JP6322525B2 - Method for producing carbon-coated graphite particles - Google Patents

Description

  The present invention relates to a method for producing carbonaceous coated graphite particles, a negative electrode material for a lithium ion secondary battery containing the obtained carbonaceous coated graphite particles, a negative electrode for a lithium ion secondary battery, and a lithium ion secondary battery using the negative electrode About.

Lithium ion secondary batteries are widely used in portable electronic devices and are expected to be used in hybrid and electric vehicles. Under such circumstances, the lithium ion secondary battery is required to have such high capacity, high-speed charge / discharge characteristics, and cycle characteristics.
The lithium ion secondary battery has a negative electrode, a positive electrode, and a non-aqueous electrolyte as main components, and acts as a secondary battery by moving lithium ions between the negative electrode and the positive electrode during a discharging process and a charging process. Currently, graphite is widely used as the negative electrode material. Graphite is roughly classified into natural graphite and artificial graphite. Natural graphite has the advantage of high crystallinity and high capacity, but due to the scale shape, the particles are oriented in one direction within the electrode, resulting in inferior high-speed charge / discharge characteristics and cycle characteristics.
In order to compensate for this, many materials have been proposed in which scaly graphite is processed into a spherical shape and surface-treated.

In Patent Document 1, a binder pitch is added to graphite particles obtained by granulating and forming natural graphite into a substantially spherical shape using a binder, heated and mixed, and then fired at 800 to 1400 ° C. in a non-oxidizing atmosphere. A method for producing a negative electrode material for a lithium ion secondary battery is disclosed. However, when the carbon coating amount is increased, the particles are fused, and a pulverization step is required, and the pulverization surface has a high reaction activity with the electrolytic solution, resulting in a decrease in charge / discharge efficiency and cycle characteristics. there were. Further, when polyvinylidene fluoride is used as a binder at the time of electrode preparation, there is a problem that the peel strength of the electrode is remarkably reduced.
Patent Document 2 discloses a method of firing in two stages in an inert atmosphere with respect to carbon coating on a graphite granulated product. As a result of investigation by the inventors of the present application, there is no need for a crushing step due to particle fusion depending on firing conditions, and there was no reduction in charge / discharge efficiency and cycle characteristics associated with crushing as described above. However, the peel strength of the electrode did not improve and remained low.
Patent Document 3 discloses a method for producing a carbon material for a negative electrode of a non-aqueous electrolyte secondary battery by subjecting a carbon material having a d ₀₀₂ value, a tap density, and a Raman R value by a specific X-ray wide angle diffraction method to mechanical processing. Have been described. However, in this method, it is difficult to obtain a specific carbon material as a raw material industrially with good reproducibility, and according to experiments by the present inventors, carbon obtained by performing mechanical treatment after one firing is obtained. When a negative electrode was produced using the raw material as a negative electrode material, the electrode peel strength was insufficient.

Japanese Patent No. 3716818 JP 2004-63321 A JP 2012-94505 A

  The present invention has been made in view of the above situation. That is, it aims at providing the negative electrode material which can obtain the outstanding battery characteristic and the outstanding electrode peeling strength, when it uses for the negative electrode material for lithium ion secondary batteries. Another object is to provide a method for producing the negative electrode material, a negative electrode containing the negative electrode material, and a lithium ion secondary battery using the negative electrode. Here, the excellent battery characteristics are at least one selected from high discharge capacity, high initial charge / discharge efficiency, and excellent high-speed charge / discharge characteristics.

The present invention provides the following.
(1) A mixing step of mixing spherical or ellipsoidal graphite particles having an average particle diameter of 20 to 50 μm and a carbonaceous precursor, and a mixture obtained in the mixing step, in an oxidizing atmosphere, at 300 ° C. to 700 ° C. A first baking step of baking in a temperature range of less than ° C., and a second baking of the first baking product obtained in the first baking step in a temperature range of 700 ° C. to 2000 ° C. in a non-oxidizing atmosphere. And the second calcined product obtained in the second calcining step are mechanochemically processed to coat the graphite particles: 70 to 91 % by mass with carbonaceous: 9 to 30% by mass. A method for producing carbonaceous coated graphite particles, comprising a compression shearing step for obtaining coated graphite particles.
(2) The carbonaceous coated graphite particles according to (1) above, wherein carbonaceous coated graphite particles obtained by coating the graphite particles: 85 to 91 mass% with carbonaceous materials: 9 to 15 mass% are obtained in the compression shearing step. Manufacturing method.
(3) The method for producing carbonaceous coated graphite particles according to (1) or (2), wherein in the first firing step, firing is performed at a temperature range of 500 ° C. or higher and lower than 700 ° C. for 3 to 50 hours.
(4) The method for producing carbonaceous coated graphite particles according to any one of (1) to (3), wherein in the mixing step, the graphite particles are spherical graphite particles having an average particle diameter of 20 to 50 μm. .

  The carbon-coated graphite particles of the present invention are at least one selected from a good discharge capacity, initial charge / discharge efficiency and fast charge / discharge characteristics as a negative electrode material for a lithium ion secondary battery, and a negative electrode having excellent electrode peel strength Material. Therefore, the lithium ion secondary battery using the negative electrode material of the present invention satisfies the recent demand for higher energy density of the battery, and is useful for downsizing and higher performance of equipment to be mounted. In addition, since the electrode peel strength is high, there are few defective products during production, and the production yield is increased.

It is sectional drawing of the evaluation battery for evaluating the battery characteristic of the negative electrode of this invention. It is sectional drawing explaining the measuring system which measures the electrode peeling strength of negative electrode material.

Hereinafter, the present invention will be described more specifically.
1. Raw material for carbon-coated graphite particles (core material)
The core material of the carbonaceous coated graphite particles of the present invention is a spherical or ellipsoidal graphite particle, preferably a graphite particle processed into a spherical or ellipsoidal shape. The average particle size is preferably 1 to 50 μm, more preferably the average particle size is 5 to 30 μm. The average aspect ratio is preferably 5 or less, and more preferably the average aspect ratio is 2 or less. The average specific surface area is preferably 10 m ² / g or less, and more preferably 8 m ² / g or less.
As the graphite particles, natural graphite particles or artificial graphite particles can be used. Natural graphite particles are preferred for reasons such as high crystallinity. Commercially available natural graphite particles processed into a spherical or ellipsoidal shape can also be used. In the case of natural graphite having a shape other than spherical or ellipsoidal shape, for example, flaky graphite particles, natural flaky graphite can be granulated and spheroidized by mechanical external force to obtain spherical graphite particles. The method of processing into a spherical or ellipsoidal shape is, for example, a method of mixing a plurality of scaly graphites in the presence of a granulating aid such as an adhesive or a resin, without using an adhesive for a plurality of scaly graphites. The method of applying mechanical external force, the combined use of both, etc. are mentioned. However, the most preferable method is to apply a mechanical external force to granulate into a spherical shape without using a granulating aid. The mechanical external force is mechanically pulverizing and granulating, and scaly graphite can be granulated and spheroidized. Examples of the pulverized graphite crushing device include a kneader such as a pressure kneader and two rolls, a rotating ball mill, a counter jet mill (manufactured by Hosokawa Micron Corporation), a current jet (manufactured by Nisshin Engineering Co., Ltd.), and the like. The device is ready for use.

  The pulverized product has an acute-angled surface, but the pulverized product may be granulated and used. Examples of granulated spheroidizing devices for pulverized products include granulators such as GRANUREX (manufactured by Freund Sangyo Co., Ltd.), Newgra Machine (manufactured by Seisin Corporation), Agromaster (manufactured by Hosokawa Micron Co., Ltd.), and hybridization. Shear compression processing devices such as [manufactured by Nara Machinery Co., Ltd.], Mechano Micros [manufactured by Nara Machinery Co., Ltd.], and mechanofusion system [manufactured by Hosokawa Micron Co., Ltd.] can be used.

Lc, which is a measured value of X-ray diffraction of graphite particles used as a core material of carbonaceous coated graphite particles, is preferably 40 nm or more, and La is preferably 40 nm or more. Here, Lc is the crystallite size Lc (002) in the c-axis direction of the graphite structure, and La is the crystallite size La (110) in the a-axis direction. The ratio I ₁₃₆₀ / I ₁₅₈₀ (R value) of ₁₃₆₀ cm ⁻¹ peak intensity (I ₁₃₆₀ ) and ₁₅₈₀ cm ⁻¹ peak intensity (I ₁₅₈₀ ) measured by Raman spectroscopy using an argon laser with d ₀₀₂ of 0.337 nm or less It is preferable that the half-value width of 0.06 to 0.30 and 1580 cm ⁻¹ band is 10 to 60.

[Carbonaceous precursor]
The core material which is the spherical or ellipsoidal graphite particles of the present invention is coated with carbonaceous matter by a production method described later using a carbonaceous precursor as a raw material. Although it does not specifically limit as a carbonaceous precursor used, Tar pitches and / or resin are illustrated. Specifically, as heavy oils, particularly tar pitches, coal tar, tar light oil, tar medium oil, tar heavy oil, naphthalene oil, anthracene oil, coal tar pitch, pitch oil, mesophase pitch, oxygen-crosslinked petroleum pitch, Examples include heavy oil. Examples of the resins include thermoplastic resins such as polyvinyl alcohol and polyacrylic acid, and thermosetting resins such as phenol resins and furan resins. It is advantageous in terms of cost if only tar pitches are used without containing resins. Any of the carbon precursors exemplified above may be used, but the coal tar pitch is particularly preferably 80% by mass or more.

2. Method for producing carbon-coated graphite particles [mixing step]
In the method for producing carbonaceous coated graphite particles of the present invention, first, the graphite particles of the core material and the carbonaceous precursor are mixed. A mixing process will not be specifically limited if it can mix uniformly, A well-known mixing method can be used. Preferably, solid graphite particles and a solid or semi-solid (including viscous liquid) carbonaceous precursor are mixed. Heavy oil is solid at room temperature. When a liquid carbonaceous precursor such as tar light oil or tar medium oil is mixed as a solvent, it is preferable to volatilize the solvent in advance at a temperature of about 200 ° C. or lower to perform the next first firing step. The mixing ratio is the ratio of the final product (carbonaceous coated graphite particles), and the raw materials are mixed so that the graphite particles are in the range of 70 to 99 mass% and carbonaceous 1 to 30 mass%. Preferably, the mixing ratio of the final product is in the range of 80 to 95% by mass of graphite particles and 5 to 20% by mass of carbonaceous material. You may perform mixing with the temperature rising process for the 1st heating process mentioned later. The heating and mixing method is not particularly limited, and examples thereof include a biaxial kneader having a heating mechanism such as a heater and a heat medium.

[First firing step]
While mixing the obtained mixture or raw material, in the first baking step, baking is performed in an oxidizing atmosphere in the range of 300 ° C. or higher and lower than 700 ° C. Within this temperature range, the carbonaceous precursor can be uniformly coated on the core material. The method for the baking treatment is not particularly limited, but baking is preferably performed while stirring, and a rotary kiln is preferable because homogeneous baking can be performed. The oxidizing atmosphere to be used is not limited, but an inert gas atmosphere containing 5 to 50% by volume of oxygen is preferable. This is because the degree of oxidation reaction is appropriate if an atmosphere containing oxygen in this range is used. It is considered that a bond is generated between carbonaceous precursors by an oxidation reaction. Examples of the inert gas include argon, helium, nitrogen, and the like. Firing in the range of 300 ° C. or higher and lower than 700 ° C. in air is preferable. This temperature range is preferred because the carbonaceous precursor can be uniformly coated on the core material. In the production method of the present invention, by performing the first firing step at a low temperature and in an oxidizing atmosphere, the carbonaceous precursor is bound to the core material of the graphite particles and small pores are increased in the carbon material of the final product, When used as a negative electrode material, it is considered that there is an effect that the adhesion strength between the negative electrode materials is increased due to an anchor effect or the like. In the first baking step, heat treatment may be performed in a plurality of stages. The firing temperature is preferably in the range of 300 to 550 ° C, and more preferably in the range of 300 to 500 ° C. When the first firing temperature is within a preferable temperature range, the carbonaceous material can be uniformly coated on the core material, so that the final carbonaceous coated graphite particles can be easily produced. The first baking time is preferably 5 minutes to 50 hours.

[Second firing step]
Next, as a second baking step, baking is performed at 700 to 2000 ° C. in a non-oxidizing atmosphere. The method for the baking treatment is not particularly limited, but baking is preferably performed while stirring, and a rotary kiln is preferable because homogeneous baking can be performed. The non-oxidizing atmosphere to be used is not limited, and examples thereof include argon, helium, and nitrogen. Firing at 700 to 2000 ° C. in a nitrogen stream is preferred. In the second baking step, heat treatment may be performed in a plurality of stages. The firing temperature is preferably in the range of 900 to 1300 ° C, and more preferably in the range of 900 to 1200 ° C.
The second baking time is preferably 5 minutes to 30 hours.
In either the first or second step, the temperature profile at the time of temperature rise and at the time of firing takes various forms such as linear temperature rise and stepwise temperature rise in which the temperature is held at a constant interval. It is possible to perform heat treatment a plurality of times.
The method for producing carbonaceous coated graphite particles of the present invention preferably does not include a pulverization step after firing.
Further, different types of graphite materials may be attached, embedded, or combined before firing. First and / or second after carbonaceous or graphite fiber, carbonaceous precursor material such as amorphous hard carbon, organic material, inorganic material is attached to, embedded in, or combined with black smoke particles of the core material You may perform a baking process.

  According to experiments by the present inventors, when a temperature exceeding 1250 ° C. is used as the second baking step, the baking time can be shortened, so that the production efficiency is increased and the discharge capacity is increased. However, if the subsequent compressive shearing step is not performed, the electrode peel strength may be lowered when the negative electrode material is used. The present inventors consider that the compressive shearing step described in the next step is a step in which the coated carbonaceous material can be repaired even when the carbonaceous material covering the graphite particles is deteriorated by high-temperature firing.

[Compression shearing process]
A compression shearing force is applied to the obtained fired product. The temperature and atmosphere at this time are not particularly limited, but are usually performed at room temperature and in air. The method for applying the compressive shearing force is also called mechanochemical treatment and is not particularly limited. Examples of devices capable of such operations include GRANUREX (manufactured by Freund Sangyo Co., Ltd.) and Newgra Machine (Seishin Enterprise Co., Ltd.). , Granulators such as Agromaster [made by Hosokawa Micron Co., Ltd.], roll mill, hybridization system [made by Nara Machinery Co., Ltd.], mechano micro system [made by Nara Machinery Co., Ltd.], mechano fusion system Examples include compression shearing processing devices such as Hosokawa Micron Corporation. When applying the compressive shear force, carbonaceous or graphite fibers, carbonaceous precursor materials such as amorphous hard carbon, organic materials, inorganic materials, and metal materials may be added.

In general, the shearing force and the compressive force are preferably such that the reduction rate of the average particle diameter of the heat-treated product by the compressive shearing treatment is suppressed to 20% or less.
The conditions of the compression shearing process vary depending on the apparatus used, and cannot be said unconditionally. For example, in the case of “Mechano-Fusion System”, the peripheral speed difference between the rotating drum and the internal member is 5 to 50 m / sec. It is preferable that the distance between them is 1 to 100 mm and the processing time is 3 to 90 min. In the case of the “hybridization system”, it is preferable that the peripheral speed difference between the fixed drum and the rotating rotor is 10 to 100 m / s, and the processing time is 30 s to 10 min.

  Unlike the pulverization process and the pulverization process, the compression shear process of the present invention is preferably such that the shear force is applied together with the compression force to suppress the reduction rate of the average particle diameter to 20% or less. By compressing and shearing, the particles are rubbed together after firing, so the resulting particles are repaired with cracks and cracks on the surface, improving battery characteristics such as discharge capacity, and rounded corners. The inventors believe that the shape, the electrode density can be improved, and the pressability can be improved. However, the function of the compression shearing process is not limited to this.

3. Carbonaceous coated graphite particles The carbonaceous proportion in the carbonaceous coated graphite particles, which is the final product of the present invention, is 1 to 30 mass%, and the proportion of the graphite particles is 70 to 99 mass%. When the carbonaceous content is less than 1% by mass, it becomes difficult to completely cover the active graphite edge surface, and the initial charge / discharge efficiency may be lowered. On the other hand, when it exceeds 30% by mass, the proportion of the carbon material having a relatively low discharge capacity is too large, and the discharge capacity of the carbonaceous coated graphite particles is lowered. In addition, the ratio of raw materials (thermosetting resins and tar pitches) for forming carbonaceous matter is large, and the carbonaceous coating is finally obtained in the coating step and the subsequent heat treatment step. Cracking and peeling may occur in a part of the carbonaceous layer of the graphite particles, and the initial charge / discharge efficiency may be reduced. The carbonaceous ratio in the carbon-coated graphite particles is particularly 1 to 20% by mass, further 1 to 15% by mass, and more preferably more than 3% by mass to 15% by mass. In addition, carbon content should just exist in the said range as an average of the whole carbonaceous covering graphite particle. It is not necessary that all the individual particles are within the above range, and some particles outside the above range may be included.

The average particle diameter of the carbon-coated graphite particles as the final product is preferably in the range of 1 to 50 μm, and more preferably in the range of 5 to 30 μm. The specific surface area measured by the BET method is preferably 5.0 m ² / g or less, and more preferably 3.0 m ² / g or less.
Further, the carbon-coated graphite particles had a ratio I ₁₃₆₀ / I ₁₅₈₀ (R value) of ₁₃₆₀ cm ⁻¹ peak intensity (I ₁₃₆₀ ) and ₁₅₈₀ cm ⁻¹ peak intensity (I ₁₅₈₀ ) measured by Raman spectroscopy using an argon laser. ) Is larger than the R value of graphite and is preferably 0.05 to 0.80.

[Negative electrode]
The present invention is also a negative electrode for a lithium ion secondary battery containing the above negative electrode material, and a lithium ion secondary battery using the negative electrode.
The negative electrode for a lithium ion secondary battery of the present invention is produced according to a normal negative electrode molding method, but is not limited as long as it is a method capable of obtaining a chemically and electrochemically stable negative electrode. When preparing the negative electrode, it is preferable to use a negative electrode mixture prepared in advance by adding a binder to the negative electrode material of the present invention. As the binder, those showing chemical and electrochemical stability with respect to the electrolyte are preferable. For example, fluorine-based resin powders such as polytetrafluoroethylene and polyvinylidene fluoride, and resin powders such as polyethylene and polyvinyl alcohol Carboxymethyl cellulose and the like are used. These can also be used together. A binder is normally used in the ratio of about 1-20 mass% in the whole quantity of a negative electrode mixture.
More specifically, first, the negative electrode material of the present invention 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 the negative electrode material of the present invention and a binder with a solvent such as water, isopropyl alcohol, N-methylpyrrolidone, dimethylformamide, etc., using a known stirrer, mixer, kneader, kneader, etc. Use to stir and mix to prepare paste. When the paste is applied to one or both sides of the current collector and dried, a negative electrode in which the negative electrode mixture layer is uniformly and firmly bonded is obtained. The film thickness of the negative electrode mixture layer is 10 to 200 μm, preferably 20 to 100 μm.
The negative electrode of the present invention can also be produced by dry-mixing the negative electrode material of the present invention and resin powders such as polyethylene and polyvinyl alcohol and hot pressing 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 producing the negative electrode is not particularly limited, but may be a foil shape, a mesh shape, a net shape such as expanded metal, or the like. The material for the current collector is preferably copper, stainless steel, nickel or the like. The thickness of the current collector is preferably about 5 to 20 μm in the case of a foil.
It should be noted that the negative electrode of the present invention can be included even if different types of graphite materials, carbonaceous materials such as amorphous hard carbon, organic substances, metals, metal compounds, and the like are mixed within a range that does not impair the object of the present invention. Alternatively, it may be coated or laminated.

[Positive electrode]
The positive electrode used in the lithium secondary battery of the present invention is formed, for example, by applying a positive electrode mixture comprising a positive electrode material, a binder and a conductive agent to the surface of the current collector. The positive electrode material (positive electrode active material) is preferably selected from materials that can occlude / release a sufficient amount of lithium, and lithium such as lithium-containing transition metal oxides, transition metal chalcogenides, vanadium oxides, and lithium compounds thereof. containing compound, the general formula _{M X Mo 6 S 8-Y} (M in the formula is a transition metal element of at least one, X is 0 ≦ X ≦ 4, Y is a number in the range from 0 ≦ Y ≦ 1) Chevrel phase compounds, activated carbon, activated carbon fibers and the like. Vanadium oxide _is one represented by _{V 2 O 5, V 6 O} 13, V 2 O 4, V 3 O 8.
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 transition metals. The composite oxide may be used alone or in combination of two or more. Specifically, the lithium-containing transition metal oxide is LiM ¹ _1-X M ² _X O ₂ (wherein M ¹ and M ² are at least one transition metal element, and X is in a range of 0 ≦ X ≦ 1. LiM ¹ _1-Y M ² _Y O ₄ (wherein M ¹ and M ² are at least one transition metal element, and Y is a value in the range of 0 ≦ Y ≦ 1). Indicated.
The transition metal elements represented by M ¹ and M ² are Co, Ni, Mn, Cr, Ti, V, Fe, Zn, Al, In, Sn, etc., preferably Co, Fe, Mn, Ti, Cr , V, Al, etc. Preferred examples include 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.
Examples of the lithium-containing transition metal oxide include lithium, transition metal oxides, hydroxides, salts, and the like as starting materials, and these starting materials are mixed in accordance with the composition of the desired metal oxide, and are mixed under an oxygen atmosphere. It can be obtained by firing at a temperature of ˜1000 ° C.
The positive electrode active material may be used alone or in combination of two or more. For example, a carbon salt such as lithium carbonate can be added to the positive electrode. Moreover, when forming a positive electrode, conventionally well-known various additives, such as a electrically conductive agent and a binder, can be used suitably.

[Production of positive electrode]
The positive electrode is produced by applying a positive electrode mixture comprising the positive electrode material, a binder, and a conductive agent for imparting conductivity to the positive electrode on both sides of the current collector to form a positive electrode mixture layer. As the binder, the same one as that used for producing the negative electrode can be used. As the conductive agent, known materials such as graphitized materials and carbon black are used.
The shape of the current collector is not particularly limited, but a foil shape or a mesh shape such as a mesh or expanded metal 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 formed in a paste by dispersing the positive electrode mixture in a solvent, 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 agent layer, pressure bonding such as press pressing may be further performed. As a result, the positive electrode mixture layer is uniformly and firmly bonded to the current collector.

[Non-aqueous electrolyte]
The non-aqueous electrolyte used in the lithium ion secondary battery of the present invention, an electrolyte salt used in the conventional non-aqueous _{electrolyte, 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 _{2 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, Lithium salts such as LiSiF ₆ can be used. From the viewpoint of oxidation stability, LiPF ₆ and LiBF ₄ are particularly preferable.
The electrolyte salt concentration in the electrolytic solution is preferably 0.1 to 5 mol / L, and more preferably 0.5 to 3.0 mol / L.
The non-aqueous electrolyte may be a liquid non-aqueous electrolyte or a polymer electrolyte such as a solid electrolyte or a gel electrolyte. In the former case, the non-aqueous electrolyte battery is configured as a so-called lithium ion secondary battery, and in the latter case, the non-aqueous electrolyte battery is configured as a polymer electrolyte battery such as a polymer solid electrolyte or a polymer gel electrolyte battery. .
As a solvent for preparing the nonaqueous electrolyte solution, 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-dioxolane, 4-methyl-1,3-dioxolane, ethers such as anisole and diethyl ether, thioethers such as sulfolane and methylsulfolane, acetonitrile, chloronitrile, propionitrile, etc. Nitrile, trimethyl borate, tetramethyl silicate, nitromethane, dimethylformamide, N-methylpyrrolidone, ethyl acetate, trimethyl orthoformate, nitrobenzene, benzoyl chloride, Benzoyl, tetrahydrothiophene, dimethyl sulfoxide, 3-methyl-2-oxazolidone, ethylene glycol, aprotic organic solvents such as dimethyl sulfite may be used.

When the non-aqueous electrolyte is a polymer electrolyte such as a polymer solid electrolyte or a polymer gel electrolyte, it is preferable to use a polymer gelled with a plasticizer (non-aqueous electrolyte) as a matrix. Examples of the polymer constituting the matrix include 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-hexafluoropropylene. It is particularly preferable to use a fluorine-based polymer compound such as a copolymer.
The polymer solid electrolyte or polymer gel electrolyte is mixed with a plasticizer, and as the plasticizer, the electrolyte salt and the non-aqueous solvent can be used. In the case of a polymer gel electrolyte, the concentration of the electrolyte salt in the nonaqueous electrolytic solution that is a plasticizer is preferably 0.1 to 5 mol / L, and more preferably 0.5 to 2.0 mol / L.
The method for producing the polymer solid electrolyte is not particularly limited. For example, a method of mixing a polymer compound constituting a matrix, a lithium salt, and a nonaqueous solvent (plasticizer) and heating to melt the polymer compound, an organic solvent A method in which a polymer compound, a lithium salt, and a non-aqueous solvent (plasticizer) are dissolved in, and an organic solvent for mixing is evaporated, a polymerizable monomer, a lithium salt, and a non-aqueous solvent (plasticizer) are mixed, and the mixture is mixed Examples thereof include a method of polymerizing a polymerizable monomer by irradiating an ultraviolet ray, an electron beam, a molecular beam or the like to obtain a polymer.
Here, the ratio of the non-aqueous solvent (plasticizer) in the solid 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]
In the lithium ion secondary battery of the present invention, a separator can also be used.
Although the material of a separator is not specifically limited, For example, a woven fabric, a nonwoven fabric, a synthetic resin microporous film, etc. can be used. As a material for the separator, a microporous membrane made of synthetic resin is suitable. Among them, a polyolefin microporous membrane is suitable in terms of thickness, membrane strength, and membrane resistance. Specifically, polyethylene and polypropylene microporous membranes, or microporous membranes composed of these are suitable.

[Manufacture of lithium ion secondary batteries]
The lithium ion secondary battery of the present invention is configured by laminating the negative electrode, the positive electrode, and the nonaqueous electrolyte having the above-described configuration in the order of, for example, the negative electrode, the nonaqueous electrolyte, and the positive electrode, and accommodating the laminate in the battery exterior material. The Further, a non-aqueous electrolyte may be disposed outside the negative electrode and the positive electrode.
In addition, the structure of the lithium ion secondary battery of the present invention is not particularly limited, and the shape and form thereof are not particularly limited, and are cylindrical, depending on the application, mounted equipment, required charge / discharge capacity, and the like. , Square shape, coin shape, button shape, and the like. In order to obtain a sealed nonaqueous electrolyte battery with higher safety, it is preferable to use a battery equipped with means for detecting an increase in the internal pressure of the battery and shutting off the current when an abnormality such as overcharging occurs.
In the case where the lithium ion secondary battery is a polymer solid electrolyte battery or a polymer gel electrolyte battery, a structure in which the lithium ion secondary battery is enclosed in a laminate film may be used.

Each physical property in this specification is measured by the following method.
1) Average particle size (μm): The particle size at which the cumulative frequency of the particle size distribution measured with a laser diffraction particle size distribution meter is 50% by volume percentage.
2) Average aspect ratio: An image analyzer (manufactured by Toyobo Co., Ltd.) was used for image processing of a scanning electron microscope 300 times the particle to be measured, and the aspect ratio (major axis direction) of any 50 graphite particles The average value of the ratio of the length to the length in the minor axis direction perpendicular to the length).

3) Electrode peel strength test: The test piece used is shown in FIG. A negative electrode mixture paste was prepared, and a part of the active material side was bonded to the aluminum plate 10 with a double-sided tape 12 in a state where the negative electrode material 15 was not pressed. The test piece was gripped by a part of the negative electrode material 15 using a tensile tester (manufactured by Shimadzu Corporation) and subjected to a tensile test in the 180 ° direction (arrow direction 17), and the average tensile test stress was defined as the peel strength.
4) Carbonaceous ratio (%): A carbonaceous precursor raw material (including a plurality of types) is provided with the same thermal history as that of carbonaceous coated graphite particles to prepare a carbonaceous carbide. The residual coal rate of the raw material was obtained. The ratio of carbonaceous matter in the carbonaceous coated graphite particles was calculated in terms of the residual carbon ratio obtained.

  EXAMPLES Next, although an Example demonstrates this invention concretely, this invention is not limited to these Examples. Further, in the following examples and comparative examples, as shown in FIG. 1, from a current collector (negative electrode) 7b having a negative electrode material 2 of the present invention attached to at least a part of the surface and a counter electrode (positive electrode) 4 made of lithium foil. A button-type secondary battery for single electrode evaluation was prepared and evaluated. An actual battery can be produced according to a known method based on the concept of the present invention.

Example 1
[Production of negative electrode material]
For 100 parts by mass of spherical natural graphite particles (average particle size 20 μm, average aspect ratio 1.4), a tar-in-oil solution of coal tar pitch (residual carbon ratio 50%) has a solid content ratio of 30 mass. And heated to 150 ° C. with a biaxial kneader and mixed for 60 minutes. The obtained mixture was subjected to heat treatment in the first firing step for 3 hours at 350 ° C. under a flow of 5 L / min using a rotary kiln (abbreviated as RK in the table). Next, this heat-treated product was subjected to a heat treatment in a second firing step for 3 hours at 1000 ° C. under a flow of nitrogen 2 L / min using a tubular furnace (abbreviated as tubular in the table).
Using a dry powder compounding device (Mechano-Fusion System, manufactured by Hosokawa Micron Co., Ltd.) for the heat treatment product of the second firing step, the peripheral speed of the rotating drum is 20 m / second, the processing time is 10 minutes, the rotating drum and the internal member The final product was obtained by applying a compressive force and a shearing force under the condition of 5 mm distance and performing mechanochemical treatment.
[Preparation of negative electrode mixture paste]
The negative electrode material (95% by mass) and polyvinylidene fluoride (5% by mass) were placed in N-methylpyrrolidone and stirred and mixed at 2000 rpm for 30 minutes using a homomixer to prepare an organic solvent-based negative electrode mixture.

[Production of working electrode (negative electrode)]
The negative electrode mixture paste was applied to a copper foil with a uniform thickness, the solvent was volatilized in a vacuum at 90 ° C., dried, and the negative electrode mixture layer was pressed by a hand press. The copper foil and the negative electrode mixture layer were punched into a cylindrical shape having a diameter of 15.5 mm to prepare a working electrode (negative electrode) composed of a current collector and a negative electrode mixture adhered to the current collector.
[Production of counter electrode (positive electrode)]
A lithium metal foil is pressed against a nickel net and punched into a circular shape with a diameter of 15.5 mm. A current collector made of nickel net and a counter electrode made of a lithium metal foil (thickness 0.5 mm) in close contact with the current collector ( Positive electrode) was prepared.
[Electrolyte, separator]
LiPF ₆ was dissolved at a concentration of 1 mol / kg in a mixed solvent of ethylene carbonate 50 wt% and propylene carbonate 50 wt% to prepare a non-aqueous electrolyte. The obtained nonaqueous electrolytic solution was impregnated into a polypropylene porous body (thickness 20 μm) to produce a separator impregnated with the electrolytic solution.

[Production of evaluation battery]
A button-type secondary battery shown in FIG. 1 was prepared as an evaluation battery.
The exterior cup 1 and the exterior can 3 were sealed by interposing an insulating gasket 6 at the peripheral portion thereof and caulking both peripheral portions. A copper foil in which a current collector 7a made of nickel net, a cylindrical counter electrode (positive electrode) 4 made of lithium foil, a separator 5 impregnated with an electrolyte, and a negative electrode material 2 are attached in that order from the inner surface of the outer can 3 This is a battery system in which current collectors 7b made of
In the evaluation battery, the separator 5 impregnated with the electrolytic solution was laminated between the current collector 7b and the counter electrode 4 in close contact with the current collector 7a, and then the current collector 7b was placed in the outer cup 1 4 is accommodated in the outer can 3, the outer cup 1 and the outer can 3 are combined, and further, an insulating gasket 6 is interposed between the outer peripheral portion of the outer cup 1 and the outer can 3, and both peripheral portions are caulked and sealed. And produced. The charge / discharge characteristics were measured by the following method. The results are shown in Tables 1 and 2.

[Charge / discharge test]
After constant current charging of 0.9 mA until the circuit voltage reaches 1 mV, switching to constant voltage charging is performed when the circuit voltage reaches 1 mV, and further, the charging capacity (unit: : MAh / g). Then, it rested for 10 minutes. Next, constant current discharge was performed at a current value of 0.9 mA until the circuit voltage reached 1.5 V, and the discharge capacity (unit: mAh / g) was determined from the amount of electricity supplied during this period. This was the first cycle. Next, charging and discharging were performed in the same manner as in the first cycle, with the charging current being 1C and the discharging current being 2C. Here, C is a unit of relative current based on the battery capacity, and 1C represents the amount of current that charges or discharges the battery capacity (mAh) in one hour (hr). The current values of 1C and 2C were calculated from the discharge capacity of the first cycle and the active material mass of the negative electrode.

The initial charge / discharge efficiency was calculated from the following equation (1).
Initial charge / discharge efficiency (%) = 100 × ((charge capacity of first cycle−discharge capacity of first cycle) / discharge capacity of first cycle) (1)
Moreover, 1C charge rate was computed from following Formula (2).
1C charge rate (%) = 100 × (charge capacity of CC (constant current) portion at 1C current value / discharge capacity of first cycle) (2)
The 2C discharge rate was calculated from the following equation (3).
2C discharge rate (%) = 100 × (discharge capacity at 2C current value / discharge capacity of first cycle) (3)
In this test, the process of occluding lithium ions in the negative electrode material was charged, and the process of detaching lithium ions from the negative electrode material was discharge.

(Examples 2 to 4, Example 5)
In Example 1, carbonaceous coated graphite particles were produced and evaluated in the same manner as in Example 1 except that the carbonaceous precursor content was changed as shown in the table.
(Examples 6 and 7)
In Example 1, carbon-coated graphite particles were produced and evaluated in the same manner as in Example 1 except that the temperature of the second firing step was changed as shown in the table.
(Example 8)
In Example 1, carbon-coated graphite particles were produced and evaluated in the same manner as in Example 1 except that the temperature of the first firing step was changed as shown in the table.
(Example 9, Example 10)
In Example 1, carbon-coated graphite particles were produced and evaluated in the same manner as in Example 1 except that the average diameter of the raw material natural graphite was changed as shown in the table.

(Comparative Example 1)
In Example 1, except that the compression shearing step was not performed after the second firing step, the production of the negative electrode material, the preparation of the negative electrode mixture, the production of the negative electrode, the production of the lithium ion secondary battery, and Characterization was performed. The evaluation results are shown in Table 1. In Comparative Example 1 in which the compression shearing process was not performed, the electrode peel strength was inferior to that in Example 1.

(Comparative Example 2)
In Comparative Example 1, carbonaceous coated graphite particles were produced and evaluated in the same manner as in Comparative Example 1 except that the carbonaceous precursor content was changed as shown in the table.
(Comparative Example 3)
An oil-in-tar solution with a coal tar pitch (residual carbon ratio of 50%) is added to 100 parts by mass of spherical natural graphite particles (particle average particle diameter of 20 μm, average aspect ratio of 1.4) with a solid content ratio of 30. It added so that it might become a mass part, and it heated to 150 degreeC with the biaxial kneader, and mixed for 60 minutes. The obtained mixture was subjected to a heat treatment in a firing process for 3 hours at 1000 ° C. under a flow of nitrogen 5 L / min using a rotary kiln. The heat-treated product was evaluated as carbon-coated graphite particles as the final product. The evaluation results are shown in Table 1.
(Comparative Example 4)
In Comparative Example 3, carbonaceous coated graphite particles were produced and evaluated in the same manner as in Comparative Example 3 except that the carbonaceous precursor content was changed as shown in the table. The evaluation results are shown in Table 1.

(Comparative Example 5)
An oil-in-tar solution of coal tar pitch (residual carbon ratio: 50%) is added to 100 parts by mass of spherically processed natural graphite particles (average particle diameter of 20 μm, average aspect ratio of 1.4). It added so that it might become a mass part, and it heated to 150 degreeC with the biaxial kneader, and mixed for 60 minutes.
The obtained mixture was subjected to a heat treatment in a firing process for 3 hours at 1000 ° C. under a flow of nitrogen 5 L / min using a rotary kiln.
Furthermore, using a dry powder compounding device (Mechano-Fusion System, manufactured by Hosokawa Micron Co., Ltd.), compression is performed under the conditions of a peripheral speed of the rotating drum of 20 m / second, a processing time of 10 minutes, and a distance of 5 mm between the rotating drum and the internal member. The final product was obtained by applying force and shear force repeatedly and performing mechanochemical treatment. The evaluation results are shown in Table 1.

(Comparative Example 6)
An oil-in-tar solution with a coal tar pitch (residual carbon ratio of 50%) is added to 100 parts by mass of spherical natural graphite particles (particle average particle diameter of 20 μm, average aspect ratio of 1.4) with a solid content ratio of 30. It added so that it might become a mass part, and it heated to 150 degreeC with the biaxial kneader, and mixed for 60 minutes.
The obtained mixture was subjected to heat treatment in a firing step for 3 hours at 1300 ° C. under a flow of nitrogen of 5 L / min using a rotary kiln.
Since the lump was generated in the heat-treated product, it was crushed so that the average particle size became 20 μm. The crushed product was evaluated as carbon-coated graphite particles as the final product. The evaluation results are shown in Table 1.
(Comparative Example 7)
An oil-in-tar solution with a coal tar pitch (residual carbon ratio of 50%) is added to 100 parts by mass of spherical natural graphite particles (average particle size 20 μm, average aspect ratio 1.4), and the solid content ratio is 57. It added so that it might become a mass part, and it heated to 150 degreeC with the biaxial kneader, and mixed for 60 minutes.
The obtained mixture was subjected to heat treatment in a firing step for 3 hours at 1300 ° C. under a flow of nitrogen of 5 L / min using a rotary kiln.
Since the lump was generated in the heat-treated product, it was crushed so that the average particle size became 20 μm.
Using a dry powder compounding device (Mechano-Fusion System, manufactured by Hosokawa Micron Co., Ltd.) for the pulverized product, a rotating drum has a peripheral speed of 20 m / second, a processing time of 10 minutes, and a distance between the rotating drum and an internal member of 5 mm. The final product was obtained by applying a compressive force and a shearing force and performing a mechanochemical treatment. The evaluation results are shown in Table 1.

[Evaluation of Examples and Comparative Examples]
Examples 1 , 5 to 7 and 10 exhibit excellent and well-balanced battery characteristics in all of discharge capacity, initial charge / discharge efficiency, and high-speed charge / discharge characteristics. Examples 1, 5 to 7 and 10 in which the average diameter of the spheroidized natural graphite particles is 20 to 25 μm and the carbonaceous content is 9 to 15% by mass are compared with Examples 2 to 4 and 8 in electrode peel strength. Is expensive. In Comparative Examples 1 and 2, the firing process is performed twice in a non-oxidizing atmosphere, but since there is no compression shearing process, the electrode peel strength is low. In Comparative Examples 3 to 7, since the high temperature firing is performed only in the first firing step, the electrode peel strength is low whether or not there is a compression shearing step.

  The negative electrode material comprising the carbonaceous coated graphite particles of the present invention can be used for the negative electrode of high performance lithium ion secondary batteries ranging from small to large, taking advantage of the characteristics.

DESCRIPTION OF SYMBOLS 1 Exterior cup 3 Exterior can 4 Counter electrode 5 Electrolyte solution impregnation separator 6 Insulation gasket 7a Current collector 10 Aluminum plate 12 Double-sided tape 15 Negative electrode material 17 Arrow direction

Claims (4)

A mixing step of mixing spherical or ellipsoidal graphite particles having an average particle diameter of 20 to 50 μm and a carbonaceous precursor;
A first baking step of baking the mixture obtained in the mixing step in an oxidizing atmosphere in a temperature range of 300 ° C. or higher and lower than 700 ° C .;
A second firing step of firing the first fired product obtained in the first firing step in a non-oxidizing atmosphere in a temperature range of 700 ° C. to 2000 ° C .;
The second calcined product obtained in the second calcining step is subjected to mechanochemical treatment to obtain carbonaceous coated graphite particles in which the graphite particles: 70 to 91 % by mass are coated with carbonaceous: 9 to 30% by mass. The manufacturing method of the carbonaceous covering graphite particle which has a compression shearing process.   The method for producing carbonaceous coated graphite particles according to claim 1, wherein in the compression shearing step, carbonaceous coated graphite particles obtained by coating the graphite particles: 85 to 91 mass% with carbonaceous materials: 9 to 15 mass% are obtained.   The method for producing carbonaceous coated graphite particles according to claim 1 or 2, wherein in the first firing step, firing is performed at a temperature range of 500 ° C or higher and lower than 700 ° C for 3 to 50 hours.   The method for producing carbonaceous coated graphite particles according to any one of claims 1 to 3, wherein in the mixing step, the graphite particles are spherical graphite particles having an average particle diameter of 20 to 50 µm.

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