JP2003263982A - Method for producing graphitic particles and negative electrode material for lithium ion secondary battery - Google Patents

Abstract

(57) [Problem] To prepare a negative electrode of a lithium ion secondary battery by mixing graphitic particles obtained by heat-treating mesophase pitch in the presence of a binder and a dispersion medium, when the dispersion medium is aqueous. Even so, the provision of graphitic particles exhibiting battery characteristics such as initial charge and discharge efficiency equivalent to that of an organic solvent system. SOLUTION: Particles having an average particle diameter smaller than the average particle diameter of the graphitic precursor particles on the surface of the graphitic precursor particles (such as mesophase small spheres) and being harder than the graphitic precursor particles (Silica, etc.) is embedded by mechanochemical treatment using, for example, a compression-shear dry-powder compounding device, and the resulting precursor particles are graphitized, and the embedded hard particles are vaporized, sublimated or decomposed. Of producing graphitic particles.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for producing graphite particles, in particular, high discharge capacity and high initial charge / discharge efficiency.
A method for producing graphite particles capable of obtaining a lithium ion secondary battery that can be charged at high speed even when an anode is produced using an aqueous binder, a negative electrode material for a lithium ion secondary battery obtained by the production method, and a lithium ion secondary battery. The present invention relates to a negative electrode for a secondary battery and a lithium ion secondary battery.

[0002]

2. Description of the Related Art In recent years, with the miniaturization and higher performance of electronic devices, there has been an increasing demand for higher energy density of batteries. Under such circumstances, a lithium ion secondary battery has been attracting attention as a battery having a high energy density and capable of high voltage. As a negative electrode material for this lithium ion secondary battery, graphite having excellent charge / discharge characteristics, high discharge capacity and potential flatness (Japanese Patent Publication No. 62-23).
No. 433, etc.) is the mainstream. As graphite (graphite particles) used as the negative electrode material, graphite particles such as natural graphite and artificial graphite, and mesophase pitch obtained by heat-treating mesophase pitch such as tar and pitch, for example, mesophase spherules, are obtained. Graphite particles can be mentioned.

The negative electrode is formed of a negative electrode material, a negative electrode material, a binder (binder resin) for binding the negative electrode material and the current collector, and a current collector. Specifically,
A negative electrode mixture paste containing the above negative electrode material and a binder is prepared, and then this paste is applied onto a current collector such as a copper foil and pressed to produce a negative electrode.

[0004]

The above-mentioned natural graphite as a negative electrode material has a high discharge capacity, but on the other hand, it tends to be oriented when the negative electrode is formed due to the scale shape, and has cycle characteristics and rate characteristics (rapid charge / discharge characteristics). ) Decreases. On the other hand, the graphitic particles obtained by heat-treating the mesophase pitch, especially the graphitic particles of mesophase spherules,
It has a spherical shape or a shape close to a spherical shape, and has good cycle characteristics and rate characteristics because it is laminated randomly at the time of forming the negative electrode, but the performance may not be sufficiently drawn out depending on the form of the binder when forming the negative electrode. . For example, when the dispersion solvent is an organic solvent, the performance of the negative electrode material can be sufficiently exhibited, but when the dispersion solvent is an aqueous solvent, battery characteristics such as charging rate may be deteriorated. In recent years,
From the viewpoint of safety and the like, in view of the situation where the use of an aqueous solvent, that is, an aqueous binder is desired, even when using an aqueous binder, make the graphite particles sufficiently exhibit the performance as a negative electrode material. The advent of a method of obtaining is desired.

[0005]

Means for Solving the Problems The present inventor has studied the problems of the negative electrode material for a graphite-based lithium ion secondary battery as described above, and found that the graphite precursor particles were formed on the surface of the graphite precursor particles. Mesophase-based graphite particles having an average particle size smaller than that of the particles and being harder than the graphite precursor particles (hereinafter also referred to as hard particles) are embedded and then graphitized. It is possible to solve the problem of battery characteristics of graphite particles, such as eliminating the solvent dependence of battery characteristics of, and being able to obtain the same battery characteristics as in the case of an organic solvent binder even in the case of an aqueous binder. I found it. Although the mechanism is not always clear, the hard particles embedded in the surface of the graphite precursor particles disappear during graphitization, and the surface of the finally obtained graphite particles is roughened. It is presumed that it enhances the adhesiveness and the absorption and desorption of lithium ions.

According to the present invention, particles having an average particle size smaller than that of the graphite precursor particles and harder than the graphite precursor particles are embedded on the surface of the graphite precursor particles. After that, it is graphitized, which is a method for producing graphite particles.

In the present invention, hard particles are vaporized during graphitization,
Particles of a metal, a metal oxide, a metal nitride, a metal boride, or a metal carbide that sublimes or decomposes are preferable.
In the present invention, it is particularly preferable that the hard particles are gas phase silica or titanium oxide.

In the present invention, it is preferable that the above-mentioned burying is performed by a mechanochemical treatment. In the present invention, the mechanochemical treatment is preferably performed by a high-speed impact type dry powder compounding device or a compression shearing dry powder compounding device.

Further, according to the present invention, particles having an average particle size smaller than that of the graphite precursor particles and harder than the graphite precursor particles are embedded on the surface of the graphite precursor particles. After that, it is a negative electrode material for lithium ion secondary batteries, which is composed of graphite particles obtained by a method of graphitizing.

Further, the present invention is a negative electrode for a lithium ion secondary battery, which uses the above-mentioned negative electrode material.

The present invention is also a lithium ion secondary battery using the above-mentioned negative electrode.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in more detail below. The graphite particles of the present invention are produced by a method of embedding hard particles on the surface of graphite precursor particles and then graphitizing.

Graphite Precursor Particle The graphite precursor particle used in the present invention is a highly crystalline carbonaceous material (soft carbon) capable of forming a graphite structure by high temperature heat treatment at 2500 ° C. or higher. The high temperature heat treatment includes all heat treatments such as heating tar or pitch to generate mesophase, carbonizing the mesophase, and the like. As the graphite precursor particles,
Mesophase calcined carbon obtained by heating petroleum-based or coal-based tar or pitch (bulk mesophase),
Examples include mesophase spheres, cokes (pitch coke, needle coke, petroleum coke, raw coke, etc.).

The graphite precursor particles include other carbon materials (including amorphous hard carbon, etc.), graphite materials (including natural graphite), organic substances, metals as long as the object of the present invention is not impaired. It may be a complex with a compound. The composite form may be any of mixing, granulating, coating, laminating and the like. In addition, the graphitic precursor particles include a liquid phase, a solid phase,
Various chemical treatments in the gas phase, heat treatments, oxidation treatments, 13
It may be one which has been previously subjected to a firing treatment at 00 ° C or less. Preferred are mesophase calcined carbon, composites of mesophase calcined carbon and graphitic material, mesophase microspheres, and particularly preferred are mesophase microspheres.
The average particle size of the graphite precursor particles used in the present invention is usually 1 to 100 μm, preferably 5 to 40 μm, and the average particle size is larger than that of the hard particles.

Hard Particles In the present invention, as described above, the hard particles are embedded in the surface of the graphite precursor particles and then graphitized. The hard particles have an average particle size smaller than the average particle size of the graphite precursor particles and are harder than the graphite precursor particles, and the form thereof is not particularly limited. Since the hard particles are harder than the graphite precursor particles, the hard particles can be embedded in the surface of the graphite precursor particles. The embedding in the present invention, the lower part of each hard particle enters the inside of the graphite precursor particles, not only when the upper part is exposed on the surface of the graphite precursor particles, a few hard particles are graphite precursor It also includes the case of being buried in body particles. The hard particles may remain partially in the graphite particles after graphitization, but from the viewpoint of roughening the surface of the graphite particles and improving the charge / discharge characteristics, the hard particles are vaporized during graphitization. (Evaporation), sublimation, or decomposition and disappearance is preferable.

The hard particles may be organic or inorganic, but the organic particles remain carbonized after graphitization and are used as a negative electrode material of a lithium ion secondary battery.
Inorganic particles are preferable because they may reduce the discharge capacity. Inorganic particles are metals, metal oxides, metal nitrides,
Particles of metal borides, metal carbides and the like are preferable, and metals and metal oxides are preferable. Silicon as the metal species,
Titanium, aluminum, iron, nickel etc. are illustrated. Specific examples thereof include particles of these metals and silica, particles of titanium oxide, alumina, iron oxide, etc., but preferred is anhydrous silica produced by a gas phase method (hereinafter,
It is also called vapor phase silica), and titanium oxide. These are 2
It is also possible to use a combination of two or more species.

In the present invention, the average particle size of the hard particles,
The surface roughness of the graphitized graphite particles can be adjusted by changing the compounding amount and the buried state / distribution. When the average particle diameter of the hard particles is 1 nm or more and 1 μm or less, preferably 10 to 200 nm and smaller than the average particle diameter of the graphite precursor particles, the degree of roughening of the graphite particle surface can be increased. The blending amount of the hard particles with respect to the graphite precursor particles is appropriately set according to the average particle diameter of the hard particles and the embedded state, but is usually 0.1 to 20% by volume,
It is preferably 1 to 10% by volume. When the hard particles are aggregates, the average particle size of the primary particles may be smaller than the average particle size of the graphite particles.

Embedding of Hard Particles In the present invention, as described above, the hard particles are embedded on the surface of the graphite precursor particles and then graphitized. For burying, a dry powder compounding device known as a surface reforming device is used, and a process in which mechanical external forces such as compressive force and shearing force are simultaneously applied to graphite precursor particles in the presence of hard particles (mechano Chemical treatment) is preferable. The compressive force and shear force of mechanochemical treatment are larger than those of normal stirring,
These mechanical stresses are preferably applied to the surface of the graphite precursor particles, and preferably do not substantially destroy the skeleton of the graphite precursor particles. Since destruction of the graphite precursor particles tends to increase the irreversible capacity, it is preferable to suppress the decrease in the average particle diameter of the graphite precursor particles due to the mechanochemical treatment to 20% or less.

When the burying is carried out by a mechanochemical treatment, as long as it is a device capable of simultaneously applying a compressive force and a shearing force to an object to be treated (graphite precursor particles and hard particles), the type of the device, The structure is not particularly limited. For example, a high-speed impact type dry powder compounding device such as a pressure kneader, a kneader for two rolls, a rotating ball mill, a hybridization system (manufactured by Nara Machinery Co., Ltd.), Mechanomicros (Nara Machinery Co., Ltd.) And a mechanofusion system (manufactured by Hosokawa Micron Co., Ltd.).

Above all, a device for simultaneously applying the shearing force and the compressing force by utilizing the difference in rotational speed is preferable. Specifically, a device having a rotating drum (rotating rotor), an internal member (inner piece) whose rotational speed is different from that of the drum, and a circulation mechanism (for example, a circulation blade) of the object to be processed (for example, FIG. ) To (b), using the Hosokawa Micron Co., Ltd. mechanofusion system whose schematic mechanism is shown in Fig. 4), the internal member rotates while applying centrifugal force to the object supplied between the rotating drum and the internal member. It is preferable to perform the mechanochemical treatment by simultaneously and repeatedly applying the compressive force and the shearing force due to the speed difference with the drum. In addition, a device for passing the object to be processed between a fixed drum (stator) and a rotating rotor rotating at a high speed to apply a compressive force and a shearing force to the object to be processed due to a speed difference between the fixed drum and the rotating rotor ( For example, a hybridization system (manufactured by Nara Machinery Co., Ltd.) whose schematic mechanism is shown in FIG. 1) is also preferable.

The conditions of the mechanochemical treatment differ depending on the apparatus used and cannot be generally stated, but it is preferable to set the reduction rate of the average particle size of the graphite particles due to the treatment to 20% or less. For example, when an apparatus including a rotating drum and an internal member is used, the peripheral speed difference between the rotating drum and the internal member: 5 to 50 m / sec, the distance between them is 1 to 10
The treatment is preferably performed under the conditions of 0 mm and the treatment time of 3 to 90 minutes. Further, in the case of an apparatus equipped with a fixed drum and a high-speed rotating rotor, it is preferable to perform processing under the conditions of a peripheral speed difference between the fixed drum and the rotating rotor of 10 to 100 m / sec and a processing time of 30 seconds to 10 minutes.

Before or during the mechanochemical treatment of the graphite precursor particles and the hard particles, a known conductive material, ion conductive material, or interface is used as long as the effect of the present invention is not impaired. Various additives such as activators and polymer compounds can be added.

Graphitization In the graphitization of the present invention, the graphite precursor particles in which the hard particles are embedded by the above-mentioned method are used in a known graphitization furnace,
If necessary, it is carried out by adjusting the shape and heat-treating (graphitizing) at a high temperature in a non-oxidizing atmosphere. The heat treatment temperature is preferably 2500 ° C. or higher, particularly preferably 2800 ° C. or higher, but 3300 ° C. is the upper limit from the viewpoint of heat resistance of the apparatus and suppression of sublimation of graphite. The time required for graphitization is 0.5 to 50 hours, preferably 2 to 20 hours.

Graphite particles The graphite particles obtained by graphitization have a lattice spacing d ₀₀₂ of 0.34 nm or less in X-ray diffraction and a true specific gravity of 2.2 or more in order to obtain a high discharge capacity. Those having a degree of graphitization are preferred. The lattice spacing d ₀₀₂ uses CuKα rays as X-rays and X with high-purity silicon as a standard substance.
Line Diffraction [Sugio Ohtani, Carbon Fiber, pp. 733-742 (1
986), modern editorial company].

At the time of graphitization, the embedded hard particles disappear due to vaporization, sublimation or decomposition to roughen the surface of the graphitic particles and increase the specific surface area. If the specific surface area is too large, the irreversible capacity increases and the safety of the battery decreases, so the nitrogen gas adsorption BET method specific surface area of the graphite particles is 20
m is ² / g or less, it is preferable that the viewpoint of exhibiting the adhesion between graphite particles and the binder 1 m ² / g or more. A particularly preferable specific surface area is 1.5 to 10 m ² / g. It is preferable that the morphology of the graphite particles maintains the morphology of the graphite precursor particles before graphitization. That is, like the graphite precursor particles, the morphology thereof is preferably spherical or nearly spherical. The average particle size is usually 1 to 100 μm,
It is preferably set to 5 to 40 μm.

The graphite particles of the present invention can be used for applications other than the negative electrode material, for example, conductive materials for fuel cell separators, graphite for refractory materials, etc., by utilizing the characteristics thereof, but especially lithium ion secondary batteries. It is suitable as a negative electrode material. Therefore, the present invention further extends to a lithium ion secondary battery negative electrode using a negative electrode material, and further to a lithium ion secondary battery.

Negative Electrode Material for Lithium Ion Secondary Battery The graphite particles of the present invention having the surface roughened as described above are mixed with a binder to prepare a negative electrode mixture paste,
It is used as a negative electrode material for lithium-ion secondary batteries, but the binder solvent is water-based (water-soluble binder and / or water-dispersible binder) without lowering the discharge capacity or increasing the irreversible capacity of the negative electrode material. The same charge / discharge characteristics as in the case of the organic solvent type can be obtained even with the agent). Therefore, the graphite particles of the present invention are optimal as a negative electrode material for lithium ion secondary batteries.

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 constituent elements, and the positive and negative electrodes each comprise a lithium ion carrier, and are non-aqueous during the charging and discharging process. Solvent entry and exit occurs between layers. Then, the battery mechanism is such that lithium ions are doped into the negative electrode during charging and dedoped from the negative electrode during discharging. The lithium ion secondary battery of the present invention is not particularly limited except that the graphite particles are used as the negative electrode material, and other battery constituents are the same as those of a general lithium ion secondary battery.

Negative Electrode Preparation of a negative electrode from the above-mentioned negative electrode material (graphite particles) in the present invention brings out the performance of the graphite particles sufficiently, has high moldability to powder, and is chemically and electrochemically stable. Any molding method can be used as long as it can obtain the negative electrode, and a usual molding method can be used. For producing the negative electrode, a negative electrode mixture obtained by adding a binder to the graphite particles may be used. By using the graphite particles, excellent charge / discharge characteristics are exhibited not only with an organic solvent-based binder that dissolves or disperses in an organic solvent but also with a water-soluble and / or water-dispersible water-based binder. A negative electrode can be obtained. Usually, the binder is preferably used in an amount of about 0.5 to 20% by mass based on the total amount of the negative electrode mixture.

The negative electrode mixture is prepared, for example, by adjusting graphite particles to have an appropriate particle size by classification or the like and mixing them with a binder. After that, for example, the negative electrode mixture is dispersed in a solvent, stirred using a stirrer, a mixer, a kneader, a kneader or the like to form a paste, the 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 adhered can be obtained. The paste is prepared, for example, by mixing and kneading graphite particles and a fluorine-based resin powder such as polytetrafluoroethylene in a solvent such as isopropyl alcohol.

Further, graphite particles, fluororesin powder such as polyvinylidene fluoride, carboxymethyl cellulose, styrene butadiene rubber and the like are stirred and mixed with a solvent such as N-methylpyrrolidone, dimethylformamide, water and alcohol to form a slurry. After that, even if the slurry is applied to one or both surfaces of the current collector and dried, the negative electrode having the negative electrode mixture layer uniformly and firmly adhered can be obtained. In consideration of safety and environmental effects in drying and removing the solvent, it is preferable to use an aqueous slurry obtained by dissolving and dispersing carboxymethyl cellulose, styrene butadiene rubber and the like in water or alcohol as a solvent.

Further, the graphite particles and resin powders such as polyethylene and polyvinyl alcohol are dry-mixed, and the mixture is hot-press molded into a current collector in a mold to obtain a laminate, which is further pressed and pressed. It is possible to manufacture a negative electrode in which the adhesive strength between the negative electrode mixture layer and the current collector is further enhanced by performing pressure bonding such as. The coating thickness of the negative electrode mixture layer is 10 to 200 μm.
Is preferred.

As the binder, those having chemical stability and electrochemical stability with respect to the electrolyte are preferable, and for example, fluorine-based resins such as polyvinylidene fluoride and polytetrafluoroethylene, polyethylene, polyvinyl alcohol, carboxymethyl cellulose. , Styrene-butadiene rubber and the like. Among them, an aqueous binder such as carboxymethyl cellulose (water-soluble), polyvinyl alcohol (water-soluble), styrene-butadiene rubber (water-dispersible) is particularly preferable in achieving the object of the present invention and maximizing the effect. . These can also be used together. In forming the negative electrode, conventionally known various additives such as a conductive agent and a binder can be appropriately used.

The shape of the current collector used for the negative electrode is not particularly limited, but may be foil, mesh, or mesh such as expanded metal. The material of the collector is copper, stainless steel,
Nickel or the like. The thickness of the current collector is 5 to 5 in the case of foil.
It is preferably 20 μm.

Using a negative electrode material composed of the above graphite particles,
The lithium ion secondary battery including the negative electrode prepared from the aqueous binder and the current collector, exhibits excellent charge and discharge characteristics,
Graphite particles that have been surface-modified by surface hydrophilicization and surface roughening adhere firmly to the water-based binder, and even after repeated charge and discharge, the graphite particles will collect each other, and the graphite particles and the water-based binder will collect electricity. Due to the strong contact with the body, and the fact that the binder is not uniformly thinned and intervenes between the graphite particles, and does not impede conductivity, ionic conductivity, electrolyte permeability, etc. It is supposed to do.

Positive Electrode 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 onto the surface of the current collector. As the binder, those exemplified for the negative electrode can be used. Graphite particles, for example, are used as the conductive agent.

The shape of the current collector is not particularly limited, but a foil shape, a mesh, a mesh shape such as expanded metal or the like is used. The material of the current collector is aluminum, stainless steel, nickel or the like. The thickness is preferably 10 to 40 μm.

Similarly to the negative electrode, the positive electrode mixture may be dispersed in a solvent to form a paste, and the paste-like positive electrode mixture may be applied to a current collector and dried to form a positive electrode mixture layer. ,
After forming the positive electrode mixture layer, pressure bonding such as pressing may be further performed. Thereby, the positive electrode material mixture layer is uniformly and firmly adhered to the current collector.

As the material of the positive electrode (positive electrode active material), it is preferable to select a material that can be doped / dedoped with a sufficient amount of lithium. Specifically, a lithium-containing transition metal oxide,
Transition metal chalcogenide, vanadium oxide (V
_{2 O 5, V 6 O 13} , V 2 O 4, etc. V ₃ O ₈₎ and lithium-containing compounds, such as the Li compound, the general formula M _X M
o ₆ S _8-Y (where X is a numerical value in the range of 0 ≦ X ≦ 4, Y is a value in the range of 0 ≦ Y ≦ 1 and M represents a metal such as a transition metal), activated carbon , Activated carbon fiber and the like.

The lithium-containing transition metal oxide is a composite oxide of lithium and a transition metal, and may be a solid solution of lithium and two or more kinds of transition metals. The lithium-containing transition metal oxide is specifically LiM (1)
_1-X M (2) _X O ₂ (where X is a numerical value in the range of 0 ≦ X ≦ 1 and M (1) and M (2) are composed of at least one transition metal element) or LiM ( 1) _2-Y M (2)
_Y O ₄ (where Y is a numerical value in the range of 0 ≦ Y ≦ 1 and M
(1) and M (2) are composed of at least one transition metal element. ). As the transition metal element represented by M, Co, Ni, Mn, Cr, Ti, V, Fe, Zn,
Al, In, Sn and the like can be mentioned, preferably Co, F
Examples thereof include e, Mn, Ti, Cr, V and Al.

As the lithium-containing transition metal oxide,
More specifically, LiCoO₂, LixNi_YM_1-YO₂(M
Is a transition metal element other than Ni, preferably Co, F
At least one selected from e, Mn, Ti, Cr, V, and Al.
One kind, 0.05 ≦ x ≦ 1.10, 0.5 ≦ Y ≦ 1.0
Is. ) Li composite oxide, LiNiO
₂, LiMnO₂, LiMn₂O_FourAnd so on.

The lithium-containing transition metal oxide as described above is prepared by using, for example, Li, a transition metal oxide, a hydroxide or a salt as a starting material, and mixing these starting materials in an oxygen atmosphere at 600 to 1000 ° C. It can be obtained by firing at a temperature. As the positive electrode active material, the above compounds may be used alone or in combination of two or more kinds. For example, a carbon salt such as lithium carbonate can be added to the positive electrode. When forming the positive electrode, various additives such as conventionally known conductive agents and binders can be appropriately used.

Electrolyte The electrolyte used in the present invention may be an electrolyte salt used in ordinary non-aqueous electrolytes, for example, L
_{iPF 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 S
O ₂ ) ₂ , LiC (CF ₃ SO ₂ ) ₃ , LiN (CF _{3
CH 2 OSO 2) 2, LiN} (CF 3 CF 2 OSO 2)
₂ , LiN (HCF ₂ CF ₂ CH ₂ OSO ₂ ) ₂ , Li
_{N ((CF 3) 2 CHOSO} 2) 2, LiB [(C 6 H
₃ ((CF ₃ ) ₂ ] ₄ , LiAlCl ₄ , LiSiF ₆
Lithium salt or the like can be used. Especially Li
PF ₆ and LiBF ₄ are preferably used from the viewpoint of oxidation stability. The electrolyte salt concentration in the electrolytic solution is 0.1 to 5 mol /
L is preferable, and 0.5 to 3.0 mol / l is more preferable.

The non-aqueous electrolyte may be a liquid non-aqueous electrolyte solution or a polymer electrolyte such as a solid electrolyte or a gel electrolyte. In the former case, the non-aqueous electrolyte battery is
It is configured as a so-called lithium ion battery, and in the latter case, the non-aqueous electrolyte battery is configured as a polymer electrolyte battery such as a polymer solid electrolyte battery and a polymer gel electrolyte battery.

When a liquid non-aqueous electrolyte solution is used, the solvent is ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate,
1,1- or 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, γ-butyrolactone, 1,3-dioxolane, 4-methyl-1,3-dioxolane, anisole, diethyl Ether, sulfolane, methylsulfolane, acetonitrile, chloronitrile, propionitrile, trimethyl borate, tetramethyl silicate, nitromethane, dimethylformamide, N-methylpyrrolidone,
Ethyl acetate, trimethyl orthoformate, nitrobenzene, benzoyl chloride, benzoyl bromide, tetrahydrothiophene, dimethyl sulfoxide, 3-methyl-2-
An aprotic organic solvent such as oxazolidone, ethylene glycol or dimethylsulfite can be used.

When the non-aqueous electrolyte is a polymer electrolyte such as a polymer solid electrolyte or a polymer gel electrolyte, it contains a matrix polymer gelled with a plasticizer (non-aqueous electrolyte solution),
Examples of the matrix polymer include ether polymers such as polyethylene oxide and cross-linked products thereof, polymethacrylate polymers, polyacrylate polymers, fluorine polymers such as polyvinylidene fluoride and vinylidene fluoride-hexafluoropropylene copolymer. Can be used alone or in combination. Among these, from the viewpoint of redox stability and the like, it is preferable to use a fluoropolymer such as polyvinylidene fluoride or vinylidene fluoride-hexafluoropropylene copolymer.

As the electrolyte salt and the non-aqueous solvent constituting the plasticizer contained in these polymer solid electrolytes and polymer gel electrolytes, any of the above can be used. In the case of a gel electrolyte, the concentration of the electrolyte salt in the non-aqueous electrolytic solution which is a plasticizer is preferably 0.1 to 5 mol / liter, and 0.5 to
2.0 mol / liter is more preferable. The method for producing such a solid electrolyte is not particularly limited, but, for example,
A method of mixing a polymer compound forming a matrix, a lithium salt and a solvent, and heating and melting the polymer compound, a lithium salt and a solvent in an appropriate organic solvent for mixing, and then mixing an organic solvent And a method of mixing a monomer, a lithium salt and a solvent, and irradiating the mixture with an ultraviolet ray, an electron beam or a molecular beam to form a polymer. The addition ratio of the solvent in the solid electrolyte is preferably 10 to 90% by mass, more preferably 30 to 80% by mass. When the content is 10 to 90% by mass, the conductivity is high, the mechanical strength is high, and a film is easily formed.

A separator may be used in the lithium ion secondary battery of the present invention. The separator is not particularly limited, and examples thereof include woven cloth, non-woven cloth, and synthetic resin microporous film. In particular, a synthetic resin microporous film is preferably used, and among them, a polyolefin microporous film is preferable in terms of thickness, film strength and film resistance. Specifically, it is a microporous membrane made of polyethylene and polypropylene, or a microporous membrane obtained by combining these.

In the lithium ion secondary battery of the present invention, since the initial charge / discharge efficiency is high, it is possible to use a gel electrolyte. The gel electrolyte secondary battery is configured by stacking a negative electrode containing graphite particles, a positive electrode, and a gel electrolyte in the order of, for example, a negative electrode, a gel electrolyte, and a positive electrode, and accommodating them in a battery exterior material. In addition to this, a gel electrolyte may be further arranged outside the negative electrode and the positive electrode. In the gel electrolyte secondary battery using such graphite particles in the negative electrode, even when propylene carbonate is contained in the gel electrolyte and the graphite particles have a small particle size such that the impedance can be sufficiently lowered. ,
Irreversible capacity can be kept small. Therefore, a large discharge capacity can be obtained and a high initial charge / discharge efficiency can be obtained.

Further, the structure of the lithium-ion secondary battery according to the present invention is arbitrary, and its shape and form are not particularly limited, and any of cylindrical, rectangular, coin-shaped, button-shaped, etc. can be selected. Can be selected. In order to obtain a more safe sealed non-aqueous electrolyte battery, it is desirable to provide a means for detecting an increase in the battery internal pressure and shutting off the current when an abnormality such as overcharge occurs. In the case of a polymer solid electrolyte battery or a polymer gel electrolyte battery, it may have a structure of being enclosed in a laminate film.

[0051]

EXAMPLES The present invention will now be specifically described with reference to examples, but the present invention is not limited to these examples.
In addition, in the following Examples and Comparative Examples, the graphite particles were evaluated by making button type secondary batteries for evaluation having a configuration as shown in FIG. 3, but the actual batteries are based on the concept of the present invention. It can be produced according to a known method. In the following examples and comparative examples, the physical properties of particles were measured as follows.

The average particle size of the graphite precursor particles, hard particles and graphite particles was measured by a laser diffraction type particle size distribution meter. The lattice spacing d ₀₀₂ of the graphite particles was determined by X-ray diffraction. The specific surface area of the graphite particles is B due to nitrogen gas adsorption.
ET specific surface area. The hardness of the graphite precursor particles and the hard particles is determined by filling 5 g of the graphite precursor particles or the graphite particles in a cylindrical container (inner diameter 20 mm), tamping 200 times, and then making the steel having the inner diameter of the cylindrical container. A round bar was pushed in from the top of the sample filling surface and a compression test was performed at a constant speed, and the load at the inflection point of the detected load (the point at which the detected load decreased due to the destruction of particles) was expressed as a relative value. That is, the inflection point load of the graphite precursor particles used in Example 1 described later was set to 1, and the relative values of the inflection point loads of the graphite precursor particles and the hard particles were shown.

Example 1 (1) Preparation of Negative Electrode Material Mesophase spheres obtained by heat treating coal tar pitch (graphite precursor particles, manufactured by Kawasaki Steel Co., Ltd., average particle size:
25 μm, relative hardness value: 1.0), corresponding to 8% by volume of vapor phase silica (“AEROSIL50”, manufactured by Nippon Aerosil Co., Ltd., average particle diameter: 30 nm, relative hardness value: 4.2) )
The resulting mixture was mixed with a dry powder compounding device (“Hybridization System”, manufactured by Nara Machinery Co., Ltd.) having a schematic structure as shown in FIG. 1, and the peripheral speed of the rotating rotor was 40 m / sec. During the treatment time of 6 minutes, while repeatedly dispersing the graphite particles, mechanical force such as impact force, compressive force including intermolecular interaction, frictional force, shearing force and the like is repeatedly applied, and mechanochemical treatment is performed. Graphite precursor particles in which vapor-phase silica is embedded on the surface of mesophase spherules (average particle size:
24 μm).

This was suspended at 3000 ° C. for 6 hours for graphitization to obtain graphite particles. Graphite particles have the initial shape (spherical shape), and the presence of pores on the surface is confirmed by SEM.
It was confirmed by observation (magnification: 50,000 times). Elemental analysis confirmed that Si element was not detected and that silica did not remain. The average particle size of the graphite particles is 22 μm, the lattice spacing d
₀₀₂ was 0.3362 nm, true specific gravity was 2.241, and specific surface area was 2.5 m ² / g.

(2) Preparation of Negative Electrode Mixture Paste Aqueous and organic solvent type negative electrode mixture pastes were prepared using the obtained graphite particles as a negative electrode material. <Preparation of water-based negative electrode mixture paste> Negative electrode material 97% by mass
Then, 1% by mass of carboxymethyl cellulose as a binder and 2% by mass of styrene-butadiene rubber were put in water and mixed by stirring for 5 minutes at 500 rpm using a homomixer to prepare an aqueous negative electrode mixture paste. <Preparation of Organic Solvent-Based Negative Electrode Mixture Paste> 90% by mass of the negative electrode material and 10% by mass of polyvinylidene fluoride as a binder are put in an N-methylpyrrolidone solvent and mixed with stirring by a homomixer at 500 rpm for 5 minutes. Then, an organic solvent-based negative electrode mixture paste was prepared.

(3) Preparation of Working Electrode (Negative Electrode) The above negative electrode mixture paste was applied on a copper foil (current collector 7b) to a uniform thickness, and the solvent was further evaporated at 90 ° C. in vacuum. Dried. Next, the negative electrode mixture applied on this copper foil was pressed by a roller press to have a diameter of 15.5.
By punching into a circular mm shape, a working electrode (negative electrode) 2 composed of a negative electrode mixture layer adhered to the current collector was produced.

(4) Preparation of Counter Electrode (Positive Electrode) As the counter electrode (positive electrode) 4, a lithium metal foil was pressed against a nickel net and punched into a circular shape having a diameter of 15.5 mm to form a current collector 7a made of the nickel net. A counter electrode 4 made of a lithium metal foil adhered to the current collector was prepared.

(5) Electrolyte 33 mol% ethylene carbonate-67 mol% methyl ethyl carbonate mixed solvent of LiPF ₆ at 1 mol / mol.
A non-aqueous electrolytic solution was prepared by dissolving it at a concentration of dm ³ .
A polypropylene porous body was impregnated with the obtained non-aqueous electrolytic solution to prepare a separator 5 impregnated with the electrolytic solution.

(6) Preparation of Evaluation Battery A button type secondary battery shown in FIG. 3 was prepared as an evaluation battery. The outer cup 1 and the outer can 3 have a hermetically sealed structure in which they are caulked at their peripheral portions via an insulating gasket 6, and inside thereof, a current collector 7a made of a nickel net, in order from the inner surface of the outer can 3, This is a battery system in which a disk-shaped counter electrode 4 made of a lithium foil, a separator 5 impregnated with an electrolyte solution, a disk-shaped working electrode 2 made of a negative electrode mixture, and a current collector 7b made of a copper foil are laminated.

In the evaluation battery, a separator 5 impregnated with an electrolyte solution was sandwiched between a working electrode 2 in close contact with a current collector 7b and a counter electrode 4 in intimate contact with a current collector 7a, and then laminated. 2 is housed in the outer cup 1, the counter electrode 4 is housed in the outer can 3, the outer cup 1 and the outer can 3 are combined, and the peripheral portions of the outer cup 1 and the outer can 3 are caulked with the insulating gasket 6 interposed therebetween. It was sealed and prepared. This evaluation battery is a battery including a working electrode 2 containing graphite particles that can be used as a negative electrode active material in an actual battery, and a counter electrode 4 made of a lithium metal foil. With respect to the evaluation battery manufactured as described above, the following charge / discharge test was performed at a temperature of 25 ° C.

(7) Charge / Discharge Test <Initial Charge / Discharge Efficiency> At a current value of 0.9 mA, the circuit voltage was 0.
Constant current charging until reaching mV, circuit voltage is 0 mV
When it reaches the threshold, it switches to constant voltage charging and the current value is 2
After continuing charging until it reached 0 μA, it was paused for 120 minutes. Next, constant current discharge was performed at a current value of 0.9 mA until the circuit voltage reached 1.5V. At this time, the charge capacity and the discharge capacity were obtained from the energization amount in the first cycle, and the initial charge / discharge efficiency was calculated from the following equation. Initial charge / discharge efficiency (%) = (discharge capacity / charge capacity) × 10
0 In this test, the process of doping lithium ions into the graphite particles was charged and the process of dedoping from the graphite particles was discharged.

<Rapid Charging Efficiency> Following the above, high speed charging was performed in the second cycle. 4.5 times the current value
As mA, constant current charging was performed until the circuit voltage reached 0 mV, the charging capacity was obtained, and the rapid charging efficiency was calculated from the following formula 1.

[0063]

[Equation 1]

Discharge capacity per 1 g of graphite particles (mAh / g
), Initial charge / discharge efficiency (%) and quick charge efficiency (%)
Is shown in Table 2.

Example 2 Instead of the mesophase spheres (average particle size: 25 μm) of Example 1, bulk mesophase (average particle size: 25 μm, relative hardness value: 1.0, specific surface area: 0.6 m) Graphite particles were produced in the same manner as in Example 1 except that ² / g) was used, a negative electrode and a lithium ion secondary battery were produced in the same manner as in Example 1, and the same evaluation test as in Example 1 was conducted. . The results are shown in Table 2.

[Example 3] Raw coke obtained by heat-treating petroleum pitch (graphite precursor particles, manufactured by Koa Oil Co., Ltd.,
Form: block, average particle size: 25 μm, relative hardness value: 1.
In 1), vapor phase titanium oxide equivalent to 5% by volume (“AERO
SILP-25 ", manufactured by Nippon Aerosil Co., Ltd., average particle size: 21
nm, relative value of hardness: 4.6), and the resulting mixture was used as a dry powder compounding device (mechanofusion system, Hosokawa Micron Co., Ltd.) with a schematic structure as shown in FIGS. 2 (a) to (b). )), The peripheral speed of the rotating drum is 20 m / sec,
Processing time 60 minutes, distance between rotating drum and internal member 5m
A compressive force and a shearing force were repeatedly applied under the condition of m, and mechanochemical treatment was carried out to produce graphite precursor particles (average particle diameter: 24 μm) in which vapor phase titanium oxide was embedded on the surface of raw coke.

This was suspended at 3000 ° C. for 6 hours for graphitization to obtain graphite particles. Graphite particles have the initial shape (spherical shape), and the presence of pores on the surface is confirmed by SEM.
It was confirmed by observation (magnification: 50,000 times). Elemental analysis confirmed that Ti element was not detected, and that titanium oxide did not remain. The average particle diameter of the graphite particles was 22 μm, the lattice spacing d ₀₀₂ was 0.3364 nm, the true specific gravity was 2.235, and the specific surface area was 2.0 m ² / g. Using the graphite particles as a negative electrode material, a negative electrode and a lithium ion secondary battery were prepared in the same manner as in Example 1, and the same evaluation test as in Example 1 was performed. The resulting battery characteristics are shown in Table 2.

As shown in Table 2, the lithium ion secondary batteries (Examples 1 to 3) using the negative electrode material containing the graphite particles of the present invention as the working electrode (corresponding to the negative electrode of the actual battery) are organic. Not only the negative electrode prepared using the solvent-based negative electrode mixture paste but also the negative electrode prepared using the aqueous negative electrode mixture paste has a high level of discharge capacity and high initial charge / discharge efficiency ( That is, it was confirmed that it has a high rapid charging efficiency together with a small irreversible capacity).

Comparative Example 1 Mesophase microspheres were mechanochemically treated in the same manner as in Example 1 except that the vapor phase silica was not added. Sphere (Average particle size: 2
3 μm) was obtained. Using this, a negative electrode mixture was prepared in the same manner as in Example 1, a negative electrode and a lithium ion secondary battery were prepared in the same manner as in Example 1, and an evaluation test was conducted in the same manner as in Example 1. The results of the battery characteristics are shown in Table 2.

[Comparative Example 2] In Example 1, a Henschel mixer (manufactured by Mitsui Mining Co., Ltd.) having a small shearing force was used in place of the mechanochemical treatment, and the stirring speed was 700 rpm.
Were mixed for 30 minutes to prepare graphitic precursor particles (average particle size: 23 μm) in which silica was adhered to the surface of the mesophase microspheres. Using this, a negative electrode mixture was prepared in the same manner as in Example 1, a negative electrode and a lithium ion secondary battery were prepared in the same manner as in Example 1, and an evaluation test was conducted in the same manner as in Example 1. The results of the battery characteristics are shown in Table 2.

As shown in Table 2, when the hard particles are not embedded in the graphite precursor particles, no pores are formed on the surface of the graphite particles obtained by graphitizing the particles, and the specific surface area is reduced. Is small. Therefore, in the lithium ion secondary battery using this as a negative electrode material, the one prepared from the organic solvent-based negative electrode mixture paste exhibits high discharge capacity, initial charge / discharge efficiency, and rapid charge efficiency as in Example 1. It can be seen that the one prepared from the water-based negative electrode mixture paste has a low rapid charging efficiency.

[0072]

[Table 1]

[0073]

[Table 2]

[0074]

The graphitic particles of the present invention have a roughened surface and a large specific surface area while maintaining the particle size. Therefore, it has a high rapid charge efficiency and maintains a high value for discharge capacity and initial charge / discharge efficiency regardless of whether an aqueous binder or an organic solvent binder is used as a binder for the negative electrode of a lithium ion secondary battery. can do. Therefore, the lithium ion secondary battery of the present invention satisfies the recent demand for higher energy density of the battery, and is effective for downsizing and high performance of the mounted device.

[Brief description of drawings]

FIG. 1 is a schematic explanatory diagram of a structure of a dry powder compounding device used in an example.

FIG. 2 (a)-(b) is a schematic explanatory view of the structure of another dry powder compounding apparatus used in the examples.

FIG. 3 is a cross-sectional view showing an evaluation battery for evaluating battery characteristics of graphite particles.

[Explanation of symbols]

1 exterior cup 2 Working electrode 3 exterior cans 4 opposite poles 5 Electrolyte solution impregnated separator 6 Insulation gasket 7a, 7b Current collector

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Hitomi Hatano             1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Made in Kawasaki             Technical Research Institute of Iron Co., Ltd. (72) Inventor Satoshi Yutani             1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Made in Kawasaki             Chiba Steel Works, Ltd. F-term (reference) 4G046 EA06 EB06 EB13 EC02 EC06                 5H029 AJ02 AJ05 AJ14 AK02 AK03                       AK05 AL07 AM03 AM04 AM05                       AM07 BJ03 BJ12 CJ02 CJ25                       CJ28 DJ14 DJ16 DJ17 HJ05                 5H050 AA02 AA07 AA19 BA17 CA02                       CA07 CA11 CB08 FA02 FA15                       FA17 FA19 GA00 GA02 GA25                       GA27 HA05

Claims (5)

[Claims] 1. After embedding particles having an average particle size smaller than the average particle size of the graphite precursor particles and harder than the graphite precursor particles on the surface of the graphite precursor particles, A method for producing graphite particles, which comprises graphitizing. 2. The method for producing graphite particles according to claim 1, wherein the burying is performed by a mechanochemical treatment. 3. A negative electrode material for a lithium ion secondary battery, comprising graphite particles obtained by the method according to claim 1. 4. A negative electrode for a lithium ion secondary battery, which uses the negative electrode material according to claim 3. 5. A lithium ion secondary battery using the negative electrode according to claim 4.