Production method of negative electrode material for lithium ion secondary battery

Abstract

A negative electrode material for lithium-ion secondary batteries is a graphite particle that satisfies the following (1) to (3): (1) specific surface area of ​​less than 2.7 m2 / g; (2) compressive pressure of more than 2.8 kN / cm2; (3) value of more than 4 expressed by elastic energy / plastic deformation energy.

Description

[Technical Field]

[0001] This disclosure relates to a negative electrode material for lithium-ion secondary batteries and its manufacturing method, a negative electrode for lithium-ion secondary batteries, and lithium-ion secondary batteries. [Previous Technology]

[0002] Lithium-ion secondary batteries effectively utilize their small size, light weight, and high energy density, and have been widely used in electronic devices such as personal computers (PCs), mobile phones, smartphones, and tablet PCs since the past. In recent years, against the backdrop of environmental problems such as global warming caused by CO2 emissions, electric vehicles (EVs) that use only batteries for driving, hybrid electric vehicles (HEVs) that combine gasoline engines and batteries, and plug-in hybrid electric vehicles (PHEVs) have gradually become popular. The development of lithium-ion secondary batteries (automotive lithium-ion secondary batteries) as batteries used in these vehicles is underway.

[0003] The performance of the negative electrode material in a lithium-ion secondary battery greatly affects the input characteristics of the battery. Carbon materials are widely used as negative electrode materials for lithium-ion secondary batteries. For example, as a material for obtaining a high-density negative electrode, a highly crystallized carbon material such as spherical natural graphite, which is formed by spheroidizing artificial graphite or flake natural graphite, has been proposed.

[0004] As an artificial graphite, for example, International Patent Publication No. 2015 / 147012 discloses a negative electrode material for lithium-ion secondary batteries, which comprises composite particles containing multiple flat graphite particles and spherical graphite particles aggregated or combined in a manner where the alignment planes are not parallel. Japanese Patent Application Publication No. 2005-302725 discloses a negative electrode active material for lithium-ion secondary batteries containing carbon powder particles, wherein the carbon powder particles have a morphology in which plate-shaped particles are assembled into a stable primary structure along the alignment planes, and micropores are formed on the surface. [Summary of the Invention]

[0005] [Problem to be Solved by the Invention] Promising applications for automotive lithium-ion secondary batteries include commercial vehicles such as cars and delivery vehicles where the driving distance is easily predictable; however, these vehicles are primarily designed for daytime driving. Furthermore, there is a desire to promote the widespread adoption of electric vehicles in tropical regions. Therefore, it is desirable to improve the high-temperature resistance of automotive lithium-ion secondary batteries. Moreover, to address the frequent charging requirements of lithium-ion secondary batteries, it is desirable to improve their fast-charging performance.

[0006] In view of the above, the present invention discloses a negative electrode material for lithium-ion secondary batteries capable of producing lithium-ion secondary batteries with excellent high-temperature resistance and fast-charging performance, a method for manufacturing the same, and a negative electrode for lithium-ion secondary batteries and a lithium-ion secondary battery made using the same negative electrode material. [Means for Solving the Problem]

[0007] Means for solving the aforementioned problems include the following: <1> A negative electrode material for lithium-ion secondary batteries, which is graphite particles that satisfy the following (1) to (3): (1) Specific surface area of ​​2.7 m2 / g or less (2) Compressive pressure of 2.8 kN / cm2 or more (3) Value expressed as elastic energy / plastic deformation energy of 4 or more <2> A negative electrode material for lithium-ion secondary batteries as described in <1>, wherein the springback coefficient is 25% or more. <3> A negative electrode material for lithium-ion secondary batteries as described in <1> or <2>, wherein the graphite particles comprise composite particles having a structure formed by stacking multiple flat graphite particles. <4> A method for manufacturing a negative electrode material for lithium-ion secondary batteries, comprising a step of graphitizing coke that satisfies the following (1) and (2). (1) The coefficient of thermal expansion after calcination at 1400°C is 2.9 × 10⁻⁶ / °C or less. (2) The Hardgrove grindability index (HGI) after calcination at 1200°C is 47 or less. <5> The method for manufacturing a negative electrode material for lithium-ion secondary batteries as described in <4>, wherein the true specific gravity of the coke after calcination at 1200°C is 2.05 or more. <6> The method for manufacturing a negative electrode material for lithium-ion secondary batteries as described in <4> or <5>, wherein the pore volume of the coke is 0.90 mL / g or less. <7> The method for manufacturing a negative electrode material for lithium-ion secondary batteries as described in any one of <4> to <6>, wherein the pore specific surface area of ​​the coke is 3.0 m² / g or less. <8> A method for manufacturing a negative electrode material for a lithium-ion secondary battery as described in any one of <4> to <7>, for manufacturing a negative electrode material for a lithium-ion secondary battery as described in any one of <1> to <3>. <9> A negative electrode for a lithium-ion secondary battery, comprising: a negative electrode material layer containing the negative electrode material for a lithium-ion secondary battery as described in any one of <1> to <3>, and a current collector. <10> A lithium-ion secondary battery, comprising a negative electrode, a positive electrode, and an electrolyte as described in <9>. [Effects of the Invention]

[0008] According to this disclosure, a negative electrode material for lithium-ion secondary batteries with excellent high temperature resistance and fast charging performance can be provided, as well as a method for manufacturing the same, and a negative electrode and a lithium-ion secondary battery made using the same negative electrode material.

Implementation Method

[0010] Hereinafter, the forms in which the present invention is implemented will be described in detail. However, the present invention is not limited to the following embodiments. In the following embodiments, unless specifically stated otherwise, structural elements (including element steps, etc.) are not essential. The same applies to numerical values ​​and their ranges; they do not limit the present invention.

[0011] In this disclosure, the term "step" includes steps independent of other steps, and even if it cannot be clearly distinguished from other steps, it is included as long as the purpose of the step is achieved. In this disclosure, the numerical range represented by "~" includes the minimum and maximum values ​​recorded before and after "~", respectively. In the numerical ranges recorded in stages in this disclosure, the upper or lower limit value recorded in one numerical range can also be replaced by the upper or lower limit value of other numerical ranges recorded in stages. In addition, in the numerical ranges recorded in this disclosure, the upper or lower limit value of the numerical range can also be replaced by the value shown in the embodiment. In this disclosure, each component may contain multiple equivalent substances. When multiple substances equivalent to each component exist in the composition, unless otherwise specified, the content or percentage of each component refers to the total content or percentage of the multiple substances present in the composition. In this disclosure, multiple particles equivalent to each component may be included. In the case where a composition contains multiple particles corresponding to each component, unless otherwise specified, the particle size of each component refers to the value of the mixture of the multiple particles present in the composition. In this disclosure, the terms "layer" or "film," when observing the region where the layer or film exists, include not only the case where it is formed entirely in that region, but also the case where it is formed only in a part of that region. In this disclosure, the term "layering" indicates overlapping of layers; two or more layers can be combined, and two or more layers can also be assembled and disassembled.

[0012] In this disclosure, the particle size distribution of primary particles contained in the negative electrode material and composite particles can be determined using a laser diffraction particle size distribution measuring device. The average particle size is the particle size (D50) that accumulates to 50% from the smallest diameter side in the volume-based particle size distribution. D90 is the particle size that accumulates to 90% from the smallest diameter side in the volume-based particle size distribution, and D10 is the particle size that accumulates to 10% from the smallest diameter side in the volume-based particle size distribution.

[0013] 《Anode Material for Lithium-ion Secondary Batteries》 The cathode material for lithium-ion secondary batteries disclosed herein (hereinafter, also referred to as cathode material) is graphite particles that satisfy the following (1) to (3): (1) Specific surface area of ​​2.7 m2 / g or less (2) Compressive pressure of 2.8 kN / cm2 or more (3) Value expressed as elastic energy / plastic deformation energy of 4 or more

[0014] Lithium-ion secondary batteries made using graphite particles that meet the above conditions as negative electrode materials exhibit excellent high-temperature resistance and fast-charging performance. The reasons may not be clear, but can be considered as follows. First, (1) the specific surface area of ​​the graphite particles is less than 2.7 m2 / g, thereby suppressing the contact area between each particle and the electrolyte. Therefore, it is believed that, for example, the decomposition reaction at the interface between the particles and the electrolyte can be suppressed, and the degradation of the battery can be suppressed even when used at high temperatures. Furthermore, (2) the compressive pressure of the graphite particles is more than 2.8 kN / cm2, and (3) the value expressed by elastic energy / plastic deformation energy is more than 4, so it is believed that the graphite particles are in a state where deformation or damage is unlikely. Therefore, it is believed that sufficient gaps can be ensured for the flow of electrolyte between the graphite particles, and rapid charging and discharging can also be coped with.

[0015] (1) Specific Surface Area: There is no particular limitation as long as the specific surface area of ​​the graphite particles is 2.7 m² / g or less. From the viewpoint of further suppressing the decomposition of the electrolyte, the specific surface area of ​​the graphite particles is preferably 2.5 m² / g or less, more preferably 2.2 m² / g or less, and even more preferably 1.8 m² / g or less. The specific surface area of ​​the graphite particles can be 0.5 m² / g or more, or 1.0 m² / g or more, or 1.2 m² / g or more. If the specific surface area is 0.5 m² / g or more, the current density applied per unit area will not increase sharply, the load can be suppressed, and therefore there is a tendency to improve the fast charge and discharge performance.

[0016] In this disclosure, the specific surface area of ​​graphite particles refers to the specific surface area (N2 specific surface area) determined by nitrogen adsorption measurement at 77 K. The N2 specific surface area can be determined using the Brunauer-Emmett-Teller (BET) method based on the adsorption isotherm obtained by nitrogen adsorption measurement at 77 K. Specifically, the specific surface area can be determined by the method described in the examples.

[0017] The specific surface area of ​​graphite particles can be adjusted according to particle size distribution, particle structure, etc. The specific surface area of ​​graphite particles can be adjusted by coating graphite particles. When it is desired to reduce the particle size, the specific surface area increases significantly due to the unevenness generated by crushing. However, by coating, the unevenness can be filled in by the coating material to make it smooth, thereby adjusting the specific surface area.

[0018] (2) Compression Pressure There is no particular limitation as long as the compression pressure of the graphite particles is 2.8 kN / cm² or higher. From the viewpoint of further suppressing deformation and damage of the graphite particles caused by pressing during the fabrication of the negative electrode, the compression pressure of the graphite particles is preferably 2.9 kN / cm² or higher, and more preferably 3.0 kN / cm² or higher. From the viewpoint of suppressing deformation of the current collector and peeling of the current collector from the active material caused by pressing during the fabrication of the negative electrode, the compression pressure of the graphite particles can be 4.5 kN / cm² or lower, or 4.3 kN / cm² or lower, or 4.0 kN / cm² or lower. In order to suppress peeling from the current collector, a structure of three or more layers with good adhesion formed between the current collector and the layer containing graphite particles can be formed, and the compression pressure can be a value greater than that described.

[0019] In this disclosure, the compression pressure of graphite particles refers to the pressure required to compress graphite particles to a specified density (1.8 g / cm3). A higher compression pressure means less likelihood of deformation or damage to the graphite particles caused by pressure. Specifically, a specified mass (e.g., 3.0 g) of graphite particles is filled into a mold, and compression is performed at a constant speed (e.g., 10 mm / min). The pressure (kN / cm2) at which the density of the compressed graphite particles reaches 1.8 g / cm3 is defined as the compression pressure of the graphite particles. In the measurement, a mold with a diameter of 15 mm is used, for example, and compression is performed using an autograph (e.g., manufactured by Shimadzu Corporation). The density of the graphite particles is calculated from the volume and mass of the graphite particles, based on the bottom area of ​​the mold (e.g., 1.767 cm2) and the distance from the bottom surface of the mold to the pressing surface of the graphite particles.

[0020] (3) Elastic Energy / Plastic Deformation Energy The value of the graphite particles expressed by elastic energy E1 / plastic deformation energy E2 is not particularly limited as long as it is 4 or more. From the viewpoint of further suppressing the deformation and damage of graphite particles caused by the pressing during the production of the negative electrode, the value of E1 / E2 is preferably 6 or more, and more preferably 7 or more. From the viewpoint that high density is easily caused by the pressing during the production of the negative electrode, the value of E1 / E2 of graphite particles can be 15 or less, or 12 or less, or 10 or less.

[0021] The value of E1 / E2 of graphite particles is a value representing the relative relationship between the property (elasticity) that the graphite particles are deformed by the application of an external force and the property that the graphite particles remain in their original state even after the external force is removed (plastic deformation).

[0022] In this disclosure, the elastic energy E1 and the plastic deformation energy E2 are determined by the differential quadrature method. Specifically, in the test for measuring the compressive pressure of the graphite particles, the pressure F (N / m2) relative to the graphite particles is recorded whenever the pressing surface moves a certain distance d, and the sum of the product of each recorded value f and the distance d (f×d) is calculated. The distance d that the pressing surface moves is set to be 2.0 μm or less (e.g., 1.67 μm).

[0023] The elastic energy E1 and plastic deformation energy E2 of the graphite particles are equivalent to the areas represented by E1 and E2 in Figure 1, respectively. In Figure 1, the vertical axis represents the pressure F (N / m2) relative to the graphite particles, and the horizontal axis represents the displacement D (m) of the pressing surface of the graphite particles. D0 represents the displacement of the pressing surface before the pressure is applied, D1 represents the displacement of the pressing surface when the density of the graphite particles reaches 1.8 g / cm3 due to the pressure, and D2 represents the displacement of the pressing surface when the pressing surface stops moving due to elasticity after the pressure is stopped.

[0024] First, the total energy E imparted until the density of the graphite particles reaches 1.8 g / cm³ is calculated. Specifically, pressure is applied until the density of the graphite particles reaches 1.8 g / cm³. The total energy E is calculated using the quadrature method based on the applied pressure F recorded from the start of pressure application until the density of the graphite particles reaches 1.8 g / cm³, i.e., until the displacement of the pressing surface reaches D1. Next, pressure application is stopped at the moment when the density of the graphite particles reaches 1.8 g / cm³. If pressure application is stopped, the pressing surface moves elastically in the opposite direction to the direction of pressure application. The displacement of the pressing surface at the point where this movement stops is defined as D2. Within the area corresponding to the total energy E calculated by the quadrature method, the area of ​​the pressing surface displacement from D0 to D2 is defined as the plastic deformation energy E2, and the area of ​​the pressing surface displacement from D2 to D1 is defined as the elastic energy E1.

[0025] In this disclosure, carbon materials with an average planar spacing (d002) of less than 0.340 nm as determined by X-ray diffraction are defined as graphite. In this disclosure, as described later, particles in which at least a portion of the surface of graphite particles is configured with low-crystallinity carbon are also defined as "graphite particles". The theoretical value of the average planar spacing (d002) of graphite crystals is 0.3354 nm, and the closer it is to this value, the more developed the graphitization. From the viewpoint of the initial charge-discharge efficiency and energy density of lithium-ion secondary batteries, the average planar spacing (d002) is preferably 0.33600 nm or less, more preferably 0.33596 nm or less, and even more preferably 0.33592 nm or less. In view of the above, the average surface spacing (d002) of graphite particles is preferably 0.3354 nm to 0.33600 nm, more preferably 0.3354 nm to 0.33596 nm, and even more preferably 0.3354 nm to 0.33592 nm.

[0026] The average surface spacing (d002) of graphite particles can be calculated using Bragg's equation based on the diffraction peaks corresponding to the carbon 002 surface that appear near the diffraction angle 2θ of 24° to 27°, obtained by irradiating the sample with X-rays (CuKα rays) and measuring the diffraction with the rays using a goniometer. The average surface spacing (d002) can be measured under the following conditions: X-ray source: CuKα rays (wavelength = 0.15418 nm) Output: 40 kV, 20 mA Sampling amplitude: 0.010° ​​Scanning range: 10° to 35° Scanning speed: 0.5° / min

[0027] Bragg's equation: 2dsinθ=nλ Here, d represents the length of one period, θ represents the diffraction angle, n represents the number of reflections, and λ represents the X-ray wavelength.

[0028] (Particle Structure of Graphite Particles) Graphite particles can comprise a state of multiple graphite particles aggregated or combined (composite particles), or composite particles having a structure formed by stacking multiple flat graphite particles (hereinafter, also referred to as specific composite particles). It is believed that specific composite particles, compared to composite particles with a structure in which the main surfaces of multiple flat graphite particles are oriented in random directions, can reduce the contact area with the electrolyte within the particles, effectively suppressing electrolyte decomposition. Furthermore, it is believed that even under pressure during the fabrication of the negative electrode, particle deformation or destruction is less likely, easily ensuring electrolyte channels between particles. In addition, the electrolyte movement and diffusion path within composite particles with a structure in which the main surfaces of multiple flat graphite particles are oriented in random directions is complex and slow. Therefore, it is believed that even active materials with more particle interfaces will experience diffusion barriers, preventing the achievement of high input / output performance. This tendency is particularly pronounced under high charge rate (C rate) input / output conditions.

[0029] The flat graphite particles contained in a specific composite particle refer to non-spherical graphite particles with anisotropic shapes. Examples of flat graphite particles include those with scaly, flake-like, and partially blocky shapes. A composite particle refers to a particle in which primary particles are aggregated or combined. That is, a specific composite particle has a structure in which multiple flat graphite particles overlap and aggregate or combine with each other on the main surface. Therefore, multiple flat graphite particles overlap in a generally parallel state to form a composite particle. Whether the flat graphite particles are layered can be confirmed by microscopic observation.

[0030] The term "state of aggregates or bonds of multiple flat graphite particles" refers to a state in which two or more flat graphite particles are aggregated or bonded. Bonding refers to a state in which the particles are chemically bonded directly or through carbonaceous materials. Aggregation refers to a state in which the particles are not chemically bonded, but retain their aggregate shape due to their form, etc. Flat graphite particles can also be aggregated or bonded through carbonaceous materials. Examples of carbonaceous materials include graphite formed from organic binders such as tar and pitch. Whether flat graphite particles are aggregated or bonded can be confirmed, for example, by observation using a scanning electron microscope.

[0031] There are no particular limitations on the flat graphite particles and their raw materials, and examples include artificial graphite, flake natural graphite, flaky natural graphite, coke, and resin. Among these, artificial graphite is preferred from the viewpoint of being less prone to deformation and having a low specific surface area. When natural graphite is used as a part of the raw material, from the viewpoint of being less prone to spheroidization and easier to obtain a layered structure, the proportion of natural graphite in the specific composite particles is preferably 40% by mass or less.

[0032] From the viewpoint of ease of assembly or combination, the average particle size of the flat graphite particles constituting the specific composite particles is preferably 5 μm to 25 μm, more preferably 8 μm to 20 μm, and even more preferably 10 μm to 15 μm. The average particle size of the flat graphite particles can be determined by any of the following methods. The average particle size of the flat graphite aggregate described later as the raw material for the specific composite particles, i.e., the particle size (D50) when the cumulative particle size distribution from the small diameter side in the volume-based particle size distribution reaches 50%, can be regarded as the average particle size of the flat graphite particles constituting the specific composite particles. Alternatively, the average particle size of the flat graphite particles can be determined by observing the cross-section of the specific composite particles using a scanning microscope, and using it as the median value of the particle size of any 100 flat graphite particles. In this case, the particle size of each flat graphite particle is set as the diameter of a circle having an area equal to the projected area, i.e., the circle equivalent diameter.

[0033] The particle size distribution D90 / D10 of the flat graphite particles contained in the specific composite particles is preferably 4.4 or less, more preferably 4.0 or less, and even more preferably 3.5 or less. It is believed that if the particle size distribution D90 / D10 of the multiple flat graphite particles is 4.4 or less, the particle size of the flat graphite particles is relatively uniform, thus further reducing the contact area with the electrolyte within the obtained specific composite particles, and further suppressing electrolyte decomposition. Furthermore, it is believed that even under pressure during the fabrication of the negative electrode, particle deformation or damage is less likely, making it easier to ensure electrolyte channels between particles. There is no particular limitation on the lower limit of the particle size distribution D90 / D10 of the multiple flat graphite particles; for example, it can be 2.0 or more.

[0034] The particle size distribution D90 / D10 of the flat graphite particles can be determined by any of the following methods. The particle size distribution D90 / D10 of the flat, graphitizable aggregate described later as a raw material for a specific composite particle, measured using a laser diffraction particle size distribution measuring device (e.g., SALD3100, Shimadzu Corporation), can be considered as the particle size distribution D90 / D10 of the flat graphite particles constituting the specific composite particle. Alternatively, the particle size distribution D90 / D10 of the flat graphite particles can be determined by observing the cross-section of the specific composite particle using a scanning microscope, and by comparing the particle size (D90) when the cumulative number of any 1000 flat graphite particles from the small diameter side reaches 90%, with the particle size (D10) when the cumulative number from the small diameter side reaches 10%. In this case, the particle size of each flat graphite particle is set as the diameter of a circle having an area equal to the projected area, i.e., the circle equivalent diameter.

[0035] When the length of the long axis of the flat graphite particles is denoted as A and the length of the short axis as B, the aspect ratio expressed by A / B is preferably 2 to 20, more preferably 4 to 10. If the aspect ratio is 2 or higher, the surface area increases further, thus reducing the applied buoyancy, and the particles tend to aggregate. During particle aggregation, in order to reduce the surface area, the surfaces with long axes are stacked on each other, thereby maximizing the contact area between the particles. In addition, the van der Waals forces acting between the particles also tend to strengthen the adhesion between the particles and stabilize them. If the aspect ratio is 20 or lower, the input and output characteristics, such as the fast charge and discharge characteristics of lithium-ion secondary batteries, tend to be further improved. Furthermore, if the aspect ratio is 20 or lower, each flat graphite particle becomes thinner, the number of stacked layers increases, the interparticle spacing increases, and the specific surface area increases, resulting in a tendency to suppress the decline in retention characteristics.

[0036] The aspect ratio is determined by observing graphite particles under a microscope, randomly selecting 100 graphite particles, measuring their respective A / B ratios, and taking the arithmetic mean of these measurements. In the observation of the aspect ratio, the length A in the major axis direction and the length B in the minor axis direction are determined as follows: In the projected image of the graphite particles observed under a microscope, two parallel tangents that are externally tangent to the outer periphery of the graphite particles are selected, namely, tangent a1 and tangent a2, which are the furthest apart, and the distance between these two tangents is set as the length A in the major axis direction. In addition, two parallel tangents that are externally tangent to the outer periphery of the graphite particles are selected, namely, tangent b1 and tangent b2, which are the furthest apart, and the distance between these two tangents is set as the length B in the minor axis direction.

[0037] When the graphite particles contain specific composite particles, all of the graphite particles may be specific composite particles, or some may be graphite particles other than specific composite particles.

[0038] When the graphite particles are a mixture of specific composite particles and particles other than specific composite particles, the proportion of specific composite particles relative to the total amount of graphite particles may be 50% by mass or more, or 60% by mass or more, or 70% by mass or more, or 80% by mass or more, or 90% by mass or more. Furthermore, the proportion of specific composite particles relative to the total amount of graphite particles may be 95% by mass or less.

[0039] Graphite particles may also be particles with low-crystallinity carbon disposed on at least a portion of their surface. If low-crystallinity carbon is disposed on at least a portion of the surface of the graphite particles, there is a tendency to further improve the charging characteristics at low temperatures when constructing a lithium-ion secondary battery. On the other hand, graphite particles may also be particles without low-crystallinity carbon disposed on their surface. If low-crystallinity carbon is not disposed on the surface of the graphite particles, there is a tendency during electrode fabrication to suppress cracking and peeling of the graphite particles, increase the decomposition activity of the electrolyte, and decrease the storage characteristics. In addition, this provides the advantage of increased freedom in manufacturing conditions. The graphite particles disclosed herein have been found to have excellent rapid input / output characteristics even when low-crystallinity carbon is not disposed on the surface. It is believed that this is because, compared with the surface structure of the graphite particles, the hardness, elasticity, etc. of the particles greatly affect the rapid input / output characteristics.

[0040] In this disclosure, "low-crystallinity carbon" refers to carbon with an R value of 0.2 or higher in its Raman spectrum. In laser Raman spectrometry with an excitation wavelength of 532 nm, when the intensity of the maximum peak appearing near 1360 cm⁻¹ is set as Id and the intensity of the maximum peak appearing near 1580 cm⁻¹ is set as Ig, the R value in the Raman spectrum is assigned as the intensity ratio of the two peaks, Id / Ig. The peak appearing near 1360 cm⁻¹ refers to the peak that is usually identified as corresponding to the amorphous structure of carbon, such as the peak observed at 1300 cm⁻¹ to 1400 cm⁻¹. The peak appearing near 1580 cm⁻¹ refers to the peak that is usually identified as corresponding to the crystalline structure of graphite, such as the peak observed at 1530 cm⁻¹ to 1630 cm⁻¹.

[0041] The R-value was determined using a Raman spectroscopy apparatus (e.g., Horiba Manufacturing Co., Ltd., XploRA PLUS). The obtained spectrum was baselined within the following range and under the following conditions: • Laser wavelength: 532 nm • Laser intensity: 100 mW or higher • Neutral density filter: 1% • Irradiation intensity: 1 mW • Measurement range: 1000 cm⁻¹ to 1800 cm⁻¹ • Irradiation time: 30 seconds • Irradiation area: 1 μm² • Baseline (D band): 1100 cm⁻¹ to 1470 cm⁻¹ • Baseline (G band): 1450 cm⁻¹ to 1710 cm⁻¹ • Number of times a single particle was measured: twice • Number of particles measured: 30 particles

[0042] (Average particle size) From the viewpoint of further improving the permeability of the electrolyte, the average particle size of the graphite particles is preferably 5 μm to 30 μm, more preferably 8 μm to 25 μm, and even more preferably 10 μm to 20 μm.

[0043] The average particle size of graphite particles can be measured using a laser diffraction particle size distribution measuring device (e.g., SALD3100, Shimadzu Corporation). The average particle size is the particle size (D50) that accumulates from the smallest diameter side in the particle size distribution on a volume basis to 50%.

[0044] Methods for determining the average particle size of graphite particles contained in the negative electrode include: fabricating a sample electrode, embedding the electrode in epoxy resin, mirror polishing it, and observing the electrode profile using a scanning electron microscope (e.g., Keyence Corporation, "VE-7800"); or fabricating an electrode profile using an ion polishing apparatus (e.g., Hitachi High-Technologies Corporation, "E-3500") and measuring it using a scanning electron microscope (e.g., Keyence Corporation, "VE-7800"). In this case, the average particle size is the median value of arbitrarily selected 100 particle sizes.

[0045] (Particle size distribution D90 / D10) The particle size distribution D90 / D10 of the graphite particles is preferably 5.0 or less, more preferably 4.0 or less, and even more preferably 3.0 or less. It is believed that if the particle size distribution D90 / D10 of the graphite particles is 5.0 or less, the electrolyte channels can be maintained well, thereby maintaining the electrolyte's liquid injection properties well. There is no particular limitation on the lower limit of the particle size distribution D90 / D10 of the graphite particles; for example, it can be 2.0 or more.

[0046] The particle size distribution D90 / D10 can be measured using a laser diffraction particle size distribution measuring device (e.g., SALD3100, Shimadzu Corporation).

[0047] As a method for determining the particle size distribution D90 / D10 of graphite particles contained in the negative electrode, the following methods can be listed: a method of preparing a sample electrode, embedding the electrode in epoxy resin, performing mirror polishing, and observing the electrode profile using a scanning electron microscope (e.g., Keyence Co., Ltd., "VE-7800"); a method of preparing an electrode profile using an ion polishing device (e.g., Hitachi High-Technologies Co., Ltd., "E-3500"), and measuring it using a scanning electron microscope (e.g., Keyence Co., Ltd., "VE-7800"). The particle size distribution D90 / D10 in this case can be obtained by the following methods. (1) Using binarization or the like, the area Sn of the projected particles is obtained (n is the particle's inherent number attached to the selected particles). (2) Assuming the particles are ideally shaped spheres, the equivalent diameter Ln = √Sn / π is obtained from the area Sn. (3) Calculate the volume of the sphere Vn = (4 / 3)π(Ln)3 from the equivalent diameter Ln of the circle. (4) Repeat (1) to (3) for the selected 100 particles. (5) Set the vertical axis to the cumulative percentage of the volume of 100 particles, and set the horizontal axis to the distribution curve of particle diameter. The particle diameter at the point that intersects the horizontal axis of 10% can be set as the 10% diameter (D10), and the particle diameter at the point that intersects the horizontal axis of 90% can be set as the 90% diameter (D90) to calculate D90 / D10.

[0048] (Standard deviation of particle size distribution) The standard deviation of the particle size distribution of graphite particles is preferably 0.30 or less, more preferably 0.25 or less, and even more preferably 0.20 or less. It is believed that if the standard deviation of the particle size distribution is 0.30 or less, the electrolyte channel can be maintained well, thereby maintaining the electrolyte injection properties well. There is no particular limitation on the lower limit of the standard deviation of the particle size distribution. The standard deviation of the particle size distribution can be measured using a laser diffraction particle size distribution measuring device (e.g., SALD3100, Shimadzu Corporation), based on a frequency distribution chart with the horizontal axis set to the logarithmic scale of particle diameter and the vertical axis set to the particle quantity (%).

[0049] (Resilience Coefficient) From the viewpoint of further suppressing deformation and damage of graphite particles caused by pressing during the production of the negative electrode, the resilience coefficient of the graphite particles is preferably 25% or more, more preferably 27% or more, and even more preferably 30% or more. From the viewpoint of the ease with which high density can be caused by pressing during the production of the negative electrode, the resilience coefficient of the graphite particles can be 50% or less, or 45% or less, or 40% or less.

[0050] The springback coefficient of graphite particles, as disclosed herein, refers to the degree of density decrease when pressure is released after graphite particles have been compressed to a reference density. A higher springback coefficient means that graphite particles deformed by compression are more likely to return to their original state. Specifically, a specified mass (e.g., 3.0 g) of graphite particles is filled into a mold and compressed at a constant speed (e.g., 10 mm / min) until the density of the graphite particles reaches a reference density (e.g., 1.8 g / cm³). Afterward, the pressure is released, and the density after pressure release is measured when the pressing surface stops moving due to elasticity. Based on the obtained value, the springback coefficient is calculated using the following formula: Springback coefficient (%) = {(Reference density - Density after pressure release) / Reference density} × 100

[0051] In the aforementioned measurement, a mold, for example, with a diameter of 15 mm, is used, and compression is performed using an automatic graphing instrument (e.g., manufactured by Shimadzu Corporation). The density of the graphite particles is calculated from the volume and mass of the graphite particles, based on the bottom area of ​​the mold (e.g., 1.767 cm²) and the distance from the bottom surface of the mold to the pressing surface of the graphite particles.

[0052] (Oil absorption) The oil absorption of the graphite particles is preferably 10 mL / 100 g to 60 mL / 100 g, more preferably 15 mL / 100 g to 45 mL / 100 g, and even more preferably 20 mL / 100 g to 40 mL / 100 g.

[0053] The oil absorption capacity of graphite particles is an indicator of the number of pores present inside and on the surface of the particles, as well as the number of gaps between the particles. It is believed that if the oil absorption capacity of graphite particles is 60 mL / 100 g or less, the number of pores present inside and on the surface of the particles is small, resulting in a sufficiently small contact area with the electrolyte. Furthermore, due to the small interface size, the amount of binder required when manufacturing the negative electrode can be reduced, which tends to lower resistance and improve battery performance. Moreover, due to the fewer pores, the amount of solvent used for drying the electrodes can be reduced, offering advantages in terms of production line costs and environmental aspects, such as equipment and power suppression during drying. If the oil absorption capacity of graphite particles is 10 mL / 100 g or more, the tendency to increase the viscosity of the slurry during mixing with binders, which occurs when the gaps between particles are too small, can be suppressed. Additionally, the binder tends to expand more easily and be easier to mix. Furthermore, it is easier to ensure sufficient gaps between particles for lithium ion movement.

[0054] In this disclosure, the oil absorption of graphite particles is determined using linseed oil (e.g., manufactured by Kanto Chemical Co., Ltd.) as the reagent liquid, in accordance with the method described in JIS K6217-4:2017 "Carbon black for rubber - basic properties - Part 4: method for determining oil absorption". Specifically, linseed oil is titrated in the target powder using a constant-speed burette, and the change in viscosity characteristics is measured using a torque detector. The amount of linseed oil added per unit mass of the target powder, corresponding to 70% of the maximum torque generated, is defined as the oil absorption (mL / 100 g). As the measuring instrument, for example, an absorption measuring device (trade name: S-500) from Asahi Research Institute Co., Ltd. can be used.

[0055] (Peak Intensity Ratio of Rhombohedral Crystal Structure) The intensity ratio (P1 / P2, also known as the peak intensity ratio of rhombohedral crystal structure) of the (101) facet of the rhombohedral crystal structure to the peak intensity ratio (P2) of the (101) facet of the hexagonal crystal structure in the X-ray diffraction pattern obtained from CuKα rays of graphite particles can be 0.15 or less, 0.10 or less, or 0.05 or less. The peak intensity ratio (P1 / P2) is preferably within the range that cannot be observed by the methods described below. If the peak intensity ratio of the rhombohedral crystal structure of graphite particles is 0.15 or less, there is a tendency for the graphite particles to be more graphitized and have a higher charge-discharge capacity.

[0056] The peak intensity ratio of the rhombohedral crystal structure of graphite particles can be calculated from the intensity ratio of the rhombohedral crystal structure diffracted rays (P1: diffraction angle 43.2°) to the hexagonal crystal structure diffracted rays (P2: diffraction angle 44.3°) in an X-ray diffraction pattern using CuKα rays. Here, the diffraction angle is represented by 2θ (θ is the Bragg angle), with the rhombohedral crystal structure (101) facet diffracted at a diffraction angle of 43.2° and the hexagonal crystal structure (101) facet diffracted at a diffraction angle of 44.3°.

[0057] (Degree of graphitization) The degree of graphitization of graphite particles, determined by X-ray diffraction, can be 93.0%–100.0%, 93.5%–99.0%, or below 94.0%–98.0%. For example, if the degree of graphitization of larger graphite particles with a particle diameter exceeding 12 μm is below 98.0%, the graphite particles have sufficiently high hardness and are less prone to deformation or breakage. When a high degree of graphitization is required, hardness can also be increased by reducing the particle diameter to less than 10 μm. For example, the particle shape can be maintained even with a degree of 100% graphitization. If the degree of graphitization of graphite particles is above 93.0%, there is a tendency for excellent discharge capacity.

[0058] The degree of graphitization of graphite particles can be determined, for example, as follows. 60 parts by mass of graphite particles and 40 parts by mass of silicon powder (e.g., Fujifilm and Kojun Pharmaceutical Co., Ltd., purity 99.9%) are mixed for 5 minutes using an agate mortar. The resulting mixture is then placed in an X-ray diffraction measurement unit. Using an X-ray diffraction measurement apparatus (e.g., Rigaku MultiFlex X-ray Diffraction Meter), the diffraction angles corresponding to the (002) plane of graphite and the (111) plane of silicon are measured using CuKα rays (2θ = 25°–29°). The observed diffraction angles of silicon and graphite are corrected using the theoretical diffraction angle of Si (2θ = 28.442°), thereby determining the correct diffraction angle of graphite. The interplanar spacing (Å) of the d(002) plane of the negative electrode material was calculated using the Bragg equation (2dsinθ=nλ), and the degree of graphitization was calculated using the following formula: Degree of graphitization = [(3.44 - interplanar spacing) / (0.086)] × 100

[0059] The method for manufacturing the negative electrode material disclosed herein includes a step of graphitizing coke that satisfies the following (1) and (2): (1) The coefficient of thermal expansion after calcination at 1400°C is 2.9 × 10⁻⁶ / °C or less; (2) The Hastelloy hardness (HGI) after calcination at 1200°C is 47 or less.

[0060] Graphite particles obtained by graphitizing coke that meets the above conditions are used as negative electrode materials in lithium-ion secondary batteries with excellent high-temperature resistance and fast charging performance. The reasons may not be clear, but can be considered as follows. First, (1) the coefficient of thermal expansion (CTE) of coke after calcination at 1400°C is less than 2.9×10-6 / °C, thereby suppressing the generation of cracks caused by the expansion and crystallization of coke particles during the graphitization step. It is believed that the graphite particles obtained by graphitizing such coke particles have a small contact area with the electrolyte, which can suppress the decomposition reaction of the electrolyte, and can suppress the degradation of the battery even when used at high temperatures. Furthermore, (2) the Hardy hardness (HGI) of coke after calcination at 1200°C is less than 47, thereby obtaining harder graphite particles. Therefore, it is believed that the graphite particles are not easily deformed or damaged, which can fully ensure the gaps for the electrolyte to flow between the graphite particles, and can also cope with rapid charging and discharging.

[0061] (1) Coefficient of thermal expansion The coefficient of thermal expansion (CTE) of the coke used in the manufacturing method after calcination at 1400°C is 2.9 × 10⁻⁶ / °C or less. The coefficient of thermal expansion of coke is related to the number of closed pores present in the coke, and there is a tendency for the smaller the coefficient of thermal expansion to be the fewer closed pores. It is believed that the smaller the coefficient of thermal expansion of coke, the smaller the degree of expansion and contraction of coke particles in the graphitization step, and the smaller the contact area between the obtained graphite particles and the electrolyte. The coefficient of thermal expansion of the coke after calcination at 1400°C is preferably 2.7 × 10⁻⁶ / °C or less, more preferably 2.5 × 10⁻⁶ / °C or less, and even more preferably 2.3 × 10⁻⁶ / °C or less. From the viewpoint of obtaining graphite particles with moderate porosity, the coefficient of thermal expansion of the coke after calcination at 1400°C can be 1.0 × 10⁻⁶ / °C or more. As a method to obtain coke with a coefficient of thermal expansion (CTE) of less than 2.9 × 10⁻⁶ / ℃ after calcination at 1400℃, one method is to convert the closed pores of coke particles into open pores. Specifically, coarse particles can be pulverized using a pulverizer such as a roller mill to convert the closed pores into open pores.

[0062] The coefficient of thermal expansion of coke was determined by the following method. 70 g of coke calcined at 1400°C was mixed with 30 g of binder pitch for 5 minutes to ensure homogeneity. 15 g of distilled oil, which is liquid at room temperature, was added to 100 g of this mixture, and the mixture was prepared by mixing for 3 minutes using a planetary mixer or similar equipment. The sample was placed in a pressure molding machine and pressed at a surface pressure of 10 MPa for 30 seconds to form the sample. The molded sample was heated from room temperature (25°C) to 1000°C in a nitrogen environment for 5 hours, held at 1000°C for 1 hour, and then cooled to obtain a calcined sample. The calcined sample was cut into 5.0 mm × 5.0 mm × 15.0 mm pieces using a precision cutting machine to obtain test pieces. For this test piece, thermal expansion was measured in the temperature range of 30°C to 500°C using a TMA (thermomechanical analysis device, such as those manufactured by Hitachi High-Technologies), and the CTE was calculated.

[0063] (2) Hardness The Hardness (HGI) of the coke used in the manufacturing method described above after calcination at 1200°C is 47 or less. The Hardness of coke is an indicator of the ease of crushing coke. The smaller the Hardness value, the harder the coke and the more difficult it is to crush. Therefore, the obtained graphite particles also tend to harden. The Hardness of coke is preferably 44 or less, and more preferably 40 or less. From the viewpoint of the ease of high-density production using graphite particles as a negative electrode, the Hardness of coke can be 20 or more.

[0064] The Hastelloy hardness of coke was determined according to JIS M 8801 (2004). Furthermore, the measured values ​​of Hastelloy hardness differ between unburned coke (raw coke) and burnt coke (burnt coke), and it is easier to grasp the effect of bulk density on hardness in the latter case.

[0065] The coke used in the manufacturing method preferably satisfies at least one of the following (3) to (5): (3) The true specific gravity of butanol after calcination at 1200°C is 2.05 or more; (4) The pore volume is 0.90 mL / g or less; (5) The pore specific surface area is 3.0 m² / g or less.

[0066] (3) Butanol True Specific Gravity The butanol true specific gravity of coke is an indicator of the number of closed pores present in the coke. There is a tendency for the higher the butanol true specific gravity value, the fewer closed pores present in the coke. Therefore, it is believed that the higher the butanol true specific gravity, the fewer cracks are generated in the graphitization step, which can reduce the contact area between the obtained graphite particles and the electrolyte. From the viewpoint of reducing the contact area between graphite particles and electrolyte, the butanol true specific gravity of coke calcined at 1200°C is preferably 2.05 or higher, more preferably 2.10 or higher, and even more preferably 2.12 or higher. From the viewpoint of adequately ensuring the contact area between graphite particles and electrolyte, the butanol true specific gravity of coke calcined at 1200°C can be 2.20 or lower. Furthermore, the measured true specific gravity of butanol differs between unburned coke (raw coke) and burnt coke (burnt coke). In the latter case, impregnation is more successful, making it easier to determine the true specific gravity.

[0067] The true butanol specific gravity of coke is determined by using the butanol specific gravity bottle method.

[0068] (4) Pore Volume The pore volume of coke is an indicator of the number of open pores present in coke. There is a tendency that the smaller the pore volume, the fewer open pores present in coke. Therefore, it is believed that the smaller the pore volume, the lower the contact area between the obtained graphite particles and the electrolyte. From the viewpoint of reducing the contact area between graphite particles and the electrolyte, the pore volume of coke is preferably 0.90 mL / g or less, more preferably 0.85 mL / g or less, and even more preferably 0.80 mL / g or less. From the viewpoint of adequately ensuring the contact area between graphite particles and the electrolyte, the pore volume of coke can be 0.50 mL / g or more.

[0069] The pore volume of coke was determined by mercury intrusion method. Specifically, the determination was performed using the following method: Using a pore distribution measuring device (e.g., an Autopore V 9620 manufactured by Shimadzu Corporation-MICROMERITICS), approximately 0.2 g to 0.3 g of sample was taken from a standard unit for powders, and the measurement was performed under an initial pressure of 9 kPa (approximately 1.3 psia, equivalent to a pore diameter of approximately 140 μm). Mercury parameters were set to a mercury contact angle of 130.0 degrees and a mercury surface tension of 485.0 dynes / cm, and calculations were performed for pore diameters ranging from 0.003 μm to 3.5 μm.

[0070] (5) Pore Specific Surface Area The pore specific surface area of ​​coke is an indicator of the number of open pores present in coke. There is a tendency that the smaller the pore specific surface area, the fewer open pores present in coke. Therefore, it is believed that the smaller the pore specific surface area, the smaller the contact area between the obtained graphite particles and the electrolyte. From the viewpoint of reducing the contact area between graphite particles and the electrolyte, the pore specific surface area of ​​coke is preferably 3.0 m2 / g or less, more preferably 2.5 m2 / g or less, and even more preferably 2.0 m2 / g or less. From the viewpoint of adequately ensuring the contact area between graphite particles and the electrolyte, the pore specific surface area of ​​coke can be 0.5 m2 / g or more.

[0071] The specific surface area of ​​the fine pores of coke was determined by mercury intrusion porosimetry. Specifically, the determination was performed using the following method. Using a fine pore distribution measuring device (e.g., an Autopore V 9620 manufactured by Shimadzu Corporation-MICROMERITICS), approximately 0.2 g to 0.3 g of sample was taken from a standard unit for powders, and the measurement was performed under an initial pressure of 9 kPa (approximately 1.3 psia, equivalent to a pore diameter of approximately 140 μm). Mercury parameters were set to a mercury contact angle of 130.0 degrees and a mercury surface tension of 485.0 dynes / cm, and calculations were performed for pore diameters ranging from 0.003 μm to 3.5 μm.

[0072] The type of coke used in the manufacturing method is not particularly limited, but examples include: liquefied coke, needle coke, embedded coke, and semi-needle coke, which has intermediate properties between needle coke and embedded coke, as well as petroleum-based or coal-based cokes. Among these, needle coke and semi-needle coke, which tend to have a low coefficient of thermal expansion, are preferred. Furthermore, semi-needle coke and needle coke have high crystallinity, making it easy to obtain flat particles. In addition, needle coke has high crystallinity, resulting in large particles that are easy to adjust in size through crushing and grading. Only one type of coke may be used, or two or more types may be used in combination. Within the range that satisfies the above conditions, needle coke and semi-needle coke may be used in combination.

[0073] There are no particular restrictions on the method for obtaining coke particles (coke particles), and known methods can be used. There are no particular restrictions on the particle size of the coke particles, and the selection can be made by considering the desired particle size, particle structure, etc. of the graphite particles.

[0074] The manufacturing method may be a method for manufacturing the negative electrode material disclosed herein. In this case, the details of the manufactured negative electrode material can be applied to the matters described for the negative electrode material.

[0075] The graphite particles manufactured by the manufacturing method can be in the form of a collection or combination of multiple graphite particles (composite particles), or they can be composite particles having a structure formed by stacking multiple flat graphite particles (specific composite particles). For example, graphite particles in the form of specific composite particles can be manufactured by processing a mixture obtained by mixing flat coke particles with a binder to produce secondary particles having a structure formed by stacking flat coke particles, and then graphitizing the obtained secondary particles.

[0076] Therefore, in a preferred embodiment, the method for manufacturing the negative electrode material sequentially includes: a step of mixing flat coke particles with a binder to obtain a mixture; a step of processing the mixture to produce secondary particles having a structure formed by stacking the flat coke particles; and a step of graphitizing the secondary particles to obtain composite particles (specific composite particles) having a structure formed by stacking multiple flat graphite particles.

[0077] In the case of manufacturing specific composite particles, the method may include the step of classifying flattened coke particles to remove at least one selected from the group consisting of fine and coarse particles. By classifying the coke particles, a denser layered structure of specific composite particles can be formed, which can further suppress the tendency for specific surface area to decrease.

[0078] In the case of manufacturing specific composite particles, the method may include the step of classifying the obtained specific composite particles to remove at least one selected from the group consisting of microparticles and coarse particles. By classifying the specific composite particles, there is a tendency to suppress deviations in the particle size of the specific composite particles and to better maintain the channels of the electrolyte.

[0079] Therefore, in a preferred embodiment, the method for manufacturing the negative electrode material sequentially includes: (a) a step of classifying flat coke particles as needed to remove at least one selected from the group consisting of fine particles and coarse particles; (b) a step of mixing the flat coke particles with a binder to obtain a mixture; (c) a step of processing the mixture to produce secondary particles having a structure formed by stacking the flat coke particles; (d) a step of graphitizing the secondary particles to obtain composite particles (specific composite particles) having a structure formed by stacking multiple flat graphite particles; and (e) a step of classifying the composite particles as needed to remove at least one selected from the group consisting of fine particles and coarse particles.

[0080] Furthermore, in this disclosure, "microparticles" refers to particles with a diameter smaller than those recovered through grading, and "coarse particles" refers to particles with a diameter larger than those recovered through grading.

[0081] Hereinafter, the steps that may be included in the manufacturing method of the negative electrode material disclosed herein will be described in detail.

[0082] [(a) Step of classifying the flattened coke particles to remove at least one of the groups consisting of fine particles and coarse particles] The flattened coke particles may also be classified before compounding to remove at least one of the groups consisting of fine particles and coarse particles. The particle size distribution D90 / D10 of the flattened coke particles may also be adjusted by classification to, for example, 2.0 to 4.4, preferably 2.0 to 4.0, and more preferably 2.0 to 3.5.

[0083] In the grading process, it is preferable to remove particles with a diameter of less than 1 μm, more preferably to remove particles with a diameter of less than 2 μm, and even more preferably to remove particles with a diameter of less than 3 μm.

[0084] In addition, during the grading process, it is preferable to remove coarse particles with a particle size of 60 μm or larger, more preferably to remove coarse particles with a particle size of 50 μm or larger, and even more preferably to remove coarse particles with a particle size of 40 μm or larger. The removed coarse particles can also be crushed and reused as raw materials.

[0085] There are no particular limitations on the classification method. Examples include: classification using sieves, classification using airflow centrifuges, and precision airflow classifiers utilizing the Coanda effect. Alternatively, the method can be adjusted by using a roller mill to apply concentrated compressive pressure to pulverize coarse particles.

[0086] [(b) Step of mixing flat coke particles with a binder to obtain a mixture] As the binder, a graphitizable binder is used. Examples of binders include: coal-based, petroleum-based, synthetic asphalt and tar, thermoplastic resins, thermosetting resins, etc. It is preferable to select a binder with low viscosity to increase the flowability of the flat particles in the mixing step. In addition, graphitizing catalysts, flowability enhancers, etc., may be added as needed. Examples of graphitizing catalysts include: silicon, iron, nickel, titanium, boron, vanadium, aluminum, and other substances that have graphitizing catalyst properties, as well as carbides, oxides, nitrides, mica clay minerals, etc.

[0087] The content of the graphitizing catalyst is not limited as long as the target material can be obtained. From the viewpoint of not excessively promoting graphitization, it is preferable not to prepare the graphitizing catalyst or to reduce the amount prepared. For example, when the graphitizing catalyst is silicon carbide (SiC), the content of silicon carbide is preferably 5% by mass or less, more preferably 3% by mass or less, and even more preferably 1% by mass or less, relative to the mass of the aggregate that can be graphitized.

[0088] There are no particular limitations on the mixing method. From the viewpoint of reducing the amount of fine pores in secondary particles, a mixing method that minimizes shear force is preferred, such as using a kiln mixer or a handle mixer. It is also preferred not to use a kneading machine or similar equipment that is also called a mixing machine.

[0089] [(c) Step of processing the mixture to produce secondary particles having a structure formed by stacking multiple flat coke particles] There are no particular limitations on the processing method of the mixture. In one embodiment, processing can be performed by heating the mixture to volatilize the volatile components of the binder. The heating temperature is preferably below 400°C. If the heating temperature is below 400°C, it is difficult to form fine pores by oxidation combustion, and particles with a small specific surface area are easily obtained. In this case, granulation is easier and better when the mixture is heated while flowing using a mixer or the like.

[0090] When the mixture is heated, the pressure in the environment can be reduced. By reducing the pressure in the environment, the binder can be easily impregnated into the coke particles, filling the gaps within the particles, and there is a tendency to easily obtain particles with fewer internal pores and layers.

[0091] Hereinafter, specific examples of the method for producing secondary particles will be described. For the mixture of coke particles and binder, the mixture is stirred for a period of time while being stirred, preferably in the temperature range where volatile components volatilize, above the softening point of the binder. By slowly degassing before graphitization (calcination), the generation of bubbles during crystallization sintering is reduced, resulting in particles with fewer pores inside and on the particle surface, tending to become hard structures with low specific surface area and excellent high-temperature resistance. In this case, if an inert gas such as nitrogen is introduced into the mixer to suppress the oxygen concentration to below 15% to prevent the volatile gases from igniting, safety is excellent, and this is preferable. Furthermore, if the temperature inside the mixer is below 400°C, it is difficult to form fine pores through oxidative combustion, making it easier to obtain particles with a small specific surface area, which is also preferable. Additionally, when using asphalt as a binder, the asphalt can absorb oxygen from the environment, preventing it from melting. This allows for good crystal development during graphitization, resulting in denser and more crystalline materials. However, since oxidation can easily cause unevenness, adjustments must be made considering factors such as specific surface area and hardness. Even better results can be achieved by performing degassing, infusibility treatment, and depressurization in an environment where the fluidity of the binder components in a softened liquid or gas phase is ensured.

[0092] [(d) Step of graphitizing secondary particles to obtain composite particles (specific composite particles) having a structure formed by stacking multiple flat graphite particles] In this step, the obtained secondary particles are graphitized. Here, the graphitizable components in the secondary particles are graphitized. Graphitization is preferably carried out in an environment where the mixture is difficult to oxidize, for example, heating in a nitrogen environment or argon. The temperature during graphitization is not particularly limited as long as it is a temperature at which the graphitizable components can be graphitized. For example, it can be above 2000°C, above 2500°C, above 2800°C, or above 3000°C. The upper limit of the temperature is not particularly limited as long as it prevents graphite from sublimating, for example, it can be below 3200°C. If the temperature is above 2000°C, crystallization will occur. If the temperature is above 2500°C, graphite crystallization develops well; if it is above 2800°C, it develops into high-capacity graphite crystals that can absorb more lithium ions. The amount of graphitized catalyst remaining after calcination is low, and there is a tendency to suppress the increase in ash content. In either case, there is a tendency for improved charge / discharge capacity and battery cycle characteristics. On the other hand, if the graphitization temperature is below 3200°C, some sublimation of graphite can be suppressed.

[0093] In this step, the degree of graphitization can be adjusted to 98.0% or less, preferably 97.0% or less, and even more preferably 96.0% or less.

[0094] The manufacturing method of the negative electrode material may also include a step of forming secondary particles into blocks or other shapes before graphitization, and a step of crushing the formed body after graphitization. By forming the secondary particles, the bulk density increases, thus increasing the loading capacity of the graphitization furnace and improving energy efficiency, allowing for energy-efficient graphitization. There are no particular limitations on the forming method; for example, the secondary particles can be placed into a container such as a mold and pressurized. It is also preferable that the manufacturing method of the negative electrode material does not include the step of forming secondary particles before graphitization (and therefore, does not include the step of crushing the formed body). In this way, it is believed that the increase of rhombohedral crystals during the forming and crushing process can be suppressed, thereby further improving the high-temperature resistance.

[0095] The bulk density of the secondary particles before graphitization is preferably 0.4 g / cm³ to 1.2 g / cm³, more preferably 0.6 g / cm³ to 1.1 g / cm³, and even more preferably 0.8 g / cm³ to 1.0 g / cm³. If the bulk density of the secondary particles before graphitization is 0.4 g / cm³ or higher, there is a tendency to obtain high-density composite particles even after graphitization due to fewer voids within the particles. If the bulk density of the secondary particles before graphitization is 1.0 g / cm³ or lower, then fragmentation after graphitization is not necessary, or fragmentation can be performed with weak force, and there is a tendency to suppress the increase of rhombohedral crystals.

[0096] In this disclosure, the bulk density of the particles can be determined by gravimetric analysis. That is, the bulk density of the particles can be determined by dividing the mass of the particles in air by their volume. Here, the mass of the particles is defined as the mass excluding the graphitized catalyst (for example, in the case of a mixture of secondary particles consisting of aggregates, binders, and graphitized catalysts, it is the total mass excluding the volatile components of the aggregates and binders).

[0097] [(e) Step of classifying the composite particles to remove at least one selected from the group consisting of microparticles and coarse particles] The obtained composite particles may also be classified to remove at least one selected from the group consisting of microparticles and coarse particles. This is believed to make the composite particles more uniform in size, thus better maintaining the electrolyte channels. The particle size distribution D90 / D10 of the composite particles can also be adjusted to the range, for example, 2.0 to 5.0, preferably 2.0 to 4.0, and more preferably 2.0 to 3.0, by classification.

[0098] There are no particular limitations on the grading method. Examples include: grading using sieves, grading using airflow centrifuges, and precision airflow classifiers utilizing the wall adhesion effect. It can also be adjusted by using a roller mill to apply concentrated compressive pressure to crush coarse particles.

[0099] When removing particulate matter in this step, it is preferable to remove particulate matter with a particle size of 1 μm or less, more preferably to remove particulate matter with a particle size of 2 μm or less, and even more preferably to remove particulate matter with a particle size of 3 μm or less. When removing coarse particles in this step, it is preferable to remove coarse particles with a particle size of 60 μm or more, more preferably to remove coarse particles with a particle size of 50 μm or more, and even more preferably to remove coarse particles with a particle size of 40 μm or more.

[0100] [Other Steps] The method for manufacturing the negative electrode material disclosed herein may include steps other than those described above. For example, the method for manufacturing the negative electrode material may include a step of attaching an organic compound to the surface of secondary particles after graphitization and performing heat treatment. By attaching the organic compound to the surface of the secondary particles and performing heat treatment, the organic compound attached to the surface is transformed into low-crystallinity carbon. This allows the low-crystallinity carbon to be disposed on at least a portion of the surface of the composite particles.

[0101] There are no particular limitations on the method of attaching organic compounds to the surface of secondary particles. Examples include: a wet method in which secondary particles are dispersed and mixed in a mixed solution in which an organic compound is dissolved or dispersed in a solvent, and then the solvent is removed and the particles are attached; and a dry method in which mechanical energy is applied to a mixture obtained by mixing secondary particles with a solid organic compound and the particles are attached.

[0102] There are no particular restrictions on organic compounds as long as they are transformed into low-crystallinity carbon (carbon precursors) through heat treatment. Examples include: petroleum-based asphalt, naphthalene, anthracene, cinnamon, coal tar, phenolic resins, polyvinyl alcohol, etc. Organic compounds may be used alone or in combination with two or more.

[0103] Regarding the heat treatment temperature for heat-treating secondary particles with organic compounds adhering to their surface, there are no particular limitations as long as the organic compounds adhering to the surface of the secondary particles change into low-crystallinity carbon; for example, 400°C to 1500°C is preferred. From the viewpoint of particularly improving high-temperature resistance, 1000°C to 1500°C is more preferred. The heat treatment is preferably carried out in an inert gas environment, such as a nitrogen environment.

[0104] "Negative Electrode for Lithium-ion Secondary Batteries" The negative electrode for lithium-ion secondary batteries disclosed herein comprises: a negative electrode material layer containing the negative electrode material for lithium-ion secondary batteries disclosed herein, and a current collector. In addition to the negative electrode material layer containing the negative electrode material for lithium-ion secondary batteries disclosed herein and the current collector, the negative electrode for lithium-ion secondary batteries may include other structural elements as needed.

[0105] For example, the negative electrode for lithium-ion secondary batteries can be prepared by mixing the negative electrode material for lithium-ion secondary batteries, binder, and solvent together to form a slurry-like composition of the negative electrode material for lithium-ion secondary batteries, and then coating it onto a current collector to form a negative electrode material layer. Alternatively, the negative electrode material composition for lithium-ion secondary batteries can be shaped into sheets, granules, or other shapes and integrated with the current collector. Mixing can be carried out using dispersion equipment such as a dispersion mixer or a planetary mixer.

[0106] There are no particular limitations on the binder used in the preparation of the negative electrode material composition for lithium-ion secondary batteries. Examples of binders include: styrene-butadiene copolymer (SBR), methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, butyl acrylate, butyl methacrylate, hydroxyethyl acrylate, hydroxyethyl methacrylate, etc., as well as homopolymers or copolymers of ethylene unsaturated carboxylic acids such as acrylic acid, methacrylic acid, itaconic acid, fumaric acid, maleic acid, polyvinylidene fluoride, polyethylene oxide, polychlorohydrin, polyphosphazene, polyacrylonitrile, polymethacrylonitrile, etc., which are polymers with high ionic conductivity. When the negative electrode material composition for lithium-ion secondary batteries contains a binder, there are no particular limitations on the content of the binder. For example, it can be 0.5 parts by mass to 20 parts by mass relative to 100 parts by mass of the total negative electrode material and binder for lithium-ion secondary batteries.

[0107] The composition of the negative electrode material for lithium-ion secondary batteries may include a thickener. As a thickener, options include: carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose, polyvinyl alcohol, polyacrylic acid or its salts, oxidized starch, phosphorylated starch, casein, etc. When the composition of the negative electrode material for lithium-ion secondary batteries includes a thickener, the content of the thickener is not particularly limited. For example, it may be 0.1 to 5 parts by weight relative to 100 parts by weight of the negative electrode material for lithium-ion secondary batteries.

[0108] The composition of the negative electrode material for lithium-ion secondary batteries may include conductive auxiliary materials. Examples of conductive auxiliary materials include: carbon materials such as carbon black, graphite, and acetylene black; oxides that exhibit conductivity; and inorganic compounds such as nitrides that exhibit conductivity. When the composition of the negative electrode material for lithium-ion secondary batteries includes conductive auxiliary materials, there is no particular limitation on the content of the conductive auxiliary materials. For example, it may be 0.5 to 15 parts by mass relative to 100 parts by mass of the negative electrode material for lithium-ion secondary batteries.

[0109] There are no particular restrictions on the material of the current collector; it can be selected from aluminum, copper, nickel, titanium, stainless steel, etc. There are no particular restrictions on the state of the current collector; it can be selected from foil, perforated foil, mesh, etc. In addition, porous materials such as porous metals (foamed metals) and carbon paper can also be used as current collectors.

[0110] When forming a negative electrode material layer by coating a negative electrode material composition for a lithium-ion secondary battery onto a current collector, the method is not particularly limited, and known methods such as metal mask printing, electrostatic coating, dip coating, spray coating, roller coating, doctor blade coating, corner wheel coating, gravure coating, and screen printing can be used. After coating the negative electrode material composition for a lithium-ion secondary battery onto the current collector, the solvent contained in the negative electrode material composition for the lithium-ion secondary battery is removed by drying. Drying can be performed using, for example, a hot air dryer, an infrared dryer, or a combination of these devices. The negative electrode material layer can also be calendered if necessary. Calendering can be performed using a flatbed press, calendering rollers, or similar methods.

[0111] When a negative electrode material composition for a lithium-ion secondary battery, formed into the shape of a sheet or granules, is integrated with a current collector to form a negative electrode material layer, the integration method is not particularly limited. For example, it can be performed by a roller, a flatbed press, or a combination of these methods. The pressure when integrating the negative electrode material composition for a lithium-ion secondary battery with the current collector is preferably, for example, about 1 MPa to 200 MPa.

[0112] There is no particular limitation on the negative electrode density of the negative electrode material layer. For example, it is preferably 1.1 g / cm3 to 1.8 g / cm3, more preferably 1.1 g / cm3 to 1.7 g / cm3, and even more preferably 1.1 g / cm3 to 1.6 g / cm3. By setting the negative electrode density to 1.1 g / cm3 or higher, there is a tendency to suppress the increase in resistance and increase the capacity. By setting it to 1.8 g / cm3 or lower, there is a tendency to suppress the decrease in input characteristics and cycle characteristics.

[0113] 《Lithium-ion Secondary Battery》 The lithium-ion secondary battery disclosed herein includes the negative electrode, positive electrode, and electrolyte for use in the lithium-ion secondary battery disclosed herein.

[0114] The positive electrode can be obtained by forming a positive electrode material layer on the current collector in the same way as the negative electrode. As the current collector, metals or alloys such as aluminum, titanium, and stainless steel can be used to form foils, perforated foils, meshes, etc.

[0115] There are no particular limitations on the cathode material used to form the cathode material layer. Examples of cathode materials include metal compounds (metal oxides, metal sulfides, etc.) and conductive polymer materials that can be doped or intercalated with lithium ions. More specifically, examples include: lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMnO2), their complex oxides (LiCoxNiyMnzO2, x+y+z=1), complex oxides containing the added element M' (LiCoaNibMncM'dO2, a+b+c+d=1, M': Al, Mg, Ti, Zr or Ge), spinel-type lithium manganese oxide (LiMn2O4), lithium vanadium compounds, V2O5, V6O13, VO2, MnO2, TiO2, MoV2O8, TiS2, V2S5, VS2, MoS2, MoS3, Cr3O8, Cr2O5, olivine-type LiMPO4 (M: Co, Ni, Mn, Fe) and other metal compounds, conductive polymers such as polyacetylene, polyaniline, polypyrrole, polythiophene, and polybenzoxene, and porous carbon, etc. The cathode material can be a single type or two or more types.

[0116] There are no particular limitations on the electrolyte; for example, an electrolyte formed by dissolving a lithium salt in a non-aqueous solvent (so-called an organic electrolyte) can be used. Examples of lithium salts include: LiClO4, LiPF6, LiAsF6, LiBF4, LiSO3CF3, etc. The lithium salt can be a single type or two or more types. Examples of non-aqueous solvents include: ethyl pentacarbonate, ethyl fluoropentadiene carbonate, ethyl chloropentadiene carbonate, propyl pentacarbonate, butyl pentacarbonate, vinyl pentacarbonate, cyclopentanone, cyclohexylbenzene, cyclobutane, propane sulpholactone, 3-methylcyclobutane, 2,4-dimethylcyclobutane, 3-methyl-1,3-oxazolidine-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-dioxacyclopentane, methyl acetate, ethyl acetate, trimethyl phosphate, triethyl phosphate, etc. Non-aqueous solvents can be a single type or two or more.

[0117] The state of the positive and negative electrodes in a lithium-ion secondary battery is not particularly limited. For example, the positive and negative electrodes, and the separator disposed between the positive and negative electrodes as needed, can be wound into a vortex shape, or they can be stacked in a flat plate shape.

[0118] There are no particular limitations on the separator; for example, resin-made non-woven fabric, cloth, microporous membrane, or combinations thereof can be used. Examples of resins include those with polyolefins such as polyethylene and polypropylene as their main components. Regarding the structure of lithium-ion secondary batteries, a separator may not be used if the positive and negative electrodes are not in direct contact.

[0119] There are no particular restrictions on the shape of lithium-ion secondary batteries. Examples include: laminated batteries, paper batteries, button batteries, coin-shaped batteries, multilayer batteries, cylindrical batteries, and square batteries.

[0120] The lithium-ion secondary battery disclosed herein is preferably used as a high-capacity lithium-ion secondary battery in electric vehicles, power tools, energy storage devices, etc. In particular, the lithium-ion secondary battery disclosed herein is preferably used as a lithium-ion secondary battery in commercial vehicles such as automobiles and delivery vehicles due to its excellent high-temperature resistance. [Example]

[0121] Hereinafter, the present disclosure will be specifically described by way of embodiments, but the present disclosure is not limited to these embodiments.

[0122] 《Production of Graphite Particles》 〔Example 1〕 As the raw material for graphite particles, petroleum-derived coke (needle coke) with the physical properties shown in Table 1 and needle-like stripe patterns observed throughout the particle cross-section was used. The coke was coarsely ground using a hammer mill. The coarsely ground material was sieved using a 3 mm mesh sieve, and the undersize material was sieved using a 1 mm mesh sieve to recover particles with a particle size of 1 mm to 3 mm.

[0123] The obtained coke particles were pulverized and classified using a roller mill (Kurimoto Iron Works Co., Ltd., K-VX mill) to obtain flat coke particles. The volumetric particle size distribution of the coke particles was measured, and the results showed that D10 was 5 μm, D50 was 14 μm, and D90 was 24 μm.

[0124] 90% by mass of the obtained coke particles and 10% by mass of coal tar pitch (softening point 100℃~150℃, quinoline insoluble component less than 15% by mass, fixed carbon 55%~75% by mass) were mixed at room temperature to obtain a mixture. Then, the mixture was heated and depressurized at 300℃~400℃ in a mixed airflow of dry air and nitrogen. At this time, if the stirring blades in the mixer are moved to facilitate gas discharge, the specific surface area will easily decrease. The endpoint of heating and stirring was determined by the phenomenon that the viscosity of the mixture decreased due to the volatilization of low molecular weight gases in the coal tar pitch, and the moment when the current value of the stirring blades decreased and stabilized was set as the point at which secondary particles with a bulk density of 0.7 g / cm³ were obtained.

[0125] Subsequently, the secondary particles were loaded into a graphitization box and graphitized at 3100°C. Afterwards, the obtained particles were sieved using a 300-mesh screen to obtain graphite particles. The volumetric particle size distribution of the graphite particles was measured, and the results showed that D10 was 11 μm, D50 was 18 μm, and D90 was 29 μm. The results of measuring the physical properties of the obtained graphite particles are shown in Table 1. Furthermore, electron microscope images of the appearance and cross-section of the obtained graphite particles are shown in Figures 2 and 3. As shown in Figures 2 and 3, the graphite particles are composite particles consisting of multiple flattened graphite particle layers.

[0126] [Example 2] The rotational speed of the roller mill (Kurimoto Iron Works Co., Ltd., K-VX mill) used in the classification of coke particles was changed, and flat coke particles were obtained in the same manner as in Example 1. The volumetric particle size distribution of the coke particles was measured, and the results showed that D10 was 6 μm, D50 was 15 μm, and D90 was 27 μm.

[0127] Using the obtained coke particles, graphite particles were prepared in the same manner as in Example 1. The volumetric particle size distribution of the graphite particles was measured, and the results showed that D10 was 8 μm, D50 was 18 μm, and D90 was 32 μm. The results of measuring the physical properties of the graphite particles are shown in Table 1.

[0128] [Example 3] The rotational speed of the roller mill (Kurimoto Iron Works Co., Ltd., K-VX mill) used in the classification of coke particles was changed, and flat coke particles were obtained in the same manner as in Example 1. The volumetric particle size distribution of the coke particles was measured, and the results showed that D10 was 4 μm, D50 was 9 μm, and D90 was 16 μm.

[0129] Using the obtained coke particles, graphite particles were prepared in the same manner as in Example 1. The volumetric particle size distribution of the graphite particles was measured, and the results showed that D10 was 8 μm, D50 was 12 μm, and D90 was 18 μm. Subsequently, the obtained graphite particles (96 parts by mass) were mixed with coal tar pitch (4 parts by mass) with 54% fixed carbon and heat-treated at 900°C to allow carbon to adhere to the surface of the graphite particles. The volumetric particle size distribution of the obtained graphite particles was measured, and the results showed that D10 was 11 μm, D50 was 16 μm, and D90 was 23 μm. Furthermore, the results obtained from the determination of physical properties are shown in Table 1.

[0130] [Comparative Example 1] As raw material for graphite particles, petroleum-derived coke (embedded coke) having the properties shown in Table 1 was used. The coke was coarsely pulverized using a hammer mill. The coarsely pulverized material was sieved using a 3 mm mesh sieve, and the undersize material was sieved using a 1 mm mesh sieve to recover particles with a particle size of 1 mm to 3 mm. Subsequently, the obtained particles were ground using a roller mill to obtain particles with an average particle size of 200 μm.

[0131] The obtained coke particles were pulverized using a counter jet mill and then classified using a high-speed cyclone classifier to obtain flat coke particles. The volumetric particle size distribution of the coke particles was measured, and the results showed that D10 was 4 μm, D50 was 15 μm, and D90 was 35 μm.

[0132] Using the obtained coke particles, graphite particles were prepared in the same manner as in Example 1. The volumetric particle size distribution of the graphite particles was measured, and the results showed that D10 was 11 μm, D50 was 23 μm, and D90 was 44 μm. The results of measuring the physical properties of the graphite particles are shown in Table 1.

[0133] [Comparative Example 2] Petroleum-derived coke (semi-needle coke) having the properties shown in Table 1 was used as the raw material for graphite particles. The coke was coarsely pulverized using a hammer mill. The coarsely pulverized material was sieved using a 3 mm mesh sieve, and the undersize material was sieved using a 1 mm mesh sieve to recover particles with a particle size of 1 mm to 3 mm. Subsequently, the obtained particles were ground using a roller mill to obtain particles with an average particle size of 200 μm.

[0134] The obtained coke particles were pulverized using a reverse jet mill and then classified using a high-speed cyclone classifier to obtain flat coke particles. The volumetric particle size distribution of the coke particles was measured, and the results showed that D10 was 6 μm, D50 was 15 μm, and D90 was 32 μm.

[0135] Using the obtained coke particles, graphite particles were prepared in the same manner as in Example 1. The volumetric particle size distribution of the graphite particles was measured, and the results showed that D10 was 13 μm, D50 was 20 μm, and D90 was 42 μm. The results of measuring the physical properties of the graphite particles are shown in Table 1.

[0136] [Comparative Example 3] As the raw material for graphite particles, petroleum-derived coke (needle coke) having the properties shown in Table 1 was used. The coke was coarsely pulverized using a hammer mill. The coarsely pulverized material was sieved using a 3 mm mesh sieve, and the undersize material was sieved using a 1 mm mesh sieve to recover particles with a particle size of 1 mm to 3 mm. Subsequently, the obtained particles were ground using a roller mill to obtain particles with an average particle size of 200 μm.

[0137] The obtained coke particles were pulverized using a reverse jet mill and then classified using a high-speed cyclone classifier to obtain flat coke particles. The volumetric particle size distribution of the coke particles was measured, and the results showed that D10 was 6 μm, D50 was 25 μm, and D90 was 55 μm.

[0138] Using the obtained coke particles, graphite particles were prepared in the same manner as in Example 1. The volumetric particle size distribution of the graphite particles was measured, and the results showed that D10 was 7 μm, D50 was 27 μm, and D90 was 58 μm. The results of measuring the physical properties of the graphite particles are shown in Table 1.

[0139] [Comparative Example 4] Spherical natural graphite with D10 of 7 μm, D50 of 16 μm, and D90 of 25 μm was used as graphite particles. The results of measuring the physical properties of the graphite particles using the method described above are shown in Table 1.

[0140] Preparation of the Negative Electrode: A slurry was prepared using graphite particles (97.6 parts by mass), carboxymethyl cellulose (CMC) (1.2 parts by mass), and styrene-butadiene rubber (SBR) (1.2 parts by mass) prepared in the mixing examples and comparative examples. This slurry was coated onto the glossy surface of an electrolytic copper foil at a coating weight of 10 g / cm², pre-dried at 90°C for 2 hours, and then adjusted to an electrode density of 1.65 g / cm³ using a roller press. Subsequently, it was dried at 120°C for 4 hours under vacuum to perform a hardening treatment, thereby obtaining a negative electrode for lithium-ion secondary batteries.

[0141] 《Fabrication of Lithium-ion Secondary Battery》Using the obtained electrode as the negative electrode, using metallic lithium as the reverse electrode, using a mixture of ethyl ester carbonate / ethyl methyl carbonate (3:7 volume ratio) and vinyl ester carbonate (VC) (1.0% by mass) containing 1 M LiPF6 as the electrolyte, using a 25 μm thick polyethylene microporous membrane as the separator, and using a 250 μm thick copper plate as the spacer, a coin cell is fabricated.

[0142] 《Characteristic Evaluation of Negative Electrode Materials and Batteries》 The characteristics of the negative electrode materials obtained in the Examples and Comparative Examples, as well as the negative electrodes and lithium-ion secondary batteries made therefrom, were determined by the following methods.

[0143] [Specific Surface Area] The negative electrode material was filled into the measuring unit. For the sample obtained by pre-treatment at 200°C while degassing under vacuum, nitrogen gas was adsorbed using a gas adsorption device (ASAP2010, manufactured by Shimadzu Corporation). The specific surface area of ​​the obtained sample was determined by BET analysis using the five-point method.

[0144] [Compression Pressure] 3.0 g of negative electrode material is filled into a mold with a diameter of 15 mm, and compressed at a constant speed of 10 mm / min using an automatic compressor (manufactured by Shimadzu Corporation). During compression, the distance from the bottom surface of the negative electrode material to the pressing surface is measured, and the density under pressure is calculated by multiplying this distance by the bottom area of ​​the mold. A force gauge is installed on the press hammer of the automatic compressor, and the pressure (kN / cm2) at which the specified density of 1.8 g / cm3 is reached is set as the pressing pressure.

[0145] 〔Elastic Energy / Plastic Deformation Energy〕 The elastic energy E1, plastic deformation energy E2, and E1 / E2 are calculated by the method described above under the same conditions as the measurement of the compression pressure. The pressure during pressurization is recorded whenever the pressing surface moves a distance of 1.67 μm.

[0146] 〔Rebound Coefficient〕The rebound coefficient (%) is calculated by dividing the absolute value of the difference between the reference density of 1.8 g / cm3 when pressed by the method described above and the density of the rebounded negative electrode material by the density of 1.8 g / cm3.

[0147] 〔Peak intensity ratio of rhombohedral structure〕 The peak intensity ratio (P1 / P2) of the rhombohedral crystal structure is calculated from the intensity ratio of the rhombohedral crystal structure's diffraction rays (P1: diffraction angle 43.2 degrees) to the hexagonal crystal structure's diffraction rays (P2: diffraction angle 44.3 degrees) in the X-ray diffraction pattern using CuKα rays.

[0148] 〔Graphitization degree〕The graphitization degree was determined using the method described above using an X-ray diffraction measuring apparatus (X-ray diffractometer MultiFlex, Rigaku Corporation).

[0149] [Average Surface Spacing (d002)] The average surface spacing (d002) is calculated using Bragg's equation based on the diffraction peaks corresponding to the carbon 002 surface that appear near the diffraction angle 2θ of 24° to 27° in the diffraction distribution obtained by irradiating the sample with X-rays (CuKα rays) and measuring the diffraction using a goniometer. The average surface spacing (d002) is measured under the following conditions: X-ray source: CuKα rays (wavelength = 0.15418 nm) Output: 40 kV, 20 mA Sampling amplitude: 0.010° ​​Scan range: 10° to 35° Scan speed: 0.5° / min

[0150] Bragg's equation: 2dsinθ=nλ Here, d represents the length of one period, θ represents the diffraction angle, n represents the number of reflections, and λ represents the X-ray wavelength.

[0151] 〔Injection Time〕 A negative electrode, pressed to an electrode density of 1.65 g / cm³, was punched into a 16ϕ circle and attached to a glass substrate using double-sided tape to create a strain-free, planar electrode surface. Using a micropipette, 3 μL of PC (polycarbonate: manufactured by Kishida Chemical Co., Ltd.) was poured into the center of the circular electrode, and the injection time until permeation was measured. Three identical electrodes were prepared, and the average value of the three measurements was calculated.

[0152] 〔Discharge Capacity〕 The prepared lithium-ion secondary battery was placed in a thermostatic bath set to 25°C and charged at a constant current of 0.2 C until the voltage reached 0 V (V vs. Li / Li+). Then, it was charged at a constant voltage of 0 V until the current reached 0.02 C. After stopping for 30 minutes, it was discharged at a constant current of 0.2 C until the voltage reached 1.5 V (V vs. Li / Li+). The discharge capacity at this point is shown in Table 1.

[0153] 〔3C Charging, Li Deposition Start〕 The prepared lithium-ion secondary battery was placed in a thermostatic bath set to 25°C. From the first to the third cycle, it was charged at a constant current of 0.1C until the voltage reached 0.005 V (V vs. Li / Li+), and then charged at a constant voltage of 0.005 V until the current reached 0.05C. After a 30-minute pause, it was discharged at a constant current of 0.2C until the voltage reached 1.5 V (V vs. Li / Li+). The discharge capacity of the third cycle was set as 1C in the Li deposition test. The fourth cycle was charged at a current density of 3C, using the discharge capacity of the third cycle, for 20 minutes, and a control voltage of 1.5V. The first inflection point in the differential distribution (dV / dQ, where V is voltage and Q is capacitance, see Figure 4) of the obtained fourth-cycle charging curve is taken as the starting point of Li deposition. The capacity at this point is expressed as a percentage of the discharge capacity of the third cycle (or the charging capacity of the fourth cycle set to the same capacity), and the tolerance to Li deposition is evaluated. The results are shown in Table 1 and Figure 4.

[0154] 〔High-Temperature Storage Retention Rate and High-Temperature Storage Recovery Rate〕 As step 1, the prepared lithium-ion secondary battery is placed in a constant temperature bath set to 25°C and charged with a constant current of 0.2 C until the voltage reaches 0 V (V vs. Li / Li+). Then, it is charged with a constant voltage of 0 V until the current reaches 0.02 C. Then, after stopping for 30 minutes, it is discharged with a constant current of 0.2 C until the voltage reaches 1.5 V (V vs. Li / Li+). This charge-discharge cycle is repeated twice. Then, it is charged with a constant current of 0.2 C until the voltage reaches 0 V (V vs. Li / Li+). Then, it is charged with a constant voltage of 0 V until the current reaches 0.02 C. The battery is then placed in a constant temperature bath set to 60°C and stored for 7 days. Next, as step 2, the lithium-ion secondary battery was placed in a constant temperature bath set at 25°C for 60 minutes, and discharged at a constant current of 0.2 C until the voltage reached 1.5 V (V vs. Li / Li+). Then, the charge-discharge cycle was repeated once under the same conditions. This 60°C storage (step 1) and capacity confirmation after storage (step 2) were repeated a total of three times. The high-temperature storage retention rate and high-temperature storage recovery rate were calculated using the following formulas: High-temperature storage retention rate (%) = (First discharge capacity at 25°C after 21 days of storage at 60°C) / (Second discharge capacity at 25°C before storage at 60°C) × 100 High-temperature storage recovery rate (%) = (Second discharge capacity at 25°C after 21 days of storage at 60°C) / (Second discharge capacity at 25°C before storage at 60°C) × 100

[0155] [Table 1] Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Comparative Example 3 Comparative Example 4 Graphite particles Specific surface area (m²) 2 / g) 1.4 1.0 2.3 4.0 3.7 1.3 3.4 Compression pressure (kN / cm) 2 ) 3.14 3.20 3.98 2.02 1.78 2.48 3.55 Elastic energy E1 (N / m) 8.04 9.20 10.61 4.90 3.07 4.44 9.43 Plastic deformation energy E2 (N / m) 0.87 1.36 2.36 2.66 2.22 0.48 2.61 E1 / E2 9.24 6.76 4.50 1.84 1.38 9.25 3.61 Rebound coefficient (%) 33.7 32.7 34.1 18.3 15.1 27.3 24.7 Peak strength ratio of rhombohedral structure Unobservable 0.22 Degree of graphitization (%) 95.5 96.7 95.5 98.5 98.5 96.7 99.8 d 002 (nm) 3.358 3.357 3.358 3.355 3.355 3.357 3.354 coke CTE (×10) -6 / ℃) 2.6 2.6 2.6 6.5 4.1 3.0 - HGI (-) 43 43 43 20 33 50 - True specific gravity of butanol (-) 2.14 2.14 2.14 1.98 1.99 2.11 - Pore ​​volume (mL / g) 0.76 0.76 0.76 1.70 1.36 0.98 - Specific surface area of ​​fine pores (m²) 2 / g) 1.1 1.1 1.1 5.2 4.2 3.9 - Total sulfur content (mass%) 0.2 0.2 0.2 0.2 0.3 0.3 - negative electrode Alignment 450 620 310 300 400 1,030 600 Injection time 50 70 55 530 430 130 170 Battery characteristics Discharge capacity (Ah / kg) 349 351 350 352 354 347 358 3C charging, Li deposition begins (%) twenty four twenty two 32 10 8 15 30 High-temperature preservation retention rate (%) 88.0 88.5 85.9 77.3 80.2 86.2 81.0 High-temperature preservation recovery rate (%) 92.9 93.1 91.6 87.8 89.2 91.4 88.9

[0156] As shown in Table 1, the lithium-ion secondary batteries made by using the graphite particles of Examples 1 to 3 as negative electrode materials have excellent high temperature resistance and fast charging performance. The disclosure of Japanese Patent Application No. 2020-208625 is incorporated herein by reference in its entirety. All documents, patent applications and technical specifications described herein are incorporated herein by reference to the same extent as those documents, patent applications and technical specifications specifically described herein and incorporated herein by reference. [Simplified Explanation of the Diagram]

[0009] Figure 1 is a graph showing elastic and plastic deformation energy. Figure 2 shows an electron microscope image of the negative electrode material of Example 1. Figure 3 shows an electron microscope image of a cross-section of the negative electrode material of Example 1. Figure 4 is an example of a graph used for evaluating Li precipitation tolerance.

Claims

1. A method for manufacturing a negative electrode material for a lithium-ion secondary battery, comprising the step of graphitizing coke that satisfies the following (1) and (2) at a temperature above 2000°C: (1) the coefficient of thermal expansion after calcination at 1400°C is less than 2.9 × 10-6 / °C; and (2) the Hardy hardness (HGI) after calcination at 1200°C is less than 47.

2. The method for manufacturing a negative electrode material for a lithium-ion secondary battery as described in claim 1, wherein the true specific gravity of the butanol after calcination of the coke at 1200°C is 2.05 or higher.

3. The method for manufacturing a negative electrode material for a lithium-ion secondary battery as described in claim 1 or claim 2, wherein the pore volume of the coke is less than 0.90 mL / g.

4. A method for manufacturing a negative electrode material for a lithium-ion secondary battery as described in claim 1 or claim 2, wherein the coke has a fine pore surface area of ​​3.0 m² / g or less.

5. A method for manufacturing a negative electrode material for a lithium-ion secondary battery as described in claim 1 or claim 2, wherein the coke comprises at least one selected from needle coke and semi-needle coke.

6. A method for manufacturing a negative electrode material for a lithium-ion secondary battery as claimed in claim 1 or claim 2, wherein the graphite particles obtained by graphitizing the coke satisfy the following (1) to (3): (1) a specific surface area of ​​0.5 m² / g or more and 2.7 m² / g or less, wherein the specific surface area is determined by nitrogen adsorption at 77 K; (2) a compressive pressure of 2.8 kN / cm² or more, wherein the compressive pressure is the pressure required to compress the graphite particles to a density of 1.8 g / cm³; and (3) a value of 4 or more in terms of elastic energy / plastic deformation energy, wherein the elastic energy / plastic deformation energy is determined by the differential quadrature method.

7. A method for manufacturing a negative electrode material for a lithium-ion secondary battery as described in claim 1 or claim 2, wherein the graphite particles obtained by graphitizing the coke have a resilience coefficient of 25% or more.

8. A method for manufacturing a negative electrode material for a lithium-ion secondary battery as claimed in claim 1 or claim 2, wherein the graphite particles obtained by graphitizing the coke include composite particles having a structure formed by stacking multiple flat graphite particles.

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