JP7240801B2 - Positive electrode mixture layer for non-aqueous electrolyte secondary battery, positive electrode for non-aqueous electrolyte secondary battery containing the same, and non-aqueous electrolyte secondary battery - Google Patents

Description

TECHNICAL FIELD The present invention relates to an electrode mixture layer for a nonaqueous electrolyte secondary battery, an electrode for a nonaqueous electrolyte secondary battery on which the mixture layer is formed, and a nonaqueous electrolyte secondary battery having the electrode. Specifically, it has a low-density electrode mixture layer for a non-aqueous electrolyte secondary battery containing a predetermined carbon-based conductive agent, an electrode for a non-aqueous electrolyte secondary battery in which the mixture layer is formed, and the electrode It relates to a non-aqueous electrolyte secondary battery.

A lithium ion secondary battery, which is a type of non-aqueous electrolyte secondary battery, is a secondary battery in which lithium ions in the electrolyte are responsible for electrical conduction. , which uses a carbon material as an electrode active material. Lithium-ion secondary batteries are characterized by high energy density among secondary batteries, so their application range is expanding from small devices such as mobile phones to large devices such as electric vehicles.

Lithium ion batteries are required to have even higher energy densities. As one method, it is conceivable to form a thick electrode mixture layer to increase the supported amount of the electrode active material. However, one of the problems associated with increasing the thickness of the electrode mixture layer is that the conductive path to the current collector is not sufficiently formed, increasing the electrical resistance of the electrode and increasing the capacity of the battery at high output. There is a problem that the retention rate decreases. Increasing the electrode density has been proposed as a method for reducing the electrode resistance. However, it is also described that if the electrode density is increased too much, the porosity decreases and diffusion of substances is inhibited, leading to an increase in resistance (see Patent Document 1). In other words, it is considered that there is a limit to the resistance reduction by optimizing the electrode density.

As one means of solving this problem, Patent Document 2 discloses that a battery with good characteristics can be obtained by fabricating an electrode by combining a low-density electrode mixture layer and a high-density electrode mixture layer. has been reported. However, since the electrodes are made using the electrode mixture layers having two different densities, there is a problem that the electrode manufacturing process is complicated.

Patent Document 3 describes that the mixture density of the positive electrode mixture layer may be 1.5 g/cc to 4 g/cc (paragraph 0014).

JP 2010-15904 A JP 2013-251213 A JP 2014-93295 A

An object of the present invention is to provide an electrode mixture layer with good properties that can be produced more easily than before, an electrode for a non-aqueous electrolyte secondary battery on which the mixture layer is formed, and a non-aqueous electrolyte having the electrode. It is to provide a secondary battery.

As a result of intensive studies in view of the above-mentioned conventional technology, the present inventors focused on the composition of the electrode mixture layer, and by using a specific carbon-based conductive additive, the electrode The inventors have found that good electrical conductivity and sufficient performance can be obtained, and have completed the present invention.

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

[1] An electrode mixture layer containing at least an electrode active material and a carbon-based conductive aid having an average diameter of 10 nm or more, wherein the carbon-based conductive aid has a volume content of 0.9 to 5.0. (%) and the density of the electrode mixture layer is 0.50 (g/cm ³ ) or more and less than 2.10 (g/cm ³ ). mixture layer.

[2] The electrode mixture layer for a non-aqueous electrolyte secondary battery according to [1], which does not contain activated carbon.

[3] The electrode mixture layer for a non-aqueous electrolyte secondary battery according to [1] or [2], wherein the carbon-based conductive additive is fibrous carbon having an average effective fiber length of 10 μm or more.

[4] The electrode mixture layer for a nonaqueous electrolyte secondary battery according to [3], wherein the fibrous carbon has a crystallite length (La) measured by X-ray diffraction of 110 to 500 nm.

[5] The electrode mixture layer for a nonaqueous electrolyte secondary battery according to any one of [1] to [4], which has a thickness of 50 to 1000 μm.

[6] a current collector;
an electrode mixture layer for a non-aqueous electrolyte secondary battery according to any one of [1] to [5] laminated on the current collector;
An electrode for a non-aqueous electrolyte secondary battery comprising:

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

According to the present invention, an electrode mixture layer for a non-aqueous electrolyte secondary battery having good electrode conductivity is obtained by a simpler process than an electrode mixture layer for a non-aqueous electrolyte secondary battery obtained by conventional techniques. be able to.

The present invention will be described in detail below.

1. Electrode mixture layer for non-aqueous electrolyte secondary battery of the present invention (hereinafter simply referred to as "positive electrode mixture layer", "negative electrode mixture layer" or "electrode mixture layer" for non-aqueous electrolyte secondary battery) ) contains at least an electrode active material and a carbon-based conductive aid, and the volume content of the carbon-based conductive aid is in the range of 0.9 to 5.0 (%). The electrode mixture layer of the present invention can be applied to both the positive electrode mixture layer and the negative electrode mixture layer.

The density (25° C.) of the electrode mixture layer of the present invention is in the range of 0.50 (g/cm ³ ) or more and less than 2.10 (g/cm ³ ). If the density of the electrode mixture layer is less than 0.50 (g/cm ³ ), the porosity is too high and sufficient battery characteristics cannot be obtained. If it is 2.10 (g/cm ³ ) or more, the porosity is too low, causing an increase in electrical resistance and poor battery characteristics. The lower limit of the density is preferably 0.95 (g/cm ³ ) or more, more preferably 1.00 (g/cm ³ ) or more. The upper limit of the density is preferably less than 2.07 (g/cm ³ ), more preferably less than 2.00 (g/cm ³ ), and less than 1.80 (g/cm ³ ). more preferably less than 1.70 (g/cm ³ ), particularly preferably less than 1.60 (g/cm ³ ).

The thickness (film thickness) of the electrode mixture layer of the present invention is preferably 50 µm or more, more preferably 60 µm or more, still more preferably 70 µm or more, and even more preferably 80 µm or more. , is particularly preferably 90 μm or more, more preferably 100 μm or more, especially more than 100 μm, and most preferably 120 μm or more. The thickness (film thickness) of the electrode mixture layer of the present invention is not particularly limited, but is preferably 1000 μm or less, more preferably less than 1000 μm, even more preferably less than 900 μm, and less than 800 μm. It is particularly preferred to have The thickness (film thickness) of the electrode mixture layer of the present invention is 50 to 1000 μm, preferably 80 to 1000 μm, more preferably 100 to 1000 μm, particularly 120 to 1000 μm. preferable.

If the film thickness of the electrode mixture layer is less than 50 μm, a large amount of separators and current collectors will be used when an attempt is made to manufacture an arbitrary capacity cell, and the volume occupation ratio of the electrode mixture layer in the cell will be reduced. decreases. This is unfavorable from an energy density point of view and considerably limits the application. In particular, it becomes difficult to apply to power supply applications that require high energy density. On the other hand, an electrode with an electrode mixture layer having a thickness of more than 1000 μm is relatively difficult to manufacture because cracks are likely to occur in the electrode mixture layer. From the viewpoint of stable production of electrodes, the thickness of the electrode mixture layer is preferably 1000 μm or less. Further, in an electrode with an electrode mixture layer having a film thickness exceeding 1000 μm, Li ion transport is likely to be inhibited, leading to an increase in resistance. Therefore, the film thickness of the electrode mixture layer is preferably 1000 μm or less from the viewpoint of resistance reduction. In addition, the thickness of the electrode mixture layer in the present invention means the thickness of the electrode mixture layer only, not including the thickness of the current collector described later.
Although the method for measuring the film thickness of the electrode mixture layer is not particularly limited, it can be measured using, for example, a micrometer.

(1-1) Carbon-Based Conductive Aid The electrode mixture layer of the present invention contains a carbon-based conductive aid at a predetermined volume content. Examples of carbon-based conductive aids include particulate carbon such as acetylene black and carbon black, single-walled carbon nanotubes, multi-walled carbon nanotubes, carbon nanofibers, carbon nanohorns, fibrous carbon such as carbon fibers, flake carbon, and graphene. , graphite and the like. The carbon-based conductive additive contained in the electrode mixture layer of the present invention is not particularly limited as long as the effect of the present invention is exhibited, and includes one or more of these carbon-based conductive additives. I wish I could.
Among the above, fibrous carbon has a good balance between handleability and conductivity. Although the shape of the fibrous carbon is not particularly limited, it preferably has a linear structure with substantially no branches. Branching means that the main axis of fibrous carbon is branched in the middle, or that the main axis of fibrous carbon has branched sub-axes. A linear structure having substantially no branches means that the degree of branching of fibrous carbon is 0.01/μm or less. The fibrous carbon may have a fibrous shape as a whole. (For example, those in which spherical carbons are linked in a beaded shape, and those in which at least one or a plurality of very short fibers are connected by fusion or the like) are also included.

The electrode mixture layer of the present invention preferably does not contain activated carbon. The reason for this is that if activated carbon is contained, the viscosity increases when forming the electrode mixture layer, which is not preferable in terms of the process.

The volume content at 25° C. of the carbon-based conductive additive in the electrode mixture layer of the present invention is 0.9 to 5.0 (%) based on the volume of the electrode mixture layer. By setting the volume content within this range, the conductivity of the electrode mixture layer can be increased. If it exceeds 5.0%, the proportion of the active material is reduced, and the capacity of the battery is also reduced. The lower limit of the volume content is preferably 1.0% or more, more preferably 1.1% or more, further preferably 1.2% or more, and 1.3% or more. is particularly preferred. Further, the upper limit of the volume content is preferably 4.0% or less, more preferably 3.0% or less, further preferably 2.5% or less, and 2.0% or less. It is particularly preferred to have

A carbon-based conductive aid having high crystallinity is desirable because it has good electrode conductivity and excellent withstand voltage. The carbon-based conductive additive used in the present invention preferably has a crystallite length (La) of 100 to 500 nm, more preferably 110 to 500 nm, as measured by an X-ray diffraction method, which is an index of crystallinity. , more preferably 150 to 500 nm, particularly preferably 200 to 500 nm. If it is less than 100 nm, the conductivity of the carbon material is not sufficient. On the other hand, the crystallite size is measured by the X-ray diffraction method, but since the measurement error increases as the crystal grows large, the practical limit of measurement is 500 nm.
In the present invention, the crystallite length (La) measured by the X-ray diffraction method is a value measured according to Japanese Industrial Standard JIS R 7651 "Method for measuring the lattice constant and crystallite size of carbon materials" (2007). Say.

The carbon-based conductive additive in the present invention preferably has a crystal plane spacing (d002), which is the distance between adjacent graphite sheets measured by X-ray diffraction, of 0.335 to 0.340 nm, preferably 0.335 to 0.335 nm. More preferably, it is 0.339 nm. Deviating from the range of 0.335 to 0.340 nm is not preferable because the conductivity of the carbon-based conductive aid is remarkably lowered.

In the carbon-based conductive additive in the present invention, the thickness (Lc) of graphene (network plane group) is preferably 1.0 to 130 nm. If it is less than 1.0 nm, the electrical conductivity of the carbon material is significantly lowered, which is not preferable.

The carbon-based conductive additive in the present invention has an average diameter of 10 nm or more. Here, the average diameter refers to the fiber diameter when the carbonaceous conductive additive is fibrous carbon, the diameter when spherical, and the minor axis when flat. That is, it refers to the length of the smallest dimension of the carbon-based conductive additive.

The carbon-based conductive additive used in the present invention is highly effective in forming a conductive network in the electrode mixture layer, and is preferably fibrous carbon from the viewpoint of improving battery output and improving battery durability. The fibrous carbon includes, for example, vapor growth carbon materials such as carbon nanotubes and carbon nanoribbons, but pitch-based carbon fibers are preferable to PAN-based carbon fibers because they must be highly crystalline carbon materials.

The average fiber diameter of the fibrous carbon as the carbon-based conductive additive used in the present invention is preferably 10 to 900 nm. The upper limit is preferably 600 nm or less, more preferably 500 nm or less, even more preferably 400 nm or less, even more preferably 300 nm or less, and particularly preferably 250 nm or less, Most preferably, it is 200 nm or less. The lower limit is preferably 50 nm or more, more preferably 100 nm or more, and even more preferably 150 nm or more.
If the thickness is less than 10 nm, the bulk density is very small and the handleability is poor. Moreover, when forming the electrode mixture layer, the electrode strength tends to decrease. If it exceeds 900 nm, gaps are likely to occur in the electrode mixture layer.
Here, the fiber diameter in the present invention means a value measured from a photographic image taken with a field emission scanning electron microscope at a magnification of 2,000.

The average effective fiber length of fibrous carbon is preferably in the range of 1 to 100 μm, more preferably 1 to 50 μm, even more preferably 5 to 20 μm. If the thickness is less than 1 μm, the conductivity in the electrode mixture layer, the strength of the electrode, and the electrolyte solution holding property are lowered, which is not preferable. If it exceeds 100 μm, the dispersibility of fibrous carbon is impaired, which is not preferable. That is, if the fibrous carbon is too long, the fibrous carbon tends to be 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. In particular, the fibrous carbon of the present invention preferably has an average effective fiber length of 10 μm or more. This is because when the average effective fiber length is 10 μm or more, conductive paths between active materials can be sufficiently formed due to the long effective fiber length even in an electrode mixture layer with a low density as in the present invention. . The average effective fiber length is preferably 12 μm or longer, more preferably 15 μm or longer.

In the present invention, the fiber length of fibrous carbon is defined not by the actual fiber length but by the effective fiber length. This is because the fibrous carbon does not necessarily contribute to the conductivity with the actual fiber length in the electrode mixture layer. For example, fibers may be bent or curled in the electrode mixture layer, and the actual length of the fibers may not contribute to conduction. In the present invention, the effective fiber length of fibrous carbon is defined as the length of the longest line segment whose both ends are in contact with a single piece of fibrous carbon. In other words, it is the maximum linear distance that a single piece of fibrous carbon can conduct. That is, if fibrous carbon has a perfectly linear structure, the effective fiber length is approximately equal to the actual fiber length. When the fibrous carbon has a branched structure or is curled, it refers to the length of the maximum line segment connecting two points on the single fibrous carbon.

The ratio (L/D) of the average effective fiber length (L) to the average fiber diameter (D) of fibrous carbon is preferably 30 or more, more preferably 50 or more. By setting the ratio (L/D) to 30 or more, conductive paths are efficiently formed in the electrode mixture layer, and the obtained battery characteristics can be improved. If it is less than 30, the formation of conductive paths in the electrode mixture layer tends to be insufficient, and the resistance value in the thickness direction of the electrode mixture layer may not sufficiently decrease. Although the upper limit of the ratio (L/D) is not particularly limited, it is generally 4,000 or less, preferably 1,000 or less, and more preferably 500 or less. If it exceeds 4000, the dispersibility of the carbon fibers is impaired, which is not preferable.

By applying the fibrous carbon as described above to the low-density electrode mixture layer, it is possible to contribute to the electrical conductivity and withstand voltage of the obtained electrode mixture layer. Generally, one of the problems associated with increasing the thickness of the electrode mixture layer is that the capacity retention rate of the battery decreases when the output is increased. In general, increasing the electrode density has been proposed as a technique for reducing electrode resistance. The present invention reduces the electrode density rather than increasing it, and uses a predetermined carbon material, that is, highly crystalline fibrous carbon, preferably with a long fiber length, to improve the capacity retention rate of the battery when the output is increased. can be raised.

(1-2) Method for Producing Carbon-Based Conductive Auxiliary Agent A method for producing carbon fiber that can be preferably used as the carbon-based conductive auxiliary agent for use in the electrode mixture layer of the present invention will be described below. The method for producing the carbon fiber is not particularly limited, but it can be produced, for example, through the steps (1) to (4) described below.
(1) A step of forming a resin composition comprising a thermoplastic resin and a carbon precursor in a molten state to fiberize the carbon precursor to obtain a resin composite fiber;
(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 the thermoplastic resin from the resin composite stabilized fiber to separate only the stabilized fiber;
(4) Carbonizing and sintering step for obtaining carbon fibers by heating the stabilized fibers in an inert atmosphere to carbonize or graphitize them.

<Thermoplastic resin>
The thermoplastic resin used in this carbon fiber manufacturing method should be capable of manufacturing a resin composite fiber and should be easily removed in the thermoplastic resin removing step. Examples of such thermoplastic resins include polyacrylate polymers such as polyolefin, polymethacrylate and polymethyl methacrylate, polystyrene, polycarbonate, polyarylate, polyester, polyamide, polyester carbonate, polysulfone, polyimide, polyetherimide, polyketone, and polylactic acid. are exemplified. Among these, polyolefin is preferably used.

Specific examples of polyolefins include polyethylene, polypropylene, poly-4-methylpentene-1, and copolymers containing these. From the viewpoint of ease of removal in the thermoplastic resin removing step, it is preferable to use polyethylene. Polyethylene includes homopolymers such as high-pressure low-density polyethylene, low-density polyethylene such as gas-phase method, solution method, and high-pressure linear low-density polyethylene, medium-density polyethylene, and high-density polyethylene, or ethylene and α-olefin. and copolymers of ethylene and other vinyl monomers, such as ethylene-vinyl acetate copolymers.

The thermoplastic resin preferably has a melt mass flow rate (MFR) of 0.1 to 10 g/10 min, more preferably 0.1 to 5 g/10 min, measured according to JIS K 7210. Particularly preferred is 0.1 to 3 g/10 min. When the MFR is within the above range, the mesophase pitch can be well micro-dispersed in the thermoplastic resin. In addition, when molding the resin composite fiber, the fiber is stretched, thereby improving the crystallinity of the carbon fiber obtained by controlling the molecular orientation of the carbon precursor, and increasing the fiber diameter of the obtained carbon fiber. can be made smaller. Since the thermoplastic resin can be easily melt-kneaded with the carbon precursor, it is preferable that the amorphous resin has a glass transition temperature of 250° C. or lower, and the crystalline resin has a melting point of 300° C. or lower.

<Carbon precursor>
Mesophase pitch is preferably used as the carbon precursor. The case of using mesophase pitch as the carbon precursor will be described below. A mesophase pitch is a pitch that can form an optically anisotropic phase (liquid crystal phase) in a molten state. Examples of the mesophase pitch to be used include those made from coal or petroleum distillation residues and those made from aromatic hydrocarbons such as naphthalene. For example, coal-derived mesophase pitch can be obtained by a treatment mainly consisting of hydrogenation and heat treatment of coal tar pitch, a treatment mainly consisting of hydrogenation, heat treatment and solvent extraction.

More specifically, it can be obtained by the following method.
First, a coal tar pitch with a softening point of 80° C. from which quinoline insoluble matter has 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. After the hydrogenated coal tar pitch is heat-treated at 480° C. under normal pressure, the pressure is reduced to remove low boiling point components to obtain crude mesophase pitch. Purified mesophase pitch can be obtained by filtering this crude mesophase pitch with a filter at a temperature of 340° C. to remove foreign matters.

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

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

<Resin composition>
A resin composition comprising a thermoplastic resin and a mesophase pitch (hereinafter also referred to as a mesophase pitch composition) used in this carbon fiber manufacturing method is a thermoplastic resin and 1 part per 100 parts by mass of the thermoplastic resin. ~150 parts by weight of mesophase pitch. More preferably, the mesophase pitch content is 5 to 100 parts by mass. If the content of the mesophase pitch exceeds 150 parts by mass, a resin conjugate fiber having a desired fiber diameter cannot be obtained, and if it is less than 1 part by mass, the target carbon fiber cannot be produced at a low cost. This is not desirable because it causes

In order to produce carbon fibers having a fiber diameter of 900 nm or less, the dispersed diameter of mesophase pitch in the thermoplastic resin is preferably 0.01 to 50 μm, more preferably 0.01 to 30 μm. If the diameter of the mesophase pitch dispersed in the thermoplastic resin is out of the range of 0.01 to 50 μm, it may be difficult to produce the desired carbon fiber. In the mesophase pitch composition, the mesophase pitch forms spherical or elliptical island phases. means shaft diameter.

The dispersion diameter of 0.01 to 50 μm preferably maintains the above range even after the mesophase pitch composition is held at 300° C. for 3 minutes, and is maintained even after holding at 300° C. for 5 minutes. It is more preferable that the temperature is maintained, and it is particularly preferable that the temperature is maintained even after the temperature is maintained at 300° C. for 10 minutes. In general, if the mesophase pitch composition is kept in a molten state, the mesophase pitch aggregates over time in the mesophase pitch composition. If the mesophase pitch aggregates and the dispersion diameter exceeds 50 μm, it may be difficult to produce the desired carbon fiber. The aggregation rate of mesophase pitch in the mesophase pitch composition varies depending on the type of 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. Melt-kneading of the thermoplastic resin and the mesophase pitch can be performed using a known apparatus. For example, one or more types selected from the group consisting of single-screw kneaders, twin-screw kneaders, mixing rolls, and Banbury mixers can be used. Among these, it is preferable to use a twin-screw kneader for the purpose of micro-dispersing the mesophase pitch in the thermoplastic resin, and it is particularly preferable to use a twin-screw kneader in which each shaft rotates in the same direction. preferable.

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

Melt-kneading is preferably performed in an inert atmosphere with an oxygen gas content of less than 10% by volume, more preferably in an inert atmosphere with an oxygen gas content of less than 5% by volume, and the oxygen gas content is It is particularly preferred to work under an inert atmosphere of less than 1% volume. The mesophase pitch used in the present invention may be denatured by reacting with oxygen during melt-kneading, which may hinder microdispersion in the thermoplastic resin. Therefore, it is preferable to melt-knead in an inert atmosphere in order 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 the desired carbon fiber can be produced, but a method of melt-spinning a mesophase pitch composition from a spinneret, and a method of melting a mesophase pitch composition from a rectangular spinneret. A film-forming method can be exemplified.

In order to obtain a highly crystalline carbon fiber, it is necessary to perform an operation to enhance the molecular orientation of the mesophase pitch contained in the resin composite fiber in the step of obtaining the resin composite fiber. As the operation for enhancing the molecular orientation of the mesophase pitch contained in the resin composite fiber, it is preferable to apply deformation to enhance the molecular orientation of the mesophase pitch in a molten state. Examples of such methods include a method of applying strain by shearing to the mesophase pitch in a molten state and a method of applying strain by elongation.

As a method of applying strain by shearing, a method of increasing the linear velocity of the mesophase pitch composition in the molten state when the mesophase pitch is in a molten state is exemplified. Specifically, there is a method of applying shear strain by increasing the passage speed of the mesophase pitch composition in a molten state as it passes through the channel.
Further, as a method of applying strain by elongation, a method of increasing the linear velocity of the mesophase pitch composition in the molten state toward the discharge side of the die while the mesophase pitch is in a molten state can be used. Specific examples include a method in which the cross-sectional area in the flow path of the die is gradually reduced toward the ejection side, and a method in which the mesophase pitch composition ejected from the die is drawn at a higher linear velocity than the ejection linear velocity. .

The temperature during the operation for enhancing the molecular orientation of the mesophase pitch must be higher than the melting temperature of the mesophase pitch, preferably 150 to 400°C, more preferably 180 to 350°C. If the temperature exceeds 400°C, the rate of deformation relaxation of the mesophase pitch increases, making it difficult to maintain the shape of the fiber.

Moreover, the manufacturing process of the resin composite fiber may have a cooling process. As the cooling step, for example, in the case of melt spinning, there is a method of cooling the atmosphere downstream of the spinneret. In the case of melt film-forming, a method of providing a cooling drum downstream of the rectangular die can be used. By providing the cooling step, the region where the mesophase pitch is deformed by elongation can be adjusted, and the strain rate can be adjusted. Moreover, by providing the cooling step, the resin conjugate fiber after being spun or formed into a film can be immediately cooled and solidified to enable stable molding.

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

As the gas component to be used, air, oxygen, or a mixed gas containing these is preferable from the viewpoint of ease of handling, and air is particularly preferable from the viewpoint of cost. The oxygen gas concentration to be used is preferably in the range of 0.5 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 reaction temperature for stabilization is preferably 30 to 350°C, more preferably 40 to 300°C. The treatment time for stabilization is preferably 10 to 1200 minutes, more preferably 10 to 600 minutes, particularly preferably 30 to 300 minutes.

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

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

Thermal decomposition of the thermoplastic resin is preferably carried out in an inert atmosphere. The inert atmosphere referred to here means a gas atmosphere such as carbon dioxide, nitrogen, argon, etc., and the oxygen concentration thereof is preferably 30 ppm by volume or less, 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.

Thermal decomposition of the thermoplastic resin can also be carried out under reduced pressure. Thermal decomposition under reduced pressure can sufficiently remove the thermoplastic resin. As a result, fusion bonding between carbon fibers or graphitized fibers obtained by carbonizing or graphitizing the stabilized fibers can be reduced. The lower the atmospheric pressure, the better, but it is preferably 50 kPa or less, more preferably 30 kPa or less, even more preferably 10 kPa or less, and particularly preferably 5 kPa or less. On the other hand, since perfect vacuum is difficult to achieve, the lower limit of pressure is generally 0.01 kPa or more.

When the thermoplastic resin is thermally decomposed under reduced pressure, a small amount of oxygen or inert gas may be present as long as the above atmospheric pressure is maintained. In particular, the presence of a small amount of inert gas is advantageous because it suppresses fusion between fibers due to thermal deterioration of the thermoplastic resin. Here, the term "trace amount of oxygen" means that the oxygen concentration is 30 volume ppm or less, and the term "trace amount of inert gas" means that the inert gas concentration is 20 volume ppm or less. The type of inert gas used is as described above.

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

The stabilizing step and the thermoplastic resin removing step are preferably carried out while holding the resin composite fiber or the resin composite stabilized fiber on the supporting substrate at a basis weight of 2000 g/m ² or less. By holding it on the support substrate, it is possible to suppress aggregation of the resin composite fiber or resin composite stabilized fiber due to heat treatment during stabilization treatment or removal of the thermoplastic resin, and it is possible to maintain breathability. .

The material of the supporting substrate must not be deformed or corroded by solvent or heat. As for the heat resistance temperature of the supporting base material, it is preferable that the supporting base material has a heat resistance of 600° C. or higher because it is necessary not to deform at the thermal decomposition temperature of 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, the shape of the support substrate is preferably a shape having air permeability in the direction perpendicular to the plane. As such a shape, a mesh structure is preferable. The opening of the mesh is preferably 0.1 to 5 mm. If the mesh opening is larger than 5 mm, the fibers tend to aggregate on the mesh lines due to the heat treatment, and the stabilization of the mesophase pitch and the removal of the thermoplastic resin may become insufficient, which is not preferable. On the other hand, if the opening of the mesh is less than 0.1 mm, the permeability of the support substrate in the direction perpendicular to the surface may decrease due to the decrease in the porosity of the support substrate, which is not preferable.

<Carbonization firing process>
Carbon fibers are obtained by carbonizing and/or graphitizing the stabilized fibers in an inert atmosphere. As a container used at that time, a crucible-shaped one made of graphite is preferable. Here, carbonization means heating at a relatively low temperature (preferably about 1000° C.), and graphitization means growing graphite crystals by heating at a higher temperature (preferably about 3000° C.). say.

Nitrogen, argon and the like are examples of the inert gas used during carbonization and/or graphitization of the stabilized fiber. The oxygen concentration in the inert atmosphere is preferably 20 ppm by volume or less, more preferably 10 ppm by volume or less. The firing temperature during carbonization and/or graphitization is preferably 500 to 3500°C, more preferably 800 to 3200°C. In particular, the firing temperature for graphitization is preferably 2000 to 3500°C, more preferably 2100 to 3200°C. If the temperature during graphitization is less than 2000° C., crystal growth is hindered, and the crystallite length becomes insufficient, which may result in a marked drop in electrical conductivity. Also, when the graphitization temperature exceeds 3500° C., it is preferable in terms of crystal growth, but the oxygen content of the carbon fibers tends to decrease. The firing time is preferably 0.1 to 24 hours, more preferably 0.2 to 10 hours.

<Pulverization processing>
The method for producing carbon fibers may include a pulverization 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, ball mill, bead mill, impeller mill, cutter mill, etc. After pulverization, classification may be performed as necessary. In the case of wet pulverization, the dispersing medium is removed after pulverization, but if significant secondary aggregation occurs at this time, subsequent handling becomes very difficult. In such a case, after drying, it is preferable to carry out a crushing operation using a ball mill, a jet mill, or the like.

(1-3) Binder A binder may be used in the electrode mixture layer of the present invention. As the binder to be used, any binder can be used as long as it is capable of forming an electrode and has sufficient electrochemical stability. Such binders include polyvinyl alcohol, polyacrylic acid, carboxymethylcellulose, 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. Especially preferable are polyvinylidene fluoride (PVDF), carboxymethylcellulose and styrene-butadiene rubber (SBR). The form when used as a binder is not particularly limited, and may be solid or liquid (e.g., emulsion). It can be appropriately selected in consideration of the nature and the like.

The content of the binder in the electrode mixture layer of the present invention is preferably 1 to 25% by mass, more preferably 3 to 20% by mass, more preferably 5 to 20% by mass, based on the mass of the electrode mixture layer. % is more preferred.

(1-4) Positive Electrode Active Material The electrode mixture layer of the present invention is preferably used mainly for nonaqueous electrolyte secondary batteries, particularly lithium ion secondary batteries. The positive electrode active material used in the lithium ion secondary battery will be described below.

As the positive electrode active material contained in the positive electrode mixture layer of the present invention, any one or more of conventionally known materials known as positive electrode active materials in non-aqueous electrolyte secondary batteries can be selected. It can be selected and used as appropriate. For example, lithium-containing metal oxides capable of intercalating and deintercalating lithium ions are suitable for lithium ion secondary batteries. 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 and Ti. 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= 2.01 to 2.3.) and the like. Preferred lithium-containing metal oxides include 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 At least one selected from the group consisting of O _z (where x, a, b and z are the same as above) can be mentioned. Note that the value of x is a value before charging/discharging is started, and increases or decreases due to charging/discharging.

The positive electrode active material may be used singly or in combination of two or more. Also, the average particle size of the positive electrode active material is preferably 10 μm or less, more preferably 0.05 to 7 μm, even more 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 decrease.

(1-5) Negative Electrode Active Material Next, the negative electrode active material used for the negative electrode mixture layer of the present invention will be described.
As the negative electrode active material used in the negative electrode mixture layer of the present invention, one or two or more of conventionally known materials known as negative electrode active materials in non-aqueous electrolyte secondary batteries are appropriately selected. It can be selected and used. In the case of a lithium ion secondary battery, for example, a carbon material, an alloy or oxide containing Si and/or Sn, or the like can be used as a material capable of intercalating and deintercalating lithium ions. Among these materials, carbon materials are preferable from the viewpoint of cost. Examples of the carbon material include natural graphite, artificial graphite produced by heat-treating petroleum-based or coal-based coke, hard carbon obtained by carbonizing resin, mesophase pitch-based carbon material, and the like.

When using natural graphite or artificial graphite, from the viewpoint of increasing the battery capacity, the lattice spacing d (002) of the (002) plane of the graphite structure according to powder X-ray diffraction is in the range of 0.335 to 0.337 nm. preferable. Natural graphite refers to a graphite material that is naturally produced as an ore. Natural graphite is classified into two types, flaky graphite with a high degree of crystallinity and earthy graphite with a low degree of crystallinity, according to its appearance and properties. Flaky graphite is further classified into flaky graphite having a leaf-like appearance and flaky graphite having a massive appearance. Natural graphite, which is a graphite material, is not particularly limited in its place of origin, properties, or type. Alternatively, natural graphite or particles produced using natural graphite as a raw material may be heat-treated before use.

Artificial graphite refers to graphite produced by a wide range of artificial methods and graphite materials that are close to perfect crystals of graphite. Typical examples are those obtained by using tar and coke obtained from the residue of dry distillation of coal and distillation of crude oil as raw materials and going through a firing process at about 500 to 1000 ° C. and a graphitization process at 2000 ° C. or higher. mentioned. Kish graphite obtained by reprecipitating carbon from molten iron is also a kind of artificial graphite.

When an alloy containing Si and/or Sn is used in addition to the carbon material as the negative electrode active material, the expansion of the electrode during charge and discharge is more pronounced than when Si and/or Sn is used alone or when the respective oxides are used. The rate becomes smaller, and the cycle characteristics become better. Among these, Si-based alloys are preferred. The Si-based alloy includes at least one element selected from the group consisting of B, Mg, Ca, Ti, Fe, Co, Mo, Cr, V, W, Ni, Mn, Zn, Cu, and the like, Si, and alloys of Specifically, _SiB4 , _SiB6 , _Mg2Si , Ni2Si, _TiSi2 , _MoSi2 , _CoSi2 , _NiSi2 , _CaSi2 , _CrSi2 , _Cu5Si , _FeSi2 , _MnSi2 , _{VSi2 ,} At least one selected from the group consisting of WSi ₂ , ZnSi ₂ and the like can be mentioned.
In the electrode mixture layer constituting the negative electrode, as the negative electrode active material, one of the materials described above may be used alone, or two or more of them may be used in combination.

Also, the average particle size of the negative electrode active material is set to 10 μm or less. If the average particle size exceeds 10 μm, the efficiency of the charge-discharge reaction under a large current is lowered. The average particle size is preferably 0.1-10 μm, more preferably 1-7 μm.

2. Electrode for Nonaqueous Electrolyte Secondary Battery The second aspect of the present invention is an electrode for a nonaqueous electrolyte secondary battery (hereinafter also simply referred to as "electrode") having the electrode mixture layer formed thereon. 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 generally used for producing an electrode having an electrode mixture layer of the present invention. One method is to mix and knead the electrode active material, the carbon-based conductive agent, preferably a binder, and other components as necessary, form a film by extrusion molding, roll and stretch the film, and then This is a method of bonding with a current collector.
Another method is to prepare a slurry by mixing the electrode active material, the carbon-based conductive agent, preferably a binder, a solvent for dissolving the binder, and other components as necessary, and use the slurry as a current collector. This is a method of applying to the body surface, removing the solvent, and then pressing.
In the case of the present invention, either method can be adopted, but the latter method is preferred, so the latter method will be described in detail below.

The solid content concentration in the slurry (refers to the ratio of the total mass of the components other than the solvent in the slurry to the total mass of the slurry) is preferably 10 to 30% by mass, and is 15 to 25% by mass. is more preferred. If the solid content concentration exceeds 30% by mass, it may be difficult to prepare a uniform slurry. 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 for the slurry is not particularly limited as long as it dissolves the binder. Specifically, one or more selected from the group consisting of N-methylpyrrolidone (NMP), dimethylacetamide (DMAc), dimethylformamide (DMF), dimethylsulfoxide (DMSO), water and the like can be mentioned, particularly NMP, DMAc or water are preferred.

If the thixotropy in the slurry is too strong when producing the electrode, it may be difficult to ensure fluidity suitable for coating. In such cases, a slurrying aid may be used. Slurry aids include one or more selected from the group consisting of polyvinylpyrrolidone, carboxymethylcellulose, polyvinylacetate, polyvinylalcohol, and the like. In particular, it is preferred to use polyvinylpyrrolidone. By adding the slurrying aid as described above, sufficient fluidity can be ensured even with a small amount of solvent, and the dispersibility of the carbon-based conductive aid is remarkably improved. In addition, it is possible to suppress the occurrence of cracks after removing the solvent. The amount of the slurrying aid added is preferably 10% by mass or less, more preferably 0.5 to 10% by mass, based on the total amount of components other than the solvent in the slurry. More preferably, it is 5 to 8% by mass. If the addition amount of the slurry-forming aid exceeds 10% by mass, the slurry viscosity may conversely decrease sharply, resulting in poor dispersion and making it 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, which will be described later. As a coating method, an appropriate coating method such as a doctor blade can be adopted. After coating, the solvent is removed by heat treatment at, for example, 60 to 100° C., preferably 75 to 85° C., for preferably 60 to 180 minutes. After that, the coated material after removing the solvent may be pressed. Preferred pressing conditions in the present invention are pressure of 10 to 30 Pa for 1 to 5 minutes.

Any conductive material can be used as the current collector that constitutes the electrode. For example, metallic materials such as aluminum, nickel, iron, stainless steel, titanium or copper can be used. In particular, aluminum, stainless steel or copper is preferred, and aluminum or carbon-coated aluminum is more preferred.
The thickness of the current collector is preferably 10 to 50 μm.

3. Non-aqueous Electrolyte Secondary Battery The third aspect of the present invention is a non-aqueous electrolyte secondary battery comprising the electrode (either positive electrode or negative electrode, or both) of the present invention.
The non-aqueous electrolyte secondary battery of the present invention is constructed using the electrode of the present invention, a separator, an electrolytic solution, and the like. A lithium ion secondary battery is exemplified as a non-aqueous electrolyte secondary battery. The case where the nonaqueous electrolyte secondary battery of the present invention is a lithium ion secondary battery will be mainly described below.

The non-aqueous electrolyte secondary battery of the present invention comprises the electrode of the present invention and an electrolyte layer containing an electrolyte, the positive electrode material layer of the positive electrode and the negative electrode material layer of the negative electrode face each other, and the positive electrode material layer and the negative electrode It is laminated so that the electrolyte layer is inserted between the material layers.

The cell shape of the non-aqueous 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 rectangular shape can be mentioned. It is also preferable to have an internal structure in which a plurality of pairs of positive and negative electrodes and separators are laminated, and in this case, it is possible to adopt a known stack lamination type, winding type, folded lamination type, or the like. Examples of exterior materials for the non-aqueous electrolyte secondary battery of the present invention include metal cans and aluminum laminated resin films. 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, electrode strength is also improved.

(3-1) Electrolyte Layer As the electrolyte layer constituting the non-aqueous electrolyte secondary battery, a known material can be used. The electrical conductivity at 25° C. of the electrolyte used in the non-aqueous electrolyte secondary battery of the present invention is preferably 1×10 ⁻² S/cm or more. When the non-aqueous electrolyte secondary battery is a lithium ion secondary battery, a non-aqueous electrolytic solution in which an electrolyte such as a lithium salt is dissolved in a non-aqueous solvent is used as the electrolyte layer.

In general, non-aqueous electrolytes are characterized in that they have a higher withstand voltage than water-based electrolytes and provide a high energy density. As the non-aqueous solvent, known solvents 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, dimethylsulfoxide, sulfolane, 3-methylsulfolane, ethyl methyl carbonate, etc. is mentioned. These non-aqueous solvents may be used singly 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 dielectric constant, and among the above solvents, those mainly containing propylene carbonate or γ-butyrolactone are preferably used.

Examples of the electrolyte used in the lithium ion 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 carboxylic acid, Lithium oxide, LiCl, LiBr, LiI, lithium chloroborane, borates, and imide salts. Borates include bis(1,2-benzenediolate(2-)-O,O')lithium borate, bis(2,3-naphthalenediolate(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. The imide salts include lithium bistrifluoromethanesulfonate imide ((CF ₃ SO ₂ ) ₂ NLi) and lithium trifluoromethanesulfonate nonafluorobutanesulfonate imide (LiN(CF ₃ SO ₂ )(C ₄ F ₉ SO ₂ ). ), lithium bispentafluoroethanesulfonic acid imide ((C ₂ F ₅ SO ₂ ) ₂ NLi), and the like. The electrolyte concentration is preferably 0.5 to 2 mol/L. One of the above electrolytes may be used alone, or two or more thereof may be used in combination. A known additive may be added to the electrolyte for the purpose of improving cycle stability and charge/discharge efficiency.
As the 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. No need to use.

(3-2) Separator When using the above non-aqueous electrolyte, a separator is generally used to prevent direct contact between the negative electrode mixture layer and the positive electrode mixture layer. As the shape of the separator, a known shape such as paper (film), porous film, or the like can be suitably adopted. 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 preferably used. Among these, cellulose paper, aromatic polyamide or aliphatic polyimide porous membranes are 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 short-circuit prevention, but in the present invention, it is possible to apply a separator having a thickness of about 5 to 20 μm, which is sufficiently thinner than conventional separators. The use of a thinner separator reduces the internal resistance derived from the separator, thereby improving the output and the energy density of the cell.

EXAMPLES The present invention will be described in more detail with reference to examples below, but the present invention is not limited by these examples. Various measurements and analyzes in the examples were carried out according to the following methods.

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

(2) X-ray diffraction measurement of carbon fiber X-ray diffraction measurement was performed using RINT-2100 manufactured by Rigaku Co., Ltd. in accordance with JIS R7651, lattice spacing (d002), crystallite size (Lc) (hexagonal mesh layer direction) and (La) (carbon hexagonal network plane direction) were measured.

(3) Effective fiber length of fibrous carbon An electrode containing fibrous carbon was observed using a scanning electron microscope (TM-3000 manufactured by Hitachi, Ltd.), and fibrous carbon was observed in the electrode mixture layer constituting the electrode. It was confirmed that the carbon was not curled. Thereafter, the electrode mixture layer in the electrode was dissolved in a solvent, and observed and photographed using a digital microscope (VHX-200 manufactured by Keyence Corporation). For the average effective fiber length of fibrous carbon, 20 locations were randomly selected from the photograph, the effective fiber length was measured, and the average value of all the measurement results (n=20) was taken as the average effective fiber length.

[Production Example 1] <Production of mesophase pitch>
Coal tar pitch with a softening point of 80° C. from which quinoline insoluble matter was removed was hydrogenated at a pressure of 13 MPa and a temperature of 340° C. in the presence of a Ni—Mo catalyst to obtain hydrogenated coal tar pitch. After heat-treating this hydrogenated coal tar pitch at 480° C. under normal pressure, the pressure was reduced to remove low boiling point components to obtain mesophase pitch. This mesophase pitch was filtered using a filter at a temperature of 340° C. to remove foreign substances in the pitch, thereby obtaining a refined mesophase pitch.

[Production Example 2] <Production of carbon nanofiber (CNF)>
84 parts by mass of linear low-density polyethylene (EVOLUE (registered trademark) SP-1510, manufactured by Prime Polymer Co., Ltd., MFR = 1 g / 10 min) as a thermoplastic resin and the thermoplastic carbon precursor obtained in Production Example 1 16 parts by mass of mesophase pitch (mesophase ratio 90.9%, softening point 303.5 ° C.) was passed through a co-directional twin-screw extruder ("TEM-26SS" manufactured by Toshiba Machine Co., Ltd., barrel temperature 300 ° C., under nitrogen flow). was melt-kneaded to prepare a resin composition. This resin composition was spun using a cylinder-type single-hole spinning machine to produce a resin composite fiber (sea-island composite fiber containing pitch as an island component). Specifically, this resin composition was formed into filaments having a fiber diameter of 100 μm with a melt spinning machine using a circular spinneret having a diameter of 0.2 mm and an introduction angle of 60°. The die temperature was 340° C., the discharge amount was 3.8 g/die/hour, the shear rate was 1000 s ⁻¹ , and the draft ratio, which is the ratio of the discharge linear velocity to the take-up velocity, was 4. Under these conditions, the elongation strain rate inside the die was 982 s ^-1 , the deformation area outside the die was 10 mm below the die, and the elongation strain rate was 9 s ^-1 .
Next, this resin composite fiber was held at 340° C. for 3 hours in an atmosphere with an oxygen concentration of 0.7% using a hot air dryer to obtain a resin composite stabilized fiber.
Next, this resin-composite stabilized fiber was placed in a vacuum gas replacement furnace, and after nitrogen replacement was performed, the pressure was reduced to 1 kPa. The temperature was raised to 500° C. at a rate of 5° C./min under reduced pressure, and held at 500° C. for 1 hour to remove the thermoplastic resin and obtain a stabilized fiber. The obtained stabilized fibers were added to a mixed solvent of ethanol/ion-exchanged water (volume ratio: 1/1) and pulverized with a mixer for 10 minutes to disperse the stabilized fibers. The resulting dispersion was filtered. The temperature of the stabilized fiber was raised from room temperature to 1000° C. at a rate of 5° C./min under nitrogen at a flow rate of 1 L/min. Carbon nanofibers were produced by raising the temperature from room temperature to 3000° C. in 3 hours. The obtained carbon nanofibers were pulverized using a dry jet mill.

The obtained carbon nanofiber has a crystallite length (La) of 110.8 nm, (d002) of 0.3372 nm, (Lc) of 62.3 nm, an average fiber diameter of 257 nm, and an average effective fiber length of 16.6 μm. Met.

[ Reference example 1]
2 parts by mass of acetylene black (AB) (Denka Black, average particle size 35 nm, manufactured by Denki Kagaku Kogyo Co., Ltd.) as a carbon-based conductive aid, and positive electrode active material (LiFePO ₄ ; SLFP-ES01, manufactured by Hosen Co., Ltd.). A slurry was prepared using 91 parts by mass, 7 parts by mass of polyvinylidene fluoride (manufactured by Kureha Co., Ltd., W#7200) as a binder, and N-methylpyrrolidone as a solvent. The produced slurry was applied to a current collector (aluminum foil having a thickness of 15 μm) to produce an electrode. The volume content (25°C) of the carbon-based conductive agent in the electrode mixture layer constituting the electrode was 1.5%, the density (25°C) of the electrode mixture layer was 1.55 g/cm ³ , and the average effective fiber The length was 0.1 μm.

Furthermore, the above electrode was used as a positive electrode, opposed to metallic lithium through a glass fiber nonwoven fabric separator, and a mixed solution of ethylene carbonate and ethyl methyl carbonate containing LiPF ₆ at a concentration of 1 mol / L (3/7 mass ratio, manufactured by Kishida Chemical Co., Ltd.) was injected into a 2032-type coin cell to prepare a coin cell for battery evaluation.

[Comparative Reference Example 1]
The same operation as in Reference Example 1 was performed except that the volume content of the carbon-based conductive agent in the electrode mixture layer was 0.8%, and the density of the electrode mixture layer was 1.58 g/cm ³ . was made.

[ Example 1 ]
Vapor growth carbon fiber (VGCF: Vapor Growth Carbon Fiber, average fiber diameter 150 nm) is used as a carbon-based conductive additive, and the volume content of the conductive additive in the electrode mixture layer is 1.5%. An electrode was produced in the same manner as in Reference Example 1, except that the density of the layer was 1.59 g/cm ³ . At this time, the average effective fiber length of the carbon-based conductive additive in the electrode mixture layer constituting the electrode was 8.5 μm.

[Comparative Example 1 ]
The same operation as in Example 1 was performed except that the volume content of the carbon-based conductive agent in the electrode mixture layer was 0.8%, and the density of the electrode mixture layer was 1.59 g/cm ³ . was made.

[ Example 2 ]
The carbon nanofiber (CNF) described in Production Example 1 was used as the carbon-based conductive additive, the volume content of the conductive additive in the electrode mixture layer was 1.5%, and the density of the electrode mixture layer was 1.5%. An electrode was produced in the same manner as in Reference Example 1 except that the density was 46 g/cm ³ .

[ Example 3 ]
An electrode was produced in the same manner as in Example 2 except that the volume content of the conductive agent in the electrode mixture layer was 0.9% and the density of the electrode mixture layer was 1.37 g/cm ³ . bottom.

[ Example 4 ]
An electrode was produced in the same manner as in Example 2 except that the volume content of the conductive agent in the electrode mixture layer was 2.1% and the density of the electrode mixture layer was 1.31 g/cm ^{3 .} bottom.

[ Example 5 ]
An electrode was produced in the same manner as in Example 2 except that the volume content of the conductive agent in the electrode mixture layer was 1.4% and the density of the electrode mixture layer was 2.06 g/cm ³ . bottom.

[ Example 6 ]
An electrode was produced in the same manner as in Example 2 except that the volume content of the conductive agent in the electrode mixture layer was 3.2% and the density of the electrode mixture layer was 2.03 g/cm ³ . bottom.

[ Example 7 ]
As a carbon-based conductive additive, the carbon nanofiber (CNF) described in Production Example 1 was pulverized (manufactured by Sugino Machine Co., Ltd., Starburst), and fibrous carbon (S-CNF) having an average effective fiber length of 5.5 μm and bottom. Reference except that this fibrous carbon (S-CNF) was used, the volume content of the conductive aid in the electrode mixture layer was 1.4%, and the density of the electrode mixture layer was 1.54 g / cm ³ An electrode was produced in the same manner as in Example 1.

<Electrode resistance measurement>
Using a potentiostat/galvanostat (HA-151 manufactured by Hokuto Denko Co., Ltd.), the electrode resistance in the film thickness direction of the electrodes prepared in Examples and Comparative Examples was measured, and the electrode conductivity calculated from the resistance value. are shown in Table 1. It can be seen that the conductivity of the electrode is good when both the volume content of the conductive aid and the electrode mixture layer density are within appropriate ranges.

Claims (7)

A positive electrode mixture layer containing at least a positive electrode active material and a carbon- based conductive aid,
The volume content of the carbon-based conductive aid based on the volume of the positive electrode mixture layer is 0.9 to 5.0 (%), and the carbon-based conductive aid has an average fiber diameter of 50 nm or more. 900 nm or less fibrous carbon containing 0.9 (%) or more based on the volume of the positive electrode mixture layer,
A positive electrode mixture layer for a non-aqueous electrolyte secondary battery, wherein the density of the positive electrode mixture layer is 0.50 (g/cm ³ ) or more and less than 2.10 (g/cm ³ ). The positive electrode material mixture layer for a non-aqueous electrolyte secondary battery according to claim 1, which does not contain activated carbon. 3. The positive electrode mixture layer for a non-aqueous electrolyte secondary battery according to claim 1, wherein said fibrous carbon has an average effective fiber length of 10 [mu]m or more. 4. The positive electrode mixture layer for a non-aqueous electrolyte secondary battery according to claim 3, wherein the fibrous carbon has a crystallite length (La) measured by X-ray diffraction of 110 to 500 nm. The positive electrode mixture layer for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 4, which has a thickness of 50 to 1000 µm. a current collector;
a positive electrode material mixture layer for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 5, which is laminated on the current collector;
A positive electrode for a non-aqueous electrolyte secondary battery comprising: A nonaqueous electrolyte secondary battery comprising the positive electrode for a nonaqueous electrolyte secondary battery according to claim 6 .