JP6285350B2 - Method for producing carbonaceous coated graphite particles and method for producing negative electrode material for lithium ion secondary battery - Google Patents

Description

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

  Lithium ion secondary batteries are widely used in portable electronic devices, and their use in hybrid vehicles and electric vehicles is also expanding. Under such circumstances, lithium ion secondary batteries are required to have higher capacity, high charge / discharge efficiency, high-speed charge / discharge characteristics, and the like.

  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 and initial charge / discharge efficiency, but due to the scale shape, the particles are oriented in one direction within the electrode, resulting in inferior high-speed charge / discharge 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 lithium ion secondary battery is characterized in that the entire particle surface of the core material covered with a basal plane (100) surface formed of a graphite skeleton and / or an amorphous covering portion is made of graphite. A negative electrode material manufacturing method is disclosed. However, in this manufacturing method, since the core material is coated with pitch and then heat treatment in three stages of infusibilization, carbonization, and graphitization is required, there is a limit in reducing the manufacturing cost. Moreover, since the final graphitization process is implemented at 2800-3200 degreeC, the crystallinity of a film may develop excessively and there exists a possibility that a high-speed charge / discharge characteristic may fall.

Japanese Patent No. 3709987

  In view of the above situation, an object of the present invention is to provide carbonaceous coated graphite particles capable of obtaining excellent battery characteristics when used as a negative electrode material for a lithium ion secondary battery. Here, the excellent battery characteristic means at least one selected from a high discharge capacity, a high initial charge / discharge efficiency, and an excellent high-speed charge / discharge characteristic.

  As a result of intensive studies to solve the above-mentioned problems, the present inventors mixed spherical or ellipsoidal graphite particles and a carbonaceous precursor, and obtained the resulting mixture with an oxygen concentration of 0.1 to 5% (v / V), a carbonaceous coating obtained by firing at a temperature of 300 ° C. or higher and lower than 700 ° C., and subjecting the fired product to graphitization at a temperature of 2000 ° C. or higher and 2800 ° C. or lower in a non-oxidizing atmosphere. It has been found that when graphite particles are used as a negative electrode material for a lithium ion secondary battery, a lithium ion secondary battery having excellent battery characteristics can be obtained, and the present invention has been completed.

That is, the present invention provides the following (1) and ( 2 ).
(1) Mixing step of mixing spherical or ellipsoidal graphite particles and carbonaceous precursor, and mixing the mixture obtained in the mixing step with an oxygen concentration of 0.1 to 5% (v / v), 300 ° C. or higher A carbonaceous coated graphite particle comprising: a firing step of heating at a temperature of less than 700 ° C; and a graphitization step of heating the fired product obtained in the firing step in a non-oxidizing atmosphere at a temperature higher than 2000 ° C and lower than 2800 ° C. Production method.
( 2) Mixing step of mixing spherical or ellipsoidal graphite particles and carbonaceous precursor, and mixing the mixture obtained in the mixing step with an oxygen concentration of 0.1 to 5% (v / v), 300 ° C. or higher A lithium ion secondary battery comprising: a baking step of heating at a temperature lower than 700 ° C .; and a graphitization step of heating the fired product obtained in the baking step in a non-oxidizing atmosphere at a temperature higher than 2000 ° C. and not higher than 2800 ° C. Manufacturing method of negative electrode material .

  Carbonaceous coated graphite particles produced by the method for producing carbonaceous coated graphite particles of the present invention are selected from good discharge capacity, initial charge / discharge efficiency, high-speed charge / discharge characteristics and cycle characteristics as a negative electrode material for lithium ion secondary batteries. A negative electrode material having at least one of the following. Therefore, the lithium ion secondary battery using the carbonaceous coated graphite particles produced by the production method of the present invention as a negative electrode material satisfies the recent demand for higher energy density of the battery, and the equipment to be mounted is reduced in size and heightened. Useful for performance.

It is sectional drawing of the evaluation battery for evaluating the battery characteristic of the negative electrode of this invention.

The method for producing carbonaceous coated graphite particles of the present invention comprises a mixing step of mixing spherical or ellipsoidal graphite particles and a carbonaceous precursor, and a mixture obtained in the mixing step, with an oxygen concentration of 0.1 to 0.1. 5% (v / v), a firing step of firing at a temperature of 300 ° C. or more and less than 700 ° C., and a fired product obtained by the firing step in a non-oxidizing atmosphere in a temperature range of more than 2000 ° C. and 2800 ° C. or less. It is characterized in that it is processed.
Hereinafter, the present invention will be described more specifically.

1. Raw material of carbonaceous coated graphite particles (1) Raw material graphite particles The raw material graphite particles that are the core material of the carbonaceous coated graphite particles of the present invention are spherical or ellipsoidal graphite particles. Any spherical or ellipsoidal shape may be used, and graphite particles that are not spherical or ellipsoidal and processed into a spherical or ellipsoidal shape may be used as the core material.

The average particle diameter of the spherical or ellipsoidal graphite particles (raw material graphite particles) is not particularly limited, but is preferably 1 to 50 μm, and more preferably 5 to 30 μm. The average aspect ratio of the raw material graphite particles is not particularly limited, but is preferably 5 or less, and more preferably 2 or less. The specific surface area of the raw graphite particles is not particularly limited, but is preferably 10 m ² / g or less, and more preferably 8 m ² / g or less.

As the spherical or ellipsoidal graphite particles (raw material 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 crusher include a kneader such as a pressure kneader and a two-roll mill, a rotating ball mill, a counter jet mill (manufactured by Hosokawa Micron), and a current jet ^(R) (manufactured by Nisshin Engineering Co., Ltd.). 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 Seishin Enterprise Co., Ltd.), Agromaster (manufactured by Hosokawa Micron Corporation), and hybridization (Nara Machinery Co., Ltd.). ), Mechanomicros (manufactured by Nara Machinery Co., Ltd.), and mechanofusion system (manufactured by Hosokawa Micron) can be used.

Lc, which is a measured value of X-ray diffraction of spherical or ellipsoidal graphite particles (raw material 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. D ₀₀₂ is 0.337 nm or less, and 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. ) Is 0.06 to 0.30, and the full width at half maximum of the 1580 cm ⁻¹ band is preferably 10 to 60.

(2) Carbonaceous precursor Spherical or ellipsoidal graphite particles (raw material graphite particles), which are the core material of carbonaceous coated graphite particles, are coated with carbonaceous material using a carbonaceous precursor as a raw material by the production method described later. Is done. Examples of the carbonaceous precursor used include tar pitches and / or resins. Specifically, heavy oils, particularly tar pitches, include 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, heavy Examples include 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 those having a coal tar pitch of 80% (w / w) or more are particularly preferred.

2. Method for Producing Carbonaceous Coated Graphite Particles (1) Mixing Step Spherical or ellipsoidal graphite particles (raw material graphite particles) and a carbonaceous precursor are mixed.
The mixing method is not particularly limited as long as the raw materials can be mixed homogeneously, and a known mixing method can be used.
Solid raw material 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 firing step.
The mixing ratio of the raw material graphite particles and the carbonaceous precursor is not particularly limited, but the ratio of the final product (carbonaceous coated graphite particles) is 70 to 99% (w / w) of graphite particles, 1 to 30% of carbonaceous matter ( It is preferable to set so as to be in the range of w / w).
You may perform mixing with the temperature rise for the baking process mentioned later. A method of mixing with increasing temperature is not particularly limited, but a method of using a biaxial kneader having heating means such as a heater and a heating medium is exemplified.

(2) Firing process A baking process is performed by heating the mixture obtained at the mixing process at a temperature of oxygen concentration 0.1 to 5% (v / v) and 300 ° C or higher and lower than 700 ° C.
The baking treatment is performed at an oxygen concentration of 0.1 to 5% (v / v), preferably an oxygen concentration of 0.8 to 4% (v / v), more preferably an oxygen concentration of 0.8 to 3% (v / v). More preferably, it is performed in an atmosphere having an oxygen concentration of 1 to 2% (v / v). When the oxygen concentration in the atmosphere during the firing treatment is within this range, the carbonaceous precursor is cross-linked by the firing treatment, and there is an effect of suppressing excessive crystal development in the graphitization step described later. As an atmosphere in the firing treatment, an inert gas atmosphere (oxygen / inert gas mixed atmosphere) containing oxygen at the above concentration is preferable. By using a slightly oxidizing atmosphere in which the oxygen concentration in the atmosphere is within the above range, the degree of oxidation reaction becomes appropriate. Examples of the inert gas include argon, helium, nitrogen, and the like. Note that the atmosphere may contain a trace amount of gas other than oxygen and inert gas as an inevitable impurity.
The baking treatment is performed at a temperature of 300 ° C. or higher and lower than 700 ° C., preferably 350 to 650 ° C. When the temperature during the firing treatment is within this range, the carbonaceous precursor can be uniformly coated on the core material.
The method for the firing treatment is not particularly limited, but it is preferably performed while stirring. Stirring using a rotary kiln is particularly preferable because uniform firing is possible.
The time for the baking treatment is not particularly limited, but is preferably 5 minutes to 50 hours.
The fired product can be used as it is in the graphitization process described later, and the carbonization treatment can be omitted, which is effective in reducing manufacturing costs.

(3) Graphitization process Graphitization is performed by heating the fired product obtained in the firing process in a non-oxidizing atmosphere at a temperature higher than 2000 ° C and not higher than 2800 ° C.
The atmosphere during the graphitization treatment is not particularly limited as long as it is a non-oxidizing atmosphere. For example, in an inert atmosphere such as argon gas, helium gas, nitrogen gas, etc., reduction in hydrogen gas, carbon monoxide gas, etc. Atmosphere is preferable, but an argon stream is particularly preferable.
The temperature during the graphitization treatment is more than 2000 ° C. and not more than 2800 ° C., preferably in the range of 2200 to 2800 ° C. When the temperature during the graphitization treatment is within this range, it is possible to prevent the crystallinity of the coating from developing excessively.
The time for the graphitization treatment is not particularly limited, but is preferably 5 minutes to 30 hours.
The method of graphitization is not particularly limited, but it is preferable to perform the treatment in a state of being enclosed in a graphite crucible or the like.

  Further, the temperature profile at the time of temperature rise and at the time of heating can take various forms such as a linear temperature rise and a stepwise temperature rise in which the temperature is held at a constant interval.

  Prior to the graphitization treatment, different types of graphite materials may be attached, embedded, or combined. The graphitization step may be performed after carbonaceous or graphite fibers, carbonaceous precursor materials such as amorphous hard carbon, organic materials, and inorganic materials are attached to, embedded in, and combined with the graphite particles of the core material.

3. Carbonaceous coated graphite particles The proportion of graphite particles in the produced carbonaceous coated graphite particles is 70 to 99% (w / w), and the proportion of carbonaceous material is 1 to 30% (w / w).
If the carbonaceous ratio in the carbonaceous coated graphite particles is less than 1% (w / w), it becomes difficult to completely coat the active graphite edge surface, and the initial charge / discharge efficiency may be lowered.
If the carbonaceous proportion in the carbonaceous coated graphite particles exceeds 30% (w / w), the proportion of the carbon material having a relatively low discharge capacity is too large, so the discharge capacity of the carbonaceous coated graphite particles decreases. To do. In addition, if the ratio of the raw materials (thermosetting resins and tar pitches) for forming the carbonaceous material is large, the particles are likely to be fused in the mixing step and the subsequent firing step and graphitization step. Cracking and peeling may occur in a part of the carbonaceous layer of the obtained carbonaceous coated graphite particles, and the initial charge / discharge efficiency may be reduced.
The carbonaceous ratio in the carbonaceous coated graphite particles is preferably 1 to 20% (w / w), more preferably 1 to 15% (w / w), and even more preferably more than 3% (w / w) 15 % (W / w) or less and preferably integrated with the remaining graphite particles. 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 produced carbonaceous coated graphite particles is not particularly limited, but is preferably in the range of 1 to 50 μm, and more preferably in the range of 5 to 30 μm.

Further, the BET specific surface area (specific surface area measured by the BET method) of the carbonaceous coated graphite particles is not particularly limited, but is preferably 8.0 m ² / g or less, and preferably 6.0 m ² / g or less. More preferred.

Further, 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 of carbonaceous coated graphite particles is It is preferably larger than the R value of graphite and 0.05 to 0.80.

4). Lithium ion secondary battery (1) Working electrode (negative electrode)
The present invention also provides a negative electrode for a lithium ion secondary battery containing the carbon-coated graphite particles as a negative electrode material and a lithium ion secondary battery using the negative electrode for the lithium ion secondary battery.
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. The binder is usually used at a ratio of about 1 to 20% (w / w) in the total amount of the 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. Although the film thickness of a negative mix layer is not specifically limited, It is preferable to exist in the range of 10-200 micrometers, and it is more preferable to exist in the range of 20-100 micrometers.

  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 a foil shape, a mesh shape, a net shape such as expanded metal, and the like are preferable. The material of the current collector is not particularly limited, but copper, stainless steel, nickel and the like are preferable. The thickness of the current collector is not particularly limited, but is preferably in the range of about 5 μm to about 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.

(2) Play (positive electrode)
The positive electrode used for 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.

  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 used for producing the positive electrode is not particularly limited, but a foil shape or a mesh shape such as a mesh or an expanded metal is preferable. Although the material of a collector is not specifically limited, Aluminum, stainless steel, nickel, etc. are preferable. The thickness of the current collector is not particularly limited, but is preferably in the range of about 10 to about 40 μm in the case of a foil.

  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.

(3) Non-aqueous electrolyte As the non-aqueous electrolyte used in the lithium ion secondary battery of the present invention, an electrolyte salt used in a normal non-aqueous electrolyte can be used, for example, 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 ( A lithium salt such as CF ₃ ) ₂ }] ₄ , LiAlCl ₄ , or LiSiF ₆ can be used. Among these, LiPF ₆ or LiBF ₄ is particularly preferable from the viewpoint of oxidation stability.

  The concentration of the electrolyte salt in the electrolytic solution is not particularly limited, but 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.

  The ratio of the nonaqueous solvent (plasticizer) in the solid electrolyte is preferably 10 to 90% (w / w), more preferably 30 to 80% (w / w). When the proportion of the nonaqueous solvent in the solid electrolyte is within this range, the electrical conductivity is high, and the mechanical strength is high, so that the film is easily formed.

(4) 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.

4). Production of Lithium Ion Secondary Battery The lithium ion secondary battery of the present invention comprises a negative electrode, a positive electrode, and a nonaqueous electrolyte having the above-described configuration, for example, laminated in the order of a negative electrode, a nonaqueous electrolyte, and a positive electrode. It is comprised by accommodating in. 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) Specific surface area [m ² / g]: The BET specific surface area by nitrogen gas adsorption was determined.
2) Average particle size [μm]: The particle size was determined so that the cumulative frequency of the particle size distribution measured with a laser diffraction particle size distribution meter was 50% (v / v).
3) Average aspect ratio: An image analyzer (manufactured by Toyobo Co., Ltd.) was used to perform image processing on 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).
4) Carbonaceous ratio [% (w / w)]: Carbonaceous precursor raw material (including plural types) is given the same thermal history as carbonaceous coated graphite particles, Carbide was prepared and the residual carbon ratio of the raw material was calculated | required. The ratio of carbonaceous matter in the carbonaceous coated graphite particles was calculated in terms of mass percentage in terms of the residual carbon ratio obtained.

  In the present invention, “% (w / w)” represents a mass percentage, and “% (v / v)” represents a volume percentage. In addition, regarding a numerical range having both ends of A and B (A and B are real numbers satisfying A <B), “A to B” represents that the entire range from A to B is included. "Less than B" indicates that the entire range excluding B from A to B is included. "Over A and below B" indicates that the entire range excluding A from A to B is included. It shall represent the whole range except A and B among A to B.

  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]
1. Production of negative electrode material For 100 parts by mass of natural graphite particles having an average aspect ratio of 1.4 processed into a spherical shape with an average particle diameter of 15 μm, a tar-in-oil solution with a coal tar pitch (residual carbon ratio of 50%) is mixed with a solid content ratio. Was added to 35 parts by mass, heated to 150 ° C. with a biaxial kneader and mixed for 60 minutes.
The obtained mixture was baked at 500 ° C. for 3 hours in an oxygen / nitrogen mixed atmosphere in which an oxygen / nitrogen mixed gas having an oxygen concentration of 1% (v / v) was circulated using a rotary kiln. Next, this treated product was graphitized at 2200 ° C. for 3 hours in an argon (Ar) atmosphere using a Tamman furnace to obtain carbon-coated graphite particles. The carbonaceous coated graphite particles thus obtained were used as a negative electrode material.

2. Preparation of Negative Electrode Mixture Paste Negative electrode material 98% (w / w), carboxymethyl cellulose 1% (w / w) and styrene butadiene rubber 1% (w / w) as binders were stirred in a negative electrode A mixture paste was prepared.

3. Production of Working Electrode (Negative Electrode) The negative electrode mixture paste was applied to a copper foil with a uniform thickness, the dispersion medium water was evaporated and dried in a vacuum at 90 ° C., and pressurized by a hand press to form a negative electrode mixture. A layer was formed. The copper foil and the negative electrode mixture layer were punched into a columnar shape with a diameter of 15.5 mm to produce a working electrode (negative electrode) composed of a current collector and a negative electrode mixture adhered to the current collector.

4). Production of Counter Electrode (Positive Electrode) A lithium metal foil was pressed against a nickel net and punched into a circular shape with a diameter of 15.5 mm, and a current collector made of nickel net and a lithium metal foil (thickness 0. 5 mm) adhered to the current collector. A counter electrode (positive electrode) made of 5 mm) was produced.

5. Electrolytic Solution and Separator LiPF ₆ was dissolved in a mixed solvent of ethylene carbonate 33% (v / v) -methyl ethyl carbonate 67% (v / v) at a concentration of 1 mol / L to prepare a nonaqueous electrolytic solution. The obtained nonaqueous electrolytic solution was impregnated into a polypropylene porous body (thickness 20 μm) to produce a separator impregnated with the electrolytic solution.

6). Production of Evaluation Battery A button type secondary battery shown in FIG. 1 was produced 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. From the inner surface of the outer can 3, a current collector 7 a made of nickel net, a cylindrical counter electrode (positive electrode) 4 made of lithium foil, a separator 5 impregnated with an electrolytic solution, and a copper foil to which a negative electrode material is attached This is a battery system in which a working electrode (negative electrode) 2 and a current collector 7b are stacked.
In the evaluation battery, the separator 5 impregnated with the electrolytic solution was sandwiched between the working electrode (negative electrode) 2 in close contact with the current collector 7b and the counter electrode (positive electrode) 4 in intimate contact with the current collector 7a. The current collector 7b is accommodated in the exterior cup 1 and the current collector 7a is accommodated in the exterior can 3. The exterior cup 1 and the exterior can 3 are combined, and further insulated at the peripheral edge between the exterior cup 1 and the exterior can 3. A gasket 6 was interposed, and both peripheral edges were caulked and sealed.
6). Evaluation of Battery Performance The discharge capacity, initial charge / discharge efficiency, and fast charge / discharge characteristics (1C charge rate, 2C discharge rate) were measured by the following methods by a charge / discharge test. The results are shown in Table 1.
(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 current applied 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 the charge / discharge 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 and 3]
Carbonaceous coated graphite particles were produced in the same manner as in Example 1 except that the temperature during graphitization was changed to 2500 ° C. (Example 2) or 2800 ° C. (Example 3).
Using the produced carbonaceous coated graphite particles as a negative electrode material, an evaluation battery was prepared and charged / discharged as in Example 1, and the discharge capacity, initial charge / discharge efficiency, and fast charge / discharge characteristics (1C charge rate, 2C discharge rate) was evaluated. The evaluation results of the battery characteristics are shown in Table 1.

[Example 4]
Carbonaceous coated graphite particles were produced in the same manner as in Example 1 except that the oxygen concentration in the atmosphere during the firing treatment was changed to 2% (v / v).
Using the produced carbonaceous coated graphite particles as a negative electrode material, an evaluation battery was prepared and charged / discharged as in Example 1, and the discharge capacity, initial charge / discharge efficiency, and fast charge / discharge characteristics (1C charge rate, 2C discharge rate) was evaluated. The evaluation results of the battery characteristics are shown in Table 1.

[Examples 5 and 6]
Carbonaceous coated graphite particles were produced in the same manner as in Example 1 except that the temperature during the firing treatment was changed to 350 ° C. (Example 5) or 650 ° C. (Example 6).
Using the produced carbonaceous coated graphite particles as a negative electrode material, an evaluation battery was prepared and charged / discharged as in Example 1, and the discharge capacity, initial charge / discharge efficiency, and fast charge / discharge characteristics (1C charge rate, 2C discharge rate) was evaluated. The evaluation results of the battery characteristics are shown in Table 1.

[Comparative Examples 1 and 4]
Except for the point that the oxygen concentration in the atmosphere during the firing treatment was 0% (v / v) (Comparative Example 1) or 20% (v / v) (Comparative Example 4), the same as in Example 2, Carbonaceous coated graphite particles were produced.
Using the produced carbonaceous coated graphite particles as a negative electrode material, an evaluation battery was prepared and charged / discharged as in Example 1, and the discharge capacity, initial charge / discharge efficiency, and fast charge / discharge characteristics (1C charge rate, 2C discharge rate) was evaluated. The evaluation results of the battery characteristics are shown in Table 1.

[Comparative Examples 2 and 3]
Carbonaceous coated graphite particles were produced in the same manner as in Example 1 except that the temperature during graphitization was changed to 2000 ° C. (Comparative Example 2) or 3000 ° C. (Comparative Example 3).
Using the produced carbonaceous coated graphite particles as a negative electrode material, an evaluation battery was prepared and charged / discharged as in Example 1, and the discharge capacity, initial charge / discharge efficiency, and fast charge / discharge characteristics (1C charge rate, 2C discharge rate) was evaluated. The evaluation results of the battery characteristics are shown in Table 1.

(Evaluation of Examples and Comparative Examples)
The lithium ion secondary battery using the carbonaceous coated graphite particles of Examples 1 to 6 as the graphite material for the negative electrode has all of the discharge capacity, initial charge / discharge efficiency, and fast charge / discharge characteristics (1C charge rate, 2C charge rate). Excellent and well-balanced battery characteristics were shown (see Table 1).
In Comparative Example 1, the firing treatment was performed in a non-oxidizing atmosphere. Therefore, the lithium ion secondary battery using the carbon-coated graphite particles of Comparative Example 1 as the graphite material for the negative electrode has higher charge / discharge characteristics (1C charge rate, 2C) than the lithium ion secondary batteries of Examples 1-6. The charging rate is inferior.
In Comparative Example 2, the temperature during graphitization was set as low as 2000 ° C. Therefore, the lithium ion secondary battery using the carbonaceous coated graphite particles of Comparative Example 2 as the graphite material for the negative electrode has a discharge capacity and high-speed charge / discharge characteristics (1C charging) as compared with the lithium ion secondary batteries of Examples 1-6. Rate and 2C charging rate) are both inferior (see Table 1).
In Comparative Example 3, the temperature during graphitization was set as high as 3000 ° C. Therefore, the lithium ion secondary battery using the carbonaceous coated graphite particles of Comparative Example 3 as the negative electrode graphite material is faster than the lithium ion secondary batteries of Examples 1 to 6 (1C charge rate, 2C). (Charge rate) is inferior (see Table 1).
In Comparative Example 4, the baking treatment was performed in an oxidizing atmosphere. Therefore, the lithium ion secondary battery using the carbon-coated graphite particles of Comparative Example 4 as the graphite material for the negative electrode has a discharge capacity, initial charge / discharge efficiency, and high-speed charge as compared with the lithium ion secondary batteries of Examples 1-6. Discharge characteristics (1C charge rate, 2C charge rate) are inferior (see Table 1).

  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.

1 exterior cup 2 working electrode (negative electrode)
3 Exterior can 4 Counter electrode (positive electrode)
5 Electrolyte solution impregnated separator 6 Insulating gasket 7a, 7b Current collector

Claims (2)

  A mixing step of mixing spherical or ellipsoidal graphite particles and a carbonaceous precursor, and a mixture obtained in the mixing step are mixed at an oxygen concentration of 0.1 to 5% (v / v), 300 ° C to 700 ° C. Production of carbonaceous coated graphite particles comprising: a firing step of heating at a temperature lower than the temperature; and a graphitization step of heating the fired product obtained in the firing step in a non-oxidizing atmosphere at a temperature of 2000 ° C. to 2800 ° C. Method.   A mixing step of mixing spherical or ellipsoidal graphite particles and a carbonaceous precursor, and a mixture obtained in the mixing step are mixed at an oxygen concentration of 0.1 to 5% (v / v), 300 ° C to 700 ° C. A negative electrode for a lithium ion secondary battery comprising: a firing step for heating at a temperature lower than the temperature; and a graphitization step for heating the fired product obtained in the firing step in a non-oxidizing atmosphere at a temperature higher than 2000 ° C. and not higher than 2800 ° C. Material manufacturing method.