JP2018133340A - Carbonaceous coated graphite particles, negative electrode for lithium ion secondary battery, and lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a negative electrode material for a lithium secondary battery which has excellent battery characteristics and can obtain excellent electrode peel strength.SOLUTION: In a carbonaceously coated graphite particle obtained by coating spherical or ellipsoidal shaped graphite particles of 70 to 85% by mass with carbonaceous matter of 15 to 30% by mass, the sum of the pore volumes of 2 nm or less as measured by a BET adsorption method of nitrogen gas is 1.0×10-2.0×10cm/g, and a BET specific surface area by adsorption of nitrogen gas is 1.00 to 0.50 m/g.SELECTED DRAWING: None

Description

  The present invention relates to carbonaceous coated graphite particles, a negative electrode for a lithium ion secondary battery containing the same, and a lithium ion secondary battery having high capacity, high charge / discharge efficiency, and excellent rate characteristics and cycle characteristics using the negative electrode.

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 higher 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 with each other, and a crushing step is required. When crushed, the crushed surface is generated, and the crushed surface has a high reaction activity with the electrolytic solution. As a result, there is a problem that the charge / discharge efficiency and cycle characteristics are lowered. Further, when polyvinylidene fluoride is used as a binder at the time of producing an electrode, there has been a problem that the peel strength of the electrode is remarkably lowered.
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.

Japanese Patent No. 3716818 JP 2004-63321 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 and cycle characteristics.

[1] Spherical or ellipsoidal graphite particles: carbon-coated graphite particles obtained by coating 70 to 85% by mass with carbonaceous: 15 to 30% by mass,
The total of pore volume of 2 nm or less measured by the nitrogen gas BET adsorption method of the carbonaceous coated graphite particles is 1.0 × 10 ^{−2 to} 2.0 × 10 ⁻² cm ³ / g,
Carbonaceous coated graphite particles having a BET specific surface area of 1.00 to 0.50 m ² / g by nitrogen gas adsorption.
[2] The carbonaceous coated graphite particle according to [1], wherein the carbonaceous coated graphite particle is a graphite material for a negative electrode of a lithium ion secondary battery.
[3] A negative electrode for a lithium ion secondary battery containing the carbonaceous coated graphite particles according to [1].
[4] A lithium ion secondary battery having the negative electrode for a lithium ion secondary battery according to [3].

  The carbon-coated graphite particles of the present invention have at least one selected from a good discharge capacity, initial charge / discharge efficiency, high-speed charge / discharge characteristics and cycle characteristics as a negative electrode material for a lithium ion secondary battery, and excellent electrode peel strength. It is a negative electrode material which has. 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.

It is sectional drawing of the evaluation battery for evaluating the battery characteristic of the negative electrode of this invention. 6 is a graph showing cumulative pore volume (Sigma Vp) versus pore diameter (dp) of carbonaceous coated graphite particles of Example 1 and Comparative Example 1 and spherically processed graphite particles. It is a figure explaining the measurement 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 preferably graphite particles having a spherical or ellipsoidal average particle diameter of 1 to 50 μm and an average particle diameter of 5 to 30 μm. The average specific surface area is more preferably 10 m ² / g or less, and particularly preferably 8 m ² / g or less. The graphite particles may be natural graphite particles or artificial graphite particles, but 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 is granulated 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 spheroidizers for pulverized products include granulators such as GRANUREX (manufactured by Freund Sangyo Co., Ltd.), Newgra Machine (manufactured by Seisin Co., Ltd.), Agromaster (manufactured by Hosokawa Micron Co., Ltd.), and hybridization. Shear compression processing apparatuses such as (manufactured by Nara Machinery Co., Ltd.), mechanomicros (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. As the carbonaceous precursor used, tar pitches and / or carbon materials that are low in crystallinity compared to graphite and are carbon materials that cannot be converted into graphite crystals even when subjected to high temperature treatment required for graphitization. Resins are exemplified. 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 such that 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% in terms of the final product. Preferably it is carbonaceous 10-25 mass%. 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]
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. while mixing the obtained mixture. 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 is preferably an inert gas atmosphere containing 5 to 50% by volume of oxygen. If the oxygen content is 0.5% by volume or less, an oxidizing atmosphere is not substantially formed. Therefore, the oxidizing atmosphere is set to an oxygen content exceeding 0.5% by volume. This is because the degree of the oxidation reaction can be appropriately performed by using an atmosphere containing oxygen in this range. 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 500 ° C, and more preferably in the range of 300 to 450 ° 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. Although the method for the firing treatment is not particularly limited, firing is preferably performed while stirring, and the use of a rotary kiln is preferred because uniform firing is possible. Examples of the non-oxidizing atmosphere 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.
Further, the temperature profile at the time of temperature rise and firing 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.
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. Carbonaceous or graphite fibers, carbonaceous precursor materials such as amorphous hard carbon, organic materials, and inorganic materials may be attached to, embedded in, or combined with the graphite particles of the core material.

3. Carbonaceous coated graphite particles The carbonaceous proportion in the carbonaceous coated graphite particles of the present invention is 1 to 30% by 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 carbonaceous coated graphite particles is particularly preferably 3 to 20% by mass, more preferably 10 to 20% 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 3.0 m ² / g or less, more preferably 1.5 m ² / g or less, and still more preferably 1.00 to 0.50 m ² / g. .
The carbon-coated graphite particles of the present invention have a total pore volume of 2 nm or less measured by a gas adsorption method by the BET method of 1.0 × 10 ^{−2 to} 2.0 × 10 ⁻² cm ³ / g. Is preferred. Further, it is more preferable that the total pore volume of 20 to 40 nm is 0.0015 cm ³ / g or less.
The pore distribution between the carbonaceous coated graphite particles obtained in Example 1 and Comparative Example 1 and the spherical natural graphite particles used in Example 1 is shown in FIG. As can be seen from FIG. 2, the pore distribution of the carbon-coated graphite particles of Example 1 is the cumulative volume of the pore volume, increasing to a pore diameter of around 2 nm, and the volume of the pores having a diameter exceeding this is the pore diameter. Regardless, it is almost constant. Therefore, when the carbon-coated graphite particles of the present invention and the pore volume of the natural graphite particles processed into a spherical shape as the core material are compared, the total pore volume becomes high, and the total pore volume is 2 nm or less. It can be seen that the ratio of the sum of the pore volumes of each is characteristically high. The total pore volume of 2 nm or less of natural graphite processed into a spherical shape is about 0.004 cm ³ / g. The carbon-coated graphite particles of the present invention have a total pore volume of 2 nm or less of 1.0 × 10 ^{−2 to} 2.0 × 10 ⁻² cm ³ / g. When the carbon-coated graphite particles of the present invention are used as a negative electrode material, the reason is not yet clear, but it is considered that the adhesion strength between the negative electrode materials and the current collector, which is a metal plate, increases due to the anchor effect of the particle surface. As a result, a negative electrode having high electrode peel strength is obtained.

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. More preferably, it is 0.45-0.75.

[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, and the like is mixed with a known stirrer, mixer, kneader, kneader. A paste is prepared by stirring and mixing using the above. 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 electrolyte salt concentration in the non-aqueous electrolyte 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) Specific surface area (m ² / g): The BET specific surface area by nitrogen gas adsorption was determined.
2) Pore diameter and pore volume: The pore diameter and the amount thereof are measured by the BET adsorption method of nitrogen gas. Using a high-accuracy gas / vapor adsorption measuring device (manufactured by Bellthorpe), the pore diameter was plotted on the horizontal axis and the pore volume (cumulative volume) was plotted on the vertical axis.
3) Average particle diameter (μm): The particle diameter at which the cumulative frequency of the particle size distribution measured with a laser diffraction particle size distribution meter is 50% in terms of volume percentage.
4) 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).
5) R value: In a Raman spectrum using an Argon laser beam with a wavelength of 514.5 nm by LabRAM ARAMIS manufactured by Horiba Scientific, the peak wavelength existing in the region of 1570 to 1630 cm ⁻¹ is defined as IG, and in the region of 1350 to 1370 cm ⁻¹ . The ID / IG ratio when the intensity of the existing peak was taken as ID was used.

6) Electrode peel strength test: A negative electrode mixture paste was prepared, and a part on the active material side was attached to the aluminum plate 10 with a double-sided tape 12 in a state where the paste was not pressed to prepare a test piece. As shown in FIG. 3, the test piece is gripped by a part of the negative electrode material 15 using a tensile tester (manufactured by Shimadzu Corp.) and subjected to a tensile test in the 180 ° direction (arrow direction 17) to peel off the average tensile test stress Strength.
7) Carbonaceous ratio (%): The carbonaceous precursor raw material (including a plurality of types) is given the same thermal history as the 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 natural graphite particles having an average aspect ratio of 1.4 processed into a sphere having an average particle size of 20 μm, a tar-in-oil solution with a coal tar pitch (residual carbon ratio of 50%) has a solid content ratio of 35 parts by 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 heat-treated in the 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) to obtain a final product.
[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. 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.

The cycle characteristics were measured as follows. After constant current charging at a current value of 0.5 C until the circuit voltage reached 1 mV, switching to constant voltage charging was continued until the current value reached 20 μA, and then rested for 10 minutes. Next, constant current discharge was performed at a current value of 0.5 C until the circuit voltage reached 1.5V. This charge / discharge was repeated 50 times, and the cycle characteristics were calculated from the obtained discharge capacity using the following equation (4).
Cycle characteristics = (discharge capacity in the 50th cycle / discharge capacity in the first cycle) × 100 (4)

(Examples 2, 3, 5-7, Example 4)
Carbonaceous coated graphite particles were produced in the same manner as in Example 1 except that the carbonaceous precursor content, the first firing step temperature, the atmosphere, and the second firing step temperature were changed as shown in the table. ,evaluated.

(Comparative Example 1)
In Example 1, except that the firing process is performed only once at 1000 ° C. under a flow of nitrogen of 5 L / min, as in Example 1, preparation of the negative electrode material, preparation of the negative electrode mixture, preparation of the negative electrode, lithium ion secondary battery And the characteristics were evaluated. The evaluation results are shown in Tables 1 and 2. Comparative Example 1 in which the firing treatment was performed only in one stage was a result inferior to Example 1 in initial charge / discharge efficiency, cycle characteristics, or electrode peel strength.

(Comparative Example 2)
In Example 1, except that the first-stage baking treatment was performed at 500 ° C. under a flow of nitrogen of 5 L / min, the production of the negative electrode material, the preparation of the negative electrode mixture, the production of the negative electrode, lithium ions were performed in the same manner as in Example 1. A secondary battery was fabricated and evaluated. The evaluation results are shown in Tables 1 and 2. In Comparative Example 2 in which the first stage baking treatment was performed in a non-oxidizing atmosphere, the initial charge / discharge efficiency, the cycle characteristics, or the electrode peel strength were inferior to those in Example 1.
(Comparative Example 3)
In Example 1, after performing the second firing step, the temperature was raised to 3000 ° C. and heated for 5 hours in the same manner as in Example 1 except that a sample was produced. Particles were produced and evaluated. The results are shown in Tables 1 and 2 (examples using comparative examples corresponding to the examples of JP-A-10-294111).
(Comparative Example 4)
Into the stirrer, the graphite particles used in Example 1 were charged with respect to the coal tar pitch in the tar-in-oil solvent, immersed at 150 ° C. for 1 hour, stirred, gradually warmed and depressurized, and after 3 hours, 200 ° C. Stirring was continued until the oil in tar was removed. The obtained coal tar pitch precursor-coated graphite particles were calcined in a firing furnace at 470 ° C. for 20 hours under an inert gas flow containing less than 0.5% by volume of oxygen and charged. A fired product having a mass of 126 parts by mass with respect to 100 parts by mass of graphite particles was obtained. This comparative example was a reproduction experiment of Example 1 described in Japanese Patent Application Laid-Open No. 2004-63321, except that the first baking treatment was performed in a slightly oxidizing atmosphere. In the same manner as in Example 1, production of a negative electrode material, preparation of a negative electrode mixture, production of a negative electrode, production of a lithium ion secondary battery, and characteristic evaluation were performed. The evaluation results are shown in Tables 1 and 2. The pore volume of 2 nm or less obtained by using a slightly oxidizing atmosphere was small as in Comparative Example 2, and the electrolytic peel strength was small (in the example of JP-A-2004-63321, a slightly oxidizing atmosphere was reduced). The case corresponding to the case of use is a comparative example).

(Comparative Example 5)
In Example 1, a negative electrode material was prepared, a negative electrode mixture was prepared, a negative electrode was prepared, lithium was prepared in the same manner as in Example 1 except that scaly natural graphite particles were used instead of the spherically processed graphite particles. An ion secondary battery was produced and evaluated. The evaluation results are shown in Tables 1 and 2. In Comparative Example 5, the initial charge / discharge efficiency, 1C charge rate, 2C discharge rate, cycle characteristics, and electrode peel strength were inferior to those of Example 1.

[Evaluation of Examples and Comparative Examples]
In Comparative Examples 1, 2, and 4 in which the first firing step was a non-oxidizing atmosphere, the electrode peel strength was inferior. In Comparative Example 3 in which additional high-temperature firing was performed in the first and second firing steps, the electrode peel strength was inferior. Comparative Example 5 in which the graphite has a flake shape is inferior in both cycle characteristics and electrode peel strength. The carbonaceous coated graphite particles of Examples 1 to 3 and 5 to 7 of the present invention are excellent in cycle characteristics and electrode peel strength. When the carbonaceous content is more than 7% by mass, the cycle characteristics are more excellent.

  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)

Spherical or ellipsoidal graphite particles: carbon-coated graphite particles obtained by coating 70 to 85% by mass with carbonaceous: 15 to 30% by mass,
The total of pore volume of 2 nm or less measured by the BET adsorption method of nitrogen gas of the carbonaceous coated graphite particles is 1.0 × 10 ^{−2 to} 2.0 × 10 ⁻² cm ³ / g,
Carbonaceous coated graphite particles having a BET specific surface area of 1.00 to 0.50 m ² / g by nitrogen gas adsorption.   The carbonaceous coated graphite particles according to claim 1, wherein the carbonaceous coated graphite particles are a graphite material for a negative electrode of a lithium ion secondary battery.   The negative electrode for lithium ion secondary batteries containing the carbonaceous covering graphite particle of Claim 1.   The lithium ion secondary battery which has a negative electrode for lithium ion secondary batteries of Claim 3.