JP2012022933A - Negative electrode material for secondary battery, negative electrode for lithium ion secondary battery and lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a negative electrode material for a secondary battery using carbon fibers which can efficiently improve initial charge-discharge efficiency and a cycle characteristic of a lithium ion secondary battery.SOLUTION: For carbon fibers composed of a plurality of microcrystalline graphite and having a fine structure shown in FIG. 3, a thin carbon fiber having a diameter of 50 nm or more and 800 nm or less, and a thick carbon fiber having a diameter of 900 nm or more and 5000 nm or less are produced. A compound made by mixing these carbon fibers is used for the negative electrode material for the lithium ion secondary battery.

Description

  The present invention relates to a negative electrode material for a secondary battery using carbon fiber, and a negative electrode for a lithium ion secondary battery and a lithium ion secondary battery using the same.

In recent years, lithium ion secondary batteries have been widely used as power sources for portable terminals and mobile communication devices. In addition, since the lithium ion secondary battery has the highest energy density among the existing secondary batteries, use of the lithium ion secondary battery as an auxiliary power source for a hybrid vehicle or a fuel cell vehicle or as a large-sized power source for stationary use has been studied. Conventional lithium ion secondary batteries generally use LiCoO ₂ as a positive electrode material and graphite as a negative electrode material.
Patent Documents 1 and 2 below describe using carbon fiber as a negative electrode material for a lithium ion secondary battery.

  In Patent Document 1, in order to improve the large current discharge characteristics and cycle characteristics of a lithium ion secondary battery, carbon fibers produced by a vapor phase growth method are added to coated graphite particles in which graphite is coated with carbonaceous matter. The use of such materials as a negative electrode active material is described. However, the actual situation is that a sufficient improvement effect is not obtained by this method. One of the causes is considered that the carbon fiber produced by the vapor phase growth method added as a conductive auxiliary material has anisotropy in conductivity.

  As shown in FIG. 5, the structure of the carbon fiber manufactured by the conventional method (arc method or vapor phase growth method) is a structure in which graphene sheets (carbon six-membered ring network) overlap each other in a specific direction. A plurality of cylindrically wound graphene sheets having different diameters are coaxially nested. 5A is called a herringbone type, FIG. 5B is called a multi-wall type (multi-walled carbon nanotube), and FIG. 5C is called a cup stack type. The carbon fiber having such a structure has directionality (anisotropy) in the degree of current or heat conduction transmitted along the graphene sheet surface.

  That is, when the carbon fiber produced by the vapor phase growth method has the structure of FIG. 5B, for example, the conductivity in the direction along the axis of the cylindrical fiber is extremely high, but from the cylindrical peripheral surface. Low conductivity. That is, since the conductivity is anisotropic, high conductivity cannot be obtained unless the contact direction of the carbon fiber with respect to the coated graphite particles is controlled. Due to this, in the case of carbon fiber produced by the vapor phase growth method, it is presumed that the effect obtained by adding as a conductive auxiliary material is not as high as expected.

In Patent Document 2, in order to obtain a lithium ion secondary battery having a small irreversible capacity and a large charge / discharge capacity, a fibrous material made of a polymer material is formed by an electrospinning method, and the fibrous material is fired. It is described that a negative electrode mainly composed of carbon fiber is used for a lithium ion secondary battery.
Specifically, polyacrylonitrile (PAN), cellulose, rayon, polycarbodiimide, polyvinyl acetate, polyvinyl alcohol, polystyrene, polyacrylic acid or the like is used as the polymer material. A polymer material-containing solution in which such a polymer material is dissolved in a solvent is sprayed toward the support to form a fibrous material deposition layer made of the polymer material on the support. The carbon fiber produced by firing this deposited layer is not only as thin as several tens to several hundreds of nanometers, but also has a three-dimensional continuous structure, ensuring a conductive path from the substrate, It is described as having a high surface area.

In the example of Patent Document 2, nylon nanofibers having a diameter of 80 to 300 nm and a length of 1 mm or more are deposited on a copper foil at a rate of 5 mg / cm ² , and then the deposited layer is attached to the copper foil with argon. A three-dimensional continuous carbon fiber is obtained by firing in a reduced-pressure atmosphere, and this is punched out together with a copper foil into a size of 16 mm in diameter and used as the negative electrode. The carbon fiber constituting the negative electrode is made of amorphous carbon and is not composed of a large number of microcrystalline graphite.

  Patent Document 3 below describes a spinning device that can perform an electrospinning method. The spinning device includes a storage container that stores a raw material melt, a melt discharge nozzle that discharges the melt stored in the storage container in a thin thread shape, a collector that is disposed to face the melt discharge nozzle, And a melt charging means for charging the melt discharged from the melt discharge nozzle by applying a voltage between the collector and the melt discharge nozzle. This melt discharge nozzle is composed of a first nozzle part for introducing a melt and a second nozzle part for containing a gas, and pressurizing with a gas placed in the second nozzle part, while in the first nozzle part The melt is discharged in the form of a fine thread.

JP 2003-168429 A JP 2007-207654 A JP 2009-275339 A

  The subject of this invention is providing the thing which can improve the initial stage charge-and-discharge efficiency and cycling characteristics of a lithium ion secondary battery effectively as a negative electrode material for secondary batteries using carbon fiber.

In order to solve the above problems, the negative electrode material for a secondary battery of the present invention is composed of a large number of microcrystalline graphite, and is composed of a thin carbon fiber having a diameter of 50 nm to 800 nm and a large number of microcrystalline graphite. And a thick carbon fiber having a diameter of 900 nm or more and 5000 nm or less.
The negative electrode material for a secondary battery of the present invention is a carbon fiber composed of a large number of microcrystalline graphite, and by using a mixture of a thin one and a thick one, the fibers can easily come into contact with each other, The initial charge / discharge efficiency and cycle characteristics of the lithium ion secondary battery can be effectively improved.

The thin carbon fiber and the thick carbon fiber are composed of a large number of randomly oriented microcrystalline graphite, and the (002) plane spacing measured by X-ray diffraction of Cu-Kα ray is 0.3400 nm or more and 0.00. It is preferable that it is 3700 nm or less.
A carbon fiber composed of a large number of randomly oriented graphites and having a (002) plane spacing of 0.3400 nm or more and 0.3700 nm or less as measured by X-ray diffraction of Cu-Kα rays is as follows. The pitch fiber spinning process, the pitch fiber infusibilization process, and the heat treatment of the infusible pitch fiber in an inert gas atmosphere shown in FIG.

(a) When discharging a melt of pitch-based material from the spinning nozzle toward the collector, a preheated gas is supplied around the spinning nozzle, and this preheated gas is parallel to the discharge direction of the melt. A spinning process of pitch fibers by an electrospinning method, characterized by spraying toward the collector.
The negative electrode for lithium ion secondary batteries of this invention has the negative electrode material for secondary batteries of this invention as the main component of the active material.
The lithium ion secondary battery having the negative electrode for a lithium ion secondary battery of the present invention has excellent initial charge / discharge efficiency and cycle characteristics.

<About the manufacturing method of the carbon fiber which makes the negative electrode material for secondary batteries of this invention>
The carbon fiber forming the negative electrode material for a secondary battery according to the present invention includes a spinning process of the pitch fiber shown in (a), an infusible process of the pitch fiber, and a process of heat-treating the infusible pitch fiber in an inert gas atmosphere. Can be manufactured.
Pitch-based materials used in the pitch fiber spinning process include anhydrous pitch and hydrogenated pitch. Anhydrous pitch is obtained by separating petroleum tar or coal tar by atmospheric distillation and vacuum distillation to separate a heavy fraction and a light fraction, heat-treating the resulting heavy fraction as it is, and subjecting it to polycondensation. It is obtained by a method of adjusting to the softening point, has optical isotropy, and is referred to as “General Purpose pitch”. Hydrogenated pitch is obtained by a method of heat-treating after increasing the thermal stability by hydrocracking the obtained heavy fraction, and is obtained by a method of adjusting to a predetermined softening point, has optical anisotropy, It is called “High Peformance Pitch” or “Liquid Crystal Pitch”.

 In the spinning process of the pitch fiber of (a), as the pitch-based material, an anhydrous pitch having a softening point exceeding 210 ° C. and less than 280 ° C. is used, or a hydrogenation having a softening point exceeding 120 ° C. and less than 250 ° C. It is preferable to use a pitch. In the case of an anhydrous pitch, it is more preferable to use a softening point of 220 ° C. or higher and 250 ° C. or lower. In the case of hydrogenated pitch, it is more preferable to use a softening point of 200 ° C. or higher and 230 ° C. or lower.

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

Since there is almost no embryo in an anhydride-added pitch having a softening point of 210 ° C. or lower, a large number of crystals are present at random even if the pitch fiber spun using this as a pitch-based material is carbonized or graphitized. Carbon fiber cannot be obtained. In addition, when spinning, the melt is discharged from the spinning nozzle, and an electrostatic attractive force is applied to the discharged melt, so that crystals grow while being oriented slightly during carbonization or graphitization. it is conceivable that.
Hydrogenated pitch with a softening point of 250 ° C or higher is easy to be oriented during spinning. Therefore, even if the pitch fiber spun using this as a pitch-based material is carbonized or graphitized, a large number of crystals are present at random. Carbon fiber to be obtained is not obtained. The crystal structure of the obtained carbon fiber is a structure in which the a axis is along (orientated) along the fiber axis. Therefore, in the case of hydrogenated pitch, the one having a softening point of less than 250 ° C is used.

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

 In the spinning method of the pitch fiber of (a), a preheated gas is supplied around the spinning nozzle, and the preheated gas is discharged in parallel with the discharge direction when the melt is discharged from the spinning nozzle. It is characterized by spraying toward the collector. It can be estimated that the pitch fiber comes to have a random fine structure by the shearing force of the sprayed preheating gas. The thickness of the pitch fiber can be controlled by adjusting the spray pressure of the preheating gas.

Then, after performing the spinning process of the pitch fiber by this method, the infusibilization process of the pitch fiber and the process of heat-treating the infusible pitch fiber in an inert gas atmosphere (carbonization process and graphitization process) are performed as usual. By carrying out by this method, the average diameter is 50 nm or more and 5000 nm or less, and it is composed of a large number of randomly oriented microcrystalline graphite, and the interplanar spacing of the (002) plane is measured by X-ray diffraction of Cu-Kα rays. A carbon fiber having a size of 0.3400 nm to 0.3700 nm can be obtained.
The temperature of the heat treatment step (carbonization step and graphitization step) is preferably 500 ° C. or higher and lower than 2700 ° C., more preferably 800 ° C. or higher and 2500 ° C. or lower.

  According to the negative electrode material for a secondary battery of the present invention, the initial charge / discharge efficiency and cycle characteristics of the lithium ion secondary battery can be effectively improved.

It is a schematic block diagram which shows the spinning apparatus used at the spinning process for producing the negative electrode material for secondary batteries corresponding to embodiment of this invention. It is sectional drawing which shows the structure of the melt discharge nozzle 5 of the apparatus of FIG. It is a schematic diagram which shows typically the microstructure of the carbon fiber obtained by No. 1 of the Example. It is a schematic diagram which shows typically the microstructure of the carbon fiber (used by No. 3 of the Example) obtained by the vapor phase growth method. It is a figure which shows the example of the fine structure of the conventional carbon fiber, Comprising: (a) is a herringbone type, (b) is a multiwall type (multilayer type carbon nanotube), (c) is a cup stack type.

Hereinafter, embodiments for carrying out the present invention will be described.
The negative electrode material for a secondary battery according to the present invention is composed of a large number of microcrystalline graphite, and is composed of a thin carbon fiber having a diameter of 50 nm to 800 nm and a large number of microcrystalline graphite, and has a diameter of 900 nm to 5000 nm. And a thick carbon fiber. These carbon fibers can be manufactured through a spinning process of pitch fibers using the spinning device of FIG. 1, an infusible process of pitch fibers, and a process of heat-treating the infusible pitch fibers in an inert gas atmosphere. it can.

  The spinning device of FIG. 1 is connected to a storage container 2 for storing the pitch-based material 1, an electric heater 3 for heating the storage container 2 to keep the pitch-based material 1 in a molten state, and the storage container 2. Nitrogen gas supply line 4, melt discharge nozzle 5 disposed at the lower end of storage container 2, flat collector 6 disposed at the position facing the tip of melt discharge nozzle 5, collector 6 and melt And a voltage generator 7 for applying a voltage to the discharge nozzle 5.

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

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

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

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

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

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

The pitch fiber thus obtained is subjected to a general infusibilization step (step of heating the pitch fiber in an oxygen-containing atmosphere), and then the infusible pitch fiber is heat-treated in an inert gas atmosphere. The carbonization step and the graphitization step to be performed are each performed by a normal method.
As a result, a carbon fiber composed of a large number of randomly oriented graphites having a (002) plane spacing of 0.3400 nm or more and 0.3700 nm or less as measured by X-ray diffraction of Cu-Kα rays. Obtainable. Moreover, the diameter of the carbon fiber obtained can be changed by changing the spraying pressure of the inert gas. Accordingly, it is possible to obtain a thin carbon fiber having a diameter of 50 nm or more and 800 nm or less and a large number of microcrystalline graphite and a thick carbon fiber having a diameter of 900 nm or more and 5000 nm or less.

And since the mixture of these carbon fibers is excellent in electroconductivity, the initial charge-and-discharge efficiency and cycling characteristics of a lithium ion secondary battery can be improved by using it as a negative electrode material of a lithium ion secondary battery.
Note that the heating temperature in the infusibilization step is set to, for example, 100 to 300 ° C. according to the softening point of the used pitch-based material. The carbonization step is performed at 500 ° C. or more and 1200 ° C. or less, and the graphitization step is performed at a temperature of 2000 ° C. or more and less than 2700 ° C. after the carbonization step. When pitch fibers after the infusibilization process are heated at 500 to 1200 ° C., a large amount of volatile substances are generated. Therefore, the carbonization process uses a furnace (carbonization furnace) having a structure capable of removing volatile substances. Do it. The graphitization step is performed by transferring the carbon fiber from which the volatile substances have been removed in the carbonization step to a furnace (graphitization furnace) having a structure with high thermal efficiency and no mixing of oxidizing substances.

Next, parts other than the features of the present invention relating to the negative electrode of the lithium ion secondary battery will be described.
<Negative electrode of lithium ion secondary battery>
For the negative electrode of a lithium ion secondary battery, for example, a negative electrode mixture composed of a negative electrode material (negative electrode active material) made of a carbon material, a binder, and an additive such as a conductive agent is applied to the surface of the current collector. Can be manufactured.
In this invention, the above-mentioned carbon fiber mixture is contained as the main component of the negative electrode active material, but in addition to this, other carbon materials such as natural graphite and artificial graphite may be contained. Artificial graphite is graphitized by heating the pitch, graphitized mesocarbon spherules obtained by heating the pitch, bulk mesophase, coke, etc., and the surface of these graphitized materials is pitch or phenolic resin. What was baked after coating | cover etc. can be illustrated. Moreover, you may use these carbon materials in mixture. Further, powdery, spherical, flake shaped, and fibrous carbon materials can be used as the negative electrode material.

Further, as the conductive agent, carbon black, carbon fiber obtained by a vapor deposition method, or the like may be used.
As the binder, those having chemical stability and electrochemical stability with respect to the electrolyte are preferable, and not only the organic binder that is dissolved and / or dispersed in the organic solvent but also dissolved and / or in the aqueous solvent. Even if the aqueous binder to disperse is used, a negative electrode exhibiting excellent charge / discharge characteristics can be obtained.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Hereinafter, the present invention will be described with reference to specific examples and comparative examples.
[Sample No.1]
Hydrogenated pitch (softening point 200 ° C.) prepared using coal tar as a raw material was used as the pitch-based material. As the spinning device of FIG. 1, a spinning device having a storage container 2 made of a stainless steel barrel (barrel container) having a capacity of 10 mL was prepared. Further, this spinning device is basically shown in FIG. 2 as the melt discharge nozzle 5, but the shape of the tip 51c of the spinning nozzle 51 is different from that shown in FIG. Are not formed with a small diameter). The tip 51c is 27G (inner diameter 0.20 mm, outer diameter 0.42 mm), the inner diameter of the outer cylinder 52 is 0.50 mm, and the nozzle port 53a of the nozzle guide 53 is 0.50 mm.

A temperature controller was connected to the electric heater 3 to control the temperature of the pitch material 1 in the storage container 2 to be 330 ° C. An electric heater was also wound around the outer side of the outer cylinder 52 of the melt discharge nozzle 5 and a temperature controller was connected to control the temperature inside the spinning nozzle 51 to be 330 ° C.
Nitrogen gas preheated to 330 ° C. was introduced into the space 5B of the melt discharge nozzle 5 from the pipe 55, and the flow rate was adjusted so that the linear velocity was 1000 mm / s at the nozzle port 53a. In this state, a voltage of 7 kV was generated by the voltage generator 7 and applied between the collector 6 and the melt discharge nozzle 5. An earth electrode was placed at a position immediately below the melt discharge nozzle 5 and at a distance of 120 mm from the nozzle port 53a.
In this state, first, spinning was performed by introducing 0.3 MPa of pressurized nitrogen into the storage container 2 from the nitrogen gas supply line 4. Thereby, spinning progressed well, and pitch fibers A having a diameter of about 200 nm were obtained.

Next, spinning was performed by introducing 0.4 MPa of pressurized nitrogen into the storage container 2 from the nitrogen gas supply line 4. As a result, spinning proceeded satisfactorily, and pitch fibers B having a diameter of about 1000 nm (1 μm) were obtained.
Next, spinning was performed by introducing 0.5 MPa of pressurized nitrogen into the storage container 2 from the nitrogen gas supply line 4. As a result, spinning proceeded satisfactorily, and pitch fibers C with a diameter of about 3000 nm (3 μm) were obtained.
The resulting pitch fiber A was heated in steps at 200 ° C. for 24 hours, then at 250 ° C. for 12 hours, and further at 300 ° C. for 5 hours to perform the infusibilization step. Next, after introducing nitrogen gas into the furnace in which the infusibilization step has been performed to form a nitrogen gas atmosphere, the temperature is raised to 1000 ° C. at a rate of 3 ° C./min, and then heat treatment (carbon is maintained at 1000 ° C. for 0.5 hours. Process).

Next, the carbon fiber subjected to the heat treatment is put into a graphitization furnace, and argon gas is introduced into the furnace to form an argon gas atmosphere. After the temperature is increased to 3000 ° C. at a rate of 3 ° C./min, the carbon fiber is heated to 3000 ° C. A heat treatment (graphitization step) was performed for 10 hours. As a result, a carbon fiber A having a relatively uniform diameter and around 200 nm was obtained.
When the same treatment was performed on pitch fiber B and pitch fiber C, carbon fiber B having a relatively uniform diameter and approximately 1000 nm (1 μm), respectively, and relatively uniform and approximately 3000 nm (3 μm) in diameter. Carbon fiber C was obtained.

The carbon fibers A to C thus obtained were each pulverized with an agate mortar so that the average fiber length was 30 μm. When these carbon fibers A to C were subjected to an X-ray diffractometer and the (002) plane spacing was measured, all were 0.3466 nm. The plane spacing was calculated from the θ angle of the peak top of the (002) plane. As a result of observing these carbon fibers A to C with a transmission electron microscope, it was found that the fiber structure was a structure in which a large number of fine crystals exist randomly as shown in FIG. As shown in FIG. 3, in such a structure, electrons (e ⁻ ) pass not only in the direction along the fiber axis but also in the direction intersecting the axis.

The carbon fibers A to C produced as described above were mixed so that A: B: C = 1: 2: 7 by mass ratio. This mixture and the polyvinylidene fluoride (PVdF) as a binder were mixed so that the mass ratio of the mixture: PVdF = 90: 10. By adding N-methylpyrrolidone to this mixture, PVdF was dissolved to obtain a pasty carbon material.
This paste-like carbon material was applied to one side of a copper foil as a current collector using a doctor blade applicator with a clearance set to 200 μm to produce an electrode plate. The electrode plate was dried at 100 ° C. for 12 minutes and then pressed so that the electrode density was 1.8 to 2.2 g / cm ³ . This was vacuum-dried at 130 ° C. for a whole day and night to obtain a test electrode.

A porous separator is interposed between the obtained test electrode having an electrode density of 1.9 g / cm ³ and a lithium foil used as a counter electrode, and the test electrode is immersed in a non-aqueous electrolyte. A battery was produced. As a non-aqueous electrolytic solution, LiPF ₆ is adjusted to a concentration of 1 mol / kg in a solvent in which ethylene carbonate (EC) and ethyl methyl carbonate (EMC) are mixed at a volume ratio of EC: EMC = 1: 2. What was dissolved in was used.

  When the battery characteristics of this test battery were examined, the discharge capacity was 346 mAh / g and the load reverse capacity was 20 mAh / g. The charge / discharge efficiency calculated from these values was 94.2%. The charge acceptance was 35.3% with a charge of 1.0 C, and the discharge rate was 92.0% with a discharge of 2.0 C. “C” is a coefficient indicating the time ratio of the current value during constant current discharge (charging), and “1.0 C charging (discharging)” is the current at which charging (discharging) is completed in one hour. It means that constant current charging (discharging) is performed by value. “Discharging (charging) at 2.0 C” means that constant current discharging (charging) is performed at a current value at which discharging is completed in 0.5 hours.

[Sample No.2]
Hydrogenated pitch (softening point 218 ° C.) prepared using coal tar as a raw material was used as the pitch-based material. The same spinning device as that of No. 1 was used as the spinning device of FIG. 1, but the preheated nitrogen gas was not introduced into the space 5B of the melt discharge nozzle 5. Further, a ground electrode was placed at a position directly below the melt discharge nozzle 5 and at a distance of 60 mm from the nozzle port 53a. Moreover, the pressure of the pressurized nitrogen introduce | transduced into the storage container 2 from the nitrogen gas supply line 4 was 0.1 MPa, 0.3 MPa, and 0.4 MPa. Except for this, spinning was performed in the same manner as No. 1.

As a result, spinning proceeded satisfactorily, and when the pressure of pressurized nitrogen was 0.1 MPa, pitch fibers D having a diameter of about 200 nm were obtained. When the pressure of the pressurized nitrogen was 0.3 MPa, pitch fibers E having a diameter of about 1000 nm (1 μm) were obtained. When the pressure of the pressurized nitrogen was 0.5 MPa, pitch fibers F having a diameter of about 3000 nm (3 μm) were obtained.
The infusibilization process, carbonization process, and graphitization process for the obtained pitch fibers D to F were performed by the same method as No. 1. As a result, a uniform carbon fiber D having a diameter of around 200 nm was obtained from the pitch fiber D. From the pitch fiber E, a uniform carbon fiber D having a diameter of about 1000 nm (1 μm) was obtained. A uniform carbon fiber F having a diameter of about 3000 nm (3 μm) was obtained from the pitch fiber F.

  The obtained carbon fibers D to F were each pulverized in an agate mortar so that the average fiber length was 30 μm. When these carbon fibers were applied to an X-ray diffractometer and the (002) plane spacing was measured by the same method as in No. 1, it was 0.3358 nm. As a result of observing these carbon fibers with a transmission electron microscope, it was similar to the structure shown in FIG. 3, but the a-axis of the graphite microcrystal was slightly oriented in the axial direction of the fiber.

The carbon fibers D to F produced as described above were mixed so that D: E: F = 1: 2: 7 by mass ratio. This mixture and the polyvinylidene fluoride (PVdF) as a binder were mixed so that the mass ratio of the mixture: PVdF = 90: 10. Using this mixture, a test electrode was obtained in the same manner as in No. 1. A porous separator is interposed between the obtained test electrode having an electrode density of 1.9 g / cm ³ and a lithium foil used as a counter electrode, and the test electrode is immersed in a non-aqueous electrolyte. A battery was produced. The same non-aqueous electrolyte as No. 1 was used.
When the battery characteristics of this test battery were examined, the discharge capacity was 344 mAh / g, and the load reverse capacity was 22 mAh / g. The charge / discharge efficiency calculated from these values was 93.6%. The charge acceptance was 33.3% with a charge of 1.0 C, and the discharge rate was 90.1% with a discharge of 2.0 C.

[Sample No.3]
Carbon fibers (VGCF) produced by the vapor phase growth method were obtained with an average diameter of 150 nm and an average fiber length of 8 μm. This carbon fiber was subjected to an X-ray diffractometer to measure the (002) plane spacing, which was 0.3383 nm. Moreover, as a result of observing this carbon fiber with a transmission electron microscope, the fiber structure was a multi-wall type as shown in FIG. As shown in FIG. 4, with such a structure, electrons (e ⁻ ) pass in the direction along the fiber axis, but do not pass in the direction intersecting the axis.

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

When the cycle characteristics of the obtained No. 1 to No. 3 test batteries were examined, it was found that the No. 1 and No. 2 test batteries were excellent in cycle characteristics.
From these test results, No. 1 and No. 2 test batteries having a test electrode made of a negative electrode material corresponding to an example of the present invention, No. 1 having a test electrode made of a negative electrode material corresponding to a comparative example. Compared with the test battery of 3, excellent battery characteristics are obtained. In comparison between No. 1 and No. 2, particularly excellent battery characteristics were obtained with the No. 1 test battery.

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

Claims (5)

  Consists of a mixture of fine carbon fibers composed of a large number of microcrystalline graphite and having a diameter of 50 nm to 800 nm and thick carbon fibers composed of a large number of microcrystalline graphite and a diameter of 900 nm to 5000 nm. A negative electrode material for a secondary battery.   The thin carbon fiber and the thick carbon fiber are composed of a large number of randomly oriented microcrystalline graphite, and the (002) plane spacing measured by X-ray diffraction of Cu-Kα ray is 0.3400 nm or more and 0.00. The negative electrode material for a secondary battery according to claim 1, which is 3700 nm or less. The thin carbon fiber and the thick carbon fiber are
When discharging the melt of pitch-based material from the spinning nozzle toward the collector, a preheated gas is supplied around the spinning nozzle, and this preheated gas is supplied to the collector in parallel with the discharge direction of the melt. Spinning process of pitch fiber by electrospinning method, characterized by spraying toward
Pitch fiber infusibilization process,
Heat treating the infusible pitch fiber in an inert gas atmosphere;
The negative electrode material for a secondary battery according to claim 2, wherein the negative electrode material is produced through a process.   The negative electrode for lithium ion secondary batteries which has the negative electrode material for secondary batteries of Claim 1 or 2 as a main component of an active material.   The lithium ion secondary battery which has a negative electrode for lithium ion secondary batteries of Claim 3.

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