a non-aqueous electrolyte secondary battery manufactured using the electrode mixture layer of the present invention, a lithium ion secondary battery is a typical battery. Hereinafter, the positive electrode active material and the negative electrode active material used for the lithium ion secondary battery will be described.

<Positive electrode active material>
As the positive electrode active material contained in the electrode mixture layer of the present invention, any one or more kinds of conventionally known materials known as positive electrode active materials in nonaqueous electrolyte secondary batteries are used. It can be appropriately selected and used. For example, in the case of a lithium ion secondary battery, a lithium-containing metal oxide capable of inserting and extracting lithium ions is suitable. The lithium-containing metal oxide is a composite oxide containing lithium and at least one element selected from the group consisting of Co, Mg, Mn, Ni, Fe, Al, Mo, V, W, Ti, and the like. Can be mentioned.

_{Specifically, Li x CoO 2, Li x} NiO 2, Li x MnO 2, Li x Co a Ni 1-a O 2, Li x Co b V 1-b O z, Li x Co b Fe 1-b _{O 2, Li x Mn 2 O} 4, Li x Mn c Co 2-c O 4, Li x Mn c Ni 2-c O 4, Li x Mn c V 2-c O 4, Li x Mn c Fe 2- _c O _4, (where, x = 0.02~1.2, a = 0.1~0.9 , b = 0.8~0.98, c = 1.2~1.96, z = At least one selected from the group consisting of 2.01 to 2.3. Preferred lithium-containing metal _{oxides, Li x CoO 2, Li x} NiO 2, Li x MnO 2, Li x Co a Ni 1-a O 2, Li x Mn 2 O 4, Li x Co b V 1-b Mention may be made of at least one selected from the group consisting of O _z (where x, a, b and z are the same as above). In addition, the value of x is a value before the start of charging / discharging, and increases / decreases by charging / discharging.

  The said positive electrode active material may be used individually by 1 type, and may be used in combination of 2 or more type. The average particle diameter of the positive electrode active material is preferably 10 μm or less, more preferably 0.05 to 7 μm, and further preferably 1 to 7 μm. If the average particle size exceeds 10 μm, the efficiency of the charge / discharge reaction under a large current may be reduced.

  The content of the positive electrode active material in the electrode mixture layer of the present invention is preferably 60% by mass or more, more preferably 70 to 98.5% by mass, and 75 to 98.5% by mass. Is more preferable. When the amount is less than 60% by mass, it may be difficult to apply to a power source application requiring high energy density. When it exceeds 98.5 mass%, the amount of the binder is too small, and cracks may be generated in the electrode mixture layer, or the electrode mixture layer may be peeled off from the current collector. Furthermore, the conductivity of the electrode mixture layer may be insufficient because the content of carbon fiber or carbon-based conductive aid is too small.

<Negative electrode active material>
As the negative electrode active material contained in the electrode mixture layer of the present invention, any one or more of conventionally known materials known as negative electrode active materials in non-aqueous electrolyte secondary batteries are used. It can be appropriately selected and used. For example, as a material capable of inserting and extracting lithium ions, a carbon material, Si and Sn, or an alloy or oxide containing at least one of them can be used. Among these, a carbon material is preferable from the viewpoint of cost and the like. Examples of the carbon material include artificial graphite produced by heat-treating natural graphite, petroleum-based or coal-based coke, hard carbon obtained by carbonizing a resin, mesophase pitch-based carbon material, and the like.

  When natural graphite or artificial graphite is used, one having a (002) plane spacing d (002) of the graphite structure by powder X-ray diffraction in the range of 0.335 to 0.337 nm from the viewpoint of increasing battery capacity. preferable. Natural graphite refers to a graphite material that is naturally produced as ore. Natural graphite is classified into two types according to its appearance and properties: scaly graphite with high crystallinity and earthy graphite with low crystallinity. The scaly graphite is further divided into scaly graphite having a leaf-like appearance and scaly graphite having a lump shape. There are no particular restrictions on the origin, properties, and types of natural graphite used as the graphite material. Further, natural graphite or particles produced using natural graphite as a raw material may be subjected to a heat treatment.

  Artificial graphite refers to graphite made by a wide variety of artificial techniques and graphite materials that are close to perfect crystals of graphite. As a typical example, a material obtained from a tar and coke obtained from dry distillation of coal, a residue obtained by distillation of crude oil, and the like, through a firing process of about 500 to 1000 ° C. and a graphitization process of 2000 ° C. or more is used. Can be mentioned. Kish graphite obtained by reprecipitation of carbon from dissolved iron is also a kind of artificial graphite.

The use of an alloy containing at least one of Si and Sn in addition to the carbon material as the negative electrode active material results in a higher electric capacity than when using each of Si and Sn alone or using each oxide. This is effective in that it can be made smaller. Among these, Si-based alloys are preferable. As the Si-based alloy, at least one element selected from the group consisting of B, Mg, Ca, Ti, Fe, Co, Mo, Cr, V, W, Ni, Mn, Zn and Cu, Si, And alloys thereof. Specifically, SiB ₄ , SiB ₆ , Mg ₂ Si, Ni ₂ Si, TiSi ₂ , MoSi ₂ , CoSi ₂ , NiSi ₂ , CaSi ₂ , CrSi ₂ , Cu ₅ Si, FeSi ₂ , MnSi ₂ , VSi ₂ , There may be mentioned at least one selected from the group consisting of WSi ₂ , ZnSi _{2 and the} like.

  In the electrode mixture layer of the present invention, the above-described materials may be used alone or in combination of two or more as the negative electrode active material. The average particle diameter of the negative electrode active material is 10 μm or less. When the average particle diameter exceeds 10 μm, the efficiency of the charge / discharge reaction under a large current is reduced. The average particle diameter is preferably 0.1 to 10 μm, and more preferably 1 to 7 μm.

<Binder>
As the binder used in the electrode mixture layer of the present invention, any binder can be used as long as it is electrode-moldable and has sufficient electrochemical stability. As such a binder, polyvinyl alcohol, polyacrylic acid, carboxymethyl cellulose, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), styrene butadiene rubber (SBR), fluoroolefin copolymer crosslinked polymer, polyimide, polyamideimide, It is preferable to use one or more selected from the group consisting of aramid, phenolic resin, etc., and polyvinylidene fluoride (PVDF) is particularly preferable. There are no particular restrictions on the form used as a binder, which may be solid or liquid (eg, emulsion), electrode manufacturing method (especially dry or wet kneading), dissolution in electrolyte It can be appropriately selected in consideration of the properties and the like.

  The binder content in the electrode mixture layer of the present invention is preferably 1 to 25% by mass, more preferably 3 to 15% by mass, and even more preferably 5 to 10% by mass. When the content is less than 1% by mass, cracks may occur in the electrode mixture layer, or the electrode mixture layer may peel from the current collector. When it exceeds 25 mass%, the amount of the active material in the electrode decreases, and the energy density of the obtained battery tends to decrease.

(Carbon-based conductive additive other than the carbon fiber of the present invention)
The electrode mixture layer of the present invention can also contain a carbon-based conductive additive in addition to the carbon fiber of the present invention. Examples of carbon-based conductive assistants other than carbon fibers include carbon black, acetylene black, carbon nanotubes, VGCF, scaly carbon, graphene, and graphite. These carbon-based conductive assistants may be used alone or in combination of two or more.

The shape of these carbon-based conductive assistants is not particularly limited, but is preferably fine particles. The average particle size (primary particle size) of the carbon-based conductive aid is preferably 10 to 200 nm, and more preferably 20 to 100 nm. The aspect ratio of these carbon-based conductive assistants is 10 or less, preferably 1 to 5, and more preferably 1 to 3.
The content of the carbon-based conductive additive other than carbon fibers in the electrode mixture layer of the present invention is preferably 0.5 to 5% by mass, more preferably 0.5 to 4% by mass. More preferably, it is -3 mass%.

3. Nonaqueous Electrolyte Secondary Battery Electrode The third aspect of the present invention is a nonaqueous electrolyte secondary battery electrode (hereinafter, also simply referred to as “electrode”) in which the electrode mixture layer is formed. This electrode is formed by forming the electrode mixture layer of the present invention on the surface of a current collector.

The following two methods are common as a method for producing an electrode including the electrode mixture layer of the present invention. One method is to mix and knead the electrode active material, the carbon fiber, the binder, and other components as necessary, form a film by extrusion, roll and stretch the film, and then bond the current collector. It is a method.
Another method is to prepare a slurry by mixing the electrode active material, the carbon fiber, the binder, a solvent for dissolving the binder, and other components as necessary, and apply the slurry to the surface of the current collector. This is a method of pressing after removing the solvent.
In the present invention, either method can be adopted, but the latter method is preferable, and the latter method will be described in detail below.

  The solid content concentration in the slurry (referring to the ratio of the total mass of components other than the solvent in the slurry to the total mass of the slurry) is preferably 10 to 30% by mass, and preferably 15 to 25% by mass. Is more preferable. When solid content concentration exceeds 30 mass%, uniform slurry preparation may be difficult. Moreover, when the solid content concentration is less than 10% by mass, the viscosity of the slurry is insufficient, and the thickness of the electrode mixture layer laminated on the current collector may become uneven.

  The solvent used in the slurry is not particularly limited as long as it is a solvent that dissolves the binder. Specifically, one or more kinds selected from the group consisting of N-methylpyrrolidone (NMP), dimethylacetamide (DMAc), dimethylformamide (DMF), dimethylsulfoxide (DMSO) and the like can be mentioned, and particularly NMP or DMAc is preferred.

  When producing an electrode, if the thixotropy in the slurry is too strong, it may be difficult to ensure fluidity suitable for coating. In such a case, a slurrying aid may be used. Examples of the slurrying aid include one or more selected from the group consisting of polyvinylpyrrolidone, carboxymethylcellulose, polyvinyl acetate, polyvinyl alcohol and the like. In particular, it is preferable to use polyvinylpyrrolidone. By adding the above-mentioned slurrying aid, sufficient fluidity can be ensured even with a small amount of solvent, and the dispersibility of the carbon-based conductive aid can be remarkably improved. Moreover, generation | occurrence | production of the crack after solvent removal can also be suppressed. The addition amount of the slurrying aid is preferably 10% by mass or less, more preferably 0.5 to 10% by mass with respect to the total of components other than the solvent in the slurry, More preferably, it is -8 mass%. If the addition amount of the slurrying aid exceeds 10% by mass, on the contrary, the slurry viscosity is drastically lowered, resulting in poor dispersion, and it may be difficult to prepare a suitable slurry. When the addition amount of the slurrying aid is less than 0.5% by mass, the effect of the slurrying aid is difficult to appear.

  The slurry is applied to the surface of a current collector described later. As a coating method, an appropriate coating method such as a doctor blade can be employed. After the application, for example, the solvent is removed by heat treatment at 60 to 100 ° C., preferably 75 to 85 ° C., preferably for 60 to 180 minutes. Then, the electrode of this invention can be manufactured by pressing the coating material after solvent removal. Preferable pressing conditions are 1 to 5 minutes under a pressure of 10 to 30 Pa.

As the current collector constituting the electrode, any conductive material can be used. For example, a metal material such as aluminum, nickel, iron, stainless steel, titanium, or copper can be used. In particular, aluminum, stainless steel or copper is preferable, and aluminum or aluminum coated with carbon is more preferably used.
The thickness of the current collector is preferably 10 to 50 μm.

4). Nonaqueous electrolyte secondary battery The fourth aspect of the present invention is a nonaqueous electrolyte secondary battery including the electrode of the present invention.
The non-aqueous electrolyte secondary battery of the present invention is configured by using the electrode of the present invention for the positive electrode and / or the negative electrode, and using a known separator or electrolyte. As the nonaqueous electrolyte secondary battery, a lithium ion secondary battery is exemplified.

  The nonaqueous electrolyte secondary battery of the present invention comprises a positive electrode in which a positive electrode material layer is formed on the surface of a current collector, an electrolyte layer containing an electrolyte, and an electrode (negative electrode) of the present invention. The layers are laminated so that the negative electrode material layer of the negative electrode layer faces the negative electrode material layer, and the electrolyte layer is inserted between the positive electrode material layer and the negative electrode material layer. Alternatively, the electrode (positive electrode) of the present invention, an electrolyte layer containing an electrolyte, and a negative electrode material layer are formed on the surface of the current collector, and the positive electrode material layer of the positive electrode and the negative electrode material layer of the negative electrode face each other. And it laminates | stacks so that an electrolyte layer may be inserted between a positive electrode material layer and a negative electrode material layer. Or it is comprised from the electrode (positive electrode) of this invention, the electrolyte layer containing electrolyte, and the electrode (negative electrode) of this invention, the positive electrode material layer of a positive electrode and the negative electrode material layer of a negative electrode face each other, and a positive electrode material layer and a negative electrode Lamination is performed such that an electrolyte layer is inserted between the material layers.

  The cell shape of the nonaqueous electrolyte secondary battery of the present invention is not particularly limited, and can be implemented in any shape. Specifically, for example, cell shapes such as a button shape, a cylindrical shape, and a square shape can be given. Further, it is also preferable to have an internal configuration in which a plurality of pairs of positive and negative electrodes and separators are laminated. In this case, a known stack lamination type, wound type, folded lamination type, or the like can be adopted. Examples of the exterior material of the nonaqueous electrolyte secondary battery of the present invention include a metal can, an aluminum laminated resin film, and the like. In the non-aqueous electrolyte secondary battery of the present invention, the predetermined carbon fiber added to the electrode mixture layer has a linear structure and has high conductivity, so that it is easy to form a conductive path. Excellent charge / discharge characteristics can be obtained. Furthermore, the electrode strength is also improved.

<Electrolyte layer>
As the electrolyte layer constituting the non-aqueous electrolyte secondary battery, a non-aqueous electrolyte solution in which an electrolyte such as a lithium salt is dissolved in a non-aqueous solvent is used.
The electric conductivity at 25 ° C. of the electrolytic solution used in the nonaqueous electrolyte secondary battery of the present invention is preferably 1 × 10 ⁻² S / cm or more.

  In general, non-aqueous electrolytes are characterized by a higher withstand voltage and higher energy density than aqueous electrolytes. As the non-aqueous solvent, known ones can be used without limitation, but propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, methyl ethyl carbonate, dimethoxyethane, γ-butyrolactone, γ-valerolactone, acetonitrile, Nitromethane, methoxyacetonitrile, nitroethane, N, N-dimethylformamide, 3-methoxypropionitrile, N-methylpyrrolidone, N, N'-dimethylimidazolidinone, dimethyl sulfoxide, sulfolane, 3-methylsulfolane, ethyl methyl carbonate, etc. Is mentioned. These nonaqueous solvents may be used alone or in combination of two or more. It is important that the solvent used in the electrolytic solution has an appropriate boiling point, melting point, viscosity, and relative dielectric constant, and among these, those mainly composed of propylene carbonate or γ-butyrolactone are preferably used.

Examples of the electrolyte used for the non-aqueous electrolyte secondary battery of the present invention include LiClO ₄ , LiBF ₄ , LiPF ₆ , LiAlCl ₄ , LiSbF ₆ , LiSCN, LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , LiAsF ₆ , lower aliphatic. Examples thereof include lithium carboxylate, LiCl, LiBr, LiI, lithium chloroborane, borates, and imide salts. Examples of borates include lithium bis (1,2-benzenediolate (2-)-O, O ') and bis (2,3-naphthalenedioleate (2-)-O, O') boric acid. Lithium, bis (2,2′-biphenyldiolate (2-)-O, O ′) lithium borate, bis (5-fluoro-2-olate-1-benzenesulfonic acid-O, O ′) lithium borate Etc. Examples of imide salts include lithium bistrifluoromethanesulfonate imide ((CF ₃ SO ₂ ) ₂ NLi), lithium trifluoromethanesulfonate nonafluorobutanesulfonate (LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ) ), Lithium bispentafluoroethanesulfonate imide ((C ₂ F ₅ SO ₂ ) ₂ NLi), and the like. The concentration of the electrolyte is preferably 0.5 to 2 mol / L. One of the above electrolytes may be used alone, or two or more electrolytes may be used in combination. For the purpose of improving cycle stability and charge / discharge efficiency, a known additive may be added to the electrolyte.
As an electrolytic solution used in the lithium ion secondary battery of the present invention, an ionic liquid typified by ethylmethylimidazolium salt can also be suitably used. In this case, the electrolyte must be dissolved in the above nonaqueous solvent. There is no need to use it.

<Separator>
When using the non-aqueous electrolyte as described above, it is common to use a separator in order to prevent the negative electrode active material layer and the electrode mixture layer of the present invention from coming into direct contact. As the shape of the separator, a known shape such as a paper shape (film shape) or a porous film shape can be suitably employed. As the material of the separator, for example, one or more materials selected from the group consisting of cellulose, aromatic polyamide, aliphatic polyimide, polyolefin, Teflon (registered trademark), polyphenylene sulfide, and the like can be suitably used. Among these, cellulose paper, aromatic polyamide or aliphatic polyimide porous membrane is preferable from the viewpoint of heat resistance and thinning. The thickness of the separator is preferably about 20 to 100 μm from the viewpoint of preventing a short circuit, but in the present invention, a separator having a thickness of about 5 to 20 μm that is sufficiently thinner than a conventional separator can be applied. When the thin separator is used, the internal resistance derived from the separator is reduced, so that the output is improved and the energy density of the cell is also improved.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further more concretely, this invention is not limited at all by this. Various measurements and analyzes in the examples were performed according to the following methods.

(1) Shape confirmation of carbon fiber Observation and photography were performed using a tabletop electron microscope (manufactured by JEOL Ltd., model NeoScope JCM-6000). The average fiber diameter of carbon fibers, etc. was measured by randomly selecting 300 locations from the obtained electron micrograph, and the average value of all the measurement results (n = 300) was taken as the average fiber diameter. . The average fiber length was similarly calculated.

(2) X-ray diffraction measurement of carbon fiber X-ray diffraction measurement is based on JIS R7651 method using RINT-2100 manufactured by Rigaku Corporation, lattice spacing (d002), crystallite size (Lc) (hexagonal mesh area layer) Direction) and (La) (carbon hexagonal network surface direction).

(3) Measuring method of powder volume resistivity The powder volume resistivity was measured using a powder resistance system (MCP-PD51) manufactured by Dia Instruments Co., Ltd. under a load of 0.25 to 2.50 kN. Measurement was performed using an electrode unit of the type. The volume resistivity was defined as the powder volume resistivity of the sample based on the relationship diagram of the volume resistivity accompanying the change in the packing density, and the value of the volume resistivity when the packing density was 0.8 g / cm ³ .

(4) Strength ratio between O—C bond and O═C bond (O═C / O—C) and measuring method of oxygen content Strength between O—C bond and O═C bond on the surface of carbon fiber The ratio (O = C / O-C) and the oxygen content were measured using XPS (Thermo Scientific K-Alpha, X-ray source; Al-Kα monochrome (1486.7 eV), X-ray spot size: 400 μm, Flood Gun on, degree of vacuum; <5 × 10 ⁻⁸ mbar, detection angle 0 degree, measurement; elemental measurement (Narrow Scan Ols).

(5) Wettability evaluation In order to confirm the affinity for the electrolytic solution, the wettability was evaluated. Propylene carbonate is used as the electrolytic solution, and the evaluation apparatus uses a dynamic wettability tester WET-6200 (RHESCA), soaking time is 1 mm / s, the immersion depth is 0.5 mm, and the immersion time is 300 sec. The wettability was evaluated.
An evaluation sample (carbon fiber) was placed in a powder sample holder and tapped until the powder height did not change to prepare a powder evaluation sample. Immersion was performed under the above conditions, and wettability was evaluated from the immersion rate method.
○: Immersion speed 0.01 or more ×: Immersion speed less than 0.01

(6) Conductivity measurement Using a potentiostat / galvanostat (HA-151 manufactured by Hokuto Denko Co., Ltd.), the electrode resistance in the film thickness direction of the prepared electrode is measured, and the electric conductivity calculated from the resistance value is measured. Asked.

[Examples 1 and 2]
High density polyethylene as thermoplastic resin (manufactured by Prime Polymer Co., Ltd., Hi-Zex 5000SR; melt viscosity of 14 Pa · s at 350 ° C., 600 s ⁻¹ ) and 90 parts by mass, and mesophase pitch AR-MPH (manufactured by Mitsubishi Gas Chemical Co., Ltd.) ) 10 parts by mass was melt kneaded with a unidirectional twin-screw extruder (TEM-26SS manufactured by Toshiba Machine Co., Ltd., barrel temperature 310 ° C., under nitrogen stream) to prepare a mesophase pitch composition. In the mesophase pitch composition obtained under these conditions, the mesophase pitch was dispersed in the thermoplastic resin with a dispersion diameter of 0.05 to 2 μm. The mesophase pitch composition was held at 300 ° C. for 10 minutes, but no aggregation of mesophase pitch was observed, and the dispersion diameter was 0.05 to 2 μm.
Next, a resin composite fiber (long fiber) was produced from the die having a diameter of 0.2 mm using the melt spinning machine under the condition of the die temperature of 390 ° C.
Next, a short fiber having a length of about 5 cm was produced from the resin composite fiber (long fiber). The short fibers were arranged in a non-woven shape on a wire mesh having an opening of 1.46 mm and a wire diameter of 0.35 mm so as to have a basis weight of 30 g / m ² .

The nonwoven fabric made of resin composite stabilized fiber was prepared by holding the nonwoven fabric made of resin composite fiber in a hot air dryer at 215 ° C. for 3 hours. Next, after performing nitrogen substitution in a vacuum gas substitution furnace, the pressure was reduced to 1 kPa, and the thermoplastic resin was removed by heating from this state. As heating conditions, the temperature was raised to 500 ° C. at a rate of temperature rise of 5 ° C./min, and then held at the same temperature for 60 minutes. The nonwoven fabric made of the stabilized fibers was added to an ethanol solvent, and the stabilized fibers were dispersed in the solvent by applying vibration for 30 minutes with an ultrasonic oscillator. A nonwoven fabric composed of the stabilized fibers was produced by filtering the stabilized fibers dispersed in the solvent.
The nonwoven fabric composed of the stabilized fibers was heated to 1000 ° C. at 5 ° C./min under a nitrogen gas flow in a vacuum gas replacement furnace, heat-treated at the same temperature for 0.5 hours, and then cooled to room temperature. Next, this non-woven fabric is placed in a graphite crucible and is heated from room temperature in a vacuum using an ultra-high temperature furnace (manufactured by Kurata Giken Co., Ltd., SCC-U-80 / 150 type, soaking section 80 mm (diameter) × 150 mm (height)). The temperature was raised to 2000 ° C. at 10 ° C./min. After reaching 2000 ° C., an atmosphere of argon gas (99.999%) of 0.05 MPa (gauge pressure) is used, and then at a temperature increase rate of 10 ° C./min, 2400 ° C. (in the case of Example 2) or 2800 ° C. (Example) 1), and graphitized by heat treatment at each temperature for 0.5 hour to obtain carbon fibers. The average fiber diameter, CV value, average aspect ratio (L / D) and crystallite size d002 of the obtained carbon fiber were measured, and the results are shown in Table 1.
Moreover, the result of the peak derived from O1s orbit of XPS measurement was shown in FIG. 1 (Example 1) and FIG. 2 (Example 2). As a result of peak separation, the peak that appears in the vicinity of 533 eV is derived from the C—O bond, and the peak in the vicinity of 533 eV is C═O. The intensity ratio is also as shown in Table 1. .

  Further, the volume resistance value and the wettability to the electrolytic solution were measured, and the results are shown in Table 2. The obtained carbon fiber was an excellent carbon fiber having high conductivity and high affinity for the electrolytic solution.

2 parts by mass of the obtained carbon fiber as a carbon-based conductive additive, 91 parts by mass of a positive electrode active material (LiFePO ₄ ; manufactured by Hosen Co., Ltd., SLFP-ES01), and polyvinylidene fluoride (manufactured by Kureha Co., Ltd., W A slurry was prepared using 7 parts by mass of # 7200) and N-methylpyrrolidone as a solvent. The prepared slurry was applied to a current collector (15 μm thick aluminum foil) to prepare an electrode. The electrode thickness was 120 μm and the electrode density was 2.5 g / cm ³ . The results of the conductivity of the obtained electrode are shown in Table 2. Also from this result, the obtained carbon fiber was excellent in electrical conductivity, and had a low CV value of electrical conductivity and high homogeneity.

[Comparative Example 1]
Carbon fibers were produced under the same conditions as in Example 1 except that the final graphitization temperature was 3000 ° C. The average effective fiber length and crystallite size of the obtained carbon fiber were measured, and the results are shown in Table 1.
Moreover, the result of the peak derived from the O1s orbit of the XPS measurement is shown in FIG. As a result of peak separation, the peak that appears in the vicinity of 533 eV is derived from the C—O bond, and the peak in the vicinity of 533 eV is C═O. The intensity ratio is also as shown in Table 1. .
The volume resistance value and wettability to the electrolyte were measured, and the results are shown in Table 2. The obtained carbon fiber was a carbon fiber having excellent conductivity but low affinity with the electrolytic solution.

  Using the obtained carbon fiber, an electrode was produced under the same conditions as in Example 1 and the conductivity was measured. The results are shown in Table 2. Also from this result, although the obtained carbon fiber was excellent in electrical conductivity, the CV value of electric conductivity was high and the homogeneity was slightly inferior.

[Comparative Example 2]
Carbon fibers were produced under the same conditions as in Example 1 except that the final graphitization temperature was 2000 ° C. The average effective fiber length and crystallite size of the obtained carbon fiber were measured, and the results are shown in Table 1. The result of the peak derived from the O1s orbit of the XPS measurement is shown in FIG. Further, the volume resistance value and the wettability to the electrolytic solution were measured, and the results are shown in Table 2. The obtained carbon fiber was a carbon fiber having excellent electrolyte affinity but low conductivity.

  Using the obtained carbon fiber, an electrode was produced under the same conditions as in Example 1 and the conductivity was measured. The results are shown in Table 2. Also from this result, the obtained carbon fiber had low conductivity.

[Reference Example 1]
The carbon fiber obtained in Comparative Example 1 was put into hydrogen peroxide solution (hydrogen peroxide concentration 30%), mixed for 3 hours, washed with ion-exchanged water, and then dried by filtration. The value of the surface O / C of the obtained carbon fiber was 0.018.

[Reference Example 2]
The carbon fiber obtained in Comparative Example 1 was put into a mixed acid (150 ml of concentrated sulfuric acid (95%) and 50 ml of concentrated nitric acid (60 to 61% mixed solution), mixed for 1 hour, washed with ion-exchanged water, Subsequently, the carbon fiber was dried by filtration, and the value of the surface O / C of the obtained carbon fiber was 0.167.

Claims (7)

The surface oxygen concentration ratio (O / C) measured by X-ray photoelectron spectroscopy is 0.02 or more and 0.15 or less,
CV value of fiber diameter is 10-50%,
A pitch-based ultrafine carbon fiber having a crystallite size (d002) measured by an X-ray diffraction method of 0.340 nm or less.   2. The pitch system according to claim 1, wherein the intensity ratio (O═C / O—C) of O—C bonds derived from O1s orbitals and O═C bonds as measured by X-ray photoelectron spectroscopy is 1.4 or more. Extra fine carbon fiber.   The pitch-based ultrafine carbon fiber according to claim 1 or 2, wherein the average fiber diameter is 100 nm or more and less than 700 nm.   The pitch ultrafine carbon fiber according to any one of claims 1 to 3, wherein the average aspect ratio is 30 to 1,000. The pitch-based ultrafine carbon fiber according to any one of claims 1 to 4,
An electrode active material;
A binder,
An electrode mixture layer for a nonaqueous electrolyte secondary battery. A current collector,
The electrode mixture layer for a nonaqueous electrolyte secondary battery according to claim 5 laminated on the current collector;
An electrode for a nonaqueous electrolyte secondary battery.   The nonaqueous electrolyte secondary battery comprised including the electrode for nonaqueous electrolyte secondary batteries of Claim 6.

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