Method for producing negative electrode material for lithium ion secondary battery and method for producing lithium ion secondary battery

Abstract

Provided is a method for manufacturing a negative electrode material for a lithium ion secondary battery, comprising: (a) a step for obtaining a mixture that contains a graphitizable aggregate, a graphitizable binder and an aromatic compound; (b) a step for obtaining a molded material having a density of 1.3 g / cm3 or less by molding the mixture; (c) a step for obtaining a graphitized material by graphitizing the molded material; and (d) a step for obtaining a pulverized material by pulverizing the graphitized material.

Description

[Technical Field]

[0001] The present disclosure relates to a method for producing a negative electrode material for a lithium ion secondary battery and a method for producing a lithium ion secondary battery. [Background technology]

[0002] Lithium-ion secondary batteries have a higher energy density than other secondary batteries such as nickel-cadmium batteries, nickel-metal hydride batteries, and lead-acid batteries, and are therefore widely used as power sources for portable electrical appliances such as laptops and mobile phones. In addition to relatively small electrical appliances, lithium-ion secondary batteries are also expected to be used in electric vehicles, power storage devices, and other applications.

[0003] Graphite is widely used as a material for the negative electrode (negative electrode material) of lithium-ion secondary batteries. For example, Patent Document 1 describes a method for producing a negative electrode material using graphite, in which a carbon material, a binder, and the like are mixed and pulverized to obtain a mixture, the pulverized product is molded using a mold and graphitized, and the graphitized product is further pulverized. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2015 / 147012 Summary of the Invention [Problem to be solved by the invention]

[0005] Demand for lithium ion secondary batteries for electrical appliances, electric vehicles, power sources for power storage, etc. is increasing, and accordingly, lithium ion secondary batteries with excellent cycle characteristics are desired.

[0006] The present inventors have found that a lithium ion secondary battery having excellent cycle characteristics can be manufactured by forming a molded product having a relatively low density when molding a mixture containing a graphitizable aggregate and a graphitizable binder.

[0007] However, when a molded product having a relatively low density is formed, the handling property during the subsequent graphitization treatment and the like is reduced, and there is room for improvement in the moldability when forming the molded product.

[0008] In view of the above circumstances, an object of the present disclosure is to provide a method for producing a negative electrode material for a lithium ion secondary battery that has excellent formability when forming a molded product and that enables the production of a lithium ion secondary battery, and a method for producing a lithium ion secondary battery that includes the method for producing this negative electrode material. [Means for solving the problem]

[0009] Specific means for solving the above problems include the following embodiments. <1> (a) obtaining a mixture comprising a graphitizable aggregate, a graphitizable binder, and an aromatic compound; (b) Molding the mixture to a density of 1.3 g / cm 3 obtaining a molded product, (c) graphitizing the molded product to obtain a graphitized product; (d) pulverizing the graphitized material to obtain a pulverized material. <2> The aromatic compound includes at least one compound selected from the group consisting of naphthalene, methylnaphthalene, acenaphthene, biphenyl, fluorene, benzopyrene, benzanthracene, dibenzanthracene, diphenylene oxide, quinoline, and isoquinoline. <1> A method for producing the negative electrode material for a lithium ion secondary battery according to claim 1. <3> The content of the aromatic compound in the mixture is 1% by mass to 20% by mass relative to 100% by mass of the total of the aggregate and the binder. <1> or <2> A method for producing the negative electrode material for a lithium ion secondary battery according to claim 1. <4> The content of the binder in the mixture is 25% by mass or less relative to 100% by mass of the total of the aggregate and the binder. <1> ~ <3> 1. A method for producing the negative electrode material for a lithium ion secondary battery according to any one of claims 1 to 9. <5> The content of the binder in the mixture is 15% by mass or less relative to 100% by mass of the total of the aggregate and the binder. <1> ~ <3> 1. A method for producing the negative electrode material for a lithium ion secondary battery according to any one of claims 1 to 9. <6> The content of the aromatic compound in the mixture is 1.5% by mass to 20% by mass relative to 100% by mass of the total of the aggregate and the binder. <1> ~ <5> 1. A method for producing the negative electrode material for a lithium ion secondary battery according to any one of claims 1 to 9. <7> The specific surface area of the pulverized material, which is a negative electrode material for lithium ion secondary batteries, is 2.8 m 2 / g or less <1> ~ <6> 1. A method for producing the negative electrode material for a lithium ion secondary battery according to any one of claims 1 to 9. <8> <1> ~ <7> 10. A method for producing a lithium ion secondary battery, comprising the step of producing a negative electrode using the negative electrode material obtained by the method for producing a negative electrode material for a lithium ion secondary battery according to any one of claims 1 to 9. <9> <1> ~ <7> 1. A method for producing a lithium ion secondary battery, comprising: a step of producing a negative electrode material for a lithium ion secondary battery by the method for producing a negative electrode material for a lithium ion secondary battery according to any one of the above-mentioned methods; and a step of producing a negative electrode using the negative electrode material for a lithium ion secondary battery. [Effects of the Invention]

[0010] According to the present disclosure, it is possible to provide a method for producing a negative electrode material for a lithium ion secondary battery that has excellent formability when forming a molded product and that enables the production of a lithium ion secondary battery, and a method for producing a lithium ion secondary battery that includes the method for producing this negative electrode material. [Brief explanation of the drawings]

[0011] [Figure 1]1 is a graph showing the relationship between the density of the compact before heat treatment and the density of the compact after heat treatment when the compacting pressure and the content of the methylnaphthalene-containing additive in Examples 5 to 8 during production of the compact are changed. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, embodiments for carrying out the present invention will be described in detail. However, the present invention is not limited to the following embodiments. In the following embodiments, components (including element steps, etc.) are not essential unless otherwise specified, or unless they are clearly considered essential in principle. The same applies to numerical values and their ranges, and they do not limit the present invention. In the present disclosure, the term "process" includes not only a process that is independent of other processes, but also a process that cannot be clearly distinguished from other processes as long as the purpose of the process is achieved. In the present disclosure, numerical ranges indicated using "to" include the numerical values before and after "to" as the minimum and maximum values, respectively. In the numerical ranges described in stages in this disclosure, the upper or lower limit value described in one numerical range may be replaced with the upper or lower limit value of another numerical range described in stages. In the numerical ranges described in this disclosure, the upper or lower limit value of the numerical range may be replaced with the value shown in the examples. In the present disclosure, each component may contain multiple substances corresponding to the component. When multiple substances corresponding to each component are present in the composition, the content or amount of each component means the total content or amount of the multiple substances present in the composition, unless otherwise specified. In the present disclosure, the composition may contain multiple types of particles corresponding to each component. When multiple types of particles corresponding to each component are present in the composition, the particle size of each component means the value for a mixture of the multiple types of particles present in the composition, unless otherwise specified. In the present disclosure, the term "layer" includes cases where the layer is formed over the entire area when the area in which the layer exists is observed, as well as cases where the layer is formed over only a portion of the area.

[0013] <Method of manufacturing negative electrode material for lithium-ion secondary batteries> The method for producing a negative electrode material for a lithium ion secondary battery (hereinafter also simply referred to as a negative electrode material) according to the present disclosure includes: (a) obtaining a mixture comprising a graphitizable aggregate, a graphitizable binder, and an aromatic compound; (b) Molding the mixture to a density of 1.3 g / cm 3 obtaining a molded product, (c) graphitizing the molded product to obtain a graphitized product; (d) pulverizing the graphitized material to obtain a pulverized material. The steps of the above method may be performed consecutively or discontinuously, and may be performed at the same location or at different locations.

[0014] In a comparative method for the manufacturing method of the present disclosure, a mixture containing a graphitizable aggregate and a graphitizable binder, but not containing an aromatic compound, is used, and the mixture is molded to produce a composite having a density of 1.3 g / cm. 3 In this comparative method, when a molded product having a relatively low density is obtained, the appearance of the molded product deteriorates, and there are problems with handling during the subsequent graphitization process, etc.

[0015] On the other hand, in the manufacturing method of the present disclosure, a mixture containing a graphitizable aggregate, a graphitizable binder, and an aromatic compound is obtained, and then the mixture is molded to produce a graphitizable aggregate having a density of 1.3 g / cm. 3The manufacturing method of the present disclosure obtains a molded product having the following properties. In the manufacturing method of the present disclosure, a mixture containing an aromatic compound is used to form the molded product. This improves the appearance of the molded product and also increases the strength of the molded product, thereby improving handleability in subsequent graphitization treatments and the like. Therefore, the manufacturing method of the present disclosure provides good moldability when forming a molded product. Conventionally, when raw coke was used, molding was difficult, and it was necessary to improve moldability by subjecting the raw coke to heat treatment or the like. On the other hand, in the manufacturing method of the present disclosure, a molded product that is easy to handle can be obtained even when raw coke is used without heat treatment. Therefore, in the manufacturing method of the present disclosure, either heat-treated raw coke or unheat-treated raw coke may be used as the graphitizable aggregate to form the molded product.

[0016] The reason why the appearance of the molded product is improved and the strength of the molded product is further increased by using a mixture containing an aromatic compound to form the molded product is presumed to be as follows. (1) By adding a liquid aromatic compound to a mixture containing graphitizable aggregate, the surface of the graphitizable aggregate becomes slippery during molding, promoting alignment of the mixture and reducing voids. (2) By dissolving a graphitizable binder in the mixture, the binder is dispersed with high uniformity. The above (1) and (2) are thought to improve the appearance and strength of the molded product.

[0017] Furthermore, the manufacturing method of the present disclosure tends to produce a lithium-ion secondary battery with excellent cycle characteristics by obtaining a molded product with a relatively low density, graphitizing the molded product, and then pulverizing the graphitized product. The reason for this is presumed to be as follows. The manufacturing method of the present disclosure allows the graphitized product to be pulverized more easily than when pulverizing a graphitized product obtained using a molded product with a relatively high density, resulting in a pulverized product with a small specific surface area. By using a negative electrode material containing a pulverized product with a small specific surface area to manufacture a lithium-ion secondary battery, the contact area of the negative electrode material with the electrolyte can be reduced. This suppresses the decomposition reaction of the electrolyte, extending the battery life and resulting in excellent cycle characteristics of the lithium-ion secondary battery.

[0018] In step (a), a mixture containing a graphitizable aggregate, a graphitizable binder, and an aromatic compound is obtained. The method for obtaining the mixture is not particularly limited, and it can be performed using a kneader or the like. The mixing is preferably performed at a temperature at which the graphitizable binder softens. Specifically, if the graphitizable binder is pitch, tar, or the like, the temperature may be 50°C to 300°C, and if it is a thermosetting resin, the temperature may be 20°C to 100°C.

[0019] The graphitizable aggregate is not particularly limited as long as it can be graphitized by a graphitization treatment. Specific examples include cokes such as fluid coke, needle coke, and mosaic coke. Among these, needle coke is preferred because it can suppress an increase in the density of the graphitized product produced, thereby making it easier to pulverize the graphitized product. The graphitizable aggregate is preferably in the form of particles.

[0020] When the graphitizable aggregate is particulate, the average particle size of the graphitizable aggregate is, for example, preferably 5 μm to 40 μm, more preferably 8 μm to 30 μm, and even more preferably 8 μm to 25 μm. The average particle size is, for example, the volume average particle size (D50) measured by the laser diffraction / scattering method described below. The volume average particle size (D50) is the particle size at 50% cumulative when a volume cumulative distribution curve is drawn from the small diameter side in the particle size distribution.

[0021] The standard deviation of the particle size distribution of the graphitizable aggregate is, for example, preferably 0.20 or less, more preferably 0.18 or less, and even more preferably 0.16 or less. By ensuring that the standard deviation of the particle size distribution of the aggregate is 0.20 or less, the particle size variation of the aggregate can be reduced, thereby suppressing the particle size variation of the resulting pulverized material. By using a negative electrode material containing pulverized material with small particle size variation in the production of a lithium-ion secondary battery, the resistance distribution within the negative electrode can be made uniform. As a result, the rapid charging performance of the lithium-ion secondary battery tends to be improved. Furthermore, by reducing the particle size variation of the aggregate, the binder function can be suitably ensured even when the content or amount of the graphitizable binder is reduced. The standard deviation of the particle size distribution is, for example, a value (volume basis) measured by the laser diffraction / scattering method described below. The lower limit of the standard deviation of the particle size distribution of the graphitizable aggregate is not particularly limited, and may be, for example, 0.05 or more, or 0.10 or more.

[0022] Methods for adjusting the average particle size of the graphitizable aggregate and the standard deviation of the particle size distribution of the graphitizable aggregate to fall within the above-mentioned ranges include sieve classification, air classification, wet classification, and the like.

[0023] The graphitizable binder is not particularly limited as long as it can be graphitized by a graphitization treatment. Specific examples include coal-based, petroleum-based, and artificial pitch and tar, thermoplastic resins, and thermosetting resins.

[0024] The content of the graphitizable binder in the mixture is preferably 25% by mass or less, more preferably 20% by mass or less, even more preferably 19% by mass or less, particularly preferably 18% by mass or less, and extremely preferably 15% by mass or less, based on 100% by mass of the aggregate and binder combined. Furthermore, the content of the graphitizable binder in the mixture is preferably 5% by mass or more, based on 100% by mass of the aggregate and binder combined. When the binder content is 5% by mass or more, the binder tends to function favorably as a binder for the graphitizable aggregate. When the binder content is 25% by mass or less, the amount of fixed carbon in the mixture is sufficiently secured, which tends to result in excellent yield and improved battery characteristics such as initial charge / discharge efficiency of the negative electrode material. Furthermore, as the binder content decreases, the increase in density of the produced graphitized product is suppressed, which tends to facilitate pulverization of the graphitized product.

[0025] The mixture may contain only one type of graphitizable aggregate and one or more types of graphitizable binder.

[0026] The aromatic compound is not particularly limited as long as it has an aromatic ring. The aromatic compound may be a compound having an aromatic ring and a molecular weight of 500 or less, or may be a compound having a molecular weight of 300 or less. Examples of aromatic compounds include naphthalene, methylnaphthalenes such as 1-methylnaphthalene and 2-methylnaphthalene, acenaphthene, biphenyl, fluorene, benzopyrene, benzanthracene, dibenzanthracene, diphenylene oxide, quinoline, and isoquinoline. The mixture may contain only one type of aromatic compound or two or more types of aromatic compounds.

[0027] Among these, methylnaphthalene, naphthalene, and the like are preferred as aromatic compounds from the viewpoint of moldability when forming a molded product.

[0028] The content of the aromatic compound in the mixture is preferably 1% by mass to 20% by mass, more preferably 1.5% by mass to 20% by mass, even more preferably 2% by mass to 18% by mass, and particularly preferably 3% by mass to 15% by mass, relative to 100% by mass of the total of the aggregate and the binder.

[0029] The mixture may contain other components in addition to the graphitizable aggregate, graphitizable binder, and aromatic compound, such as graphite, a dispersant, and a graphitization catalyst.

[0030] The mixture may contain graphite. Examples of graphite include natural graphite and artificial graphite. The graphite is preferably in particulate form. The mixture may contain only one type of graphite or two or more types of graphite.

[0031] From the viewpoint of facilitating dispersion of the components in the mixture in step (b), the mixture preferably contains a dispersant. By including a dispersant in the mixture, it is possible to suppress variation in particle size of the pulverized material obtained by pulverizing the graphitized material, and it becomes easier to obtain pulverized material with a uniform particle size. As a result, the rapid charging performance of the lithium-ion secondary battery tends to be improved. Furthermore, the inclusion of a dispersant in the mixture leads to a reduction in the amount of graphitizable binder, which is expected to improve battery characteristics such as the initial charge / discharge efficiency of the negative electrode material.

[0032] The type of dispersant is not particularly limited. Specific examples include hydrocarbons such as liquid paraffin, paraffin wax, and polyethylene wax; fatty acids such as stearic acid, oleic acid, erucic acid, and 12-hydroxystearic acid; fatty acid metal salts such as zinc stearate, lead stearate, aluminum stearate, calcium stearate, and magnesium stearate; fatty acid amides such as stearic acid amide, oleic acid amide, erucic acid amide, methylene bisstearic acid amide, and ethylene bisstearic acid amide; fatty acid esters such as stearic acid monoglyceride, stearyl stearate, and hydrogenated oil; and higher alcohols such as stearyl alcohol. Among these, fatty acids are preferred, and stearic acid is more preferred, because they do not affect the performance of the negative electrode material, are easy to handle because they are solid at room temperature, melt at the temperature of step (a) and therefore disperse uniformly, disappear in the process up to the graphitization treatment, and are inexpensive.

[0033] When the mixture contains a dispersant, the amount thereof is not particularly limited. For example, the content of the dispersant relative to the total mixture may be 0.1% by mass to 20% by mass, 0.5% by mass to 10% by mass, or 0.5% by mass to 5% by mass.

[0034] From the viewpoint of promoting graphitization of the graphitizable aggregate or binder, the mixture preferably contains a graphitization catalyst. The type of graphitization catalyst is not particularly limited. Specific examples include substances having graphitization catalytic activity such as silicon, iron, nickel, titanium, and boron, as well as carbides of these substances, oxides of these substances, and nitrides of these substances.

[0035] When the mixture contains a graphitization catalyst, the amount thereof is not particularly limited. For example, the content of the graphitization catalyst relative to the total mass of the mixture may be 0.1 to 50 mass%, 0.5 to 40 mass%, or 0.5 to 30 mass%.

[0036] In step (b), the mixture obtained in step (a) is molded to obtain a molded product. Preferably, the mixture is molded into a predetermined shape using a uniaxial press or the like. By molding the mixture in this manner, it is possible to increase the amount of material packed into a graphitization furnace when graphitizing the mixture, thereby improving productivity and improving the effectiveness of the graphitization catalyst.

[0037] The method for molding the mixture in step (b) is not particularly limited, and examples thereof include a molding method in which the mixture is placed in a container such as a metal mold and pressurized in a uniaxial direction, a vibration molding method in which the mixture is placed in a container such as a metal mold, a weight is placed on top, and vibration and impact are applied to the metal frame to mold the mixture, and an extrusion molding method in which the mixture is extruded from a nozzle or the like using a horizontal press to mold the mixture.

[0038] In step (b), the density of the resulting molded product is 1.3 g / cm 3 From the viewpoint of productivity of the negative electrode material and cycle characteristics of the lithium ion secondary battery, it is preferable that the density is 0.8 g / cm. 3 ~1.3g / cm 3 is preferred, and 1.0 g / cm 3 ~1.25g / cm 3 more preferably 1.05 g / cm 3 ~1.2g / cm 3 is more preferable. This allows a molded product with a relatively low density to be obtained, graphitized, and then pulverized. Therefore, the graphitized product can be pulverized more easily than when a graphitized product obtained using a molded product with a relatively high density is pulverized. As a result, a pulverized product with a small specific surface area tends to be obtained. By using a negative electrode material containing a pulverized product with a small specific surface area in the production of a lithium ion secondary battery, the contact area of the negative electrode material with the electrolyte can be reduced. As a result, the decomposition reaction of the electrolyte can be suppressed, the battery life can be extended, and a lithium ion secondary battery with excellent cycle characteristics tends to be produced.

[0039] The molded product obtained in step (b) is preferably subjected to a heat treatment before being graphitized in step (c). By performing the heat treatment, organic components contained in the mixture that do not contribute to graphitization are removed, and gas generation during the graphitization process tends to be suppressed.

[0040] The temperature of the heat treatment is not particularly limited, and is preferably lower than the temperature of the heat treatment in step (c), and may be, for example, within the range of 500°C to 1000°C.

[0041] In step (c), the molded product obtained in step (b) is graphitized. The method for graphitizing the molded product is not particularly limited as long as the conditions are such that the graphitizable components contained in the mixture can be graphitized. For example, a method of heat treating the mixture in an atmosphere in which the mixture is unlikely to be oxidized can be used. The atmosphere in which the mixture is unlikely to be oxidized is not particularly limited, and examples include an inert atmosphere such as nitrogen or argon, and a vacuum.

[0042] The temperature of the heat treatment for graphitization may be, for example, 1500°C or higher, 2000°C or higher, 2500°C or higher, or 2800°C or higher. The upper limit of the heat treatment temperature is not particularly limited, but may be, for example, 3200°C or lower. When the heat treatment temperature is 1500°C or higher, crystal changes tend to occur, making graphitization more likely to progress. When the heat treatment temperature is 2000°C or higher, the development of graphite crystals tends to be better. On the other hand, when the heat treatment temperature for graphitization is 3200°C or lower, sublimation of part of the graphite tends to be suppressed.

[0043] In step (d), the graphitized material obtained in step (c) is pulverized to obtain a pulverized product. The pulverization method is not particularly limited, and can be performed by a known method using a jet mill, vibration mill, pin mill, hammer mill, or the like. The particle size of the pulverized product may be adjusted to a desired size. The method for adjusting the particle size is not particularly limited, and examples include a method using the above-mentioned pulverizing device and a method using a sieve.

[0044] If necessary, the pulverized material obtained in step (d) may be subjected to steps such as (e) disposing low-crystalline carbon on at least a portion of the surface of the pulverized material, and (f) mixing the pulverized material with other negative electrode active materials.

[0045] In step (e), low-crystalline carbon can be disposed on at least a portion of the surface of the pulverized material by, for example, mixing the pulverized material with a substance (such as a resin) that can be converted to low-crystalline carbon by heat treatment, followed by heat treatment. Disposing low-crystalline carbon on at least a portion of the surface of the pulverized material can improve the input / output characteristics, such as rapid charge / discharge characteristics, of a lithium-ion secondary battery using the pulverized material as a negative electrode material.

[0046] The method for mixing the pulverized material with other negative electrode active materials in step (f) is not particularly limited. By mixing the pulverized material with other negative electrode active materials, the desired characteristics of the lithium ion secondary battery may be improved compared to when the pulverized material is used alone as the negative electrode active material. Examples of other negative electrode active materials include, but are not limited to, graphite particles such as natural graphite and artificial graphite, and particles containing elements capable of absorbing and desorbing lithium ions. Examples of elements capable of absorbing and desorbing lithium ions include, but are not limited to, Si, Sn, Ge, and In.

[0047] The pulverized material obtained in step (f) may contain particles in which a plurality of flat graphite particles are aggregated or bonded together, or may contain particles in which a plurality of flat graphite particles are aggregated or bonded together so that the main surfaces of the graphite particles are non-parallel to each other (hereinafter also referred to as secondary graphite particles).

[0048] When the pulverized material is in the form of graphite secondary particles, the phenomenon in which the particles of the negative electrode material are oriented along the direction of the current collector when pressed to densify the negative electrode is suppressed, and there tends to be sufficient pathways for lithium ions to enter and exit the negative electrode material.

[0049] Furthermore, by containing particles in which a plurality of flat graphite particles are aggregated or bonded together, the voids present between the plurality of flat graphite particles reduce the effect of the pressure applied during pressing on the individual graphite particles, which tends to suppress the destruction of graphite particles, the generation of cracks, etc.

[0050] In the present disclosure, the term "flat graphite particles" refers to non-spherical graphite particles having an anisotropic shape. Examples of flat graphite particles include graphite particles having a scaly, flake, or partially lumpy shape.

[0051] The flat graphite particles preferably have an aspect ratio, expressed as A / B, where A is the length in the major axis direction and B is the length in the minor axis direction, of 1.2 to 20, more preferably 1.3 to 10. When the aspect ratio is 1.2 or more, the contact area between particles increases, tending to further improve conductivity. When the aspect ratio is 20 or less, input / output characteristics such as rapid charge / discharge characteristics of the lithium ion secondary battery tend to further improve.

[0052] The aspect ratio is determined by observing graphite particles under a microscope, randomly selecting 100 graphite particles, measuring the A / B ratio for each, and then calculating the arithmetic mean of these measurements. In observing the aspect ratio, the major axis length A and the minor axis length B are measured as follows. That is, in a projected image of a graphite particle observed under a microscope, two parallel tangents circumscribing the periphery of the graphite particle are selected, with tangents a1 and a2 having the greatest distance between them, and the major axis length A is defined as the distance between these tangents a1 and a2. Two parallel tangents circumscribing the periphery of the graphite particle are selected, with tangents b1 and b2 having the smallest distance between them, and the minor axis length B is defined as the distance between these tangents b1 and b2.

[0053] In the present disclosure, the phrase "main surfaces are non-parallel" of graphite secondary particles means that the surfaces (main surfaces) of the plurality of flat graphite particles with the largest cross-sectional areas are not aligned in a fixed direction. Whether the main surfaces of the plurality of flat graphite particles are non-parallel to each other can be confirmed by observation with a microscope. When the plurality of flat graphite particles are aggregated or bonded together with their main surfaces non-parallel to each other, the orientation of the main surfaces of the flat graphite particles in the negative electrode is suppressed, and expansion of the negative electrode during charging is suppressed, which tends to further improve the cycle characteristics of the lithium-ion secondary battery. The secondary graphite particles may partially include a structure in which a plurality of flat graphite particles are aggregated or bonded together with their respective main faces parallel to one another.

[0054] In the present disclosure, the phrase "a state in which a plurality of flat graphite particles are aggregated or bonded" refers to a state in which two or more flat graphite particles are aggregated or bonded. "Bonded" refers to a state in which the particles are chemically bonded to each other directly or via a carbon substance. "Aggregated" refers to a state in which the particles are not chemically bonded to each other but maintain an aggregate shape due to their shape, etc. The flat graphite particles may be aggregated or bonded via a carbon substance. Examples of carbon substances include graphitized products of graphitizable binders. From the viewpoint of mechanical strength, it is preferable that two or more flat graphite particles are bonded via a carbon substance. Whether the flat graphite particles are aggregated or bonded can be confirmed, for example, by observation using a scanning electron microscope.

[0055] From the viewpoint of ease of aggregation or bonding, the average particle size of the flat graphite particles is, for example, preferably 1 μm to 50 μm, more preferably 1 μm to 25 μm, and even more preferably 1 μm to 15 μm. Methods for measuring the average particle size of the flat graphite particles include a method using a scanning electron microscope, and the average particle size of the flat graphite particles is, for example, the arithmetic mean value of the particle sizes of 100 flat graphite particles.

[0056] The flat graphite particles and the raw material thereof are not particularly limited, and examples thereof include artificial graphite, scaly natural graphite, scaly natural graphite, coke, resin, tar, pitch, etc. Among these, graphite obtained from artificial graphite, natural graphite, or coke tends to be soft particles with high crystallinity, which makes it easier to increase the density of the negative electrode.

[0057] The negative electrode material may contain spherical graphite particles. When the negative electrode material contains spherical graphite particles, the spherical graphite particles themselves have a high density, so that the pressing pressure required to obtain a desired electrode density tends to be reduced.

[0058] Examples of spherical graphite particles include spherical artificial graphite and spherical natural graphite. From the viewpoint of increasing the density of the negative electrode, the spherical graphite particles are preferably high-density graphite particles. Specifically, spherical natural graphite that has been subjected to a particle spheroidization treatment to enable a high tap density is preferred. Furthermore, a negative electrode material layer containing spherical natural graphite has excellent peel strength and tends to be less likely to peel off from the current collector even when pressed with a strong force.

[0059] When the negative electrode material contains spherical graphite particles, it may contain both the above-mentioned flat graphite particles and spherical graphite particles. When the negative electrode material contains the above-mentioned flat graphite particles and spherical graphite particles, the ratio between the two is not particularly limited and can be set depending on the desired electrode density, pressure conditions during pressing, desired battery characteristics, etc.

[0060] When the negative electrode material contains flat graphite particles and spherical graphite particles, it may be in a state where flat graphite particles and spherical graphite particles are mixed, or in a state where flat graphite particles and spherical graphite particles are bonded together (hereinafter also referred to as composite particles), etc. Examples of composite particles include particles in which flat graphite particles and spherical graphite particles are bonded together via an organic carbide.

[0061] The composite particles can be produced, for example, by using a mixture containing flat graphite particles or raw materials thereof and spherical graphite particles in the step (a).

[0062] The average particle size of the negative electrode material produced by the above method is not particularly limited. For example, it is preferably 5 μm to 40 μm, more preferably 10 μm to 30 μm, and even more preferably 10 μm to 25 μm. The average particle size may be measured, for example, by a scanning electron microscope in the same manner as the average particle size of the above-mentioned flat graphite particles, or it may be the volume average particle size (D50) measured by a laser diffraction / scattering method.

[0063] When an electrode (negative electrode) is manufactured using a negative electrode material, the average particle size can be measured by preparing a sample electrode, embedding the electrode in epoxy resin, mirror-polishing the electrode, and observing the electrode cross section with a scanning electron microscope (e.g., Keyence Corporation, "VE-7800"); or by preparing an electrode cross section using an ion milling device (e.g., Hitachi High-Technologies Corporation, "E-3500") and measuring the particle size with a scanning electron microscope (e.g., Keyence Corporation, "VE-7800"). In this case, the average particle size is the median of particle sizes of 100 particles randomly selected from the particles observed.

[0064] The sample electrode can be produced, for example, by preparing a dispersion liquid by adding water to a mixture of 98 parts by mass of a negative electrode material, 1 part by mass of styrene-butadiene resin as a binder, and 1 part by mass of carboxymethyl cellulose as a thickener as solid content, coating the dispersion liquid onto a 10 μm-thick copper foil to a thickness of about 70 μm (when coated), and then drying at 105° C. for 1 hour.

[0065] The negative electrode material may have an orientation of 40 or less, 20 or less, or 15 or less when made into a negative electrode (a negative electrode after pressing, if a pressing step is involved in the production of the negative electrode). The orientation may be 10 or more. The orientation of the negative electrode material is an index that indicates the degree of orientation of the particles of the negative electrode material contained in the negative electrode. A low orientation means that the particles of the negative electrode material are oriented in random directions. In other words, this means that the pressure applied during pressing prevents the graphite particles from orienting along the surface of the current collector.

[0066] In the present disclosure, the orientation of the negative electrode is determined by measuring the surface of the sample electrode using an X-ray diffractometer with CuKα radiation as an X-ray source. Specifically, the X-ray diffraction pattern of the surface of the sample electrode is measured, and the orientation is determined from the intensities of the carbon (004) plane diffraction peak detected at a diffraction angle 2θ of approximately 53° to 56° and the carbon (110) plane diffraction peak detected at a diffraction angle 2θ of approximately 70° to 80° using the following formula (1): (004) plane diffraction peak intensity / (110) plane diffraction peak intensity Equation (1)

[0067] The specific surface area of the negative electrode material is set to 3.0 m from the viewpoint of cycle characteristics and storage characteristics. 2 / g or less, and 2 / g or less is more preferable, and 0.5m 2 / g~2.5m 2 / g, more preferably 0.7m 2 / g~2.0m 2 / g is particularly preferred. The specific surface area of the negative electrode material can be calculated by the BET method by measuring nitrogen adsorption at liquid nitrogen temperature (77 K) using a specific surface area / pore distribution measuring device (e.g., FlowSorb III 2310, Shimadzu Corporation) and a mixed gas of nitrogen and helium (nitrogen:helium = 3:7) by the single-point method at a relative pressure of 0.3.

[0068] <Method of manufacturing lithium-ion secondary batteries> A first embodiment of the method for producing a lithium ion secondary battery according to the present disclosure includes a step of producing a negative electrode using the negative electrode material obtained by the method for producing a negative electrode material described above. A second embodiment of the method for manufacturing a lithium ion secondary battery of the present disclosure includes a step of manufacturing a negative electrode material by the above-described method for manufacturing a negative electrode material, and a step of fabricating a negative electrode using the negative electrode material.

[0069] The method for producing a negative electrode using the negative electrode material is not particularly limited, and examples thereof include a method in which a negative electrode material layer is formed on a current collector using a composition containing the negative electrode material, a binder, and a solvent, and then heat treatment, pressing treatment, or the like is performed as necessary.

[0070] The binder contained in the composition is not particularly limited. Examples include styrene-butadiene rubber, polymeric compounds containing ethylenically unsaturated carboxylic acid esters (methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, hydroxyethyl (meth)acrylate, etc.) as polymerization components, polymeric compounds containing ethylenically unsaturated carboxylic acids (acrylic acid, methacrylic acid, itaconic acid, fumaric acid, maleic acid, etc.) as polymerization components, and polymeric compounds such as polyvinylidene fluoride, polyethylene oxide, polyepichlorohydrin, polyphosphazene, polyacrylonitrile, polyimide, and polyamideimide. In the present disclosure, (meth)acrylate refers to either or both of methacrylate and acrylate.

[0071] The solvent contained in the composition is not particularly limited, and specifically, organic solvents such as N-methylpyrrolidone, dimethylacetamide, dimethylformamide, and γ-butyrolactone, and water may be used.

[0072] The composition may contain a thickener to adjust the viscosity, if necessary, such as carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose, polyvinyl alcohol, polyacrylic acid and its salts, oxidized starch, phosphated starch, casein, etc.

[0073] The composition may contain a conductive additive, if necessary. Examples of the conductive additive include carbon black, graphite, acetylene black, conductive oxides, and conductive nitrides.

[0074] The material and shape of the current collector used to prepare the negative electrode are not particularly limited. For example, materials such as strip-shaped foil, perforated foil, and mesh made of metal or alloy such as aluminum, copper, nickel, titanium, and stainless steel can be used. Porous materials such as porous metal (foamed metal) and carbon paper can also be used.

[0075] The method for forming a negative electrode material layer on a current collector using the composition is not particularly limited, and can be any known method such as metal mask printing, electrostatic coating, dip coating, spray coating, roll coating, doctor blade coating, gravure coating, screen printing, etc. When the negative electrode material layer and the current collector are integrated, they can be integrated by any known method such as rolling, pressing, or a combination thereof.

[0076] After the negative electrode material layer is formed on the current collector, it may be subjected to a heat treatment (drying). The heat treatment removes the solvent contained in the negative electrode material layer, hardens the binder, and increases the strength, improving the adhesion between particles and between the particles and the current collector. The heat treatment may be performed in an inert atmosphere such as helium, argon, or nitrogen, or in a vacuum atmosphere, to prevent oxidation of the current collector during the treatment.

[0077] After the negative electrode material layer is formed on the current collector, a press treatment may be performed. By performing the press treatment, the electrode density of the negative electrode can be adjusted. The electrode density of the negative electrode is not particularly limited, but is preferably 1.5 g / cm. 3 ~1.9g / cm 3 1.6 g / cm 3 ~1.8g / cm 3 The higher the electrode density, the more the volumetric capacity of the negative electrode is improved, the more the adhesion of the negative electrode material layer to the current collector is improved, and the more the cycle characteristics tend to improve. It is preferable to perform the pressing treatment before the heat treatment.

[0078] The lithium-ion secondary battery manufactured by the above method may include a negative electrode, a positive electrode, and an electrolyte produced by the above method. The lithium-ion secondary battery may be configured such that, for example, the negative electrode and the positive electrode are arranged to face each other through a separator, and an electrolytic solution containing an electrolyte is injected.

[0079] Similar to the negative electrode, the positive electrode may be manufactured by forming a positive electrode layer on the surface of a current collector. As the current collector, materials such as strip foils, strip perforated foils, and strip meshes made of metals or alloys such as aluminum, titanium, and stainless steel can be used.

[0080] The positive electrode material contained in the positive electrode layer is not particularly limited. Examples include metal compounds, metal oxides, metal sulfides, and conductive polymer materials capable of doping or intercalating lithium ions. Further examples include lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganate (LiMnO2), and their complex oxides (LiCo x Ni y Mn z O2, x + y + z = 1, 0 < x, 0 < y; LiNi 2-x Mn x O4, 0 < x ≤ 2), lithium manganese spinel (LiMn2O4), lithium vanadium compounds, V2O5, V6O 13 , VO2, MnO2, TiO2, MoV2O8, TiS2, V2S5, VS2, MoS2, MoS3, Cr3O8, Cr2O5, olivine-type LiMPO4 (M: Co, Ni, Mn, Fe), conductive polymers such as polyacetylene, polyaniline, polypyrrole, polythiophene, and polyacene, porous carbon, etc. can be used alone or in combination of two or more. Among them, lithium nickel oxide (LiNiO2) and its complex oxides (LiCo x Ni y Mn z O2, x + y + z = 1, 0 < x, 0 < y; LiNi 2-x Mn x O4, 0 < x ≤ 2) are suitable as positive electrode materials because of their high capacity.

[0081] Examples of the separator include nonwoven fabrics, cloths, microporous films, and combinations thereof, each of which is primarily made of a polyolefin such as polyethylene or polypropylene. If the lithium ion secondary battery has a structure in which the positive electrode and the negative electrode do not come into contact with each other, there is no need to use a separator.

[0082] As the electrolyte, a so-called organic electrolyte can be used, which is obtained by dissolving a lithium salt such as LiClO4, LiPF6, LiAsF6, LiBF4, or LiSO3CF3 in a non-aqueous solvent such as ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, fluoroethylene carbonate, cyclopentanone, sulfolane, 3-methylsulfolane, 2,4-dimethylsulfolane, 3-methyl-1,3-oxazolidin-2-one, γ-butyrolactone, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, butyl methyl carbonate, ethyl propyl carbonate, butyl ethyl carbonate, dipropyl carbonate, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, methyl acetate, or ethyl acetate, either alone or as a mixture of two or more components. Among these, an electrolyte solution containing fluoroethylene carbonate is preferred because it tends to form a stable SEI (solid electrolyte interface) on the surface of the negative electrode material, significantly improving cycle characteristics.

[0083] The form of the lithium ion secondary battery is not particularly limited, and examples thereof include a paper-type battery, a button-type battery, a coin-type battery, a laminated battery, a cylindrical battery, a prismatic battery, etc. The negative electrode material for lithium ion secondary batteries can be applied to all electrochemical devices other than lithium ion secondary batteries, such as hybrid capacitors, which use the insertion and desorption of lithium ions as a charge and discharge mechanism. [Example]

[0084] The above embodiment will be described in more detail below based on examples, but the above embodiment is not limited to the following examples.

[0085] [Example 1] (1) Formation of molded products The calcined needle coke used as the raw material was finely pulverized using a free pulverizer manufactured by Nara Machinery Works, Ltd., and then air-classified using a Turboplex (registered trademark) manufactured by Hosokawa Micron Corporation to prepare the aggregates listed below. The following materials were also used as raw materials. The average particle size of the aggregate and anode material, and the standard deviation of the aggregate particle size distribution were measured using a particle size distribution analyzer (Shimadzu Corporation, SALD-3000) that utilizes laser diffraction and scattering methods. The standard deviation of the aggregate particle size distribution is the standard deviation defined on a logarithmic scale. Aggregate: Needle coke with an average particle size of 14 μm and a standard deviation of particle size distribution of 0.15 Binder: Tar pitch Aromatic compounds: Methylnaphthalene-containing additives (total aromatic compound content of 77% or more by mass) Dispersant: stearic acid

[0086] (Evaluation of test force at crushing) The crushing test force (N) of the test piece was measured as follows. Specifically, 10 g of a mixture having the composition shown in Table 1 was used to prepare a test piece having a long side of 52.1 mm and a short side of 11.4 mm at room temperature under a surface pressure of 75.8 MPa. The density (g / cm 3 ) are shown in Table 1. Using the prepared test specimen and an autograph manufactured by Shimadzu Corporation, a three-point bending test was carried out at a test speed of 10 mm / min and a support distance of 7 mm, and the test force at collapse (N) was measured. The results are shown in Table 1.

[0087] (Evaluation of formability) The moldability of the molded product obtained in Example 1 was evaluated based on the following evaluation criteria, where the higher the value, the better the moldability. The results are shown in Table 1. -Evaluation criteria- 1: It easily crumbled when touched with hands and was impossible to handle. 2: The film peeled off easily when touched with hands, making it impossible to handle. 3: It was possible to handle, but it was brittle and crumbled easily when picked up by hand. 4: It was easy to handle and did not crumble even when picked up by hand, but the surface was easily peeled off. 5: It was easy to handle, and even when picked up by hand, it did not crumble and the surface did not peel off.

[0088] [Comparative Example 1] A molded product was obtained in the same manner as in Example 1, except that no aromatic compound was used. Furthermore, evaluation of the crushing test force and moldability were carried out in the same manner as in Example 1. The results are shown in Table 1. In Comparative Example 1, a block-shaped molded product could not be formed well, and therefore the density of the test piece could not be evaluated.

[0089] Comparative Example 2 A molded product was obtained in the same manner as in Example 1, except that water was used instead of the aromatic compound in Example 1. Furthermore, evaluation of the crushing test force and moldability were performed in the same manner as in Example 1. The results are shown in Table 1.

[0090] Comparative Example 3 A molded product was obtained in the same manner as in Example 1, except that acetone was used instead of the aromatic compound in Example 1. Furthermore, evaluation of the crushing test force and moldability were performed in the same manner as in Example 1. The results are shown in Table 1.

[0091] [Table 1]

[0092] [Example 2] A mixture was prepared in the same manner as in Example 1, except that methylnaphthalene was used as the aromatic compound, and 3% by mass of methylnaphthalene was contained relative to 100% by mass of the aggregate and binder combined. The resulting mixture was then molded at room temperature under a surface pressure of 10 MPa for a processing time of 10 seconds to obtain a molded product with a long side of 140 mm and a short side of 30 mm.

[0093] [Example 3] In Example 2, a mixture was prepared in the same manner as in Example 1 except that 10% by mass of methylnaphthalene was contained relative to 100% by mass of the total of the aggregate and binder, and a molded product was obtained.

[0094] [Example 4] In Example 2, a mixture was prepared in the same manner as in Example 1 except that 15% by mass of methylnaphthalene was contained relative to 100% by mass of the total of the aggregate and binder, and a molded product was obtained.

[0095] The density of the molded product in Example 2 was 1.13 g / cm 3 The density of the molded product in Example 3 was 1.21 g / cm 3 The density of the molded product in Example 4 was 1.23 g / cm 3 It was. Furthermore, the moldability of the molded products of Examples 2 to 4 was evaluated based on the evaluation criteria for the above item (Evaluation of moldability). All of Examples 2 to 4 were scored as 5, indicating good moldability.

[0096] [Example 5] (1) Formation of molded products Instead of calcined needle coke, raw needle coke that had not been heat-treated was used as the raw material to prepare an aggregate with an average particle size of 9 μm and a standard deviation of particle size distribution of 0.15. Furthermore, the same binder, aromatic compound (methylnaphthalene-containing additive), and dispersant as in Example 1 were used as raw materials and mixed in the blending ratios shown in Table 2 to prepare mixtures. The resulting mixtures were then molded at room temperature under the molding pressure (unit: N) shown in Figure 1 to obtain three types of molded products with long sides of 52.1 mm and short sides of 11.4 mm. The densities (g / cm) of the three molded products in Example 5 were3 ) are shown by dots in Fig. 1. In Fig. 1, the vertical axis represents the density of the molded product before or after the heat treatment, and the horizontal axis represents the molding pressure during the preparation of the molded product, i.e., the molding pressure before the heat treatment.

[0097] In order to remove organic components contained in the molded products that do not contribute to graphitization, the three molded products obtained in Example 5 were each heat-treated at 900°C. The densities (g / cm) of the three molded products after the heat treatment in Example 5 were 3 ) are shown as dots in Figure 1.

[0098] [Example 6] The same test as in Example 5 was carried out except that the content of the methylnaphthalene-containing additive was changed to 7% by mass relative to 100% by mass of the total of the aggregate and binder, and the molding pressure of the mixture was changed as shown in Figure 1. The densities (g / cm) of the molded products before and after the three types of heat treatment in Example 6 were 3 ) are shown as dots in Figure 1.

[0099] [Example 7] The same test as in Example 5 was carried out except that the content of the methylnaphthalene-containing additive was changed to 15% by mass relative to 100% by mass of the total of the aggregate and binder, and the molding pressure of the mixture was changed as shown in Figure 1. The densities (g / cm3) of the molded products before and after the three types of heat treatment in Example 7 were 3 ) are shown as dots in Figure 1.

[0100] [Example 8] The same test as in Example 5 was carried out except that the content of the methylnaphthalene-containing additive was changed to 30% by mass relative to 100% by mass of the total of the aggregate and binder, and the molding pressure of the mixture was changed as shown in Figure 1. The density (g / cm) of the molded product before and after the two types of heat treatment in Example 8 was 3 ) are shown as dots in Figure 1.

[0101] As shown in Figure 1, when the content of the methylnaphthalene-containing additive was the same, the density of the molded product after heat treatment tended to increase as the molding pressure increased. As shown in Figure 1, the density of the compacted product before heat treatment at the same compacting pressure tended to increase as the amount of methylnaphthalene-containing additive increased. On the other hand, under the same compacting pressure, the density of the compacted product after heat treatment did not tend to increase even when the amount of methylnaphthalene-containing additive increased. In both cases, the density was below 1.0 g / cm3, as indicated by the dotted line. 3 It was as follows. From the above results, it can be seen that even if the density of the molded product before heat treatment is significantly increased by adding a methylnaphthalene-containing additive or by increasing the amount of the additive, the density of the molded product after heat treatment does not significantly increase. Furthermore, it is presumed that the density of the graphitized product will not significantly increase even if a graphitization treatment is subsequently performed. Therefore, it is thought that the increase in density of the molded product before heat treatment does not have an adverse effect on the grinding process of the graphitized product (for example, the adverse effect of requiring a greater force to grind the graphitized product, which increases the specific surface area of the ground product).

[0102] [Table 2]

[0103] [Examples 9 to 11] The binder, methylnaphthalene-containing additive, and dispersant shown in Example 1 were used as raw materials, except that unheat-treated raw mosaic coke with an average particle diameter of 14 μm and a standard deviation of particle size distribution of 0.22 was used instead of calcined needle coke as the aggregate. These raw materials were mixed in the amounts (parts by mass) shown in Table 3 using a kneader to obtain a mixture. The resulting mixture was then molded using a uniaxial press at room temperature to obtain a molded product. The resulting molded product was then heat-treated in a nitrogen atmosphere at 800°C to 850°C for 8 hours, and then graphitized at 2600°C to 3100°C for 30 hours to obtain a graphitized product. The obtained graphitized product was then pulverized to obtain graphite powders (negative electrode materials for lithium-ion secondary batteries) of Examples 9 to 11.

[0104] The density (g / cm) of the molded product obtained in the above process 3 ), density of graphite (g / cm 3) and D10 of the negative electrode material, D50 of the negative electrode material, D90 of the negative electrode material, specific surface area of the negative electrode material (m 2 / g), saturated tap density (g / cm 3 ) are shown in Table 3. D10 and D90 are the particle sizes at 10% and 90% of the cumulative volume distribution curve when plotting the particle size distribution from the smallest diameter side. D10 and D90 are values measured by laser diffraction / scattering.

[0105] The saturated tap density of the negative electrode material was determined as follows. 3 100 cm of sample powder was placed in a graduated flat-bottom test tube (Kuramochi Scientific Instruments Manufacturing Co., Ltd., KRS-406). 3 The graduated flat-bottom test tube was dropped 250 times from a height of 5 cm, and the value calculated from the mass and volume of the sample powder was taken as the saturated tap density of the negative electrode material.

[0106] (Preparation of negative electrode) 96 parts by mass of the prepared negative electrode material, 1.5 parts by mass of styrene butadiene rubber (BM-400B, manufactured by Zeon Corporation), 1.5 parts by mass of carboxymethyl cellulose (CMC1380, manufactured by Daicel Corporation), and 1.0 part by mass of carbon black (SuperP, manufactured by Imerys GC) were mixed, and then water was added to adjust the viscosity. The resulting composition was applied to a current collector (copper foil with a thickness of 10 μm) at a coating amount of 10 mg / cm. 2 The composition layer was applied so that the density after vacuum drying was 1.60 g / cm. 2 It was done so that it would be like this.

[0107] (Preparation of evaluation cells) The negative electrode obtained above was placed on a plate with an electrode area of 1.54 cm 2The evaluation cell was a 2016-type coin cell, which was punched into a circular shape and fabricated using metallic lithium as the positive electrode, a mixed solution of ethylene carbonate / ethyl methyl carbonate (3 / 7 volume ratio) containing 1.0 M LiPF6 and vinylene carbonate (0.5 mass%) as the electrolyte, a 25 μm thick polyethylene microporous membrane as the separator, and a 230 μm thick copper plate as the spacer.

[0108] (Evaluation of battery characteristics) The fabricated evaluation cell was used to measure the initial discharge capacity (Ah / kg) and initial charge / discharge efficiency (%). Specifically, the evaluation cell was placed in a thermostatic chamber maintained at 25°C and charged at a constant current of 0.53 mA until the voltage reached 0 V. It was then further charged at a constant voltage of 0 V until the current decayed to a value equivalent to 0.026 mA, and the initial charge capacity was measured. After charging, the cell was rested for 30 minutes and then discharged. The initial discharge capacity was measured at 0.53 mA until the voltage reached 1.5 V. The capacity was converted to a value per mass of the negative electrode material used. The initial charge / discharge efficiency (%) was calculated by dividing the initial discharge capacity by the initial charge capacity and multiplying this value by 100. The results are shown in Table 3.

[0109] [Table 3]

[0110] As shown in Table 3, it was confirmed that the initial discharge capacity and the initial charge / discharge efficiency of the battery fabricated using the negative electrode material tended to increase as the amount of binder in the mixture decreased. Furthermore, by reducing the amount of binder in the mixture, it is possible to suppress an increase in the density of the graphitized material when the molded product is graphitized. Since the increase in the density of the graphitized material is suppressed, it is possible to easily pulverize the graphitized material, which is thought to suppress the variation in particle size of the resulting pulverized material. It is presumed that the use of a negative electrode material containing pulverized material with small variation in particle size in the production of a lithium-ion secondary battery tends to improve the rapid charging performance of the lithium-ion secondary battery.

[0111] [Reference examples 1~4] (1) Preparation of negative electrode material The raw material, needle coke, was finely pulverized using a free pulverizer manufactured by Nara Machinery Works, Ltd., and then air-classified using a Turboplex (registered trademark) manufactured by Hosokawa Micron Corporation to prepare the aggregates listed below. Furthermore, the following materials were used as raw materials. The average particle size of the aggregate and anode material, and the standard deviation of the aggregate particle size distribution were measured using a particle size distribution analyzer (Shimadzu Corporation, SALD-3000) that utilizes laser diffraction and scattering methods. The standard deviation of the aggregate particle size distribution is the standard deviation defined on a logarithmic scale. Aggregate: Needle coke with an average particle size of 14 μm and a standard deviation of particle size distribution of 0.15 Binder: Tar pitch Dispersant: stearic acid

[0112] The raw materials shown in Table 4, such as aggregate and binder, were mixed in the amounts (parts by mass) shown in Table 4 using a kneader to obtain a mixture. The mixture was then molded at room temperature using a uniaxial press to obtain a molded product. The molded product was then heat-treated in a nitrogen atmosphere at 800°C to 850°C for 8 hours, and then graphitized at 2600°C to 3100°C for 30 hours to obtain a graphitized product. The graphitized product was then pulverized to obtain graphite powders (negative electrode materials for lithium-ion secondary batteries) of Reference Examples 1 to 4.

[0113] The density (g / cm) of the molded product obtained in the above process 3 ), density of graphite (g / cm 3 ) and the average particle size (μm) and specific surface area (m 2 / g), saturated tap density (g / cm 3 ) are shown in Table 4. The pulverized material obtained in each of the Reference Examples contained secondary particles in which the graphite particles, which were graphitized aggregates, were aggregated or bonded together so that the main surfaces of the graphite particles were not parallel to each other.

[0114] (2) Preparation of negative electrode and evaluation of orientation 96 parts by mass of the prepared negative electrode material, 1.5 parts by mass of styrene butadiene rubber (BM-400B, manufactured by Zeon Corporation), 1.5 parts by mass of carboxymethyl cellulose (CMC1380, manufactured by Daicel Corporation), and 1.0 part by mass of carbon black (SuperP, manufactured by Imerys GC) were mixed, and then water was added to adjust the viscosity. The resulting composition was applied to a current collector (copper foil with a thickness of 10 μm) at a coating amount of 10 mg / cm. 2 The composition layer was applied so that the density after vacuum drying was 1.60 g / cm. 2 The orientation of the produced negative electrode was evaluated by the method described above.

[0115] (3) Preparation of evaluation cells The negative electrode obtained above was placed on a plate with an electrode area of 1.54 cm 2 The evaluation cell was a 2016-type coin cell, which was punched into a circular shape and fabricated using metallic lithium as the positive electrode, a mixed solution of ethylene carbonate / ethyl methyl carbonate (3 / 7 volume ratio) containing 1.0 M LiPF6 and vinylene carbonate (0.5 mass%) as the electrolyte, a 25 μm thick polyethylene microporous membrane as the separator, and a 230 μm thick copper plate as the spacer.

[0116] (4) Evaluation of battery characteristics The fabricated evaluation cell was used to measure the initial discharge capacity (Ah / kg) and initial charge / discharge efficiency (%). Specifically, the evaluation cell was placed in a thermostatic chamber maintained at 25°C and charged at a constant current of 0.53 mA until the voltage reached 0 V. It was then further charged at a constant voltage of 0 V until the current decayed to a value equivalent to 0.026 mA, and the initial charge capacity was measured. After charging, the cell was rested for 30 minutes and then discharged. The initial discharge capacity was measured at 0.53 mA until the voltage reached 1.5 V. The capacity was converted to a value per mass of the negative electrode material used. The initial charge / discharge efficiency (%) was calculated by dividing the initial discharge capacity by the initial charge capacity and multiplying this value by 100. The results are shown in Table 4.

[0117] (5) Evaluation of quick charging capacity retention rate The negative electrode obtained above was placed on a plate with an electrode area of 2.00 cm. 2 A 2016-type coin cell was fabricated using a cathode made of aluminum foil coated with lithium cobalt oxide, an electrolyte solution of a mixture of ethylene carbonate / ethyl methyl carbonate (3 / 7 volume ratio) containing 1.0 M LiPF6 and vinylene carbonate (0.5 mass%), a 25 μm-thick polyethylene microporous membrane as the separator, and a spring spacer as the spacer. This coin cell was used as the evaluation cell.

[0118] The fabricated evaluation cell was subjected to an aging treatment before a rapid charge test. Specifically, the evaluation cell was placed in a thermostatic chamber maintained at 25°C and charged at a constant current of 0.92 mA to 4.2 V, and then further charged at a constant voltage of 4.2 V until the current decayed to a value equivalent to 0.046 mA. After charging, a 10-minute pause was taken, followed by discharging at 0.92 mA to 2.75 V. This charge-discharge cycle was repeated five times.

[0119] A rapid charge test was conducted using the aged evaluation cell. Specifically, the evaluation cell was placed in a thermostatic chamber maintained at 25°C and charged at a constant current of 0.92 mA to 4.2 V, and the charge capacity (1) was measured. After charging, the cell was allowed to rest for 10 minutes before discharging. Discharging was continued at 4.6 mA to 2.75 V. Further, constant current charging was continued at 6.9 mA to 4.2 V, and the charge capacity (2) was measured. After charging, the cell was allowed to rest for 10 minutes before discharging at 4.6 mA to 2.75 V. The rapid charge capacity retention rate (%) was calculated by dividing the charge capacity (2) by the charge capacity (1) and multiplying the result by 100. The results are shown in Table 4.

[0120] (6) Battery life evaluation An evaluation cell was fabricated and aged as described above in (5). A cycle test was performed using the aged evaluation cell to evaluate its battery life. Specifically, the evaluation cell was placed in a thermostatic chamber maintained at 25°C and charged at a constant current of 4.6 mA to 4.2 V. It was then further charged at a constant voltage of 4.2 V until the current decayed to a value equivalent to 0.046 mA. After a 10-minute pause, the cell was discharged at 4.6 mA to 2.75 V, and the initial discharge capacity was measured. This charge-discharge cycle was repeated 299 times, and the discharge capacity at the 300th cycle was measured. The battery life maintenance rate (%) was calculated by dividing the discharge capacity at the 300th cycle by the initial discharge capacity and multiplying the result by 100. The results are shown in Table 4.

[0121] [Table 4]

[0122] As shown in Table 4, the evaluation cells prepared using the molded articles with lower densities in Reference Examples 1 to 3 were evaluated as having better rapid charge capacity retention and battery life retention than the evaluation cell prepared using the molded article with higher density in Reference Example 4.

[0123] The disclosure of PCT / JP2021 / 003588, filed February 1, 2021, is incorporated herein by reference in its entirety. All publications, patent applications, and technical standards mentioned in this specification are herein incorporated by reference to the same extent as if each individual publication, patent application, or technical standard was specifically and individually indicated to be incorporated by reference.

Claims

1. (a) obtaining a mixture comprising a graphitizable aggregate, a graphitizable binder, and an aromatic compound; (b) molding the mixture to obtain a molded product having a density of 1.3 g / cm ３ obtaining a molded product, (c) graphitizing the molded product to obtain a graphitized product; (d) pulverizing the graphitized material to obtain a pulverized material, The content of the binder in the mixture is 5% by mass to 20% by mass relative to 100% by mass of the total of the aggregate and the binder, the content of the aromatic compound in the mixture is 1% by mass to 20% by mass relative to 100% by mass of the total of the aggregate and the binder; The method for producing a negative electrode material for a lithium ion secondary battery, wherein the aromatic compound is at least one compound selected from the group consisting of methylnaphthalene and naphthalene.

2. 2. The method for producing a negative electrode material for a lithium ion secondary battery according to claim 1, wherein a content of the binder in the mixture is 15% by mass or less relative to 100% by mass of the total of the aggregate and the binder.

3. 3. The method for producing a negative electrode material for a lithium ion secondary battery according to claim 1, wherein the content of the aromatic compound in the mixture is 1.5% by mass to 20% by mass relative to 100% by mass of the aggregate and the binder.

4. The specific surface area of the pulverized material, which is a negative electrode material for a lithium ion secondary battery, is 2.8 m ２ The method for producing a negative electrode material for a lithium ion secondary battery according to any one of claims 1 to 3, wherein the solubility is 0.01 to 0.15 wt % or less.

5. A method for producing a negative electrode material for a lithium ion secondary battery according to any one of claims 1 to 4, wherein the mixture further contains a dispersant.

6. A method for producing a negative electrode material for a lithium ion secondary battery as described in Claim 5, wherein the content of the dispersant in the entire mixture is 0.5 mass% to 20 mass%.

7. A method for producing a lithium ion secondary battery, comprising the step of producing a negative electrode using the negative electrode material obtained by the method for producing a lithium ion secondary battery negative electrode material according to any one of claims 1 to 6.

8. 7. A method for producing a lithium ion secondary battery, comprising: a step of producing a negative electrode material for a lithium ion secondary battery by the method for producing a negative electrode material for a lithium ion secondary battery according to any one of claims 1 to 6; and a step of producing a negative electrode using the negative electrode material for a lithium ion secondary battery.

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