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

Abstract

The present invention provides carbonaceous material-coated graphite particles which exhibit excellent cycle capacity retention properties if used as a negative electrode material of a lithium ion secondary battery. The carbonaceous material-coated graphite particles each comprise a graphite particle and a carbonaceous material which covers at least a part of the surface of the graphite particle, and the elastic modulus as determined using a scanning probe microscope is 10 GPa or more.

Description

[Technical Field]

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

[0002] A lithium-ion secondary battery has a negative electrode, a positive electrode, and a non-aqueous electrolyte as its main components. It functions as a secondary battery when lithium ions move between the negative and positive electrodes during the discharge and charging processes. Conventionally, spheroidized graphite particles (spheroidized graphite) have sometimes been used as the negative electrode material for lithium-ion secondary batteries (Patent Document 1). [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2014-146607 [Overview of the project] [Problems that the invention aims to solve]

[0004] When conventional graphite particles (e.g., spheroidized graphite) are used as the negative electrode material for lithium-ion secondary batteries, battery characteristics such as cycle capacity maintenance may be insufficient. As a result of their investigation, the inventors found that these battery characteristics can be improved by using carbonaceous coated graphite particles, in which the surface of graphite particles is coated with carbonaceous material.

[0005] Therefore, the present invention aims to provide carbonaceous coated graphite particles that exhibit excellent cycle capacity maintenance when used as a negative electrode material for lithium-ion secondary batteries. [Means for solving the problem]

[0006] As a result of diligent research by the inventors, we discovered that by optimizing the firing conditions when producing carbonaceous coated graphite particles, carbonaceous material with specific crystallinity can be obtained, thereby achieving the above objective, and thus completing the present invention.

[0007] In other words, the present invention provides the following [1] to [7]. [1] Carbonaceous coated graphite particles comprising graphite particles and a carbonaceous material covering at least a portion of the surface of the graphite particles, wherein the elastic modulus determined using a scanning probe microscope is 10 GPa or more. [2] Particle size is 5.0 to 15.0 μm and specific surface area is 3.0 to 15.0 m² 2 Carbonaceous coated graphite particles as described in [1] above, in a quantity of / g. [3] The carbonaceous coated graphite particles according to [1] or [2] above, wherein the carbonaceous content is 1.0 to 30.0% by mass relative to the total mass of the carbonaceous coated graphite particles. [4] Carbonaceous coated graphite particles according to any of [1] to [3] above, which are used as a negative electrode material for lithium-ion secondary batteries. [5] A negative electrode for a lithium-ion secondary battery, containing carbonaceous coated graphite particles as described in any of [1] to [3] above. [6] A lithium-ion secondary battery having the negative electrode described in [5] above. [7] A method for producing carbonaceous coated graphite particles according to any of [1] to [3] above, comprising: attaching a carbonaceous precursor, which is a precursor of the carbonaceous material, to graphite particles to obtain precursor-attached graphite particles; performing a first calcination in a non-oxidizing atmosphere at a temperature of more than 900°C and less than or equal to 1500°C; after the first calcination, performing an intermediate exposure in an oxidizing atmosphere at a temperature of 50 to 100°C on the precursor-attached graphite particles; and after the intermediate exposure, performing a second calcination in a non-oxidizing atmosphere at a temperature of 1100 to 1500°C on the precursor-attached graphite particles. [Effects of the Invention]

[0008] According to the present invention, it is possible to provide carbon-coated graphite particles that are excellent in cycle capacity retention when used as a negative electrode material of a lithium-ion secondary battery.

Brief Description of the Drawings

[0009] [Figure 1] It is a cross-sectional view of an evaluation battery fabricated to evaluate battery characteristics in Examples and Comparative Examples.

Modes for Carrying Out the Invention

[0010] In this specification, when a range is expressed using "~", the range includes both ends of "~". For example, the range of A~B includes A and B.

[0011] [Carbon-Coated Graphite Particles] The carbon-coated graphite particles of the present embodiment (hereinafter, also referred to as "the present carbon-coated graphite particles") include graphite particles and a carbonaceous material that coats at least a part of the surface of the graphite particles, and the elastic modulus determined using a scanning probe microscope is 10 GPa or more. When the present carbon-coated graphite particles are used as a negative electrode material of a lithium-ion secondary battery, the cycle capacity retention is excellent.

[0012] 〈Elastic Modulus〉 Regarding the present carbon-coated graphite particles, the elastic modulus (hereinafter, also simply referred to as "elastic modulus") determined using a scanning probe microscope (SPM) is 10 GPa or more, more preferably 12 GPa or more, still more preferably 14 GPa or more, and particularly preferably 16 GPa or more because the cycle capacity retention is excellent.

[0013] The reason for the excellent cycle capacity retention is not clear, but it is presumed that when the elastic modulus is less than the above range, the carbonaceous material coating the surface of the graphite particles is insufficient, whereas when the elastic modulus is within the above range, the surface of the graphite particles is sufficiently coated with the carbonaceous material.

[0014] On the other hand, the elastic modulus of these carbonaceous coated graphite particles is preferably 25 GPa or less, more preferably 24 GPa or less, and even more preferably 22 GPa or less, for the reason that it has excellent rapid charging characteristics.

[0015] The measurement of elastic modulus using a scanning probe microscope (SPM) is performed as follows: First, a thin layer of epoxy resin is applied to a silicon substrate, and the sample (carbonaceous coated graphite particles) is scattered and fixed on top of it. Next, a force curve is obtained by scanning the surface of the particles with a probe under the following conditions (a pressing load of 200 nN for each particle). The DMT model is fitted to the obtained force curve to obtain the elastic modulus (unit: GPa) of each particle. The average value of the obtained elastic moduli is determined as the elastic modulus of the sample (carbonaceous coated graphite particles).

[0016] • SPM: Dimension ICON (manufactured by Bruker) • SPM controller: NanoScopeV (manufactured by Bruker) • Probe: Silicone cantilever (manufactured by Bruker) • Probe (cantilever) specifications: Single-crystal Si, radius of curvature 8nm • Measurement range: 2μm × 2μm • Number of measurements: 512 x 512 points • Measurement mode: QNM Scan mode (mode for measuring shape and modulus of elasticity) • Measurement atmosphere: Argon atmosphere

[0017] <Carbonate content> The carbonaceous content is preferably 1.0% by mass or more, more preferably 3.0% by mass or more, even more preferably 3.5% by mass or more, and particularly preferably 4.5% by mass or more, relative to the total mass of the carbonaceous coated graphite particles. If the carbonaceous content is within this range, the active edge surfaces of the graphite particles are more easily coated, resulting in superior initial charge-discharge characteristics.

[0018] On the other hand, the carbonaceous content is preferably 30.0% by mass or less, more preferably 25.0% by mass or less, even more preferably 20.0% by mass or less, even more preferably 15.0% by mass or less, particularly preferably 10.0% by mass or less, and most preferably 8.0% by mass or less. If the carbonaceous content is within this range, the amount of carbonaceous material with relatively low discharge capacity will decrease, resulting in superior discharge capacity. Furthermore, when the carbonaceous content is within this range, the amount of carbonaceous precursor used (described later) is reduced, which makes fusion less likely to occur during the mixing and calcination processes (described later). This suppresses cracking and peeling of the final carbonaceous material, resulting in superior initial charge-discharge characteristics.

[0019] The carbonaceous content only needs to be within the above range as an average value for the entire carbonaceous-coated graphite particle. Not all individual carbonaceous-coated graphite particles need to be within the above range; some carbonaceous-coated graphite particles outside this range may be included. The carbonaceous content is determined by calcining only the carbonaceous precursor under the same conditions as those used to calcine the precursor-attached graphite particles described later, and then calculating the amount of residual carbon.

[0020] In the following, for convenience, the carbonaceous content relative to the total mass of carbonaceous-coated graphite particles may simply be referred to as "carbonaceous content."

[0021] <Particle size> The particle size of these carbonaceous coated graphite particles is preferably 5.0 μm or larger, preferably 6.0 μm or larger, and more preferably 6.5 μm or larger. On the other hand, the particle size of the carbonaceous coated graphite particles is preferably 15.0 μm or less, more preferably 12.0 μm or less, even more preferably 10.0 μm or less, and particularly preferably 9.0 μm or less. The particle size is defined as the median diameter at which the cumulative frequency of the particle size distribution, determined using a laser diffraction particle size analyzer (LMS2000e, manufactured by Seishin Corporation), reaches 50% by volume.

[0022] <Specific surface area> The specific surface area of ​​these carbonaceous coated graphite particles is 3.0 m². 2 Preferably 4.0 m2 More preferably, it is 4.5 m 2 / g or more, and even more preferably 4.0 m 2 / g or more is particularly preferable. On the other hand, the specific surface area of the carbonaceous-coated graphite particles is preferably 20.0 m 2 / g or less, more preferably 18.0 m 2 / g or less, still more preferably 15.0 m 2 / g or less, and even more preferably 14.5 m 2 / g or less is particularly preferable. The specific surface area is determined by the BET method. More specifically, it is determined by nitrogen gas adsorption in accordance with JIS Z 8830:2013 (Method for Measuring Specific Surface Area of Powder (Solid) by Gas Adsorption).

[0023] [Method for Producing Carbonaceous-Coated Graphite Particles] Next, the method for producing the carbonaceous-coated graphite particles of the present embodiment (hereinafter also referred to as "the present production method") will be described. The present production method is a method for producing the above-described carbonaceous-coated graphite particles. Generally, a carbonaceous precursor is attached to graphite particles, and then the first firing, intermediate exposure, and second firing (hereinafter, these are collectively simply referred to as "firing") described below are carried out in this order.

[0024] Hereinafter, first, the graphite particles and the carbonaceous precursor used in the present production method will be described, and then each step in the present production method will be described.

[0025] 〈Graphite Particles〉 The method for producing the graphite particles used in the present production method is not particularly limited. For example, a method of processing the raw material into a spherical shape can be mentioned. Here, the raw material is graphite having a shape other than a spherical shape (including an ellipsoidal shape), for example, flaky graphite. The graphite may be either natural graphite or artificial graphite, but natural graphite is preferred because of its high crystallinity and other reasons. Methods for processing raw materials into a spherical shape (methods for spheroidizing raw materials) include, for example, stirring the raw materials in the presence of granulation aids such as adhesives or resins; applying mechanical force to the raw materials without using granulation aids; and using a combination of both methods. Of these methods, the method of applying mechanical force to the raw materials without using granulation aids is preferred. This method will be explained in more detail below.

[0026] More specifically, the raw material (for example, flaky graphite) is crushed and granulated by applying mechanical force using a crushing device. In this way, the raw material is spheroidized to obtain spheroidized graphite. Examples of grinding equipment include rotary ball mills, counter jet mills (manufactured by Hosokawa Micron Corporation), current jets (manufactured by Nisshin Engineering Co., Ltd.), hybridization systems (manufactured by Nara Machine Works Co., Ltd.), CF mills (manufactured by Ube Industries, Ltd.), mechanofusion systems (manufactured by Hosokawa Micron Corporation), and Theta Composers (manufactured by Tokuju Kogyo Co., Ltd.), among which the hybridization system (manufactured by Nara Machine Works Co., Ltd.) is preferred.

[0027] In this manufacturing method, it is preferable to arrange multiple grinding devices in series and to allow the raw material to pass through these devices continuously. In other words, it is preferable to arrange multiple grinding devices in series so that after the raw material passes through one grinding device, it is immediately ground and granulated in the next grinding device.

[0028] In this case, the number of crushing 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 crushing devices is preferably 10 or less, more preferably 8 or less, and even more preferably 7 or less.

[0029] In a single grinding device, the time for grinding and granulating the raw material (hereinafter also referred to as "grinding 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 a single grinding device is preferably 60 minutes or less, more preferably 50 minutes or less, and even more preferably 40 minutes or less.

[0030] 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, and more preferably 160 minutes or less.

[0031] Crushing devices typically have a built-in rotor. The peripheral speed of the rotor in each crushing device 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 crushing device is preferably 100 m / s or less, and more preferably 80 m / s or less.

[0032] To facilitate the application of shear and compressive forces to the raw materials, it is preferable to fill each crushing device with a small amount of raw material.

[0033] <Carbonaceous precursor> The carbonaceous precursor is the carbonaceous precursor described above, which is carbonized through calcination (described later) to become the carbonaceous material that coats the graphite particles. Examples of carbonaceous precursors include resins, which are carbon materials that have lower crystallinity than graphite and cannot become graphite crystals even when subjected to the high-temperature treatment required for graphitization. Examples of resins include polyvinyl alcohol, polyacrylic acid, phenolic resins, furan resins, cellulose resins, polystyrene resins, polyimide resins, and epoxy resins. From the viewpoint of the battery characteristics of the resulting battery, phenolic resin is preferred. The carbonaceous precursor is preferably in powder form to facilitate spreading on the surface of the graphite particles. The median diameter (D) of the powdered carbonaceous precursor is... 50 The thickness is not particularly limited, but for example, it is 1 to 50 μm.

[0034] <Preparation of precursor-attached graphite particles> In this manufacturing method, first, a carbonaceous precursor is attached to graphite particles. This yields precursor-attached graphite particles in which the carbonaceous precursor is attached to at least a portion of the surface of the graphite particles. One method for attaching a carbonaceous precursor to graphite particles is to mix the graphite particles with the carbonaceous precursor. The mixing method is not particularly limited, but for example, a method of mixing graphite particles with a carbonaceous precursor that is in powder form or heated and melted into a liquid form using a kneader or the like. In this case, a dispersion liquid in which graphite particles are dispersed in a dispersion medium may be used. As for the kneader, for example, a pressure kneader or a double-roll kneader may be used.

[0035] Amount of carbonaceous precursor added The amount of carbonaceous precursor added (the amount of carbonaceous precursor attached to the graphite particles) is adjusted as appropriate according to the desired amount of carbonaceous material obtained in the end.

[0036] <Firing> Next, the obtained precursor-attached graphite particles are subjected to calcination (first calcination, intermediate exposure, and second calcination). This carbonizes the carbonaceous precursor, converting it into carbonaceous material. In this way, carbonaceous coated graphite particles are obtained in which at least a portion of the surface of the graphite particles is coated with carbonaceous material. The resulting carbonaceous coated graphite particles have an elastic modulus that satisfies the range described above.

[0037] First firing In the first firing, the precursor-attached graphite particles are heated in a non-oxidizing atmosphere at a temperature between 900°C and 1500°C (hereinafter also referred to as the "first firing temperature"). The reason for using a non-oxidizing atmosphere during the first firing is that in an oxidizing atmosphere, carbonaceous materials burn and disappear. Examples of non-oxidizing atmospheres include nitrogen, argon, helium, and vacuum atmospheres. Alternatively, a coke breeze, which oxidizes itself, can be placed to lower the oxygen concentration of the atmosphere, effectively creating a non-oxidizing atmosphere.

[0038] The first firing temperature is above 900°C, preferably 950°C or higher, and more preferably 1000°C or higher. On the other hand, the first firing temperature is 1500°C or lower, preferably 1400°C or lower, and more preferably 1250°C or lower.

[0039] The time required for the first calcination (the time for heating the precursor-attached graphite particles at the first calcination temperature in a non-oxidizing atmosphere) 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, for example, 30 hours, preferably 10 hours, and more preferably 5 hours.

[0040] It is preferable to cool the precursor-attached graphite particles, which have undergone the first calcination, to the intermediate exposure temperature described later, while maintaining the atmosphere of the first calcination (non-oxidizing atmosphere).

[0041] 《Intermediate exposure》 In the intermediate exposure, the precursor-attached graphite particles, which have undergone a first calcination (and subsequently cooled), are exposed to an oxidizing atmosphere at a temperature of 50-100°C (hereinafter also referred to as the "intermediate exposure temperature"). Examples of oxidizing atmospheres include those containing oxygen, ozone, and carbon dioxide, but from a convenience standpoint, an atmospheric atmosphere is preferred.

[0042] 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, the intermediate exposure temperature is 100°C or lower, preferably 90°C or lower, and more preferably 80°C or lower.

[0043] The duration of the intermediate exposure (the time during which the precursor-attached 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, for example, 120 minutes, preferably 90 minutes, and more preferably 60 minutes.

[0044] Second firing In the second calcination, the precursor-attached graphite particles that have undergone 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 calcination temperature"). The reason for using a non-oxidizing atmosphere for the second firing, and preferred examples of non-oxidizing atmospheres, are the same as for the first firing.

[0045] The second firing temperature is 1100°C or higher, preferably 1150°C or higher, and more preferably 1200°C or higher. It is also preferable that the second firing temperature is higher than or equal to the first firing temperature. On the other hand, the second firing temperature is 1500°C or lower, preferably 1400°C or lower, and more preferably 1300°C or lower.

[0046] The time required for the second calcination (the time for heating the precursor-attached graphite particles in a non-oxidizing atmosphere at the second calcination 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; for example, it is 30 hours, preferably 10 hours, and preferably 5 hours.

[0047] [Negative electrode for lithium-ion secondary batteries] The negative electrode for the lithium-ion secondary battery in this embodiment contains the 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 the lithium-ion secondary battery may also be simply referred to as the "negative electrode."

[0048] The negative electrode in this embodiment is manufactured in accordance with the method for a normal negative electrode. When preparing the negative electrode, it is preferable to use a negative electrode mixture that has been prepared in advance by adding a binder to the negative electrode material. The negative electrode mixture may also contain active materials and conductive materials other than the negative electrode material. As a binder, one that exhibits chemical and electrochemical stability with respect to the electrolyte is preferred. Examples include fluororesins such as polytetrafluoroethylene and polyvinylidene fluoride; resins such as polyethylene, polyvinyl alcohol, and styrene-butadiene rubber; carboxymethylcellulose; and two or more of these can be used in combination. The binder is typically used in a proportion of about 1 to 20% by mass of the total amount of the negative electrode mixture. More specifically, first, the negative electrode material is optionally adjusted to the desired particle size by classification or other means. Then, the negative electrode material is 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, isopyrropyr alcohol, N-methylpyrrolidone, and dimethylformamide. Known stirrers, mixers, kneaders, etc., are used for mixing and dispersion. Apply the prepared paste to one or both sides of the current collector and allow it to dry. The application amount should be 3-15 mg / cm². 2 Preferably, 5-15 mg / cm³ 2 This is more preferable. In this way, a negative electrode mixture layer (negative electrode) that adheres uniformly and firmly to the current collector is obtained. The thickness of the negative electrode mixture layer is preferably 10 to 200 μm, and more preferably 20 to 100 μm. After forming the negative electrode mixture layer, applying pressure such as by pressing can further increase the adhesion strength between the negative electrode mixture layer (negative electrode) and the current collector. The shape of the current collector is not particularly limited, but examples include foil, mesh, or a network of expanded metal. Preferred materials for the current collector include copper, stainless steel, and nickel. The thickness of the current collector is preferably about 5 to 20 μm in the case of a foil.

[0049] <Coated electrode density> In the negative electrode of this embodiment, the coating electrode density is 1.10 g / cm³. 3 The above is preferable, 1.20 g / cm³ 3 The above is more preferable, while 2.00 g / cm³ is preferable. 3 The following is preferable: 1.90 g / cm³ 3 The following are preferable. The coating 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 out over a certain area are measured. Next, the masses of 10 current collectors punched out over the same area are measured, and the average value is taken as the mass of the current collector. Furthermore, the thickness of the current collector is determined from the density of the metal in the current collector. Then, the coating electrode density of the negative electrode is determined using the following formula. Coating density of the negative electrode = (mass of the negative electrode - mass of the current collector) / (thickness of the negative electrode - thickness of the current collector) × (punching area)

[0050] [Lithium-ion rechargeable battery] The lithium-ion secondary battery of this embodiment has the negative electrode of this embodiment, and further includes a positive electrode and a non-aqueous electrolyte, etc. The lithium-ion secondary battery of this embodiment is constructed, for example, by stacking a negative electrode, a non-aqueous electrolyte, and a positive electrode in this order and housing them within an outer casing. The type of lithium-ion secondary battery in this embodiment can be arbitrarily selected from cylindrical, prismatic, coin-type, button-type, etc., depending on the application, the equipment it is mounted on, the required charge / discharge capacity, etc.

[0051] <Positive electrode> The positive electrode material (positive electrode active material) should preferably be one that can intercept / deintercept a sufficient amount of lithium. Possible positive electrode active materials include lithium, as well as lithium-containing compounds such as lithium-containing transition metal oxides, transition metal chalcogenides, vanadium oxides, and their lithium compounds; general formula M X Mo6S 8-Y Examples include Schevrel phase compounds represented by the formula (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 fibers; etc. Vanadium oxides include V2O5, V6O 13 These are represented by V2O4 and V3O8. Lithium-containing transition metal oxides are composite oxides of lithium and transition metals, and may also be solid solutions of lithium and two or more transition metals. These composite oxides may be used individually or in combination of two or more types. Lithium-containing transition metal oxides are specifically LiM 1 1-X M 2 X O2 (M in the formula 1 M 2 (where X is at least one transition metal element, and X is a number in the range 0 ≤ X ≤ 1), or LiM 1 1-YM 2 Y O4 (M in the formula 1 M 2 (where is at least one transition metal element, and Y is a numerical value in the range 0 ≤ Y ≤ 1) M 1 M 2 The transition metal elements represented by are Co, Ni, Mn, Cr, Ti, V, Fe, Zn, Al, In, Sn, etc., with Co, Fe, Mn, Ti, Cr, V, Al, etc. Preferred specific examples are LiCoO2, LiNiO2, LiMnO2, LiNi 0.9 Co 0.1 O2, LiLiLi 0.5 Co 0.5 Examples include O2. Lithium-containing transition metal oxides can be obtained by using lithium, transition metal oxides, hydroxides, salts, etc., as starting materials, mixing these starting materials according to the desired metal oxide composition, and firing them at a temperature of 600 to 1000°C under an oxygen atmosphere. The positive electrode active material may be one of the above-mentioned compounds 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. When forming the positive electrode, various additives such as conventionally known conductive agents and binders can be used as appropriate. The positive electrode is manufactured, for example, by applying a positive electrode mixture, which consists 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 a binder, the same binder used in the fabrication of the negative electrode can be used. Known conductive agents such as graphite and carbon black are used as conductive materials. The shape of the current collector is not particularly limited, but examples include foil-like or mesh-like shapes. The material of the current collector is aluminum, stainless steel, nickel, etc. The thickness of the current collector is preferably 10 to 40 μm. Similar to the negative electrode, the positive electrode may also be prepared by applying a paste-like positive electrode mixture to the current collector, drying it, and then applying pressure such as by pressing.

[0052] <Non-aqueous electrolytes> The non-aqueous electrolyte may be a liquid non-aqueous electrolyte (non-aqueous electrolyte solution), or it may be a polymer electrolyte such as a solid electrolyte or gel electrolyte. As non-aqueous electrolytes, lithium salts such as LiPF6, LiBF4, LiAsF6, LiClO4, LiB(C6H5), LiCl, LiBr, LiCF3SO3, LiCH3SO3, LiN(CF3SO2)2, LiC(CF3SO2)3, LiN(CF3CH2OSO2)2, LiN(CF3CF2OSO2)2, LiN(HCF2CF2CH2OSO2)2, LiN((CF3)2CHOSO2)2, LiB[{C6H3(CF3)2}]4, LiAlCl4, and LiSiF6 are used, which are electrolyte salts commonly used in non-aqueous electrolyte solutions. From the viewpoint of oxidation stability, LiPF6 and LiBF4 are preferred. The concentration of the electrolyte salt in the non-aqueous electrolyte solution is preferably 0.1 to 5.0 mol / L, and more preferably 0.5 to 3.0 mol / L. Examples of solvents for preparing non-aqueous electrolyte solutions 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 for 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 acts as a plasticizer, is preferably 0.1 to 5.0 mol / L, and more preferably 0.5 to 2.0 mol / L. In polymer electrolytes, the proportion of plasticizer is preferably 10 to 90% by mass, and more preferably 30 to 80% by mass.

[0053] <Separator> In the lithium-ion secondary battery of this embodiment, a separator can also be used. The material of the separator is not particularly limited, but for example, woven fabrics, nonwoven fabrics, and microporous membranes made of synthetic resins can be used. Of these, microporous membranes made of synthetic resins are preferred, and among them, polyolefin-based microporous membranes are more preferred in terms of thickness, membrane strength, and membrane resistance. Suitable examples of polyolefin-based microporous membranes include polyethylene microporous membranes, polypropylene microporous membranes, and microporous membranes made by combining these. [Examples]

[0054] The present invention will be specifically described below with reference to examples. However, the present invention is not limited to the examples described below.

[0055] <Example 1> Preparation of graphite particles The raw material, flaky natural graphite (particle size: 8 μm), was continuously passed through five pulverizers (hybridization system, manufactured by Nara Machine Works Co., Ltd.) arranged in series. In each pulverizer, the pulverization time was 30 minutes, and the rotor peripheral speed was 50 m / second. By pulverizing and granulating the raw material in this way, graphite particles, which are spheroidized graphite, were obtained.

[0056] Preparation of Precursor-Attached Graphite Particles The obtained graphite particles (spheroidized graphite) were mixed with a pulverized phenolic resin carbonaceous precursor and mixed at room temperature for 30 minutes using a twin-screw kneader. The amount of carbonaceous precursor added was such that the final carbonaceous content was 4.0% by mass. 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).

[0057] Preparation of carbonaceous coated graphite particles Next, the obtained precursor-attached graphite particles were subjected to calcination (first calcination, intermediate exposure, and second calcination) using a tubular furnace. Specifically, in the first calcination, a nitrogen atmosphere was created by flowing 5 L / min (a non-oxidizing atmosphere), and the precursor-attached graphite particles were heated at a first calcination temperature of 950°C for 60 minutes. After that, the mixture was cooled to the intermediate exposure temperature described later without changing the atmosphere. Next, in the intermediate exposure, cooled precursor-attached graphite particles were exposed to an air atmosphere (oxidizing atmosphere) for 30 minutes at an intermediate exposure temperature of 80°C. Subsequently, in the second firing, the atmosphere was maintained by circulating nitrogen at 2 L / min (a non-oxidizing atmosphere), and the precursor-attached graphite particles were heated at a second firing temperature of 1100°C for 60 minutes. In this way, the carbonaceous precursor (phenol resin) becomes carbonaceous, and carbonaceous coated graphite particles are obtained in which the surface of graphite particles (spheroidized graphite) is coated with carbonaceous material. The physical properties of the obtained carbonaceous coated graphite particles were determined using the method described above. The results are shown in Table 1 below.

[0058] Fabrication of the 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 carboxymethylcellulose (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 (thickness: 16 μm) 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 with a hand press at a pressure of 120 MPa. After that, the copper foil and negative electrode mixture layer were punched out in a circular shape with a diameter of 15.5 mm. In this way, a negative electrode (thickness: 60 μm, coating electrode density: 1.20 g / cm³) was formed in close contact with a current collector made of copper foil. 3 ) was created.

[0059] 《Fabrication of the positive electrode》 A lithium metal foil was pressed onto a nickel mesh and punched out in a circular shape with a diameter of 15.5 mm. This created a positive electrode made of lithium metal foil (thickness: 0.5 mm) that was in close contact with a current collector made of nickel mesh.

[0060] Fabrication of evaluation batteries As an evaluation battery, we fabricated a button-type rechargeable battery as shown in Figure 1. Figure 1 is a cross-sectional view showing a button-type rechargeable battery. In the button-type rechargeable battery shown in Figure 1, the peripheral edges of the outer cup 1 and the outer 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 stacked in order from the inner surface of the outer can 3 toward the inner surface of the outer cup 1.

[0061] The button-type rechargeable battery shown in Figure 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 vol%) and methyl ethyl carbonate (67 vol%). Separator 5 impregnated with the non-aqueous electrolyte solution was fabricated by impregnating a polypropylene porous material (thickness: 20 μm) with the obtained non-aqueous electrolyte solution. Next, the fabricated separator 5 was stacked by sandwiching it between the negative electrode 2, which was in close contact with the current collector 7b made of copper foil, and the positive electrode 4, which was in close contact with the current collector 7a made of nickel mesh. Then, the current collector 7b and the negative electrode 2 were housed inside the outer cup 1, and the current collector 7a and the positive electrode 4 were housed inside the outer can 3, and the outer cup 1 and outer can 3 were joined together. Furthermore, the peripheral edges of the outer cup 1 and outer can 3 were crimped and sealed with an insulating gasket 6 interposed therebetween. In this way, a button-type secondary battery was manufactured.

[0062] The characteristics of the fabricated button-type rechargeable battery (evaluation battery) were evaluated by conducting the tests described below. The results are shown in Table 1 below. In the following tests, the process of intercalating lithium ions into the negative electrode material was defined as charging, and the process of deintercalating lithium ions from the negative electrode material was defined as discharging.

[0063] Test 1: Discharge Capacity and Initial Charge / Discharge Efficiency (Initial Charge / Discharge Characteristics) First, constant current charging was performed at a current of 0.9 mA until the circuit voltage reached 0 mV. Once the circuit voltage reached 0 mV, the charging was switched to constant voltage charging and continued until the current reached 20 μA. The charging capacity (in mAh) was calculated from the amount of current supplied during this period. After that, the system was left idle for 120 minutes. Next, constant current discharge was performed at a current of 0.9 mA until the circuit voltage reached 1.5 V. The discharge capacity (in mAh) was calculated from the amount of current supplied during this period. This was defined as the first cycle. The initial charge-discharge efficiency (in %) was calculated from the charge and discharge capacities in the first cycle using the following formula. The results are shown in Table 1 below. A higher value indicates superior initial charge-discharge characteristics. Initial charge / discharge efficiency = (discharge capacity / charge capacity) × 100

[0064] Test 2: Input resistivity at 25°C (input characteristics) Under a temperature atmosphere of 25°C, constant current charging at 1.0C was performed until the circuit voltage reached 3.82V. After that, the temperature atmosphere was adjusted to 0°C and the system was left idle for 3 hours. Next, the battery was charged at 0.5C for 10 seconds, then left to rest for 10 minutes, and then discharged at 0.5C for 10 seconds, followed by another 10 minutes of rest. Next, the battery was charged at 1.0C for 10 seconds, then left to rest for 10 minutes, discharged at 0.5C for 20 seconds to bring the State of Charge (SOC) to 50%, and then left to rest for 10 minutes. Next, I charged it at 1.5C for 10 seconds, then let it rest for 10 minutes, discharged it at 0.5C for 30 seconds to bring the State of Charge (SOC) to 50%, and let it rest for another 10 minutes. Next, I charged it at 2.0C for 10 seconds, then let it rest for 10 minutes, discharged it at 0.5C for 40 seconds to bring the SOC to 50%, and let it rest for another 10 minutes. The discharge capacity (in mAh) obtained in Test 1 was multiplied by each C rate (0.5C, 1.0C, 1.5C, 2.0C) to calculate the current value. The voltage (10-second value) obtained when charging at each C rate was also determined. The results for each C rate were plotted with the current value as the x-coordinate and the voltage as the y-coordinate, and the slope of the linear approximation line was calculated using the least squares method. This slope was defined as the input resistance (in ohms). A smaller value indicates better input characteristics. Furthermore, the relative values ​​(ratios) of the input resistance of each example to the input resistance of Example 1 were determined using the following formula, and these were defined as the 25°C input resistivity. The results are shown in Table 1 below. Input resistivity at 25°C = (Input resistance of each example / Input resistance of Example 1) × 100

[0065] Test 3: 45°C cycle endurance (cycle capacity maintenance) Under a temperature of 45°C, constant current charging at 0.5C was performed until the circuit voltage reached 4.1V. Afterward, the system was left idle for 10 minutes. Next, the circuit was discharged at 0.5C until the voltage reached 2.5V, and then left to rest for 10 minutes. This charging and discharging process was repeated a total of 100 times, and the ratio of the discharge capacity after the 100th cycle to the discharge capacity after the initial discharge (45°C cycle endurance rate, in %) was calculated. The results are shown in Table 1 below. A higher value indicates better cycle capacity maintenance.

[0066] <Example 2> Carbonaceous coated graphite particles were prepared and evaluated in the same manner as in Example 1, except that the amount of carbonaceous precursor added was changed to an amount that resulted in a final carbonaceous content of 5.0% by mass. The results are shown in Table 1 below.

[0067] <Example 3> Carbonaceous coated graphite particles were prepared and evaluated in the same manner as in Example 1, except that the amount of carbonaceous precursor added was changed to an amount that resulted in a final carbonaceous content of 6.0% by mass. The results are shown in Table 1 below.

[0068] <Example 4> Carbonaceous coated graphite particles were prepared and evaluated in the same manner as in Example 2, except that the intermediate exposure time was changed to 10 minutes. The results are shown in Table 1 below.

[0069] <Example 5> Carbonaceous coated graphite particles were prepared and evaluated in the same manner as in Example 2, except that the first firing temperature was changed to 1000°C. The results are shown in Table 1 below.

[0070] <Example 6> Carbonaceous coated graphite particles were prepared and evaluated in the same manner as in Example 1, except that the intermediate exposure temperature was set to 60°C. The results are shown in Table 1 below.

[0071] <Comparative Example 1> In Comparative Example 1, without attaching a carbonaceous precursor (phenol resin) to the graphite particles (spheroidized graphite), calcination (first calcination, intermediate exposure, and second calcination) was performed only on the graphite particles (spheroidized graphite) in the same manner as in Example 5.

[0072] <Comparative Example 2> Preparation of graphite particles Graphite particles, which are spheroidized graphite, were obtained in the same manner as in Example 1.

[0073] Preparation of Precursor-Attached Graphite Particles Precursor-attached graphite particles were obtained in which a carbonaceous precursor (phenol resin) was attached to the surface of graphite particles (spheroidized graphite) in the same manner as in Example 1.

[0074] Preparation of carbonaceous coated graphite particles The obtained precursor-attached graphite particles were placed in a graphite-lidded container and calcined using a tubular furnace. Specifically, first, a non-oxidizing atmosphere was created by flowing nitrogen at 2 L / min, and then the precursor-attached graphite particles were heated at a first calcination temperature of 950°C for 60 minutes. Next, without performing cooling (intermediate exposure), the mixture was heated at a second firing temperature of 1100°C for 60 minutes in an atmosphere with nitrogen flowing at 2 L / min (non-oxidizing atmosphere). In this way, the carbonaceous precursor (phenol resin) becomes carbonaceous, and carbonaceous coated graphite particles are obtained in which the surface of graphite particles (spheroidized graphite) is coated with carbonaceous material. The physical properties of the obtained carbonaceous coated graphite particles were determined using the method described above. The results are shown in Table 1 below.

[0075] <Comparative Example 3> Preparation of graphite particles Graphite particles, which are spheroidized graphite, were obtained in the same manner as in Example 1.

[0076] Preparation of Precursor-Attached Graphite Particles Precursor-attached graphite particles were obtained in which a carbonaceous precursor (phenol resin) was attached to the surface of graphite particles (spheroidized graphite) in the same manner as in Example 1.

[0077] Preparation of carbonaceous coated graphite particles The obtained precursor-attached graphite particles were placed in a graphite-lidded container and calcined using a tubular furnace. Specifically, the atmosphere was created by flowing nitrogen at 2 L / min (a non-oxidizing atmosphere), and then the precursor-attached graphite particles were heated at a calcination temperature of 1100°C for 60 minutes. In this way, the carbonaceous precursor (phenol resin) becomes carbonaceous, and carbonaceous coated graphite particles are obtained in which the surface of graphite particles (spheroidized graphite) is coated with carbonaceous material. The physical properties of the obtained carbonaceous coated graphite particles were determined using the method described above. The results are shown in Table 1 below.

[0078] <Comparative Example 4> Preparation of graphite particles Graphite particles, which are spheroidized graphite, were obtained in the same manner as in Example 1.

[0079] Preparation of Precursor-Attached Graphite Particles In the same manner as in Example 1, precursor-coated graphite particles were obtained in which a carbonaceous precursor (phenol resin) was attached to the surface of graphite particles (spheroidized graphite) so that the carbonaceous content in the final carbonaceous coated graphite particles was 2.0 parts by mass.

[0080] Preparation of carbonaceous coated graphite particles The obtained precursor-attached graphite particles were placed in a graphite-lidded container and calcined using a tubular furnace. Specifically, first, the first firing 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 to obtain an intermediate exposed sample. Subsequently, a second application of the carbonaceous precursor was carried out. More specifically, phenolic resin was applied to the intermediate exposure material under the same conditions as in Example 1, "Preparation of Precursor-Coated Graphite Particles." At this time, the amount of phenolic resin applied was such that the carbonaceous content in the final carbonaceous-coated graphite particles was 2.0 parts by mass. Finally, a second firing was performed in the same manner as in Example 1. In this way, the carbonaceous precursor (phenolic resin) became carbonaceous, and carbonaceous coated graphite particles were obtained in which the surface of graphite particles (spheroidized graphite) was coated with carbonaceous material. The carbonaceous content in the obtained carbonaceous coated graphite particles was 4.0% by mass. The physical properties of the obtained carbonaceous coated graphite particles were determined using the method described above. The results are shown in Table 1 below.

[0081] [Table 1]

[0082] <Summary of Evaluation Results> As shown in Table 1 above, Examples 1-6, in which graphite particles were coated with carbonaceous material and had an elastic modulus of 10 GPa or higher, exhibited better cycle capacity retention compared to Comparative Examples 1-4, which did not meet these conditions. Furthermore, Examples 1-6 also showed good initial charge / discharge characteristics and input characteristics. [Explanation of Symbols]

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

Claims

1. Graphite particles and, A carbonaceous coated graphite particle comprising a carbonaceous material that covers at least a portion of the surface of the graphite particle, The elastic modulus of the carbonaceous coated graphite particles, as determined using a scanning probe microscope, is 10 GPa or more and 25 GPa or less. Carbonaceous coated graphite particles wherein the carbonaceous content is 1.0 to 30.0% by mass relative to the total mass of the carbonaceous coated graphite particles.

2. The particle size is 5.0 to 15.0 μm, and the specific surface area is 3.0 to 15.0 m². ２ Carbonaceous coated graphite particles according to claim 1, wherein the particle size is / g.

3. Carbonaceous coated graphite particles according to claim 1 or 2, which are a negative electrode material for a lithium-ion secondary battery.

4. A negative electrode for a lithium-ion secondary battery, comprising carbonaceous coated graphite particles as described in claim 1 or 2.

5. A lithium-ion secondary battery having the negative electrode described in claim 4.

6. A method for producing carbonaceous coated graphite particles according to claim 1 or 2, To obtain precursor-attached graphite particles, the carbonaceous precursor is attached to the graphite particles. The aforementioned precursor-attached graphite particles are subjected to a first calcination process in which they are heated in a non-oxidizing atmosphere at a temperature between 900°C and 1500°C. After the first calcination, the precursor-attached graphite particles are subjected to an intermediate exposure in an oxidizing atmosphere at a temperature of 50 to 100°C. A method for producing carbonaceous coated graphite particles, comprising the above-mentioned intermediate exposure followed by a second calcination in which the precursor-coated graphite particles are heated in a non-oxidizing atmosphere at a temperature of 1100 to 1500°C.

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