Negative electrode material for lithium ion secondary battery, evaluation method thereof, and manufacturing method thereof, negative electrode for lithium ion secondary battery, and lithium ion secondary battery

Abstract

A negative electrode material for lithium-ion secondary batteries is graphite particles that satisfy the following formula (1): [Resilience coefficient × 7.6] + [Compressive load (kN / cm 2)] ≧ 4.2··· (1)

Description

[Technical Field]

[0001] This disclosure relates to a negative electrode material for lithium-ion secondary batteries, its evaluation method and 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, Patent Document 1 discloses, for example, a negative electrode material for lithium-ion secondary batteries, which comprises composite particles containing multiple flat graphite particles aggregated or bonded in a manner where the alignment planes are not parallel, and spherical graphite particles. Furthermore, Patent Document 2 discloses a negative electrode active material for lithium-ion secondary batteries containing carbon powder particles, wherein the carbon powder particles have a morphology of plate-shaped particles assembled into a stable primary structure along the alignment planes, and micropores are formed on the surface. [Prior Art Documents] [Patent Documents]

[0005] [Patent Document 1] International Publication No. 2015 / 147012 [Patent Document 2] Japanese Patent Application Publication No. 2005-302725 [Summary of the Invention]

[0006] [The problem the invention aims to solve]

[0007] In lithium-ion secondary batteries, during repeated charge-discharge processes, the gaps between active material particles disappear, resulting in areas where the electrolyte and active material cannot contact each other. As a result, the decline in cycle characteristics, such as cycle capacity retention, caused by increased battery resistance, becomes a problem.

[0008] In view of the above, the present invention discloses an anode material for lithium-ion secondary batteries capable of producing lithium-ion secondary batteries with excellent cycle characteristics, an evaluation method thereof, a manufacturing method thereof, an anode material for lithium-ion secondary batteries made using the anode material, and a lithium-ion secondary battery. [Means for Solving the Problem]

[0009] Means for solving the aforementioned problem include the following forms. <1> A negative electrode material for a lithium-ion secondary battery, which is graphite particles satisfying the following formula (1). [Resilience coefficient × 7.6] + [Compression load (kN / cm2)] ≥ 4.2··· (1) <2> The negative electrode material for a lithium-ion secondary battery as described in <1>, wherein the compression load of the graphite particles is 2.0 kN / cm2 or more. <3> The negative electrode material for a lithium-ion secondary battery as described in <1> or <2>, wherein the resilience coefficient of the graphite particles is 0.25 or more. <4> The negative electrode material for a lithium-ion secondary battery as described in any one of <1> to <3>, wherein the graphite particles comprise composite particles having a structure formed by stacking multiple flat graphite particles. <5> An evaluation method for a negative electrode material for lithium-ion secondary batteries, using a regression formula obtained by multiple regression analysis to evaluate the negative electrode material for lithium-ion secondary batteries. In the multiple regression analysis, the rebound coefficient of graphite particles and the compressive load (kN / cm2) of the graphite particles, which are used as the negative electrode material for lithium-ion secondary batteries, are set as explanatory variables, and the liquid absorption time of the negative electrode containing the negative electrode material for lithium-ion secondary batteries is set as the target variable. <6> A method for manufacturing a negative electrode material for lithium-ion secondary batteries, manufacturing the negative electrode material for lithium-ion secondary batteries based on the evaluation results obtained using the evaluation method for lithium-ion secondary battery negative electrode materials as described in <5>. <7> A method for manufacturing a negative electrode material for lithium-ion secondary batteries, manufacturing the negative electrode material for lithium-ion secondary batteries as described in any one of <1> to <4>, wherein the method for manufacturing the negative electrode material for lithium-ion secondary batteries includes a step of graphitizing coke. <8> A method for manufacturing a negative electrode material for a lithium-ion secondary battery as described in <7>, wherein the coefficient of thermal expansion of the coke after calcination at 1400°C is 2.30 × 10⁻⁶ / °C or higher. <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 <4>, 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]

[0010] According to this disclosure, a negative electrode material for lithium-ion secondary batteries capable of producing lithium-ion secondary batteries with excellent cycle characteristics, an evaluation method thereof and a manufacturing method thereof, as well as a negative electrode for lithium-ion secondary batteries and a lithium-ion secondary battery made using the negative electrode material, can be provided.

Implementation Method

[0012] 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.

[0013] 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 a value relating to 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 portion 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.

[0014] 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.

[0015] 《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 formula (1). [Resilience coefficient × 7.6] + [Compression load (kN / cm2)] ≧ 4.2··· (1)

[0016] The inventors have focused on the fact that during repeated charge-discharge cycles of lithium-ion secondary batteries, the gaps between graphite particles disappear, creating areas where the electrolyte and the negative electrode active material cannot contact each other, leading to a decrease in the liquid absorption of the negative electrode and a decrease in cycle capacity retention. They believe that if the electrolyte can also easily flow within the negative electrode during repeated charge-discharge cycles of a lithium-ion secondary battery, the liquid absorption of the negative electrode can be improved, thereby suppressing the decrease in cycle capacity retention and resulting in better cycle characteristics.

[0017] In view of the above, the inventors have studied the physical properties of the negative electrode material and the negative electrode itself, which are conducive to the liquid absorption of the negative electrode. As a result, they found that the combination of the resilience coefficient and compressive load of the graphite particles used as the negative electrode material is strongly correlated with the liquid absorption.

[0018] Moreover, the inventors have found that by adjusting the combination of the resilience coefficient and the compressive load of the graphite particles to an appropriate range, i.e., the graphite particles satisfy the above formula (1), the liquid absorption of the negative electrode can be improved, and the cycle characteristics of the lithium-ion secondary battery can be maintained well.

[0019] The reason why the combination of the resilience coefficient and compressive load of graphite particles has a strong correlation with the liquid absorption is not necessarily clear, but it can be considered as follows.

[0020] The high compressive load of graphite particles means that graphite particles are not easily deformed by compression. Therefore, under the condition of high compressive load of graphite particles, the gaps between graphite particles are not easily eliminated, and the flow path of electrolyte can be better ensured in the negative electrode containing the graphite particles, thereby suppressing the generation of parts where the electrolyte and the negative electrode active material cannot contact each other.

[0021] The high resilience of graphite particles means that they are not easily broken or damaged by compression. Therefore, with a high resilience, a portion of the graphite particles is less likely to break, and the gaps between them are less likely to disappear. As a result, the flow path of the electrolyte can be better ensured in the negative electrode containing these graphite particles, thereby suppressing the formation of portions where the electrolyte and the negative electrode active material cannot contact each other.

[0022] Based on the above, it is believed that the combination of the rebound coefficient and the compressive load of graphite particles has a strong correlation with the liquid absorption. By increasing the rebound coefficient and the compressive load of graphite particles, the liquid absorption of the negative electrode can be improved.

[0023] (Compression Load) The compression load of the graphite particles is not particularly limited as long as the graphite particles can satisfy the value of Equation (1). For example, from the viewpoint of suppressing the deformation of graphite particles caused by pressing during the production of the negative electrode, and from the viewpoint of further improving the cycle characteristics of the lithium-ion secondary battery, the compression load of the graphite particles is preferably 2.0 kN / cm2 or more, more preferably 2.4 kN / cm2 or more, even more preferably 2.8 kN / cm2 or more, and particularly preferably 3.0 kN / cm2 or more.

[0024] From the viewpoint of suppressing the deformation of the current collector caused by the pressing during the production of the negative electrode and the peeling of the current collector from the active material, the compressive load of the graphite particles can be 4.5 kN / cm2 or less, or 4.3 kN / cm2 or less, or 4.0 kN / cm2 or less.

[0025] In this disclosure, the compressive load of graphite particles is the pressure required to compress them to a density of 1.7 g / cm³. A higher compressive load 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 1.7 g / cm³ is defined as the compressive load 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.

[0026] (Rebound Coefficient) The rebound coefficient of the graphite particles is not particularly limited as long as the graphite particles can satisfy the value of Equation (1). For example, from the viewpoint of further suppressing the damage of graphite particles caused by pressing during the production of the negative electrode, the rebound coefficient of the graphite particles is preferably 0.25 or more, more preferably 0.27 or more, and even more preferably 0.30 or more.

[0027] From the viewpoint that high density can be easily caused by pressing during the production of negative electrode, the springback coefficient of graphite particles can be 0.50 or less, or 0.45 or less, or 0.40 or less.

[0028] The resilience 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 resilience 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., 1.7 g / cm³). Afterward, the pressure is released, and the density (g / cm³) after pressure release is measured when the pressed surface stops moving due to elasticity. Based on the obtained value, the resilience coefficient is calculated using the following formula: Resilience coefficient = {(Reference density - Density after pressure release) / Reference density} In the aforementioned measurement, a mold, for example, with a diameter of 15 mm, is used, and compression is performed using an automatic measuring 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.

[0029] The compressive load and resilience coefficient of graphite particles can be adjusted by changing the physical properties and composition of the raw material of the graphite particles (e.g., needle coke) or by changing the graphitization conditions.

[0030] In this disclosure, from the viewpoint of superior cycle characteristics of lithium-ion secondary batteries, graphite particles can satisfy the following equation (2). [Resilience coefficient × 7.6] + [Compression load (kN / cm2)] ≥ 4.5··· (2)

[0031] 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.

[0032] 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

[0033] 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.

[0034] (Specific Surface Area) The specific surface area of ​​graphite particles is not particularly limited. For example, from the viewpoint of further suppressing electrolyte decomposition, the specific surface area of ​​graphite particles is preferably 2.7 m² / g or less, more preferably 2.5 m² / g or less, and even more preferably 2.2 m² / g or less. The specific surface area of ​​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.

[0035] 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.

[0036] 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.

[0037] (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.

[0038] The flat graphite particles included 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 in a state where they face 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.

[0039] 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 shape as an aggregate 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.

[0040] 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.

[0041] 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.

[0042] 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 uniform particle size of the flat graphite particles can further reduce the contact area with the electrolyte within the obtained specific composite particles, thereby 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.

[0043] 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.

[0044] 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 layers increases, the gaps within the particles increase, and the specific surface area increases, resulting in a tendency to suppress the decline in retention characteristics.

[0045] 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 have the largest distance between them, 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 have the smallest distance between them, and the distance between these two tangents is set as the length B in the minor axis direction.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] 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.

[0050] 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 per particle: twice • Number of particles measured: 30 particles

[0051] (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.

[0052] 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 to 50% from the smallest diameter side in the particle size distribution on a volume basis.

[0053] 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, performing mirror polishing, 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.

[0054] (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.

[0055] The particle size distribution D90 / D10 can be measured using a laser diffraction particle size distribution measuring device (e.g., SALD3100, Shimadzu Corporation).

[0056] 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 particle size distribution curve. The particle size at the point that intersects the horizontal axis of 10% can be set to the 10% diameter (D10), and the particle size at the point that intersects the horizontal axis of 90% can be set to the 90% diameter (D90) to calculate D90 / D10.

[0057] (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 size and the vertical axis set to the particle amount (%).

[0058] (Oil absorption) The oil absorption of graphite particles can be 10 mL / 100 g to 60 mL / 100 g, or 15 mL / 100 g to 45 mL / 100 g, or 20 mL / 100 g to 40 mL / 100 g.

[0059] 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.

[0060] 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, the absorption measuring device (trade name: S-500) of Asahi Research Institute Co., Ltd. can be used.

[0061] (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 following methods. 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.

[0062] 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°.

[0063] (Degree of graphitization) The degree of graphitization of graphite particles, determined by X-ray diffraction, can be 90.0%–100.0%, 90.5%–99.0%, or 91.0%–98.0%. For example, if the degree of graphitization of larger graphite particles with a particle size 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 size 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 90.0%, there is a tendency for excellent discharge capacity.

[0064] The degree of graphitization of graphite particles can be determined, for example, as follows. Using an agate mortar, 60 parts by mass of graphite particles are mixed with 40 parts by mass of silicon powder (e.g., Fujifilm and Kojun Pharmaceutical Co., Ltd., purity 99.9%) for 5 minutes, and the resulting mixture is placed in an X-ray diffraction measurement cell. Using an X-ray diffraction measurement apparatus (e.g., Rigaku Corporation's X-ray diffractometer MultiFlex), X-ray diffraction measurements using CuKα rays (2θ = 25° to 29°) are performed to measure the diffraction angles corresponding to the (002) plane of graphite and the (111) plane of silicon. Using the theoretical diffraction angle of Si (2θ = 28.442°), the observed diffraction angles of silicon and graphite are corrected, 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

[0065] Evaluation Method for Negative Electrode Materials for Lithium-ion Secondary Batteries The evaluation method for negative electrode materials for lithium-ion secondary batteries disclosed herein uses a regression formula obtained by multiple regression analysis to evaluate the negative electrode materials for lithium-ion secondary batteries. In the multiple regression analysis, the rebound coefficient of graphite particles and the compressive load (kN / cm2) of the graphite particles, which are used as negative electrode materials for lithium-ion secondary batteries, are set as explanatory variables, and the liquid absorption time of the negative electrode containing the negative electrode material for lithium-ion secondary batteries is set as the target variable.

[0066] The inventors have discovered that the combination of the resilience coefficient and compressive load of graphite particles, which are used as negative electrode materials, is strongly correlated with liquid absorption. Therefore, using a regression formula obtained through multiple regression analysis as described above, the liquid absorption time of the negative electrode can be predicted based on the resilience coefficient and compressive load of the graphite particles. Since the cycle characteristics of a lithium-ion secondary battery containing a negative electrode can be evaluated based on the liquid absorption time of the negative electrode, the performance of negative electrode materials for lithium-ion secondary batteries can be easily evaluated using the evaluation method disclosed herein.

[0067] For example, by using the following formula (2) as a regression formula, the liquid absorption time of the negative electrode using graphite particles with a specific rebound coefficient and compressive load can be deduced. (Liquid absorption time) = ab × (rebound coefficient) - c × (compression load) ... (2) In formula (2), a, b, and c represent constants respectively.

[0068] The number of samples for the rebound coefficient, compression load, and liquid absorption time of graphite particles in the multivariate regression analysis is not particularly limited as long as it is the usual number of samples for the multivariate regression analysis. For example, it can be more than 20 or more than 30. In the above formula (2), the liquid absorption time can be the converted value when the liquid absorption time of a specific sample is set to 100.

[0069] Manufacturing Method 1 for Anode Material of Lithium-ion Secondary Batteries The manufacturing method 1 for anode material of lithium-ion secondary batteries disclosed herein is based on the evaluation results of the anode material of lithium-ion secondary batteries obtained using the evaluation method for anode material of lithium-ion secondary batteries disclosed herein, and manufactures the anode material of lithium-ion secondary batteries. The manufacturing method 1 disclosed herein may include the step of graphitizing coke.

[0070] For example, in manufacturing method 1 of this disclosure, graphite particles having a rebound coefficient and compressive load generally below a specific value for the calculated liquid absorption time can be manufactured based on the relationship of the above equation (2), i.e., a negative electrode material for lithium-ion secondary batteries. The preferred conditions for manufacturing a negative electrode material for lithium-ion secondary batteries using manufacturing method 1 of this disclosure are the same as those described in manufacturing method 2 of this disclosure below.

[0071] "Method for Manufacturing Negative Electrode Material for Lithium-ion Secondary Batteries 2" The method for manufacturing negative electrode material for lithium-ion secondary batteries disclosed herein is a method for manufacturing negative electrode material for lithium-ion secondary batteries, including the step of graphitizing coke.

[0072] In the manufacturing method 2 disclosed herein, coke is graphitized to manufacture a negative electrode material for lithium-ion secondary batteries as artificial graphite. Among the graphite particles as artificial graphite, the springback coefficient tends to increase compared with natural graphite, and there is a tendency to easily satisfy the above formula (1).

[0073] The type of coke used in manufacturing methods 1 and 2 (hereinafter also referred to as "manufacturing methods") is not particularly limited, and 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 small coefficient of thermal expansion, are preferred. Furthermore, semi-needle coke and needle coke have high crystallinity, so it is easy to obtain flat particles. In addition, needle coke has high crystallinity, so the particles are large, and the particle size can be easily adjusted by 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 the above conditions are met, needle coke and semi-needle coke may be used in combination.

[0074] There are no particular limitations on the method for obtaining coke particles (coke particles), and known methods can be used. There are no particular limitations 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.

[0075] From the viewpoint of increasing the compressive load of graphite particles, the coefficient of thermal expansion of coke after calcination at 1400°C is preferably 2.30 × 10⁻⁶ / °C or higher. More preferably, the coefficient of thermal expansion of coke after calcination at 1400°C is 2.50 × 10⁻⁶ / °C to 4.00 × 10⁻⁶ / °C, and even more preferably 2.80 × 10⁻⁶ / °C to 3.70 × 10⁻⁶ / °C.

[0076] 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.

[0077] 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). Regarding graphite particles in the form of specific composite particles, for example, a mixture obtained by mixing flat coke particles with a binder can be processed to produce secondary particles having a structure formed by stacking flat coke particles, and then the obtained secondary particles can be graphitized.

[0078] 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.

[0079] 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.

[0080] 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 the deviation of the particle size of the specific composite particles and maintain the channel of the electrolyte better.

[0081] Therefore, in a preferred embodiment, the method for manufacturing the negative electrode material sequentially includes: (a) a step of classifying 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 flattened 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 flattened coke particles; (d) a step of graphitizing the secondary particles to obtain composite particles (specific composite particles) having a structure formed by stacking multiple flattened 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.

[0082] Furthermore, in this disclosure, "microparticles" refers to particles with a diameter smaller than those recovered by grading, and "coarse particles" refers to particles with a diameter larger than those recovered by grading.

[0083] Hereinafter, the steps that may be included in the method for manufacturing the negative electrode material disclosed herein will be described in detail.

[0084] [(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.

[0085] 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.

[0086] 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.

[0087] 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.

[0088] [(b) Step of mixing flat coke particles with a binder to obtain a mixture] As the binder, a graphitizable binder is used. Examples include: coal-based, petroleum-based, synthetic asphalt and tar, thermoplastic resins, thermosetting resins, starch, 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 with graphitizing catalyst properties, as well as carbides, oxides, nitrides, mica clay minerals, etc.

[0089] 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.

[0090] 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 imparts as little shear force as possible is preferred, such as using a kiln mixer or a paddle mixer. It is also preferred not to use a kneading machine or similar equipment that is accompanied by kneading, which is called a mixing machine.

[0091] [(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.

[0092] 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.

[0093] Hereinafter, specific examples of the method for producing secondary particles will be described. Above the softening point of the binder, preferably in the temperature range where volatile components volatilize, the mixture of coke particles and binder is stirred for a period of time while being stirred until the volatile components are removed. 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 therefore 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.

[0094] [(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 atmosphere. 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 2000°C or higher, 2500°C or higher, 2800°C or higher, or 3000°C or higher. The upper limit of the temperature is not particularly limited as long as it prevents graphite from sublimating, for example, it can be 3200°C or lower. If the temperature is 2000°C or higher, 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.

[0095] The method for manufacturing the negative electrode material may also include a step of forming secondary particles into blocks or the like before graphitization, and a step of disintegrating the formed body after graphitization. By forming secondary particles, the bulk density increases, thus increasing the loading amount in the graphitization furnace and improving energy efficiency, allowing for energy-efficient graphitization. Furthermore, when graphitizing a formed body with secondary particles in blocks or the like, compared to graphitizing powdered secondary particles by loading them into a box or the like, there is a tendency to easily increase the compressive load of the graphite particles. In particular, from the viewpoint of further increasing the compressive load of graphite, it is preferable to use coke with a coefficient of thermal expansion of 2.80 × 10⁻⁶ / ℃ or higher after calcination at 1400°C as raw material to graphitize the formed body with secondary particles in blocks or the like. Furthermore, when graphitizing a preformed object in the shape of blocks or similar shapes from secondary particles, compared to graphitizing powdered secondary particles by filling them into a box or similar container, it is easier to obtain graphite with high crystallinity, resulting in superior discharge capacity of the lithium-ion secondary battery. The forming method is not particularly limited; for example, the secondary particles can be placed in a container such as a mold and pressurized. The manufacturing method of the negative electrode material is also preferably one that does not include the step of forming the secondary particles before graphitization (and therefore, does not include the step of crushing the preformed object). This is believed to suppress the increase of rhombohedral crystals during the forming and crushing process, thereby further improving high-temperature resistance.

[0096] 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.

[0097] 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).

[0098] [(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 have a uniform particle 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.

[0099] 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. Alternatively, the method can be adjusted by using a roller mill to apply concentrated compressive pressure to pulverize coarse particles.

[0100] 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.

[0101] [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 then 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.

[0102] 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.

[0103] 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.

[0104] 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.

[0105] "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.

[0106] 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.

[0107] 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.

[0108] 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.

[0109] 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 exhibiting conductivity; and inorganic compounds such as nitrides exhibiting 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.

[0110] 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.

[0111] 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 a 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, a calendering roller, or similar methods.

[0112] 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.

[0113] 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.

[0114] Lithium-ion secondary battery The lithium-ion secondary battery disclosed herein includes a negative electrode, a positive electrode, and an electrolyte for use in lithium-ion secondary batteries as disclosed herein.

[0115] 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.

[0116] 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.

[0117] There are no particular limitations on the electrolyte. For example, an electrolyte formed by dissolving a lithium salt as an electrolyte 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.

[0118] 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 made into a flat plate and stacked.

[0119] There are no particular limitations on the separator; for example, resin-based 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 contact.

[0120] 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 prismatic batteries.

[0121] In particular, previous cylindrical batteries used in electric vehicles (EVs) and the like have a high-density negative electrode material layer filled into a cylindrical casing, thus greatly suppressing the decline in cycle capacity retention caused by the interruption of conduction between graphite particles due to repeated charging and discharging. However, because the negative electrode material layer is high-density, the gaps between graphite particles disappear during repeated charging and discharging, resulting in areas where the electrolyte and the negative electrode active material cannot contact each other. As a result, the decline in cycle capacity retention caused by the increase in battery resistance becomes a problem.

[0122] On the other hand, in the lithium-ion secondary battery disclosed herein, a negative electrode material capable of producing a negative electrode with excellent liquid absorption is used. Therefore, in the lithium-ion secondary battery disclosed herein, even when the negative electrode material layer is made denser, as in a cylindrical battery, the decrease in cycle capacity retention caused by the interruption of conduction between graphite particles and the decrease in cycle capacity retention caused by the generation of portions where the electrolyte and the negative electrode active material cannot contact each other are suppressed, thus exhibiting excellent cycle characteristics.

[0123] 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. [Example]

[0124] Hereinafter, the present disclosure will be specifically described by way of embodiments, but the present disclosure is not limited to these embodiments.

[0125] Determination of the Correlation between Liquid Absorption Time and Powder Properties: As raw material for graphite particles, petroleum-derived coke, i.e., needle coke, 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 diameter of 1 mm to 3 mm.

[0126] The obtained coke particles were crushed and classified using a roller mill (Kurimoto Iron Works Co., Ltd., K-VX mill) to obtain flat coke particles.

[0127] 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 mixing container containing the mixture under 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 is easily reduced. The endpoint of heating and stirring was determined by the phenomenon that the viscosity of the mixture decreases 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 decreases and stabilizes was set as the point at which secondary particles with a bulk density of 0.7 g / cm³ were obtained.

[0128] Subsequently, the secondary particles were loaded into a graphitization box and graphitized at 3100°C. Afterward, the obtained particles were sieved using a 300-mesh screen to obtain graphite particles.

[0129] In addition to the graphite particles, ten types of graphite particles were prepared together with the coke particles by changing the conditions for crushing and classifying the coke particles, the stirring conditions of the mixer, and the graphitization conditions. Using these ten types of graphite particles, the D50, standard deviation of particle size distribution, specific surface area, knock density, compression load, and resilience coefficient were calculated respectively. Then, the negative electrode was manufactured using these graphite particles as follows, and the orientation of the graphite particles in the negative electrode and the liquid absorption time of the negative electrode were calculated.

[0130] (Tap Tightness) 100 cm³ of graphite particle sample powder is placed into a graduated flat-bottomed test tube (manufactured by Kuramochi Scientific Instruments Co., Ltd., KRS-406) with a capacity of 150 cm³, and the graduated flat-bottomed test tube is plugged. Then, the graduated flat-bottomed test tube is dropped from a height of 5 cm 250 times, and the tap tightness is determined based on the mass and volume of the sample powder afterward.

[0131] (Determination of the orientation of graphite particles in the negative electrode) A slurry was prepared by mixing 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). The 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 1.70 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.

[0132] The orientation of graphite particles was determined by measuring the surface of a negative electrode for a lithium-ion secondary battery, which serves as a sample electrode, using an X-ray diffraction apparatus with CuKα rays as the X-ray source. Specifically, the X-ray diffraction pattern of the sample electrode surface was measured, and the orientation of the graphite particles was determined by the intensity of the diffraction peak of the carbon (002) plane detected near the diffraction angle 2θ = 20° to 20° and the diffraction peak of the carbon (110) plane detected near the diffraction angle 2θ = 70° to 80° using the following equation (X). (002) plane diffraction peak intensity / (110) plane diffraction peak intensity ... Equation (X)

[0133] The physical properties other than the knock density and the orientation of graphite particles in the negative electrode are determined by the method described above.

[0134] (Determination of liquid absorption time) A lithium-ion secondary battery negative electrode, obtained in the same manner as (determination of the orientation of graphite particles in the negative electrode), was cut into a square of 2.0 cm × 2.0 cm and attached to a glass substrate using double-sided tape to form a strain-free planar electrode surface. Using a micropipette, 3 μL of PC (polycarbonate: manufactured by Kishida Chemical Co., Ltd.) was allowed to drip down to the center of the square electrode, and the liquid absorption time until penetration was measured. Six identical electrodes were prepared, and the average of the six measurements was calculated. Next, the liquid absorption time of the lithium-ion secondary battery negative electrode manufactured using graphite particles produced under the stated conditions with a compressive load of 2.804 kN / cm² and a resilience coefficient of 0.277 was set as a reference value of 100. Based on this reference value, the liquid absorption time values ​​measured in each lithium-ion secondary battery negative electrode were converted, and the resulting value was set as the liquid absorption time index.

[0135] For ten types of graphite particles, simple regression analysis and multiple regression analysis were used to determine the correlation between powder properties and liquid absorption time index as described above. The results of simple regression analysis are shown in Table 1, and the results of multiple regression analysis are shown in Table 2.

[0136] [Table 1] D50 Standard deviation of particle size distribution Specific surface area Tightness Compression load rebound coefficient Alignment The correlation coefficient R with the aspiration time index 0.688 -0.276 0.024 -0.339 -0.811 -0.174 0.435

[0137] [Table 2] D50 and compression load Resilience coefficient and compression load Compression load and alignment Coefficient of determination R of the aspiration time index 2 0.659 0.998 0.905

[0138] As shown in Tables 1 and 2, by using the rebound coefficient and compression load as explanatory variables and the liquid absorption time index as the target variable, a multiple regression analysis was performed to obtain the regression formula with the strongest correlation. The obtained regression formula is shown below. Regression formula...(liquid absorption time index) = 337.7 - 369 × (rebound coefficient) - 48.4 × (compression load)

[0139] When the liquid absorption time index is about 135 or less, the liquid absorption of lithium-ion secondary batteries becomes good, and the tendency for cycle characteristics to decrease during repeated charge and discharge can be suppressed. Therefore, regarding the regression formula, it is assumed that the cycle characteristics of lithium-ion batteries become good when the right side is 135 or less, and the following formula (1) is obtained. [Rebound coefficient × 7.6] + [Compression load (kN / cm2)] ≧ 4.2··· (1)

[0140] Hereinafter, graphite particles were prepared in each embodiment and each comparative example, and the compressive load and rebound coefficient of the prepared graphite particles were measured. Then, based on the regression formula, the liquid absorption time index of the negative electrode obtained by using the graphite particles prepared in each embodiment and each comparative example was calculated, and the cycle capacity maintenance index was calculated based on the calculated liquid absorption time index.

[0141] [Example 1] As the raw material for graphite particles, petroleum-derived coke, namely needle coke, 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.

[0142] 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 8.9 μm, D50 was 15.8 μm, and D90 was 23.5 μm. The coefficient of thermal expansion of the coke particles after calcination at 1400℃ was 3.52×10-6 / ℃.

[0143] 85% by mass of the obtained coke particles and 15% 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 mixing container containing the mixture under 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 is easily reduced. The endpoint of heating and stirring is determined by the phenomenon that the viscosity of the mixture decreases due to the volatilization of low molecular weight gases in the coal tar pitch; this is set as the moment when the current value of the stirring blades decreases and stabilizes, thereby obtaining secondary particles.

[0144] 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 9.0 μm, D50 was 16.0 μm, and D90 was 24.9 μm. The results obtained from measuring the compressive load and resilience coefficient of the obtained graphite particles are shown in Table 3.

[0145] [Example 2] Needle coke, a type of coke from a different petroleum source than that used in Example 1, was used as the raw material for graphite particles. Flat coke particles were obtained following the same procedure as in Example 1. The volumetric particle size distribution of the coke particles was measured, and the results showed that D10 was 8.2 μm, D50 was 13.2 μm, and D90 was 20.8 μm. The coefficient of thermal expansion of the coke particles after calcination at 1400°C was 3.57 × 10⁻⁶ / °C.

[0146] 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 8.5 μm, D50 was 14.6 μm, and D90 was 22.5 μm. The results obtained from measuring the compressive load and resilience coefficient of the graphite particles are shown in Table 3.

[0147] [Example 3] Using the coke particles obtained in Example 2, secondary particles were obtained in the same manner as in Example 1. The secondary particles were then shaped using a uniaxial press at room temperature to obtain a block-shaped molded article. The molded article was then heat-treated in a nitrogen environment at 800°C–850°C, followed by graphitization at 2600°C–3100°C to obtain graphitized material. The obtained graphitized material was pulverized, and the 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 9.5 μm, D50 was 17.5 μm, and D90 was 31.2 μm. The results of measuring the compressive load and resilience coefficient of the obtained graphite particles are shown in Table 3.

[0148] [Example 4] Graphite particles were obtained using the coke particles obtained in Example 1, in the same manner as in Example 3. The volumetric particle size distribution of the graphite particles was measured, and the results showed that D10 was 10.0 μm, D50 was 18.4 μm, and D90 was 33.1 μm. The results of measuring the compressive load and resilience coefficient of the obtained graphite particles are shown in Table 3.

[0149] [Example 5] Needle coke, a type of coke from a different petroleum source than that used in Example 1, was used as the raw material for graphite particles. Flat coke particles were obtained following the same procedure as in Example 1. The volumetric particle size distribution of the coke particles was measured, and the results showed that D10 was 9.4 μm, D50 was 15.8 μm, and D90 was 21.4 μm. The coefficient of thermal expansion of the coke particles after calcination at 1400°C was 3.90 × 10⁻⁶ / °C.

[0150] 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 9.6 μm, D50 was 16.1 μm, and D90 was 23.4 μm. The results obtained from measuring the compressive load and resilience coefficient of the graphite particles are shown in Table 3.

[0151] [Example 6] Needle coke, a type of coke from a different petroleum source than that used in Example 1, was used as the raw material for graphite particles. Flat coke particles were obtained following the same procedure as in Example 1. The volumetric particle size distribution of the coke particles was measured, and the results showed that D10 was 6.4 μm, D50 was 12.5 μm, and D90 was 18.0 μm. The coefficient of thermal expansion of the coke particles after calcination at 1400°C was 2.30 × 10⁻⁶ / °C.

[0152] Using the coke particles obtained in Example 6, graphite particles were obtained in the same manner as in Example 3. The volumetric particle size distribution of the graphite particles was measured, and the results showed that D10 was 11.4 μm, D50 was 19.2 μm, and D90 was 33.1 μm. The results of measuring the compressive load and resilience coefficient of the obtained graphite particles are shown in Table 3.

[0153] [Example 7] Needle coke, a type of coke from a different petroleum source than that used in Example 1, was used as the raw material for graphite particles. Flat coke particles were obtained following the same procedure as in Example 1. The volumetric particle size distribution of the coke particles was measured, and the results showed that D10 was 7.3 μm, D50 was 13.9 μm, and D90 was 25.2 μm. The coefficient of thermal expansion of the coke particles after calcination at 1400°C was 2.72 × 10⁻⁶ / °C.

[0154] Using the coke particles obtained in Example 7, graphite particles were obtained in the same manner as in Example 3. The volumetric particle size distribution of the graphite particles was measured, and the results showed that D10 was 8.8 μm, D50 was 18.8 μm, and D90 was 36.4 μm. The results obtained by measuring the compressive load and resilience coefficient of the obtained graphite particles are shown in Table 3.

[0155] [Example 8] The same needle-shaped coke as in Example 7 was used as the raw material for graphite particles. Flat coke particles were obtained according to the same procedure as in Example 1. The volumetric particle size distribution of the coke particles was measured, and the results showed that D10 was 3.3 μm, D50 was 10.4 μm, and D90 was 22.3 μm.

[0156] 81% by mass of the coke particles obtained in Example 8 were used as raw materials, and 9% by mass and 10% by mass of coal tar and starch (α degree ≥ 95%) were used as binders, respectively. Otherwise, graphite particles were obtained in the same manner as in Example 3. The volumetric particle size distribution of the graphite particles was measured, and the results showed that D10 was 5.5 μm, D50 was 12.5 μm, and D90 was 21.4 μm. The results of measuring the compressive load and resilience coefficient of the obtained graphite particles are shown in Table 3.

[0157] [Example 9] The same needle-shaped coke as in Example 7 was used as the raw material for graphite particles. Flat coke particles were obtained according to the same procedure as in Example 1. The volumetric particle size distribution of the coke particles was measured, and the results showed that D10 was 5.1 μm, D50 was 10.1 μm, and D90 was 17.7 μm.

[0158] 90% by mass of the coke particles obtained in Example 9 were used as raw materials, and 10% by mass of starch (α-degree of oxidation ≥ 95%) was used as a binder. Otherwise, graphite particles were obtained in the same manner as in Example 3. The volumetric particle size distribution of the graphite particles was measured, and the results showed that D10 was 6.4 μm, D50 was 14.0 μm, and D90 was 29.6 μm. The results of measuring the compressive load and resilience coefficient of the obtained graphite particles are shown in Table 3.

[0159] [Example 10] Needle coke, a type of coke from a different petroleum source than that used in Example 1, was used as the raw material for graphite particles. Flat coke particles were obtained following the same procedure as in Example 1. The volumetric particle size distribution of the coke particles was measured, and the results showed that D10 was 8.8 μm, D50 was 13.9 μm, and D90 was 19.0 μm. The coefficient of thermal expansion of the coke particles after calcination at 1400°C was 2.25 × 10⁻⁶ / °C.

[0160] 85 parts by weight of coke particles and 15 parts by weight of coal tar pitch (softening point 100℃~150℃, quinoline insoluble content less than 15% by weight, fixed carbon 55%~75% by weight) were mixed at room temperature to obtain a mixture. Then, using the obtained mixture, graphite particles were obtained in the same manner as in Example 3. The volumetric particle size distribution of the graphite particles was measured, and the results showed that D10 was 12.0 μm, D50 was 19.5 μm, and D90 was 28.6 μm. The results of measuring the compressive load and resilience coefficient of the obtained graphite particles are shown in Table 3.

[0161] [Comparative Examples 1 to 3] Spherical natural graphite with a D10 of 15.1 μm, D50 of 22.0 μm, and D90 of 30.5 μm was used as graphite particles in Comparative Example 1; spherical natural graphite with a D10 of 2.3 μm, D50 of 4.0 μm, and D90 of 6.3 μm was used as graphite particles in Comparative Example 2; and spherical natural graphite with a D10 of 8.2 μm, D50 of 10.5 μm, and D90 of 13.6 μm was used as graphite particles in Comparative Example 3. The results obtained by measuring the compressive load and resilience coefficient of the graphite particles using the method described above are shown in Table 3.

[0162] [Comparative Example 4] Needle coke, a type of coke from a different petroleum source than that used in Example 1, was used as the raw material for graphite particles. Flat coke particles were obtained following the same procedure as in Example 1. The volumetric particle size distribution of the coke particles was measured, and the results showed that D10 was 8.8 μm, D50 was 13.9 μm, and D90 was 19.0 μm. The coefficient of thermal expansion of the coke particles after calcination at 1400°C was 2.25 × 10⁻⁶ / °C.

[0163] 85 parts by mass of coke particles, 15 parts by mass of coal tar pitch (softening point 100℃~150℃, quinoline insoluble content less than 15% by mass, fixed carbon 55%~75% by mass), and 6 parts by mass of silicon carbide as a graphitization catalyst were mixed at room temperature to obtain a mixture. Then, using the obtained mixture, graphite particles were obtained in the same manner as in Example 3. The volumetric particle size distribution of the graphite particles was measured, and the results showed that D10 was 11.4 μm, D50 was 19.7 μm, and D90 was 30.3 μm. The results obtained from measuring the compressive load and resilience coefficient of the obtained graphite particles are shown in Table 3.

[0164] [Comparative Example 5] 85 parts by mass of coke particles obtained in Comparative Example 4, 15 parts by mass of coal tar pitch (softening point 100°C to 150°C, quinoline insoluble content less than 15% by mass, fixed carbon 55% to 75% by mass), and 12 parts by mass of silicon carbide as a graphitization catalyst were mixed at room temperature to obtain a mixture. Then, using the obtained mixture, graphite particles were obtained in the same manner as in Example 3. The volumetric particle size distribution of the graphite particles was measured, and the results showed that D10 was 11.0 μm, D50 was 19.9 μm, and D90 was 31.7 μm. The results obtained by measuring the compressive load and resilience coefficient of the obtained graphite particles are shown in Table 3.

[0165] (Compression Load) 3.0 g of negative electrode material was filled into a mold with a diameter of 15 mm, and compressed at a constant speed of 10 mm / min using an automatic compressive testing machine (manufactured by Shimadzu Corporation). During compression, the distance from the bottom surface of the negative electrode material to the pressing surface was measured, and the density under pressure was calculated by multiplying this distance by the bottom area of ​​the mold. A force gauge was installed on the press hammer of the automatic compressive testing machine, and the force applied when the specified density of 1.7 g / cm3 was reached was set as the compression load.

[0166] (Rebound coefficient) The rebound coefficient is calculated by dividing the absolute value of the difference between the reference density of 1.7 g / cm3 when pressed using the method described above and the density of the rebounded negative electrode material by the density of 1.7 g / cm3.

[0167] (Calculation of Liquid Absorption Time Index) Using the results of the compression load and rebound coefficient of the graphite particles obtained in each embodiment and comparative example, and the regression formula below, the calculated value of the liquid absorption time index of the negative electrode was obtained. The results are shown in Table 3. Regression formula...(Liquid Absorption Time Index) = 337.7 - 369 × (Rebound Coefficient) - 48.4 × (Compression Load)

[0168] (Relationship between aspiration time index and circulating volume maintenance rate)

[0169] 《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 battery is fabricated.

[0170] Battery Characteristic Evaluation The cycle characteristics of the lithium-ion secondary battery manufactured as described are evaluated as follows. Specifically, the manufactured lithium-ion secondary battery is placed in a thermostatic bath maintained at 25°C and charged at a constant current of 4.6 mA until it reaches 4.2 V. Then, it is further charged at a constant voltage of 4.2 V until the current decays to the equivalent of 0.046 mA. After charging, it is stopped for 10 minutes and then discharged at 4.6 mA until it reaches 2.75 V. The initial discharge capacity is measured. This charge and discharge cycle is repeated 299 times. At this time, the discharge capacity of the 100th cycle and the discharge capacity of the 300th cycle are measured. The value obtained by dividing the discharge capacity after each cycle by the initial discharge capacity is multiplied by 100, and the result is set as the cycle capacity retention rate (%). The results are shown in Table 3.

[0171] The change in capacity retention rate during repeated charge-discharge cycles is evaluated by comparing the capacity retention rate (%) after 100 cycles with the capacity retention rate (%) after 300 cycles. Specifically, the change rate (%) of capacity retention rate from 100 cycles to 300 cycles is calculated as shown in Equation (3) below. A larger change rate means that the cycle characteristics have decreased significantly due to repeated charge-discharge cycles. Change rate (%) = [(Capacity retention rate after 100 cycles - Capacity retention rate after 300 cycles) / Capacity retention rate after 100 cycles] × 100··· (3) The results are shown in Table 3.

[0172] [Table 3] Coefficient of thermal expansion of coke [×10] -6 / ℃] SiC (parts by mass) in the mixture D50[μm] Compression load [kN / cm] 2 ] rebound coefficient Aspiration time index (calculated value) [Resilience coefficient × 7.6] + [Compression load] Capacity retention rate (%) after 100 cycles Capacity retention rate (%) after 300 cycles Rate of change (%) Example 1 3.52 0 16.0 2.493 0.337 93 5.056 98.0 92.8 5.3 Example 2 3.57 0 14.6 2.673 0.353 78 5.359 98.2 93.2 5.1 Example 3 3.57 0 17.5 3.478 0.388 26 6.427 97.8 95.0 2.9 Example 4 3.52 0 18.4 3.433 0.377 32 6.298 98.0 94.6 3.5 Example 5 3.9 0 16.1 2.804 0.277 100 4.912 97.8 92.0 5.9 Example 6 2.3 0 19.2 2.051 0.338 114 4.622 98.1 91.3 6.9 Example 7 2.72 0 18.8 2.513 0.275 114 4.605 97.7 91.0 6.9 Example 8 2.72 0 12.5 2.032 0.293 131 4.261 97.6 82.3 15.7 Example 9 2.72 0 14.0 2.429 0.315 104 4.822 97.9 92.4 5.6 Example 10 2.25 0 19.5 2.068 0.291 130 4.280 98.0 87.4 10.8 Comparative Example 1 - 0 22.0 1.748 0.060 231 2.204 96.0 45.0 53.1 Comparative Example 2 - 0 3.0 3.234 0.115 139 4.105 97.4 74.3 23.7 Comparative Example 3 - 0 10.5 2.386 0.076 194 2.963 96.4 47.0 51.2 Comparative Example 4 2.25 6 19.7 1.868 0.275 146 3.960 97.9 72.1 26.4 Comparative Example 5 2.25 12 19.9 1.731 0.257 159 3.686 98.0 65.6 33.1

[0173] As shown in Table 3, in each embodiment, the condition of Equation (1) shown below is met, and the result of the capacity retention rate, especially the result of the capacity retention rate after 300 cycles, is better than that of the comparative examples. [Rebound coefficient × 7.6] + [Compression load (kN / cm2)] ≥ 4.2··· (1) Furthermore, as shown in Table 3, it can be confirmed that in each embodiment, the rate of change (%) of the capacity retention rate from 100 cycles to 300 cycles is smaller than that of the comparative examples. It can be seen that by the decrease of the absorption time exponent and the increase of the value on the left side of Equation (1), the decrease of the cycle capacity retention rate is slower even during repeated charge-discharge cycles.

[0174] The disclosure of PCT / JP2021 / 003589, filed on February 1, 2021, is incorporated herein by reference in its entirety. All documents, patent applications, and technical specifications described herein are incorporated herein by reference to the specific and individual documents, patent applications, and technical specifications described herein. [Simplified Explanation of the Diagram]

[0011] None

Claims

1. A negative electrode material for lithium-ion secondary batteries is graphite particles that satisfy the following formula (1), wherein the compressive load of the graphite particles is 2.0 kN / cm2 or more, the resilience coefficient of the graphite particles is 0.25 or more, and [resilience coefficient × 7.6] + [compressive load (kN / cm2)] ≥ 4.2··· (1).

2. The negative electrode material for lithium-ion secondary batteries as claimed in claim 1, wherein the graphite particles comprise composite particles having a structure formed by stacking multiple flat graphite particles.

3. An evaluation method for negative electrode materials for lithium-ion secondary batteries, wherein a regression formula obtained by multiple regression analysis is used to evaluate the negative electrode materials for lithium-ion secondary batteries, wherein the rebound coefficient of graphite particles and the compressive load (kN / cm2) of the graphite particles used as negative electrode materials for lithium-ion secondary batteries are set as explanatory variables, and the liquid absorption time of the negative electrode containing the negative electrode material for lithium-ion secondary batteries is set as the target variable.

4. A method for manufacturing a negative electrode material for a lithium-ion secondary battery, comprising manufacturing a negative electrode material for a lithium-ion secondary battery based on an evaluation result of the negative electrode material for a lithium-ion secondary battery obtained using the evaluation method for a negative electrode material for a lithium-ion secondary battery as described in claim 3.

5. A method for manufacturing a negative electrode material for a lithium-ion secondary battery, comprising the method of manufacturing a negative electrode material for a lithium-ion secondary battery as described in claim 1 or claim 2, wherein the method includes a step of graphitizing coke.

6. The method for manufacturing a negative electrode material for a lithium-ion secondary battery as claimed in claim 5, wherein the coefficient of thermal expansion of the coke after calcination at 1400°C is 2.30 × 10⁻⁶ / °C or higher.

7. 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 claim 1 or claim 2, and a current collector.

8. A lithium-ion secondary battery comprising a negative electrode, a positive electrode, and an electrolyte as described in claim 7.