Carbonaceous coated graphite particles, negative electrode for lithium ion secondary battery, lithium ion secondary battery, and method for producing carbonaceous coated graphite particles

Abstract

Provided are carbon material–coated graphite particles that, when used as a negative electrode material for a lithium ion secondary battery, produce excellent output characteristics, rapid charging characteristics, and cycle characteristics. The carbon material–coated graphite particles comprise graphite particles and a carbon material that coats at least a portion of the surface of the graphite particles. When the carbon material–coated graphite particles are heated under an atmosphere of steam, the temperature at which a decrease in mass begins is 800°C–980°C. The carbon material content of the carbon material–coated graphite particles is 0.1–15.0 parts by mass per 100.0 parts by mass of the graphite particles.

Description

[Technical Field]

[0001] The present invention relates to carbonaceous-coated graphite particles, a negative electrode for a lithium ion secondary battery, a lithium ion secondary battery, and a method for producing carbonaceous-coated graphite particles. [Background technology]

[0002] Lithium-ion secondary batteries have a negative electrode, a positive electrode, and a non-aqueous electrolyte as their main components. Lithium ions move between the negative electrode and the positive electrode during charging and discharging, thereby functioning as a secondary battery. BACKGROUND ART Conventionally, carbonaceous-coated graphite particles, in which the surfaces of graphite particles are coated with carbonaceous material, have sometimes been used as negative electrode materials for lithium ion secondary batteries (Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2021 / 260964 Summary of the Invention [Problem to be solved by the invention]

[0004] Lithium-ion secondary batteries are expected to be widely used in automobiles (hybrid automobiles, electric automobiles, etc.) in the future. For example, in hybrid vehicles, acceleration is important, so batteries with excellent output characteristics (rapid discharge characteristics) are required. Furthermore, electric vehicles require batteries that have excellent rapid charging characteristics and whose capacity does not deteriorate easily even after repeated charging and discharging (that is, excellent cycle characteristics).

[0005] Therefore, an object of the present invention is to provide carbonaceous-coated graphite particles that, when used as a negative electrode material for lithium ion secondary batteries, are excellent in all of output characteristics, rapid charging characteristics, and cycle characteristics. [Means for solving the problem]

[0006] As a result of extensive research, the inventors have found that by optimizing the firing conditions when producing carbonaceous-coated graphite particles, carbonaceous material having specific crystallinity can be obtained, and as a result, the above-mentioned object can be achieved, and have completed the present invention.

[0007] That is, the present invention provides the following [1] to [5]. [1] Carbonaceous-coated graphite particles comprising graphite particles and a carbonaceous material coating at least a portion of the surface of the graphite particles, the carbonaceous material having a mass loss initiation temperature of 800 to 980°C when heated in a water vapor atmosphere, and a carbonaceous material content of 0.1 to 15.0 parts by mass per 100.0 parts by mass of the graphite particles. [2] The carbonaceous-coated graphite particles according to [1] above, which are a negative electrode material for a lithium-ion secondary battery. [3] A negative electrode for a lithium ion secondary battery, containing the carbonaceous-coated graphite particles according to [1] above. [4] A lithium ion secondary battery having the negative electrode according to [3] above. [5] A method for producing the carbonaceous-coated graphite particles according to [1] above, comprising: adhering a carbonaceous precursor, which is a precursor of the carbonaceous material, to the graphite particles to obtain precursor-adhered graphite particles; performing a first firing in which the precursor-adhered graphite particles are heated in a non-oxidizing atmosphere at a temperature higher than 900°C and not higher than 1500°C; after the first firing, performing an intermediate exposure in which the precursor-adhered graphite particles are exposed to an oxidizing atmosphere at a temperature of 50 to 100°C; and after the intermediate exposure, performing a second firing in which the precursor-adhered graphite particles are heated in a non-oxidizing atmosphere at a temperature of 1100 to 1500°C. [Effects of the Invention]

[0008] According to the present invention, it is possible to provide carbonaceous-coated graphite particles that, when used as a negative electrode material for a lithium ion secondary battery, are excellent in all of output characteristics, rapid charging characteristics, and cycle characteristics. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 2 is a cross-sectional view of an evaluation battery fabricated to evaluate battery characteristics in Examples and Comparative Examples. DETAILED DESCRIPTION OF THE INVENTION

[0010] In this specification, when a range is expressed using "to", the range is inclusive. For example, a range from A to B includes A and B.

[0011] [Carbon-coated graphite particles] The carbonaceous-coated graphite particles of this embodiment (hereinafter also referred to as "the present carbonaceous-coated graphite particles") comprise graphite particles and carbonaceous matter that coats at least a portion of the surface of the graphite particles, and have a mass loss starting temperature of 800 to 980°C when heated in a water vapor atmosphere, and a content of the carbonaceous matter of 0.1 to 15.0 parts by mass relative to 100.0 parts by mass of the graphite particles. When the carbonaceous-coated graphite particles are used as a negative electrode material for a lithium ion secondary battery, the battery exhibits excellent output characteristics, rapid charging characteristics, and cycle characteristics.

[0012] <Mass reduction start temperature> The mass loss initiation temperature (also simply referred to as the "mass loss initiation temperature") when carbonaceous-coated graphite particles are heated in a water vapor atmosphere reflects the crystallinity of the carbonaceous material coating the graphite particles; the higher the mass loss initiation temperature, the higher the crystallinity. The mass loss starting temperature is 800°C or higher, preferably 840°C or higher, and more preferably 880°C or higher, which results in excellent rapid charging characteristics and output characteristics. This is because the higher the crystallinity of the carbonaceous material coating the graphite particles, the better the conductivity of lithium ions and electrons. On the other hand, if the mass loss starting temperature is too high, the carbonaceous crystals grow too much. In this case, the crystal grain boundaries are reduced, and as a result, the paths for lithium ions to diffuse into the carbonaceous material are reduced, resulting in deterioration of the rapid charging characteristics and output characteristics. Therefore, the mass loss starting temperature is preferably 980°C or lower, more preferably 950°C or lower, and even more preferably 935°C or lower.

[0013] The mass loss starting temperature is determined as follows. First, the carbonaceous-coated graphite particles are heated in a water vapor atmosphere. More specifically, thermogravimetric analysis is carried out under the following conditions to obtain a graph showing the relationship between the mass loss rate and temperature. The thermogravimetric analysis is carried out under the following conditions. Equipment: Steam gasification carbon analyzer CASGa (Rigaku Corporation) Atmosphere: 20% water vapor - N2 balance (water vapor partial pressure P H2O =20.2kPa) Heating rate: 10℃ / min Measurement method: Rate-controlled thermogravimetric analysis (rate control value: 0.001% / sec) Next, from the obtained graph (mass loss rate vs. temperature graph), the temperature at which the mass loss rate is 0.50% is read, and this temperature is determined as the mass loss starting temperature.

[0014] The reason for carrying out the thermogravimetric analysis in a water vapor atmosphere is as follows. At high temperatures, carbon is oxidized by oxidizing agents such as oxygen, carbon dioxide, and water vapor to form carbon dioxide or carbon monoxide gas. The mass loss temperature of carbon varies depending on the type of oxidizing agent and the crystallinity of the carbon oxidized by the oxidizing agent. In other words, if the atmosphere surrounding the carbon during thermogravimetric analysis is not constant, the mass loss temperature cannot be related to the crystallinity of the carbon. Of the above oxidizing agents, water vapor is most suitable for evaluating the crystallinity of carbon from the viewpoints of ease of handling, purity, concentration control, and the like. Therefore, the atmosphere surrounding the carbon during thermogravimetric analysis is set to a water vapor atmosphere, which allows the mass loss starting temperature to be related to the crystallinity of the carbon.

[0015] <Carbon content> For reasons of excellent cycle characteristics, the content of the carbonaceous material is 0.1 parts by mass or more, preferably 0.3 parts by mass or more, and more preferably 0.5 parts by mass or more, per 100.0 parts by mass of graphite particles. On the other hand, for reasons of excellent rapid charging characteristics and output characteristics, the carbonaceous content is 15.0 parts by mass or less, preferably 10.0 parts by mass or less, more preferably 8.0 parts by mass or less, and even more preferably 5.0 parts by mass or less, per 100.0 parts by mass of graphite particles.

[0016] The carbonaceous content in the carbonaceous-coated graphite particles is determined as follows. First, the residual carbon ratio of the carbonaceous precursor (described later) is determined. The residual carbon ratio is the ratio (unit: mass%) of the remaining amount to the charged amount when the carbonaceous precursor alone is subjected to the same thermal history as the carbonaceous-coated graphite particles to form a carbonaceous material. The carbonaceous content is determined from the obtained carbon residue rate of the carbonaceous precursor and the amount of the carbonaceous precursor added (described later). For example, consider a case where the amount of carbonaceous precursor with a residual carbon content of 34% by mass is 8.0 parts by mass relative to 100.0 parts by mass of graphite particles. In this case, the carbonaceous content in the resulting carbonaceous-coated graphite particles is 2.7 parts by mass (=8.0×0.34) relative to 100.0 parts by mass of graphite particles.

[0017] Hereinafter, for convenience, the content of carbonaceous matter relative to 100.0 parts by mass of graphite particles may be simply referred to as the "content of carbonaceous matter."

[0018] <Particle size> The particle size of the present carbonaceous-coated graphite particles is preferably 5.0 μm or more, more preferably 6.5 μm or more, while the particle size of the present carbonaceous-coated graphite particles is preferably 15.0 μm or less, more preferably 12.0 μm or less. The particle size is the median size at which the cumulative frequency of the particle size distribution determined using a laser diffraction particle size distribution analyzer (LMS2000e, manufactured by Seishin Enterprise Co., Ltd.) is 50% in volume percentage.

[0019] [Method of manufacturing carbonaceous coated graphite particles] Next, a method for producing carbonaceous-coated graphite particles according to this embodiment (hereinafter also referred to as "the present production method") will be described. This manufacturing method is a method for manufacturing the above-mentioned carbonaceous-coated graphite particles, and roughly involves attaching a carbonaceous precursor to graphite particles, followed by the first firing, intermediate exposure, and second firing described below (hereinafter, these are also collectively referred to as "firing"), in that order.

[0020] In the following, first, the graphite particles and carbonaceous precursor used in this production method will be described, and then each step in this production method will be described.

[0021] <Graphite particles> The method for producing the graphite particles used in the present production method is not particularly limited, but may be, for example, a method in which raw materials are processed into spherical shapes. Here, the raw material is graphite having a shape other than spherical (including ellipsoidal), for example, flake graphite. The graphite may be either natural graphite or artificial graphite, but natural graphite is preferred because of its high crystallinity. Specific examples of methods for processing raw materials into spherical shapes (methods for spheronizing raw materials) include a method of stirring raw materials in the presence of a granulation aid such as an adhesive or a resin; a method of applying a mechanical external force to raw materials without using a granulation aid; and a method of using both methods in combination. Of these, the method of applying a mechanical external force to the raw material without using a granulation aid is preferred. This method will be described in more detail below.

[0022] More specifically, a raw material (for example, flake graphite) is crushed and granulated by applying a mechanical external force using a crushing device, thereby spheroidizing the raw material to obtain spherical graphite. Examples of grinding devices include a rotary ball mill, a counter jet mill (manufactured by Hosokawa Micron Corporation), a current jet (manufactured by Nisshin Engineering Co., Ltd.), a hybridization system (manufactured by Nara Machinery Works, Ltd.), a CF mill (manufactured by Ube Industries, Ltd.), a mechanofusion system (manufactured by Hosokawa Micron Corporation), and a Theta Composer (manufactured by Tokuju Kosakusho Co., Ltd.), and among these, the hybridization system (manufactured by Nara Machinery Works, Ltd.) is preferred.

[0023] In this manufacturing method, it is preferable to arrange a plurality of pulverizers in series and have the raw material pass through these pulverizers continuously, i.e., it is preferable to arrange the plurality of pulverizers in series so that after the raw material passes through one pulverizer, it is immediately pulverized and granulated in the next pulverizer.

[0024] In this case, the number of grinding devices is, for example, two or more, preferably three or more, more preferably four or more, even more preferably five or more, and particularly preferably six or more. On the other hand, the number of pulverizers is preferably 10 or less, more preferably 8 or less, and even more preferably 7 or less.

[0025] The time required for pulverizing and granulating the raw material in one pulverizer (hereinafter also referred to as "pulverization time") is preferably 8 minutes or more, more preferably 13 minutes or more, and even more preferably 18 minutes or more. On the other hand, the grinding time in one grinding device is preferably 60 minutes or less, more preferably 50 minutes or less, and even more preferably 40 minutes or less.

[0026] The product of the number of grinding devices and the grinding time in one grinding device (hereinafter also referred to as "total grinding time") is preferably 30 minutes or more, more preferably 50 minutes or more, and even more preferably 90 minutes or more. On the other hand, the total grinding time is preferably 180 minutes or less, more preferably 160 minutes or less.

[0027] The grinding device usually contains a rotor. The peripheral speed of the rotor in each pulverizer is preferably 30 m / sec or more, more preferably 40 m / sec or more, and even more preferably 60 m / sec or more. On the other hand, the peripheral speed of the rotor in each pulverizer is preferably 100 m / sec or less, more preferably 80 m / sec or less.

[0028] In order to make it easier to apply shear force and compression force to the raw material, it is preferable that the amount of raw material charged into each crushing device be small.

[0029] <Carbonaceous precursor> The carbonaceous precursor is a precursor of the above-mentioned carbonaceous material, and is carbonized through firing, which will be described later, to become the carbonaceous material that coats the graphite particles. Examples of carbonaceous precursors include tar pitches and / or resins, which are carbon materials that have lower crystallinity than graphite and cannot be transformed into graphite crystals even after high-temperature treatment required for graphitization. Examples of tar pitches include coal tar, light tar oil, medium tar oil, heavy tar oil, naphthalene oil, anthracene oil, coal tar pitch, pitch oil, mesophase pitch, oxygen-crosslinked petroleum pitch, and heavy oil. Examples of resins include polyvinyl alcohol, polyacrylic acid, phenol resin, furan resin, cellulose resin, polystyrene resin, polyimide resin, and epoxy resin. From the viewpoint of the battery characteristics to be obtained, tar pitches, phenol resins, etc. are preferred. The carbonaceous precursor is preferably in powder form so that it can be easily spread on the surface of the graphite particles. The median diameter (D 50 ) is not particularly limited, but is, for example, 1 to 50 μm.

[0030] <Preparation of precursor-attached graphite particles> In this production method, first, a carbonaceous precursor is adhered to graphite particles, thereby obtaining precursor-adhered graphite particles in which the carbonaceous precursor is adhered to at least a portion of the surface of the graphite particles. As a method for attaching the carbonaceous precursor to the graphite particles, for example, a method of mixing the graphite particles with the carbonaceous precursor can be mentioned. The mixing method is not particularly limited, and examples thereof include a method of mixing graphite particles with a powdered or heated-melted carbonaceous precursor in a liquid state using a kneader or the like. In this case, a dispersion in which graphite particles are dispersed in a dispersion medium may be used. Examples of the kneader that may be used include a pressure kneader and a two-roll mill.

[0031] <<Amount of carbonaceous precursor added>> The amount of carbonaceous precursor added (the amount of carbonaceous precursor attached to the graphite particles) is adjusted appropriately depending on the desired amount of carbonaceous material to be finally obtained.

[0032] <Firing> Next, the obtained precursor-adhered graphite particles are subjected to firing (first firing, intermediate exposure, and second firing). This carbonizes the carbonaceous precursor into a carbonaceous material. This results in carbonaceous-coated graphite particles, in which at least a portion of the surface of the graphite particles is coated with a carbonaceous material. In this case, intermediate exposure in an oxidizing atmosphere is carried out between the first firing and the second firing, which are carried out in a non-oxidizing atmosphere, thereby improving the crystallinity of the resulting carbonaceous material.

[0033] First firing In the first firing, the precursor-adhered graphite particles are heated in a non-oxidizing atmosphere at a temperature higher than 900° C. and equal to or lower than 1500° C. (hereinafter also referred to as the "first firing temperature"). The reason why the atmosphere for the first firing is a non-oxidizing atmosphere is that carbonaceous materials will burn and disappear in an oxidizing atmosphere. Examples of the non-oxidizing atmosphere include a nitrogen atmosphere, an argon atmosphere, a helium atmosphere, a vacuum atmosphere, etc. A coke breeze, which oxidizes itself, may be placed to reduce the oxygen concentration in the atmosphere, thereby making it a substantially non-oxidizing atmosphere.

[0034] If the first firing temperature is too low, the mass loss starting temperature of the obtained carbonaceous-coated graphite particles will be low, so the first firing temperature is above 900°C, preferably 950°C or higher, and more preferably 1000°C or higher. On the other hand, if the first firing temperature is too high, the mass loss starting temperature of the obtained carbonaceous-coated graphite particles becomes too high. Therefore, the first firing temperature is 1500°C or lower, preferably 1400°C or lower, and more preferably 1250°C or lower.

[0035] The time for carrying out the first firing (the time for heating the precursor-adhered graphite particles in a non-oxidizing atmosphere at the first firing temperature) is preferably 0.5 hours or more, more preferably 1 hour or more, and even more preferably 2 hours or more. There is no particular upper limit, and it is, for example, 30 hours, preferably 10 hours, and more preferably 5 hours.

[0036] The precursor-adhered graphite particles that have been subjected to the first firing are preferably cooled to an intermediate exposure temperature, which will be described later, while still in the atmosphere (non-oxidizing atmosphere) of the first firing.

[0037] 《Intermediate exposure》 In the intermediate exposure, the precursor-adhered graphite particles that have been subjected to the first firing (and then cooled) are exposed to an oxidizing atmosphere at a temperature of 50 to 100° C. (hereinafter also referred to as "intermediate exposure temperature"). Examples of the oxidizing atmosphere include an atmosphere containing oxygen, ozone, carbon dioxide, etc., and from the viewpoint of convenience, the air atmosphere is preferred.

[0038] If the intermediate exposure temperature is too low, the mass loss starting temperature of the obtained carbonaceous-coated graphite particles will decrease, so the intermediate exposure temperature is 50°C or higher, preferably 55°C or higher, and more preferably 60°C or higher. On the other hand, if the intermediate exposure temperature is too high, the mass loss starting temperature of the obtained carbonaceous-coated graphite particles becomes too high. Therefore, the intermediate exposure temperature is 100°C or less, preferably 90°C or less, and more preferably 80°C or less.

[0039] The time for which the intermediate exposure is carried out (the time for which the precursor-adhered graphite particles are exposed to an oxidizing atmosphere at the intermediate exposure temperature) is preferably 10 minutes or more, more preferably 20 minutes or more, and even more preferably 30 minutes or more. There is no particular upper limit, and it is, for example, 120 minutes, preferably 90 minutes, and more preferably 60 minutes.

[0040] Second firing In the second firing, the precursor-adhered graphite particles that have been subjected to intermediate exposure are heated in a non-oxidizing atmosphere at a temperature of 1100° C. to 1500° C. (hereinafter also referred to as the "second firing temperature"). The reasons for using a non-oxidizing atmosphere for the second firing and suitable examples of the non-oxidizing atmosphere are the same as those for the first firing.

[0041] If the second firing temperature is too low, the mass loss starting temperature of the obtained carbonaceous-coated graphite particles will be low, so the second firing temperature is 1100°C or higher, preferably 1150°C or higher, and more preferably 1200°C or higher. The second baking temperature is preferably equal to or higher than the first baking temperature. On the other hand, if the second firing temperature is too high, the mass loss starting temperature of the obtained carbonaceous-coated graphite particles becomes too high. Therefore, the second firing temperature is 1500°C or lower, preferably 1400°C or lower, and more preferably 1300°C or lower.

[0042] The time for carrying out the second firing (the time for heating the precursor-adhered graphite particles in a non-oxidizing atmosphere at the second firing temperature) is preferably 0.5 hours or more, more preferably 1 hour or more, and even more preferably 2 hours or more. There is no particular upper limit, and it is, for example, 30 hours, preferably 10 hours, and more preferably 5 hours.

[0043] [Anode for lithium-ion secondary batteries (negative electrode)] The negative electrode for a lithium ion secondary battery of this embodiment contains the present carbonaceous-coated graphite particles described above. Hereinafter, the "carbonaceous-coated graphite particles" may be referred to as the "negative electrode material." The negative electrode for a lithium ion secondary battery may also be simply referred to as the "negative electrode."

[0044] The negative electrode of this embodiment is produced in the same manner as a normal negative electrode. When preparing the negative electrode, it is preferable to use a negative electrode mixture prepared in advance by adding a binder to a negative electrode material. The negative electrode mixture may contain an active material and a conductive material other than the negative electrode material. The binder is preferably one that is chemically and electrochemically stable with respect to the electrolyte, and examples thereof include fluorine-based resins such as polytetrafluoroethylene and polyvinylidene fluoride; resins such as polyethylene, polyvinyl alcohol and styrene-butadiene rubber; and carboxymethyl cellulose; and two or more of these can also be used in combination. The binder is usually used in an amount of about 1 to 20 mass % of the total amount of the negative electrode mixture. More specifically, the negative electrode material is first optionally adjusted to the desired particle size by classification or the like. The negative electrode material is then mixed with a binder, and the resulting mixture is dispersed in a solvent to prepare a paste-like negative electrode mixture. Examples of solvents include water, isopropyl alcohol, N-methylpyrrolidone, and dimethylformamide. Known agitators, mixers, kneaders, and the like are used for mixing and dispersion. The prepared paste is applied to one or both sides of the current collector and dried. The application amount is 3 to 15 mg / cm. 2 is preferred, and 5 to 15 mg / cm 2 In this way, a negative electrode mixture layer (negative electrode) that is uniformly and firmly adhered to the current collector is obtained. The thickness of the negative electrode mixture layer is preferably 10 to 200 μm, more preferably 20 to 100 μm. After forming the negative electrode mixture layer, pressure bonding such as pressing can be performed to further increase the adhesive strength between the negative electrode mixture layer (negative electrode) and the current collector. The shape of the current collector is not particularly limited, but may be, for example, a foil, a mesh, or a net-like shape such as expanded metal. Preferred materials for the current collector include copper, stainless steel, nickel, etc. The thickness of the current collector in the case of a foil shape is preferably about 5 to 20 μm.

[0045] <Coated electrode density> In the negative electrode of this embodiment, the coating electrode density is 1.10 g / cm 3 More than 1.20 g / cm is preferable. 3 More preferably, 2.00 g / cm 3 Preferably less than 1.90 g / cm 3 The following is more preferred: The coating electrode density of the negative electrode is determined as follows. The mass (using an electronic balance) and thickness (using a micrometer) of a negative electrode punched to a fixed area are measured. Next, the mass of 10 current collectors punched to the same area is measured, and the average value is taken as the mass of the current collector. Furthermore, the thickness of the current collector is calculated from the metal density of the current collector. The coated electrode density of the negative electrode is then calculated using the following formula. Coated electrode density of negative electrode = (mass of negative electrode - mass of current collector) / (thickness of negative electrode - thickness of current collector) x (punching area)

[0046] [Lithium-ion secondary battery] The lithium ion secondary battery of this embodiment has the negative electrode of this embodiment, and further has a positive electrode, a non-aqueous electrolyte, and the like. The lithium ion secondary battery of this embodiment is constructed by, for example, stacking a negative electrode, a non-aqueous electrolyte, and a positive electrode in this order and housing them in an exterior material. The type of the lithium ion secondary battery of this embodiment can be arbitrarily selected from cylindrical, prismatic, coin, button, etc. depending on the application, mounted equipment, required charge / discharge capacity, etc.

[0047] <Positive electrode> It is preferable to select a material for the positive electrode (positive electrode active material) that can absorb / desorb a sufficient amount of lithium. Positive electrode active materials include, in addition to lithium, lithium-containing compounds such as lithium-containing transition metal oxides, transition metal chalcogenides, vanadium oxides and lithium compounds thereof; X Mo6S 8-Y (wherein M is at least one transition metal element, X is a number in the range of 0≦X≦4, and Y is a number in the range of 0≦Y≦1), activated carbon, activated carbon fiber, etc. Examples of vanadium oxide include VO, VO 13 , V2O4, V3O8. 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. The lithium-containing transition metal oxide is specifically LiM 1 1-X M 2 X O2 (M in the formula 1 , M 2 is at least one transition metal element, and X is a number in the range of 0≦X≦1), or LiM 1 1-Y M 2 Y O4 (M in the formula 1 , M 2 is at least one transition metal element, and Y is a number in the range of 0≦Y≦1). M 1 , M 2 The transition metal element represented by is Co, Ni, Mn, Cr, Ti, V, Fe, Zn, Al, In, Sn, etc., and preferred are Co, Fe, Mn, Ti, Cr, V, Al, etc. Preferred specific examples are LiCoO2, LiNiO2, LiMnO2, LiNi 0.9 Co 0.1 O2, LiNi 0.5 Co 0.5 O2, etc. The lithium-containing transition metal oxide can be obtained by, for example, using lithium, oxides, hydroxides, salts, etc. of transition metals as starting materials, mixing these starting materials according to the composition of the desired metal oxide, and firing the mixture at a temperature of 600 to 1000°C in an oxygen atmosphere. The positive electrode active material may be one of the above compounds or a combination of two or more of them. For example, a carbonate such as lithium carbonate may be added to the positive electrode. When forming the positive electrode, various additives such as a conventional conductive agent and a binder may be appropriately used. The positive electrode is produced, for example, by applying a positive electrode mixture composed of a positive electrode active material, a binder, and a conductive agent for imparting conductivity to the positive electrode to both sides of a current collector to form a positive electrode mixture layer. As the binder, the binder used in the production of the negative electrode can be used. As the conductive agent, known conductive agents such as graphitized materials and carbon black are used. The shape of the current collector is not particularly limited, but examples include foil, mesh, etc. The current collector is made of aluminum, stainless steel, nickel, etc. The thickness of the current collector is preferably 10 to 40 μm. As with the negative electrode, the positive electrode may also be formed by applying a paste-like positive electrode mixture to a current collector, drying it, and then pressing it together by pressing or the like.

[0048] <Non-aqueous electrolyte> The non-aqueous electrolyte may be a liquid non-aqueous electrolyte (non-aqueous electrolyte solution), or may be a polymer electrolyte such as a solid electrolyte or a gel electrolyte. Examples of non-aqueous electrolytes include lithium salts such as LiPF, LiBF, LiAsF, LiClO, LiB(CH), LiCl, LiBr, LiCFSO, LiCHSO, LiN(CFSO), LiC(CFSO), LiN(CFCHOSO), LiN(CFCFOSO), LiN(HCFCFCHOSO), LiN((CF)CHOSO), LiB[{CH(CF)}], LiAlCl, and LiSiF. From the standpoint of oxidation stability, LiPF and LiBF are preferred. The concentration of the electrolyte salt in the nonaqueous electrolyte solution is preferably 0.1 to 5.0 mol / L, more preferably 0.5 to 3.0 mol / L. Examples of solvents for preparing the non-aqueous electrolyte solution include carbonates such as ethylene carbonate, propylene carbonate, dimethyl carbonate, and diethyl carbonate; ethers such as 1,1- or 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, γ-butyrolactone, 1,3-dioxolane, 4-methyl-1,3-dioxolane, anisole, and diethyl ether; thioethers such as sulfolane and methylsulfolane; nitriles such as acetonitrile, chloronitrile, and propionitrile; and aprotic organic solvents such as trimethyl borate, tetramethyl silicate, nitromethane, dimethylformamide, N-methylpyrrolidone, ethyl acetate, trimethyl orthoformate, nitrobenzene, benzoyl chloride, benzoyl bromide, tetrahydrothiophene, dimethyl sulfoxide, 3-methyl-2-oxazolidone, ethylene glycol, and dimethyl sulfite. When the non-aqueous electrolyte is a polymer electrolyte such as a solid electrolyte or a gel electrolyte, it is preferable to use a polymer gelled with a plasticizer (non-aqueous electrolyte solution) as the matrix. Suitable polymers constituting the matrix include ether-based polymer compounds such as polyethylene oxide and its crosslinked products; poly(meth)acrylate-based polymer compounds; and fluorine-based polymer compounds such as polyvinylidene fluoride and vinylidene fluoride-hexafluoropropylene copolymer. The concentration of the electrolyte salt in the non-aqueous electrolyte solution, which is a plasticizer, is preferably 0.1 to 5.0 mol / L, and more preferably 0.5 to 2.0 mol / L. In the polymer electrolyte, the proportion of the plasticizer is preferably 10 to 90 mass %, more preferably 30 to 80 mass %.

[0049] <Separator> In the lithium ion secondary battery of this embodiment, a separator can also be used. The separator is not particularly limited in material, but examples thereof include woven fabric, nonwoven fabric, and synthetic resin microporous membranes. Of these, synthetic resin microporous membranes are preferred, and polyolefin microporous membranes are more preferred in terms of thickness, membrane strength, and membrane resistance. Suitable polyolefin microporous membranes include polyethylene microporous membranes, polypropylene microporous membranes, and composite microporous membranes thereof. [Example]

[0050] The present invention will be specifically described below with reference to examples, but the present invention is not limited to the examples described below.

[0051] Example 1 <Preparation of graphite particles> The raw material, flake-like natural graphite (particle diameter: 8 μm), was continuously passed through four pulverizers (hybridization system, manufactured by Nara Machinery Works, Ltd.) arranged in series. In each pulverizer, the pulverization time was 10 minutes, and the rotor peripheral speed was 60 m / s. In this way, the raw material was pulverized and granulated to obtain graphite particles, which were spherical graphite.

[0052] <<Preparation of precursor-attached graphite particles>> A carbonaceous precursor, which was a phenolic resin pulverized into powder using an impact pulverizer, was added to the obtained graphite particles (spheroidized graphite), and the mixture was mixed for 15 minutes at 25° C. using a drum mixer. In this way, precursor-attached graphite particles were obtained, in which the carbonaceous precursor (phenolic resin) was attached to the surface of the graphite particles (spheroidized graphite).

[0053] <<Preparation of carbonaceous-coated graphite particles>> Next, the obtained precursor-adhered graphite particles were placed in a graphite container with a lid, and fired (first firing, intermediate exposure, and second firing) using a tubular furnace. Specifically, in the first firing, nitrogen was circulated at 2 L / min in an atmosphere (non-oxidizing atmosphere), and the precursor-attached graphite particles were then heated at the first firing temperature of 1100°C for 60 minutes, and then cooled to the intermediate exposure temperature described below without changing the atmosphere. Next, in the intermediate exposure step, the cooled precursor-attached graphite particles were exposed to an air atmosphere (oxidizing atmosphere) at an intermediate exposure temperature of 70°C for 30 minutes. Thereafter, in the second firing, the atmosphere was changed to a non-oxidizing atmosphere in which nitrogen was circulated at 2 L / min, and the precursor-adhered graphite particles were heated at a second firing temperature of 1100° C. for 60 minutes. In this way, the carbonaceous precursor (phenolic resin) became carbonaceous, and carbonaceous-coated graphite particles were obtained in which the surfaces of the graphite particles (spheroidized graphite) were coated with the carbonaceous material. The physical properties of the obtained carbonaceous-coated graphite particles were determined by the methods described above, and the results are shown in Table 1 below.

[0054] <<Preparation of negative electrode>> A negative electrode mixture paste was prepared by adding 98 parts by mass of carbonaceous coated graphite particles (negative electrode material), 1 part by mass of carboxymethyl cellulose (binder), and 1 part by mass of styrene butadiene rubber (binder) to water and stirring. The prepared negative electrode mixture paste was applied to a copper foil in a uniform thickness and then dried in a vacuum at 90°C to form a negative electrode mixture layer. Next, this negative electrode mixture layer was pressed at a pressure of 120 MPa using a hand press. Thereafter, the copper foil and the negative electrode mixture layer were punched out into a circular shape with a diameter of 15.5 mm. In this way, a negative electrode (coated electrode density: 1.50 g / cm) adhered to a current collector made of copper foil was obtained. 3 ) was produced.

[0055] <<Preparation of the positive electrode>> The positive electrode consisted of LiCoO (93% by mass), a conductive agent (4% by mass), and a binder (3% by mass). The conductive agent was flake graphite particles, and the binder was styrene-butadiene rubber.

[0056] <<Preparation of Evaluation Battery>> A button-type secondary battery shown in FIG. 1 was fabricated as an evaluation battery. Fig. 1 is a cross-sectional view of a button-type secondary battery. In the button-type secondary battery shown in Fig. 1, the peripheral edges of an exterior cup 1 and an exterior can 3 are crimped together via an insulating gasket 6, forming a sealed structure. Inside the sealed structure, a current collector 7a, a positive electrode 4, a separator 5, a negative electrode 2, and a current collector 7b are layered in this order from the inner surface of the exterior can 3 toward the inner surface of the exterior cup 1.

[0057] The button-type secondary battery shown in FIG. 1 was fabricated as follows. First, a non-aqueous electrolyte solution was prepared by dissolving LiPF6 at a concentration of 1 mol / L in a mixed solvent of ethylene carbonate (33% by volume) and methyl ethyl carbonate (67% by volume). The obtained non-aqueous electrolyte solution was impregnated into a porous polypropylene body (thickness: 20 μm) to produce a separator 5 impregnated with the non-aqueous electrolyte solution. Next, the prepared separator 5 was sandwiched and laminated between a negative electrode 2 adhered to a current collector 7b made of copper foil and a positive electrode 4 adhered to a current collector 7a made of nickel net. Thereafter, the current collector 7b and the negative electrode 2 were placed inside an exterior cup 1, and the current collector 7a and the positive electrode 4 were placed inside an exterior can 3, and the exterior cup 1 and the exterior can 3 were then joined together. Furthermore, the peripheral portions of the exterior cup 1 and the exterior can 3 were crimped and sealed with an insulating gasket 6 interposed therebetween. In this manner, a button-type secondary battery was produced.

[0058] The produced button-type secondary batteries (evaluation batteries) were used to carry out the tests described below to evaluate the battery characteristics. The results are shown in Table 1 below. In the following tests, the process of absorbing lithium ions into the negative electrode material was referred to as charging, and the process of desorbing lithium ions from the negative electrode material was referred to as discharging.

[0059] Test 1: Output characteristics The battery was charged at a constant current of 0.1 C until the circuit voltage reached 1 mV, then switched to constant voltage charging and continued charging until the current reached 20 μA. The charge capacity (unit: mAh) was calculated from the amount of current flowing during this period. After this, a 10-minute break was allowed. Next, the battery was discharged at a constant current of 0.2 C until the circuit voltage reached 1.5 V. The initial discharge capacity (unit: mAh) was calculated from the amount of current flowing during this period. Next, constant current charging was performed at a current value of 0.1 C until the circuit voltage reached 1 mV, after which it was switched to constant voltage charging and charging continued until the current value reached 20 μA. After a 10-minute break, constant current discharging was performed at a current value of 2.0 C until the circuit voltage reached 1.5 V. The rapid discharge capacity (unit: mAh) was calculated from the amount of current flowing during this period. From the obtained discharge capacity, the output characteristics (unit: %) were calculated based on the following formula. Output characteristics = 100 x (rapid discharge capacity / initial discharge capacity)

[0060] Test 2: Rapid charging characteristics The battery was charged at a constant current of 0.1 C until the circuit voltage reached 1 mV, then switched to constant voltage charging and continued charging until the current reached 20 μA. The charge capacity (unit: mAh) was calculated from the amount of current flowing during this period. After this, a 10-minute break was allowed. Next, the battery was discharged at a constant current of 0.2 C until the circuit voltage reached 1.5 V. The initial discharge capacity (unit: mAh) was calculated from the amount of current flowing during this period. Next, constant current discharge was carried out at a current value of 2.0 C until the circuit voltage reached 1.5 V. The rapid charge capacity (unit: mAh) was calculated from the amount of current flowing during this period. From the obtained charge capacity, the rapid charge characteristic (unit: %) was calculated based on the following formula. Quick charge characteristics = 100 × (quick charge capacity / initial discharge capacity)

[0061] Test 3: Cycle characteristics The battery was charged at a constant current of 0.1 C until the circuit voltage reached 1 mV, then switched to constant voltage charging and continued charging until the current reached 20 μA. The charge capacity (unit: mAh) was calculated from the amount of current flowing during this period. After this, a 10-minute break was allowed. Next, the battery was discharged at a constant current of 0.2 C until the circuit voltage reached 1.5 V. The discharge capacity (unit: mAh) was calculated from the amount of current flowing during this period. This charge / discharge cycle was repeated 10 times (from the first cycle to the tenth cycle), and the cycle characteristics (unit: %) were calculated from the obtained discharge capacities according to the following formula. Cycle characteristics = 100 × (discharge capacity at 10th cycle / discharge capacity at 1st cycle)

[0062] Example 2 Carbonaceous-coated graphite particles were prepared and evaluated in the same manner as in Example 1, except that the second baking temperature was changed to 1200° C. The results are shown in Table 1 below.

[0063] Example 3 Except for changing the time for carrying out the intermediate exposure to 10 minutes, carbonaceous-coated graphite particles were prepared and evaluated in the same manner as in Example 1. The results are shown in Table 1 below.

[0064] Example 4 Except for changing the carbonaceous content to 1.0 part by mass, carbonaceous-coated graphite particles were prepared and evaluated in the same manner as in Example 1. The results are shown in Table 1 below.

[0065] Example 5 Except for changing the carbonaceous content to 10.0, carbonaceous-coated graphite particles were prepared and evaluated in the same manner as in Example 1. The results are shown in Table 1 below.

[0066] Example 6 Except for changing the carbonaceous precursor from phenolic resin to coal tar pitch, carbonaceous-coated graphite particles were prepared and evaluated in the same manner as in Example 1. The results are shown in Table 1 below.

[0067] Comparative Example 1 Carbonaceous-coated graphite particles were prepared and evaluated in the same manner as in Example 1, except that the first firing temperature was changed to 800° C. The results are shown in Table 1 below.

[0068] Comparative Example 2 Except for not carrying out the intermediate exposure and second baking, carbonaceous-coated graphite particles were prepared and evaluated in the same manner as in Example 1. The results are shown in Table 1 below.

[0069] Comparative Example 3 Carbonaceous-coated graphite particles were prepared and evaluated in the same manner as in Example 1, except that the second baking temperature was changed to 1000° C. The results are shown in Table 1 below.

[0070] Comparative Example 4 Carbonaceous-coated graphite particles were prepared and evaluated in the same manner as in Example 1, except that the first firing temperature was changed to 1600° C. The results are shown in Table 1 below.

[0071] Comparative Example 5 Except for changing the carbonaceous content to 0.04 parts by mass, carbonaceous-coated graphite particles were prepared and evaluated in the same manner as in Example 1. The results are shown in Table 1 below.

[0072] Comparative Example 6 Except for changing the carbonaceous content to 16.0 parts by mass, carbonaceous-coated graphite particles were prepared and evaluated in the same manner as in Example 1. The results are shown in Table 1 below.

[0073] Comparative Example 7 Carbonaceous-coated graphite particles were prepared and evaluated in the same manner as in Example 1, except that the intermediate exposure temperature was changed to 30° C. The results are shown in Table 1 below.

[0074] Comparative Example 8 Carbonaceous-coated graphite particles were prepared and evaluated in the same manner as in Example 1, except that the intermediate exposure temperature was changed to 150° C. The results are shown in Table 1 below.

[0075] Comparative Example 9 <Preparation of graphite particles> In the same manner as in Example 1, graphite particles, which were spherical graphite, were obtained.

[0076] <<Preparation of precursor-attached graphite particles>> Precursor-attached graphite particles were obtained in the same manner as in Example 1, in which a carbonaceous precursor (phenolic resin) was attached to the surface of graphite particles (spheroidized graphite), so that the carbonaceous content in the finally obtained carbonaceous-coated graphite particles was 5.0 parts by mass.

[0077] <<Preparation of carbonaceous-coated graphite particles>> The obtained precursor-attached graphite particles were placed in a graphite container with a lid, and calcined using a tubular furnace. Specifically, first, the first baking was carried out in the same manner as in Example 1. Next, intermediate exposure was carried out in the same manner as in Example 1, except that the intermediate exposure temperature was changed to 80°C, to obtain an intermediately exposed body. Thereafter, a second deposition of the carbonaceous precursor was carried out. More specifically, the phenolic resin was deposited on the intermediate exposed body under the same conditions as in "Preparation of precursor-deposited graphite particles" in Example 1. At this time, the phenolic resin was deposited so that the carbonaceous content in the finally obtained carbonaceous-coated graphite particles was 5.0 parts by mass. Finally, the second baking was carried out in the same manner as in Example 1. In this way, the carbonaceous precursor (phenolic resin) became carbonaceous, and the surfaces of the graphite particles (spheroidized graphite) were coated with the carbonaceous to obtain carbonaceous-coated graphite particles. The carbonaceous content in the obtained carbonaceous-coated graphite particles was 10.0 parts by mass. The physical properties of the obtained carbonaceous-coated graphite particles were determined by the methods described above, and the results are shown in Table 1 below.

[0078] Comparative Example 10 <Preparation of graphite particles> In the same manner as in Example 1, graphite particles, which were spherical graphite, were obtained.

[0079] <<Preparation of precursor-attached graphite particles>> In the same manner as in Example 1, precursor-adhered graphite particles were obtained.

[0080] <<Preparation of carbonaceous-coated graphite particles>> The obtained precursor-attached graphite particles were placed in a graphite container with a lid, and calcined using a tubular furnace. Specifically, first, nitrogen was circulated at 2 L / min in an atmosphere (non-oxidizing atmosphere), and then the precursor-adhered graphite particles were heated at a first baking temperature of 700° C. for 60 minutes. Next, without cooling (intermediate exposure), the mixture was heated at a second firing temperature of 1300° C. for 60 minutes in an atmosphere (non-oxidizing atmosphere) where nitrogen was circulated at 2 L / min. In this way, the carbonaceous precursor (phenolic resin) became carbonaceous, and carbonaceous-coated graphite particles were obtained in which the surfaces of the graphite particles (spheroidized graphite) were coated with the carbonaceous material. The physical properties of the obtained carbonaceous-coated graphite particles were determined by the methods described above, and the results are shown in Table 1 below.

[0081] [Table 1]

[0082] <Summary of evaluation results> As shown in Table 1 above, compared to Comparative Examples 1 to 4 and 7 to 8, which had mass loss onset temperatures outside the range of 800 to 980°C, Examples 1 to 6, which had mass loss onset temperatures within the range of 800 to 980°C, had good output characteristics, fast charging characteristics, and cycle characteristics. Comparative Example 5, in which the carbonaceous content was less than 0.1 parts by mass, had insufficient cycle characteristics. In Comparative Example 6, in which the carbonaceous content was more than 15.0 parts by mass, the output characteristics and rapid charging characteristics were insufficient. In Comparative Example 9, in which a second deposition of a carbonaceous precursor was carried out after intermediate exposure, the mass loss starting temperature was less than 800° C., and the output characteristics and rapid charging characteristics were insufficient. In Comparative Example 10, in which only the first and second firings were performed without intermediate exposure, the mass loss starting temperature was over 980° C., and the output characteristics, rapid charging characteristics, and cycle characteristics were insufficient. [Explanation of symbols]

[0083] 1: Exterior cup 2: Negative electrode 3: Outer can 4: Positive electrode 5: Separator 6: Insulation gasket 7a: Current collector 7b: Current collector

Claims

1. Graphite particles; a carbonaceous material that coats at least a portion of the surface of the graphite particles, The mass loss starting temperature when heated in a water vapor atmosphere is 800 to 935°C, The carbonaceous-coated graphite particles have a carbonaceous content of 0.1 to 15.0 parts by mass per 100.0 parts by mass of the graphite particles.

2. The carbonaceous-coated graphite particles according to claim 1, which are used as a negative electrode material for a lithium-ion secondary battery.

3. A negative electrode for a lithium ion secondary battery, comprising the carbonaceous-coated graphite particles according to claim 1.

4. A lithium ion secondary battery comprising the negative electrode according to claim 3.

5. Graphite particles, a carbonaceous material that coats at least a portion of the surface of the graphite particles, The mass loss starting temperature when heated in a water vapor atmosphere is 800 to 980°C, A method for producing carbonaceous-coated graphite particles, wherein the carbonaceous content is 0.1 to 15.0 parts by mass per 100.0 parts by mass of the graphite particles, a carbonaceous precursor, which is a precursor of the carbonaceous material, is attached to the graphite particles to obtain precursor-attached graphite particles; A first firing step is performed in which the precursor-deposited graphite particles are heated at a temperature of more than 900°C and not more than 1500°C in a non-oxidizing atmosphere; After the first firing, an intermediate exposure is carried out in which the precursor-deposited graphite particles are exposed to an oxidizing atmosphere at a temperature of 50 to 100°C; After the intermediate exposure, the precursor-deposited graphite particles are subjected to a second firing in a non-oxidizing atmosphere at a temperature of 1100 to 1500°C.

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