JP7108372B2 - Electrode mixture layer for non-aqueous electrolyte secondary battery, electrode for non-aqueous electrolyte secondary battery, and non-aqueous electrolyte secondary battery - Google Patents

Description

TECHNICAL FIELD The present invention relates to an electrode mixture layer for a non-aqueous electrolyte secondary battery, an electrode for a non-aqueous electrolyte secondary battery using the same, and a non-aqueous electrolyte secondary battery using the same.

From the viewpoint of improving the functionality of non-aqueous electrolyte secondary batteries represented by lithium ion secondary batteries, specifically from the viewpoint of increasing input and output, fibrous carbon, specifically multi-walls with an average fiber diameter of 10 to 150 nm. A technique of adding a carbon nanotube (MWCNT) as a conductive aid to an electrode mixture layer is known. For example, Patent Document 1 discloses a technique of adding MWCNT having an average fiber diameter of 150 nm to an electrode mixture layer. Further, Patent Document 2 describes a technique of using a composite of a granular conductive additive and fibrous carbon.

In the case of the method using MWCNT described in Patent Documents 1 and 2, the dispersibility of MWCNT is poor, and when dispersing it in a paste (slurry) for electrode coating, it is necessary to pulverize the fiber, or the fiber diameter is small. Therefore, the fibers often shrink and curl. Therefore, the effective length of the fibers in the electrode mixture layer is shortened, making it difficult to form a long-distance electron conduction path in spite of the use of a fibrous conduction aid, resulting in long-distance electronic conduction. It has been difficult to ensure sufficient electron conductivity in a thick electrode layer that requires . Against this background, there is a demand for the development of an electrode mixture layer that achieves both the surface current collecting property of the electrode active material particles and the long-distance electron conductivity in the electrode mixture layer.

JP 2009-16265 A JP 2014-160590 A

An object of the present invention is to provide an electrode mixture layer for a non-aqueous electrolyte battery that achieves both surface current collection and long-distance electron conductivity of the electrode material in the electrode mixture layer, an electrode using the same, and an electrode using the same. An object of the present invention is to provide a non-aqueous electrolyte secondary battery.

As a result of extensive studies in view of the above-described conventional technology, the present inventors have found that a coated positive electrode active material obtained by coating the surface of a positive electrode active material with a conductive material and fibrous carbon having a long effective length and a large aspect ratio By using together, it is possible to form a long-distance electronic conductive network in the electrode mixture layer, and to produce an electrode mixture layer that achieves both the surface current collecting property of the positive electrode active material and long-distance electronic conductivity. rice field. Furthermore, by combining a predetermined fibrous carbon and a coated positive electrode active material, an electrode mixture layer having high electrical conductivity per amount added and excellent long-distance electronic conductivity even in a thick film electrode can be obtained. The present inventors have found that it can be produced, and have completed the present invention.

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

[1] A coated positive electrode active material comprising a positive electrode active material and a conductive material covering the surface of the positive electrode active material;
fibrous carbon having an average fiber diameter of 200 to 900 nm and an aspect ratio of 30 or more;
An electrode mixture layer for a non-aqueous electrolyte secondary battery, comprising:

[2] The electrode combination for a non-aqueous electrolyte secondary battery according to [1], wherein the fibrous carbon contains 48% or less of fine fibrous carbon having an effective length longer than the average effective length. agent layer.

[3] The electrode mixture layer for a non-aqueous electrolyte secondary battery according to [1] or [2], wherein the fibrous carbon is pitch-based carbon.

[4] The powder volume resistance of the coated positive electrode active material at a packing density of 2.0 g/cm3 at ³ o'clock is 1.0×10 ⁶ Ω·cm or less. Electrode mixture layer for non-aqueous electrolyte secondary battery.

[5] The electrode mixture layer for a non-aqueous electrolyte secondary battery according to any one of [1] to [4], wherein the conductive material is carbon.

[6] The electrode mixture layer for a nonaqueous electrolyte secondary battery according to any one of [1] to [5], wherein the fibrous carbon content is 0.5 to 10% by mass.

[7] The electrode composition for a non-aqueous electrolyte secondary battery according to any one of [1] to [6], which contains 5% by mass or less of a carbon-based conductive agent having an average particle size (primary particle size) of 10 to 100 nm. agent layer.

[8] a current collector;
an electrode mixture layer for a non-aqueous electrolyte secondary battery according to any one of [1] to [7] laminated on the current collector;
An electrode for a non-aqueous electrolyte secondary battery comprising:

[9] A nonaqueous electrolyte secondary battery comprising the electrode mixture layer for a nonaqueous electrolyte secondary battery according to any one of [1] to [7].

According to the present invention, it is possible to form an electrode mixture layer in which a long-distance conductive path is formed, compared with a conventional electrode mixture layer using MWCNT as a conductive agent, and a high-performance non-aqueous An electrolyte secondary battery can be provided.

4 is a graph showing the density-volume resistivity of positive electrode active materials A, B, and C used in Examples. 1 is a photograph (digital microscope image, magnification: 3,000) of an electrode mixture layer containing fibrous carbon of Example 1 dissolved in a solvent and observed after drying. 4 is a graph (histogram) showing the effective length of fibrous carbon obtained by dismantling the electrode mixture layer of Example 1. FIG. It is the graph which plotted the value of the electrical conductivity with respect to an aspect-ratio.

The present invention will be described below.

1. Electrode mixture layer for non-aqueous electrolyte secondary battery The electrode mixture layer for non-aqueous electrolyte secondary battery of the present invention comprises:
(1) a coated positive electrode active material comprising a positive electrode active material and a conductive material covering the surface of the positive electrode active material;
(2) fibrous carbon having an average fiber diameter of 200 to 900 nm and an aspect ratio of 30 or more;
contains
It is preferable that the electrode mixture layer for a non-aqueous electrolyte secondary battery of the present invention further contains a particulate carbon-based conductive aid.
Hereinafter, the electrode mixture layer for non-aqueous electrolyte secondary batteries is also simply referred to as "electrode mixture layer".

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 60 μm or more, further preferably 70 μm or more, and 80 μm or more. More preferably, it is particularly preferably 90 μm or more, and most preferably 100 μm or more. Although the upper limit of the film thickness is not particularly limited, it is generally less than 1000 μm, particularly preferably less than 800 μm. If the film thickness 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 decrease. This is unfavorable from an energy density point of view and considerably limits the application. If the film thickness is 1000 μm or more, cracks are likely to occur in the electrode mixture layer, making production relatively difficult. Moreover, when the film thickness is 1000 μm or more, the transport of Li ions is likely to be inhibited, and the resistance is likely to increase. 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. 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.

The density (25° C.) of the electrode mixture layer varies depending on the positive electrode active material used, but is preferably 1.5 to 3.5 g/cm ³ and more preferably 2.0 to 3.0 g/cm ³ . is more preferred. The electrode mixture layer preferably has a porosity of 15 to 60%, more preferably 20 to 50%. If the porosity is less than 15%, the movement of ions is hindered, which is not preferable from the viewpoint of high output. Moreover, when the porosity exceeds 60%, the capacity density per volume becomes low, which is not preferable.

1-1. Coated Positive Electrode Active Material In the present invention, the coated positive electrode active material is composed of a positive electrode active material and a conductive material covering part or all of the surface of the positive electrode active material. In the coated positive electrode active material, 20% or more of the entire surface of the positive electrode active material is preferably coated with the conductive material, and 50% or more is more preferably coated.

The volume resistance of the powder when the coated positive electrode active material is packed at a packing density of 2.0 g/cm ³ is preferably 1.0×10 ⁶ Ω·cm or less, and ranges from 1.0×10 ⁻² to 1.0×10 6 Ω·cm. It is more preferably 0×10 ⁶ Ω·cm, further preferably 1.0×10 ⁻¹ to 1.0×10 ⁵ Ω·cm, and further preferably 1.0 to 1.0×10 ⁴ Ω·cm. cm is particularly preferred. If it exceeds 1.0×10 ⁶ Ω·cm, it is difficult to obtain sufficient battery characteristics even if it is used in combination with fibrous carbon having a long effective length, which will be described later.

The average particle size of the coated 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.

(Positive electrode active material)
As the positive electrode active material contained in the 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 used. 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.

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 preferred. If it is less than 60% by mass, it may be difficult to apply it to power supply applications that require high energy density. If it exceeds 98.5% by mass, the amount of the binder is too small, and cracks may occur in the electrode mixture layer, or the electrode mixture layer may separate from the current collector. Furthermore, the content of fibrous carbon or carbon-based conductive aid may be too small, resulting in insufficient conductivity of the electrode mixture layer.

(Conductive material)
All or part of the positive electrode active material contained in the electrode mixture layer of the present invention is coated with a conductive material. Metal particles, conductive polymers, carbon materials, and the like can be used as the conductive material. Among these, it is preferable to use a carbon material from the viewpoint of workability and conductivity. A case where a carbon material is used as the conductive material will be described in detail below.

The method of coating the surface of the positive electrode active material with a carbon material is not particularly limited, but a method of mixing the positive electrode active material and fine carbon particles (carbon black) and mechanically processing (mechanofusion), chemical A vapor deposition method such as vapor deposition (CVD), a method of applying a carbon-forming polymer such as phenol resin or polyvinyl alcohol and then baking it to carbonize it (solution method) can be used.

The coating amount (% by mass) of the carbon material with respect to the positive electrode active material is 0.1 to 5% by mass, preferably 0.5 to 4% by mass, more preferably 1 to 3% by mass. If it is less than 0.1% by mass, the surface conductivity of the positive electrode active material is insufficient, and good battery characteristics cannot be obtained. If it exceeds 5% by mass, the volume energy density of the positive electrode active material is lowered, which is not preferable.

1-2. Fibrous Carbon The fibrous carbon contained in the electrode mixture layer of the present invention is fibrous carbon having an average fiber diameter of 200 to 900 nm and an aspect ratio of 30 or more.

Such fibrous carbon is not particularly limited as long as the effect of the present invention is exhibited, but natural graphite, artificial graphite produced by heat-treating petroleum-based or coal-based coke, non-graphitizable carbon, and easily graphitizable curable carbon and the like. Among these, easily graphitizable carbon is preferable. Graphitizable carbon is a carbon raw material that easily forms a graphite structure having a three-dimensional stacking regularity by heat treatment at a high temperature of 2500° C. or higher. Graphitizable carbon is also called soft carbon, soft carbon, and the like.

Examples of easily graphitizable carbon include those made from petroleum coke, coal pitch coke, polyvinyl chloride, 3,5-dimethylphenol formaldehyde resin, and the like. In particular, pitch-based carbon obtained from a mesophase pitch or a mixture thereof capable of forming an optically anisotropic phase (liquid crystal phase) in a molten state is preferable because high crystallinity and high conductivity are expected.

Mesophase pitch includes petroleum-based mesophase pitch obtained mainly by hydrogenation and heat treatment of residual petroleum oil, and by hydrogenation, heat treatment and solvent extraction; Coal tar pitch is mainly hydrogenated and heat treated. Coal-based mesophase pitch obtained by a method mainly consisting of hydrogenation, heat treatment, and solvent extraction; super strong acids (e.g., HF, BF ₃ ) using aromatic hydrocarbons such as naphthalene, alkylnaphthalene, or anthracene as raw materials Synthetic liquid crystal pitch obtained by polycondensation in the presence thereof, and the like can be mentioned. Among these, synthetic liquid crystal pitch is more preferable because it does not contain impurities.

From the viewpoint of conductivity, the fibrous carbon preferably has an interplanar spacing d (002) between the (002) planes of the graphite structure determined by powder X-ray diffraction in the range of 0.335 to 0.340 nm, and is preferably 0.335. A range of ∼0.339 nm is more preferred, and a range of 0.335 to 0.3385 nm is even more preferred.

The fibrous carbon preferably has high crystallinity, preferably has a crystallite length (La) of 10 to 500 nm, more preferably 30 to 500 nm, even more preferably 50 to 500 nm, 100 to 500 nm is even more preferred, and 120 to 500 nm is particularly preferred. If it is less than 10 nm, the conductivity of fibrous carbon 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.

The fibrous carbon preferably has a graphene (network plane group) thickness (Lc) of 1.0 to 130 nm, more preferably 20 to 130 nm, even more preferably 30 to 130 nm, and 40 nm. ~130 nm is particularly preferred, and 50 nm to 130 nm is most preferred. If it is less than 1.0 nm, the conductivity of the fibrous carbon is significantly lowered, which is not preferable.

The average fiber diameter of fibrous carbon is 200 to 900 nm, preferably 200 to 600 nm, more preferably 200 to 500 nm, even more preferably 220 to 400 nm, and 250 to 350 nm. Most preferred. If the average fiber diameter is less than 200 nm, the fibrous carbon tends to be folded or curled, and the effective length contributing to electrical conductivity is likely to be shortened. On the other hand, when the average fiber diameter exceeds 900 nm, the number of fibers per unit mass decreases. As a result, formation of conductive paths may be insufficient. In the present invention, the average fiber diameter, average particle diameter and aspect ratio mean values measured from photographs taken using a field emission scanning electron microscope.

The average effective length of the fibrous carbon is 3 μm or more, preferably 3 to 100 μm, more preferably 10 to 100 μm, even more preferably 12 to 80 μm, and 15 to 70 μm. is particularly preferred. The longer the average effective length of the fibrous carbon, the better the electrical conductivity in the electrode for non-aqueous electrolyte secondary battery, the strength of the electrode, and the ability to retain the electrolyte solution. However, if it 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. Therefore, the average effective length of fibrous carbon in the present invention is preferably within the above range.

In the present invention, the length of fibrous carbon is defined by the effective length rather than the actual 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 length of fibrous carbon is defined as the length of the longest line segment with both ends 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 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.

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 manner, 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 aspect ratio of the fibrous carbon (the value obtained by dividing the effective length of the fibrous carbon by the fiber diameter) is 30 or more, preferably 40 or more, more preferably over 50, and 55 or more. is more preferable, and 60 or more is more preferable. If the aspect ratio is less than 30, it is difficult to form a conductive path in the electrode, and it is difficult to obtain the effect of reducing the resistance of the electrode by adding fibrous carbon. Although the upper limit of the aspect ratio is not particularly limited, it is generally 4000 or less, preferably 1000 or less, more preferably 500 or less. If it exceeds 4000, the dispersibility of fibrous carbon is impaired, which is not preferable.

The electrode mixture layer of the present invention contains a plurality of fibrous carbon fibers having an average fiber diameter of 200 to 900 nm and an average aspect ratio of 30 or more (hereinafter also referred to as "fibrous carbon aggregate"). . In the fibrous carbon aggregates, the content of fibrous carbon having an effective length longer than the average effective length is preferably 48% or less based on the number of fibrous carbon aggregates. When it is 48% or less, it contains a relatively large amount of long fibrous carbon having an effective length exceeding the average effective length, which is advantageous for forming a long-distance electron conduction path. The content of fibrous carbon whose effective length is longer than the average effective length is more preferably 45% or less, further preferably 42% or less, even more preferably 40% or less, and 35%. It is particularly preferably 30% or less, particularly preferably 30% or less. The content of fibrous carbon having an effective length longer than the average effective length is preferably 5% or more, more preferably 10% or more.

In the fibrous carbon aggregates of the present invention, it is preferable that the mode of the effective length of the fibrous carbon aggregates divided into 5 μm sections is smaller than the average effective length of the fibrous carbon aggregates. That is, in the effective length distribution (number basis) of aggregates of fibrous carbon, the effective length of the highest peak top is preferably smaller than the average effective length. Moreover, it is preferable that there is one peak top in the effective length distribution (based on the number of lines). By using fibrous carbon having such a distribution, a long-distance conductive path is formed by fibrous carbon with a long effective length, and fibrous carbon with a long effective length spreads in-plane with fibrous carbon with a short effective length. It is possible to enhance the action of inhibiting the orientation in the direction. As a result, it becomes easier to form a conductive path in the thickness direction of the electrode mixture layer.

The method for producing such fibrous carbon is not particularly limited, and known methods can be used, and an example is shown below.

The fibrous carbon can be produced, for example, through the following steps (1) to (4).
(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) A charcoal-firing step in which the stabilized fiber is heated in an inert atmosphere to carbonize or graphitize to obtain fibrous carbon.

<Thermoplastic resin>
The thermoplastic resin used in this fibrous carbon production method should be capable of producing resin composite fibers and should be easily removed in the thermoplastic resin removal 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. As polyethylene, 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 (1999). is more preferred, and 0.1 to 3 g/10 min is particularly preferred. When the MFR is within the above range, the mesophase pitch can be well micro-dispersed in the thermoplastic resin. Further, when molding the resin composite fiber, the fiber is stretched to improve the crystallinity of the fibrous carbon obtained by controlling the molecular orientation of the carbon precursor, and the obtained fibrous carbon fiber The diameter 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.

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>
The resin composition comprising a thermoplastic resin and mesophase pitch (hereinafter also referred to as a mesophase pitch composition) used in this method for producing fibrous carbon comprises 100 parts by mass of thermoplastic resin and 1 to 150 parts by mass of mesophase pitch. It preferably comprises: More preferably, the mesophase pitch content is 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 intended fibrous carbon cannot be produced at a low cost. is not preferable because

In order to produce fibrous carbon with 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 dispersed 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 the desired fibrous carbon. In the mesophase pitch composition, the mesophase pitch forms spherical or elliptical island phases. means its long axis diameter.

The dispersion diameter of 0.01 to 50 μm is preferably maintained within the above range even after the mesophase pitch composition is held at 300° C. for 3 minutes, and is maintained even after the mesophase pitch composition is held at 300° C. for 5 minutes. It is more preferable that the temperature is maintained even after holding 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 fibrous carbon. 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 fibrous carbon 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 this production method is modified 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 the resin composite fiber from the above mesophase pitch composition is not limited as long as the desired fibrous carbon can be produced, but a method of melt spinning the mesophase pitch composition from a spinneret, and a method of spinning the mesophase pitch composition from a rectangular spinneret. A method of melt film formation can be exemplified.

In order to obtain fibrous carbon with high crystallinity, it is effective to perform an operation to increase the molecular orientation of the mesophase pitch contained in the resin composite fiber in the step of obtaining the resin composite fiber. As an operation to increase the molecular orientation of the mesophase pitch contained in the resin composite fiber, it is effective to add deformation to increase the molecular orientation of the mesophase pitch in the molten state. A method of applying strain by shearing to the pitch and a method of applying strain by elongation can be exemplified.

As a method of applying strain by shearing, the linear velocity of the mesophase pitch composition in the molten state is increased while the mesophase pitch is in the molten state so that the mesophase pitch composition in the molten state passes through the nozzle flow path. A method of applying shear strain by increasing the passing speed can be mentioned.
Moreover, as a method of applying strain by elongation, a method of increasing the linear velocity of the mesophase pitch composition in a molten state toward the discharge side in a state where the mesophase pitch is melted can be used. Specific examples include a method in which the cross-sectional area in the nozzle flow channel is gradually reduced toward the discharge side, and a method in which the mesophase pitch composition discharged from the nozzle is drawn at a higher linear velocity than the discharge linear velocity.

The temperature at which the mesophase pitch undergoes the operation for enhancing the molecular orientation 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 viewpoint 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 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 reaction temperature for stabilization is preferably 50 to 350°C, more preferably 60 to 300°C, even more preferably 100 to 300°C, and particularly preferably 200 to 300°C. The treatment time for stabilization is preferably 10 to 1200 minutes, more preferably 10 to 600 minutes, still more preferably 30 to 300 minutes, and particularly preferably 60 to 210 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 fibrous carbon. preferable.

<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 between fibrous carbon 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>
Fibrous carbon is obtained by carbonizing and/or graphitizing the stabilized fiber 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 fibrous carbon 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 fibrous carbon 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.

The content of fibrous carbon contained in the electrode mixture layer of the present invention is preferably 0.5 to 10% by mass, more preferably 0.5 to 5% by mass, and 1 to 3% by mass. is more preferable. If it is less than 0.5% by mass, the formation of conductive paths tends to be insufficient, and the resistance value in the thickness direction of the electrode mixture layer may not sufficiently decrease. If it exceeds 10% by mass, the amount of active material in the electrode is reduced, and the energy density of the obtained battery tends to be lowered.

1-3. Other substances (carbon-based conductive aids other than fibrous carbon)
The electrode mixture layer of the present invention preferably contains a carbon-based conductive aid in addition to the above fibrous carbon. Examples of carbon-based conductive aids other than fibrous carbon include carbon black, acetylene black, scale-like carbon, graphene, and graphite. These carbon-based conductive aids may be used alone, or two or more of them may be used in combination.

Although the shape of these carbon-based conductive aids is not particularly limited, fine particles are preferred. The average particle size (primary particle size) of the carbon-based conductive aid is preferably 10 to 100 nm, more preferably 20 to 100 nm. The aspect ratio of these carbon-based conductive aids is 10 or less, preferably 1-5, more preferably 1-3.

The content of the carbon-based conductive agent other than fibrous carbon in the electrode mixture layer of the present invention is preferably 5% by mass or less, more preferably 0.5 to 4% by mass, and 1 to 3% by mass. % by mass is more preferred.

(binder)
The electrode mixture layer of the present invention preferably contains a binder. As the binder, 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 and the like, and polyvinylidene fluoride (PVDF) is particularly preferable.

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 15% by mass, even more preferably 5 to 10% by mass. If it is less than 1% by mass, cracks may occur in the electrode mixture layer, or the electrode mixture layer may separate from the current collector. If it exceeds 25% by mass, the amount of active material in the electrode is reduced, and the energy density of the obtained battery tends to be lowered.

2. Electrode for Nonaqueous Electrolyte Secondary Battery Hereinafter, an electrode for a nonaqueous electrolyte secondary battery comprising the electrode mixture layer of the present invention will be described.
Hereinafter, the electrode for non-aqueous electrolyte secondary battery is also simply referred to as "electrode".

The electrode mixture layer of the present invention is laminated on the surface of the current collector that constitutes the electrode. 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 coated positive electrode active material, the fibrous carbon, the binder, and other components as necessary, form a film by extrusion molding, roll and stretch the film, and then form a current collector. It is a method of pasting together.
Another method is to prepare a slurry by mixing the coated positive electrode active material, the fibrous carbon, the binder, a solvent that dissolves the binder, and other components as necessary, and apply the slurry to the surface of the current collector. In this method, the solvent is removed by coating, followed by 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 80% by mass, and 20 to 70% by mass. is more preferred. If the solid content concentration exceeds 80% by mass, it may be difficult to prepare a uniform slurry. Further, 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 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) and the like can be mentioned, particularly NMP or DMAc is 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 slurrying aid added is preferably 10% by mass or less, more preferably 0.5 to 10% by mass, more preferably 0.5% by mass, based on the total amount of components other than the solvent in the slurry. More preferably, it is up 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 150° C., preferably 80 to 120° C., preferably for 60 to 180 minutes. Thereafter, by pressing the applied material after removing the solvent, an electrode comprising the electrode mixture layer of the present invention can be manufactured.

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. Nonaqueous Electrolyte Secondary Battery The nonaqueous electrolyte secondary battery of the present invention comprises the electrode mixture layer of the present invention. Examples of nonaqueous electrolyte secondary batteries include lithium ion secondary batteries, lithium batteries, and lithium ion polymer batteries.

The non-aqueous electrolyte secondary battery of the present invention comprises a negative electrode having a negative electrode active material layer formed on the surface of a current collector, an electrolyte layer containing an electrolyte, and an electrode mixture layer of the present invention formed on the surface of the current collector. It is composed of a positive electrode formed by The negative electrode active material layer and the electrode mixture layer of the present invention are laminated so that the electrolyte layer is inserted between the negative electrode active material layer and the electrode mixture layer of the present invention. .

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 fibrous carbon added to the electrode mixture layer has a linear structure and high conductivity, so that it forms a conductive path. It is easy to use, and excellent charge/discharge characteristics can be obtained. Furthermore, electrode strength is also improved.

(Electrolyte layer)
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 constituting the non-aqueous electrolyte secondary battery. 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.

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 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 and lower aliphatic Examples include lithium carboxylate, 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), 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.

(Negative electrode active material)
As the negative electrode active material constituting the non-aqueous electrolyte secondary battery, one or more of conventionally known materials known as negative electrode active materials (materials capable of intercalating and deintercalating lithium ions) in lithium-based batteries Two or more kinds can be selected and used. For example, carbon materials, alloys and oxides containing Si and/or Sn, and the like can be used as materials 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 and 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 non-aqueous electrolyte secondary battery of the present invention, the above materials may be used singly or in combination of two or more as the negative electrode active material. 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.

(separator)
When using the above non-aqueous electrolyte, a separator is generally used to prevent direct contact between the negative electrode active material layer and the electrode mixture layer of the present invention. As the shape of the separator, a known shape such as paper (film), porous film, or the like can be suitably adopted. As the separator material, for example, one or more materials selected from the group consisting of cellulose, aromatic polyamide, aliphatic polyimide, polyolefin, Teflon (registered trademark), and polyphenylene sulfide 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 fibrous carbon Observation and photographing were performed using a desktop electron microscope (manufactured by JEOL Ltd., model NeoScope JCM-6000). The average fiber diameter of fibrous carbon or the like is obtained by randomly selecting 300 locations from the obtained electron micrograph and measuring the fiber diameter, and the average value of all the measurement results (n = 300) is the average fiber diameter. and The average effective length was similarly calculated.

(2) X-ray diffraction measurement of fibrous carbon X-ray diffraction measurement was performed using RINT-2100 manufactured by Rigaku Co., Ltd. in accordance with the JIS R7651 method (2007), lattice spacing (d002), crystallite length (La). , the thickness (Lc) (hexagonal mesh plane layer direction) of graphene (network plane group) was measured.

(3) Volume resistivity measurement method Volume resistivity is measured using a powder resistance system (MCP-PD51) manufactured by Dia Instruments Co., Ltd., under a load of 0.50 to 10.00 kN. Measured using units. As for the volume resistivity, the value of the volume resistivity when the filling density is 2.0 g/cm ³ was taken as the volume resistivity of the sample from the relationship diagram of the volume resistivity accompanying the change of the filling density.

[Production Example 1] <Production of fibrous carbon>
90 parts by mass of high-density polyethylene (HI-ZEX (registered trademark) ^5000SR , manufactured by Prime Polymer Co., Ltd.; melt viscosity of 14 Pa s at 350°C and 600 s-1) as thermoplastic resin and synthetic mesophase pitch as thermoplastic carbon precursor 10 parts by mass of AR MPH (manufactured by Mitsubishi Gas Chemical Co., Ltd.) is melted and kneaded with a co-directional twin-screw extruder ("TEM-26SS" manufactured by Toshiba Machine Co., Ltd., barrel temperature 310 ° C., under a nitrogen stream). A resin composition was prepared.
The dispersed diameter of the mesophase pitch in the thermoplastic resin was 0.05 to 2 μm. Further, when this mesophase pitch composition was held at 300° C. for 10 minutes, aggregation of mesophase pitch was not observed, and the dispersion diameter was 0.05 to 2 μm.
Next, this mesophase pitch composition was formed into a sheet having a thickness of 60 μm using a rectangular die having a slit width of 0.2 mm, a slit length of 100 mm and an introduction angle of 60°. The nozzle temperature was 340° C., the discharge rate was 2.4 kg/hour, the shear rate was 1360 s ⁻¹ , the draft ratio, which is the ratio of the discharge linear velocity to the take-up speed, was 25, and the distance from the discharge port to the cooling drum was 50 mm. there were. Under these conditions, the elongation strain rate inside the spinneret was 95 s ^-1 and the elongation strain rate outside the spinneret was 208 s ^-1 . Using the obtained planar body, it was arranged in the form of a non-woven fabric on a wire mesh having an opening of 1.46 mm and a wire diameter of 0.35 mm so that the basis weight of the short fibers was 30 g/m ² .
A resin composite stabilized fiber was obtained by holding the nonwoven fabric made of this resin composite fiber at 215° C. for 3 hours.

Next, the above-mentioned resin-composite stabilized fiber was placed in a vacuum gas replacement furnace, and after nitrogen replacement, 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 dispersed by pulverizing with a mixer for 10 minutes. The resulting dispersion was filtered. This stabilized fiber was heated from room temperature to 1,000° C. at a rate of 5° C./min under nitrogen at a flow rate of 1 l/min, and after reaching 1,000° C., held for 30 minutes for carbonization. Furthermore, fibrous carbon was produced by raising the temperature from room temperature to 3000° C. in 3 hours in an argon gas atmosphere. The obtained fibrous carbon was pulverized by a dry jet mill.
The fiber diameter of the fibrous carbon obtained through the above graphitization treatment is 200 to 600 nm (average fiber diameter 300 nm), and the aspect ratio calculated from the ratio of the effective fiber length to the fiber diameter is 101. It was a fibrous carbon (carbon fiber) with extremely excellent dispersibility. Further, from the results measured by the X-ray diffraction method, the lattice spacing (d002) of fibrous carbon was 0.3367 nm, the crystallite length (La) was 185 nm, and the thickness of the mesh plane group (Lc002) was 80 nm. , was fibrous carbon with high crystallinity.

[Production Example 2] <Production of coated positive electrode active material>
Lithium iron phosphate was produced by a hydrothermal method according to a known technique. Cathode active material A was produced by allowing butane gas to flow at 100 ml/min, treating at 700° C. for 5 minutes, and coating the surfaces of lithium iron phosphate particles with carbon. A positive electrode active material B was produced in the same manner as the positive electrode active material A, except that the butane gas flow time was set to 10 minutes. Positive electrode active material C was prepared without carbon coating. Table 1 shows the amount of coated carbon of the positive electrode active materials A, B, and C and the powder volume resistance at a bulk density of 2.0 g/cm ³ . The larger the amount of coated carbon, the lower the powder volume resistance.
Moreover, the results of the powder volume resistance measurement of the positive electrode active materials A, B, and C are shown in FIG.

[Example 1] <Production of electrodes>
2 parts by mass of fibrous carbon (CNF) of Production Example 1, 91 parts by mass of coated positive electrode active material A of Production Example 2, 7 parts by mass of polyvinylidene fluoride (manufactured by Kureha Co., Ltd., W # 7200) as a binder, solvent A slurry was prepared using N-methylpyrrolidone as the An electrode was produced by applying the prepared slurry to a current collector (aluminum foil having a thickness of 15 μm) and then drying it at 120° C. for 3 hours. The thickness of the electrode mixture layer constituting the electrode was 112 μm, and the density was 2.5 g/cm ³ .
The electrode mixture layer of this electrode was dissolved in a solvent, dried, and a representative photograph taken using a digital microscope (VHX-200 manufactured by Keyence Corporation) is shown in FIG. The average effective length of fibrous carbon was 19.6 μm (aspect ratio=65). FIG. 3 shows a histogram of fibrous carbon aggregates whose effective lengths were measured. The content of fibrous carbon having an effective length longer than the average effective length was 37.5% based on the number of all fibrous carbon.
Also, in FIG. 4, the electrical conductivity values are plotted against the aspect ratio.

[Example 2] <Production of electrodes>
An electrode was produced in the same manner as in Example 1 except that 1.5 parts by mass of the fibrous carbon (CNF) of Production Example 1 and 91.5 parts by mass of the coated positive electrode active material A of Production Example 2 were used. did. The thickness of the electrode mixture layer constituting the electrode was 110 μm, and the density was 2.5 g/cm ³ .

[Example 3] <Production of electrodes>
1.8 parts by mass of the fibrous carbon (CNF) of Production Example 1 and 0.2 parts by mass of acetylene black (average particle size = 36 nm) as a carbon-based conductive additive were used in the same manner as in Example 1. was performed to prepare an electrode. The thickness of the electrode mixture layer constituting the electrode was 112 μm, and the density was 2.5 g/cm ³ .

[Example 4] <Production of electrodes>
An electrode was produced in the same manner as in Example 1, except that the coated positive electrode active material B of Production Example 2 was used instead of the coated positive electrode active material A of Production Example 2. The thickness of the electrode mixture layer constituting the electrode was 122 μm, and the density was 2.4 g/cm ³ .

[Example 5] <Production of electrodes>
An electrode was produced in the same manner as in Example 4 except that 1.5 parts by mass of fibrous carbon (CNF) of Production Example 1 and 91.5 parts by mass of coated positive electrode active material B of Production Example 2 were used. did. The thickness of the electrode mixture layer constituting the electrode was 125 μm, and the density was 2.4 g/cm ³ .

[Example 6] <Production of electrodes>
An electrode was produced in the same manner as in Example 4, except that 1 part by mass of the fibrous carbon (CNF) of Production Example 1 and 92 parts by mass of the coated positive electrode active material B of Production Example 2 were used. The thickness of the electrode mixture layer constituting the electrode was 118 μm, and the density was 2.4 g/cm ³ .

[Comparative Example 1] <Production of electrode>
The same operation as in Example 1 was performed except that MWCNT (average fiber diameter = 150 nm, average effective length = 7.5 µm, aspect ratio = 50) was used instead of fibrous carbon (CNF) in Production Example 1. was made. The thickness of the electrode mixture layer constituting the electrode was 116 μm, and the density was 2.5 g/cm ³ .
Also, in FIG. 4, the electrical conductivity values are plotted against the aspect ratio.

[Comparative Example 2] <Production of electrode>
An electrode was produced in the same manner as in Example 1, except that 1.5 parts by mass of MWCNT and 91.5 parts by mass of the coated positive electrode active material A of Production Example 2 were used. The thickness of the electrode mixture layer constituting the electrode was 115 μm, and the density was 2.5 g/cm ³ .

[Comparative Example 3] <Production of electrode>
An electrode was produced in the same manner as in Example 1, except that acetylene black (average particle size = 36 nm, aspect ratio = 1) was used instead of fibrous carbon (CNF) in Production Example 1. The thickness of the electrode mixture layer constituting the electrode was 110 μm, and the density was 2.5 g/cm ³ .

[Comparative Example 4]
An electrode was produced in the same manner as in Comparative Example 3 except that 1.5 parts by mass of acetylene black and 91.5 parts by mass of the coated positive electrode active material A of Production Example 2 were used. The thickness of the electrode mixture layer constituting the electrode was 110 μm, and the density was 2.5 g/cm ³ .

[Comparative Example 5]
Instead of the fibrous carbon (CNF) of Production Example 1, the fibrous carbon (CNF) was pulverized (manufactured by Sugino Machine Co., Ltd., Starburst), and fibrous carbon (S-CNF) with an average effective length of 5.5 μm , aspect ratio=18). An electrode was produced in the same manner as in Example 1, except that this fibrous carbon was used. The thickness of the electrode mixture layer constituting the electrode was 123 μm, and the density was 2.5 g/cm ³ .

[Comparative Example 6]
It does not contain the fibrous carbon (CNF) of Production Example 1, 93 parts by mass of the coated positive electrode active material B of Production Example 2, and 7 parts by mass of polyvinylidene fluoride (manufactured by Kureha Co., Ltd., W # 7200) as a binder. Except for this, the same operation as in Example 1 was performed to prepare an electrode. The thickness of the electrode mixture layer constituting the electrode was 123 μm, and the density was 2.4 g/cm ³ .

[Comparative Example 7]
An electrode was produced in the same manner as in Example 1, except that instead of the coated positive electrode active material A of Production Example 2, the positive electrode active material C of Production Example 2, which was not coated, was used. The thickness of the electrode mixture layer constituting the electrode was 123 μm, and the density was 2.4 g/cm ³ .

<Electrode resistance measurement>
Using a potentiostat / galvanostat (HA-151 manufactured by Hokuto Denko Co., Ltd.), the results of measuring the electrode resistance in the film thickness direction of the produced electrode and the electrical conductivity calculated from the resistance value are shown in Table 2. . In addition, the relationship between the aspect ratio of the carbon-based conductive aid and the electrical conductivity of the electrode/the amount of the conductive aid added is shown in FIG. 4. From Table 2 and FIG. 4, when using the carbon-coated coated positive electrode active material A or B and using fibrous carbon (CNF) having an aspect ratio of 30 or more, the electrical conductivity of the electrode (electrode conductivity) is It was extremely expensive.

<Expected effects of batteries>
Table 1 and FIG. 1 suggest that the surface current collecting property of the coated positive electrode active material is good. On the other hand, from Table 2 and FIG. 4, it can be seen that the electrical conductivity of the electrode mixture layer using fibrous carbon with an aspect ratio of 30 or more is very good, and the coated positive electrode active material is used as the positive electrode active material. Thus, it can be seen that better electrical conductivity can be achieved.
Therefore, by forming an electrode mixture layer in which the coated positive electrode active material and fibrous carbon having an aspect ratio of 30 or more are combined, the surface current collecting property of the active material and the long-distance electron conduction network in the electrode mixture layer It is possible to produce an electrode mixture layer that satisfies both the formation and the formation. By using such an electrode mixture layer, a thick-film electrode that enables high capacity and long-distance electronic conductivity that simultaneously achieves high output can be obtained. An electrolyte secondary battery can be provided.

Claims (9)

a coated positive electrode active material comprising a positive electrode active material and a conductive material covering the surface of the positive electrode active material;
fibrous carbon having an average fiber diameter of 220 to 400 nm, an average effective length of 10 to 70 μm , and an Lc of 50 to 130 nm ;
An electrode mixture layer for a non-aqueous electrolyte secondary battery, comprising: 2. The electrode mixture layer for a non-aqueous electrolyte secondary battery according to claim 1, wherein the content of fibrous carbon having an effective length longer than the average effective length is 48% or less on a number basis. 3. The electrode mixture layer for a non-aqueous electrolyte secondary battery according to claim 1, wherein said fibrous carbon is pitch-based carbon. 4. The non-aqueous electrolyte according to any one of claims 1 to 3, wherein the powder volume resistance of the coated positive electrode active material at a packing density of 2.0 g/cm ³ is 1.0 × 10 ⁶ Ω·cm or less. Electrode mixture layer for secondary batteries. 5. The electrode mixture layer for a non-aqueous electrolyte secondary battery according to claim 1, wherein said conductive material comprises carbon. 6. The electrode mixture layer for a non-aqueous electrolyte secondary battery according to claim 1, wherein the content of said fibrous carbon is 0.5 to 10% by mass. The electrode mixture layer for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 6, containing 5% by mass or less of a carbon-based conductive additive having an average particle size (primary particle size) of 10 to 100 nm. a current collector;
an electrode mixture layer for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 7, which is laminated on the current collector;
An electrode for a non-aqueous electrolyte secondary battery comprising: A non-aqueous electrolyte secondary battery comprising the electrode mixture layer for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 7.

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