Negative electrode material for lithium-ion secondary battery, negative electrode for lithium-ion secondary battery, lithium-ion secondary battery, and method for producing negative electrode material for lithium-ion secondary battery

Abstract

A negative electrode material for a lithium-ion secondary battery includes a plurality of graphite particles, and when binarizing SEM images of cross sections of the graphite particles into void portions and regions other than the void portions, and assigning 0 to the void portions and assigning 1 to the regions other than the voids, a plurality of images of graphite particles having at least 10,000 pixels are selected from the binarized images, and the average value of the amount of voids of the selected images of graphite particles is 0.700 to 0.940.

Description

Anode material for lithium ion secondary battery, anode for lithium ion secondary battery, lithium ion secondary battery, and method for producing anode material for lithium ion secondary battery

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

[0002] Taking advantage of their characteristics of small size, light weight, and high energy density, lithium-ion secondary batteries have been widely used in electronic devices such as notebook personal computers (PCs), mobile phones, smartphones, and tablet PCs. ２ Against the backdrop of environmental problems such as global warming caused by emissions, electric vehicles such as clean electric vehicles (EVs) that run solely on batteries, hybrid electric vehicles (HEVs) that combine gasoline engines with batteries, and plug-in hybrid electric vehicles (PHEVs) are becoming more common, and development of lithium-ion secondary batteries (automotive lithium-ion secondary batteries) to be installed in these vehicles is underway.

[0003] The input characteristics of lithium-ion secondary batteries are significantly affected by the performance of their negative electrode materials. Carbon materials are widely used as negative electrode materials for lithium-ion secondary batteries. For example, highly crystalline carbon materials such as artificial graphite and spherical natural graphite, which is made by spheroidizing scaly natural graphite, have been proposed as materials for obtaining high-density negative electrodes.

[0004] As an example of artificial graphite, Patent Document 1 discloses a negative electrode material for lithium ion secondary batteries that includes composite particles containing a plurality of flat graphite particles that are aggregated or bonded so that their orientation planes are non-parallel, and spherical graphite particles.

[0005] International Publication No. 2015 / 147012

[0006] The rapid growth of the EV market is driving demand for anode materials for lithium-ion secondary batteries. Repeated charge and discharge cycles of lithium-ion secondary batteries cause the anode to expand, which can lead to issues such as peeling of the anode material and deterioration of cycle characteristics.

[0007] An object of the present disclosure is to provide a negative electrode material for lithium ion secondary batteries that can be used to manufacture lithium ion secondary batteries in which expansion of the negative electrode is suppressed during repeated charge and discharge, as well as a negative electrode for lithium ion secondary batteries that includes the same, and a lithium ion secondary battery.An object of the present disclosure is to provide a method for manufacturing a negative electrode material for lithium ion secondary batteries that can be used to manufacture lithium ion secondary batteries in which expansion of the negative electrode is suppressed during repeated charge and discharge.

[0008] Means for solving the above problems include the following aspects: <1> An anode material for a lithium ion secondary battery according to <1>, comprising: a plurality of graphite particles; wherein an SEM image of a cross section of the graphite particle is binarized into void portions and regions other than the void portions, the void portions being set to 0 and the regions other than the void portions being set to 1, and a plurality of images of the graphite particles having a pixel count of 10,000 or more are selected from the images obtained by the binarization; and the selected images of the graphite particles have an average void volume of 0.700 to 0.940. <2> An anode material for a lithium ion secondary battery according to <1>, wherein a plurality of images of the graphite particle having a pixel count of 10,000 or more are selected from the images obtained by the binarization; and the number of pixels corresponding to the void portions and the number of pixels corresponding to the regions other than the void portions are calculated for the selected images of the graphite particles, and the total number of pixels in the selected images of the graphite particles is 100,000 or more, and the average number of pixels corresponding to the void portions in one image of the graphite particle is 1,000 to 20,000. <3> The negative electrode material for a lithium ion secondary battery according to <1> or <2>, wherein when a plurality of images of graphite particles each having a pixel count of 10,000 or more are selected from images obtained by binarization processing, and the number of pixels corresponding to the void portions and the number of pixels corresponding to areas other than the void portions are calculated for each of the selected images of graphite particles, the total number of pixels in the selected images of graphite particles is 100,000 or more, and an average value of a ratio of the number of pixels corresponding to the void portions in one image of the graphite particle to the total number of pixels corresponding to the void portions and the number of pixels corresponding to areas other than the void portions in one image of the graphite particle is 0.030 to 0.300. <4> The negative electrode material for a lithium ion secondary battery according to any one of <1> to <3>, wherein, with respect to the standard deviation and dispersion of the hues of the graphite particles obtained from a birefringence image of the cross section of the graphite particles, the total number of pixels in the birefringence image is 10,000 or more, the average value of the standard deviation of the hues of the graphite particles is 0.210 to 0.400, and the average value of the dispersion of the hues of the graphite particles is 0.0550 to 0.1000. <5> The true density of the graphite particles is 2.20 g / cm ３The negative electrode material for a lithium ion secondary battery according to any one of <1> to <4>, wherein: <6> the graphite particles include artificial graphite particles, and the artificial graphite particles are particles obtained by graphitizing a mixture containing a graphitizable aggregate and a graphitizable binder, and the binder includes a water-soluble or water-absorbent polymer compound. <7> the negative electrode material for a lithium ion secondary battery according to any one of <1> to <6>, wherein the graphite particles have a circularity of 91.0% or more. <8> the negative electrode material for a lithium ion secondary battery according to any one of <1> to <7>, wherein the graphite particles have an oil absorption of 35 mL / 100 g or less. <9> the negative electrode material for a lithium ion secondary battery according to any one of <1> to <8>, wherein the graphite particles have a graphitization degree of 93.5% or less. <10> The compressive load of the graphite particles is 2.50 kN / cm ２ The negative electrode material for a lithium ion secondary battery according to any one of <1> to <9>. <11> A negative electrode for a lithium ion secondary battery, comprising: a negative electrode material layer including the negative electrode material for a lithium ion secondary battery according to any one of <1> to <10>; and a current collector. <12> A lithium ion secondary battery, comprising: the negative electrode for a lithium ion secondary battery according to <11>, a positive electrode, and an electrolyte. <13> A method for producing the negative electrode material for a lithium ion secondary battery according to any one of <1> to <10>, comprising: (a) obtaining a mixture containing a graphitizable aggregate and a graphitizable binder, the graphitizable binder containing a water-soluble or water-absorbent polymer compound; (b) molding the mixture to obtain a molded product; (c) graphitizing the molded product to obtain a graphitized product; and (d) pulverizing the graphitized product to obtain a pulverized product. <14> The method for producing a negative electrode material for a lithium ion secondary battery according to <13>, further comprising the step of spheronizing the graphitizable aggregate before the step (a).

[0009] The present disclosure can provide a negative electrode material for lithium ion secondary batteries that can produce lithium ion secondary batteries in which expansion of the negative electrode can be suppressed during repeated charge and discharge, as well as a negative electrode for lithium ion secondary batteries that includes the same, and a lithium ion secondary battery.The present disclosure can provide a method for producing a negative electrode material for lithium ion secondary batteries that can produce lithium ion secondary batteries in which expansion of the negative electrode can be suppressed during repeated charge and discharge.

[0010] Hereinafter, embodiments for carrying out the present invention will be described in detail. However, the present invention is not limited to the following embodiments. In the following embodiments, components (including element steps, etc.) are not essential unless otherwise specified. The same applies to numerical values ​​and their ranges, and do not limit the present invention.

[0011] In the present disclosure, the term "process" includes not only processes that are independent of other processes, but also processes that cannot be clearly distinguished from other processes as long as the purpose of the process is achieved. In the present disclosure, numerical ranges indicated using "to" include the numerical values ​​before and after "to" as the minimum and maximum values, respectively. In the numerical ranges described in stages in the present disclosure, the upper or lower limit value described in one numerical range may be replaced with the upper or lower limit value of another staged numerical range. Furthermore, in the numerical ranges described in the present disclosure, the upper or lower limit value of that numerical range may be replaced with a value shown in the examples. In the present disclosure, each component may contain multiple corresponding substances. When multiple substances corresponding to each component are present in the composition, the content or amount of each component means the total content or amount of the multiple substances present in the composition, unless otherwise specified. In the present disclosure, multiple types of particles corresponding to each component may be present. When multiple types of particles corresponding to each component are present in the composition, the particle size of each component means the value for a mixture of the multiple types of particles present in the composition, unless otherwise specified. In the present disclosure, the term "layer" or "film" includes cases where the layer or film is formed over the entire area when the area where the layer or film is present is observed, as well as cases where the layer or film is formed over only a part of the area. In the present disclosure, the term "laminate" refers to stacking layers, and two or more layers may be bonded together, or two or more layers may be detachable.

[0012] In the present disclosure, the particle size distribution of the negative electrode material can be measured using a laser diffraction particle size distribution analyzer. The average particle size of the particles is the particle size (D50) at which the cumulative total from the small diameter side in the volume-based particle size distribution is 50%. D90 is the particle size at which the cumulative total from the small diameter side in the volume-based particle size distribution is 90%, and D10 is the particle size at which the cumulative total from the small diameter side in the volume-based particle size distribution is 10%.

[0013] <<Negative Electrode Material for Lithium-Ion Secondary Battery>> The negative electrode material for lithium-ion secondary batteries (hereinafter also simply referred to as the negative electrode material) of the present disclosure includes a plurality of graphite particles, and an SEM image of a cross section of the graphite particle is binarized into void portions and regions other than the void portions, with the void portions being set to 0 and the regions other than the void portions being set to 1; a plurality of images of graphite particles having a pixel count of 10,000 or more are selected from the image obtained by the binarization; and the selected images of the graphite particles have an average void volume of 0.700 to 0.940.

[0014] An SEM image of the cross section of a graphite particle is acquired, and then the SEM image is binarized into void portions and regions other than the void portions (also referred to as solid portions). At this time, a threshold is set so that the void portions are 0 (e.g., black) and the solid portions are 1 (e.g., white). An image of the graphite particle is extracted from the binarized image, and a distribution of 0, which represents the void portion, and 1, which represents the solid portion, is obtained from the extracted image of the graphite particle to determine the void volume of the extracted graphite particle. This void volume refers to the average value of the distribution of 0 and 1 when the distribution of 0 and 1 is obtained for each pixel in one direction of the graphite particle (e.g., the two-dimensional X-axis direction). The void volumes of multiple graphite particles (e.g., a total of eight graphite particles) are determined as described above, and the arithmetic average value is used as the average void volume.

[0015] The use of the negative electrode material of the present disclosure makes it possible to manufacture a lithium-ion secondary battery in which expansion of the negative electrode can be suppressed during repeated charge and discharge. The reason for this is presumed to be as follows: The larger the average void volume in the negative electrode material, the smaller the proportion of closed voids (e.g., hollow structures) within the graphite particles and the greater the proportion of solid structures. Therefore, a void volume of 0.940 or less allows for an appropriate amount of closed voids (e.g., hollow structures) to be present within the graphite particles, thereby suppressing expansion of the negative electrode during repeated charge and discharge. Furthermore, such graphite particles can be easily obtained by spheronizing the graphitizable aggregate before obtaining a mixture containing the graphitizable aggregate and the graphitizable binder, and then graphitizing the mixture containing the spheronized aggregate. The reason for this is that by spheronizing the aggregate in advance, closed voids (e.g., hollow structures) can be more easily formed within the graphite particles due to structural contraction associated with crystal growth within the spheronized aggregate during graphitization.

[0016] When an SEM image of a cross section of a graphite particle is binarized into a void portion and a solid portion, a plurality of images of the graphite particle having a pixel count of 10,000 or more are selected from the images obtained by the binarization (e.g., eight images are selected), and the number of pixels corresponding to the void portion and the number of pixels corresponding to the solid portion are calculated for each of the selected images of the graphite particle. Preferably, the total number of pixels in the selected images of the graphite particle is 100,000 or more, and the average number of pixels corresponding to the void portion included in one image of the graphite particle is 1,000 to 20,000. When the average number of pixels corresponding to the void portion is 1,000 or more, closed voids (e.g., hollow structure) are more appropriately present inside the graphite particle, and expansion of the negative electrode can be suitably suppressed during repeated charge and discharge.

[0017] When an SEM image of a cross section of a graphite particle is binarized into a void portion and a solid portion, a plurality of images of the graphite particle having a pixel count of 10,000 or more are selected from the images obtained by the binarization, and the number of pixels corresponding to the void portion and the number of pixels corresponding to the solid portion are calculated for each of the selected images of the graphite particle, the total number of pixels in the selected images of the graphite particle is 100,000 or more, and the average value of the ratio of the number of pixels corresponding to the void portion in one image of the graphite particle to the total number of pixels corresponding to the void portion and the solid portion in one image of the graphite particle is preferably 0.030 to 0.300. When the aforementioned average value, i.e., (number of pixels corresponding to the void portion / [number of pixels corresponding to the void portion + number of pixels corresponding to the solid portion]) is 0.030 or more, closed voids (e.g., hollow structure) are more appropriately present inside the graphite particle, and expansion of the negative electrode can be suitably suppressed during repeated charge and discharge.

[0018] With regard to the standard deviation and variance of the hue of the graphite particles obtained from a birefringence image of the cross section of the graphite particles, it is preferable that the total number of pixels in the birefringence image is 10,000 or more, the average value of the standard deviation of the hue of the graphite particles is 0.210 to 0.400, and the average value of the variance of the hue of the graphite particles is 0.0550 to 0.1000. The standard deviation and variance of the hue of the graphite particles, and their average values, can be obtained by the methods described in the Examples.

[0019] From the viewpoint of Li precipitation resistance in lithium ion secondary batteries, the graphite particles preferably include artificial graphite particles. The artificial graphite particles are preferably particles obtained by graphitizing a mixture containing a graphitizable aggregate and a graphitizable binder. From the viewpoint of Li precipitation resistance in lithium ion secondary batteries, the graphitizable binder preferably includes a water-soluble or water-absorbent polymer compound, and more preferably includes an aqueous binder containing a water-soluble or water-absorbent polymer compound.

[0020] The water-soluble or water-absorbent polymer compound is not particularly limited, and may include, for example, at least one selected from the group consisting of starch, amylose, amylopectin, polyacrylic acid, carboxymethyl cellulose, polyvinyl alcohol, and water-soluble protein.

[0021] (Average particle size) The particle size at which the cumulative total from the small diameter side in the volume-based particle size distribution of graphite particles measured by a laser diffraction method is 50% (hereinafter also referred to as "average particle size" or "D50") may be 10.0 μm to 30.0 μm.

[0022] From the viewpoint of further improving Li precipitation resistance, the average particle size of the graphite particles may be 28.0 μm or less or 25.0 μm or less, and may be 12.0 μm or more, 15.0 μm or more, or 20.0 μm or more.

[0023] The average particle size of the graphite particles can be measured using a laser diffraction particle size distribution measuring device (for example, SALD3100, Shimadzu Corporation).

[0024] Examples of methods for measuring the average particle size of graphite particles contained in a negative electrode include a method in which a sample electrode is prepared, embedded in epoxy resin, mirror-polished, and the cross section of the electrode is observed with a scanning electron microscope (for example, "VE-7800" manufactured by Keyence Corporation), and a method in which a cross section of the electrode is prepared using an ion milling device (for example, "E-3500" manufactured by Hitachi High-Technologies Corporation) and then measured with a scanning electron microscope (for example, "VE-7800" manufactured by Keyence Corporation). In this case, the average particle size is the median value of the particle sizes of 100 arbitrarily selected particles.

[0025] (D10) The D10 of the graphite particles may be 1.0 μm to 20.0 μm, 3.0 μm to 18.0 μm, 5.0 μm to 17.0 μm, or 7.0 μm to 17.0 μm.

[0026] The D10 of the graphite particles can be measured using a laser diffraction particle size distribution analyzer (for example, SALD3100, Shimadzu Corporation).

[0027] (Particle size distribution D90 / D10) The particle size distribution D90 / D10 of the graphite particles is not particularly limited and may be 2.0 to 5.0, or 2.0 to 4.0. The particle size distribution D90 / D10 can be measured using a laser diffraction particle size distribution analyzer (for example, SALD3100, Shimadzu Corporation).

[0028] Examples of methods for measuring the particle size distribution D90 / D10 of graphite particles contained in a negative electrode include a method in which a sample electrode is prepared, embedded in epoxy resin, mirror-polished, and the electrode cross section is observed with a scanning electron microscope (e.g., Keyence Corporation, "VE-7800"); and a method in which an electrode cross section is prepared using an ion milling device (e.g., Hitachi High-Technologies Corporation, "E-3500") and measured with a scanning electron microscope (e.g., Keyence Corporation, "VE-7800"). The particle size distribution D90 / D10 in this case can be determined by the following method. (a) Using a binarization method or the like, the area Sn of the imaged particle (n is a particle unique number assigned to the selected particle) is determined. (b) Assuming that the particle is an ideally shaped perfect sphere, the circle-equivalent diameter Ln = √Sn / π is determined from the area Sn. (c) From the circle-equivalent diameter Ln, the sphere volume Vn = (4 / 3)π(Ln) ３ (d) Repeat steps (a) to (c) for the selected 100 particles. (e) Using the distribution curve, where the vertical axis is the cumulative percentage of the volume of 100 particles and the horizontal axis is the particle size, as the particle size at the point where it intersects with the horizontal axis at 10%, the 10% diameter (D10) can be calculated, and the particle size at the point where it intersects with the horizontal axis at 90%, as the 90% diameter (D90), D90 / D10 can be calculated.

[0029] (I ００２ / I １１０ A negative electrode is formed using the negative electrode material of the present disclosure, and from the viewpoint of input characteristics in a lithium ion secondary battery, the negative electrode is pressed at 0.8 t / cm. When the negative electrode is subjected to X-ray diffraction measurement using CuKα rays, the peak intensity (I ００２ ) and the peak intensity of the 110 diffraction line (I １１０ ) and I ００２ / I １１０(also referred to as electrode orientation) may be 1100 or less, 1000 or less, 500 or less, or 400 or less. ００２ / I １１０ The lower limit of I is not particularly limited, and may be 200 or more. ００２ / I １１０ can be determined by the method described in the Examples. ００２ / I １１０ When measuring the electrode orientation, the components other than the graphite particles, the ratio thereof, and the manufacturing conditions of the negative electrode are the same as those described in the measuring method of electrode orientation in the Examples.

[0030] (Compression Load) The compression load of the graphite particles is set to 2.25 kN / cm from the viewpoint of suitably suppressing deformation of the graphite particles due to pressing during the production of the negative electrode and achieving excellent input / output characteristics. ２ Preferably, it is 2.50 kN / cm or more. ２ Preferably, it is 2.75 kN / cm or more. ２ More preferably, it is 3.00 kN / cm or more. ２ More preferably, it is equal to or greater than this.

[0031] In order to prevent deformation of the current collector due to pressing during the preparation of the negative electrode and separation of the current collector from the active material, the compressive load of the graphite particles is set to 6.00 kN / cm. ２ or less, and 5.00 kN / cm ２ It may be the following:

[0032] In the present disclosure, the compression load of graphite particles can be determined as follows: A predetermined 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). When the density of the compressed graphite particles is 1.7 g / cm ３ The pressure (kN / cm ２ ) is the compression load of the graphite particles. In the above measurement, a mold having a diameter of, for example, 15 mm is used, and compression is performed using an autograph (for example, manufactured by Shimadzu Corporation). The density of the graphite particles is calculated by multiplying the area of ​​the base of the mold (for example, 1.767 cm ２The compressive load is calculated from the volume of the graphite particles calculated from the mass of the graphite particles and the distance from the bottom of the mold to the pressing surface of the graphite particles. The greater the compressive load on the graphite particles, the less likely the graphite particles are to be deformed, broken, or the like by pressure.

[0033] (Springback ratio) The springback ratio of the graphite particles is not particularly limited. For example, from the viewpoint of ease of densification by pressing during negative electrode production, it may be 25% or less, or may be 23% or less. From the viewpoint of further suppressing breakage of the graphite particles by pressing during negative electrode production, the springback ratio of the graphite particles may be 15% or more, or may be 17% or more.

[0034] In the present disclosure, the springback rate of graphite particles refers to the degree to which density decreases when graphite particles are compressed to a reference density and then pressure is released. The greater the springback rate, the more easily graphite particles deformed by compression return to their original state. Specifically, a mold is filled with a predetermined mass (e.g., 3.0 g) of graphite particles, and the density of the graphite particles is reduced to the reference density (e.g., 1.7 g / cm). ３ ) is reached. Thereafter, the pressure is released, and when the press surface stops moving due to its elasticity, the density after pressure release is measured. From the obtained value, the springback rate is calculated using the following formula: Springback rate (%) = {(reference density - density after pressure release) / reference density} x 100 In the above measurement, for example, a mold having a diameter of 15 mm is used, and compression is carried out using an autograph (for example, manufactured by Shimadzu Corporation). The density of the graphite particles is calculated based on the base area of ​​the mold (for example, 1.767 cm ２ ) and the volume of the graphite particles calculated from the distance from the bottom surface of the mold to the pressing surface of the graphite particles, and the mass of the graphite particles.

[0035] The compression load and springback rate of the graphite particles can be adjusted by changing the physical properties, composition, etc. of the raw material of the graphite particles (e.g., needle coke) or by changing the graphitization conditions, etc.

[0036] (0-time tap density) The 0-time tap density of the graphite particles is set to 0.65 g / cm from the viewpoint of easily improving the input / output characteristics and compressive load of the lithium ion secondary battery. ３ ~0.80 g / cm ３ and preferably 0.65 g / cm ３ ~0.75g / cm ３ It is more preferable that:

[0037] The zero tap density of graphite particles is ３ 100 cm of graphite powder sample was placed in a graduated flat-bottom test tube (e.g., KRS-406, manufactured by Kuramochi Scientific Instruments Manufacturing Co., Ltd.). ３ The density is the value determined from the mass and volume of the sample powder before the stoppered, graduated, flat-bottomed test tube is dropped.

[0038] (30-time tap density) The 30-time tap density of the graphite particles is set to 0.86 g / cm from the viewpoint of easily improving the input / output characteristics and compressive load of the lithium ion secondary battery. ３ ~1.10 g / cm ３ and preferably 0.87 g / cm ３ ~1.05g / cm ３ It is more preferable that:

[0039] The 30 tap density of graphite particles is 150 cm ３ 100 cm of graphite powder sample was placed in a graduated flat-bottom test tube (e.g., KRS-406, manufactured by Kuramochi Scientific Instruments Manufacturing Co., Ltd.). ３ is added to the graduated flat-bottomed test tube, the graduated flat-bottomed test tube is then stoppered, and the graduated flat-bottomed test tube is then dropped from a height of 5 cm 30 times, after which the density is determined from the mass and volume of the sample powder.

[0040] (250 tap density) The 250 tap density of the graphite particles is 1.05 g / cm from the viewpoint of easily improving the input / output characteristics and energy density of the lithium ion secondary battery. ３ ~1.25g / cm ３ It is preferable that the density is 1.10 g / cm ３ ~1.25g / cm ３ It is more preferable that:

[0041] The 250 tap density of graphite particles is 150 cm ３ 100 cm of graphite powder sample was placed in a graduated flat-bottom test tube (e.g., KRS-406, manufactured by Kuramochi Scientific Instruments Manufacturing Co., Ltd.). ３ is added to the graduated flat-bottomed test tube, the graduated flat-bottomed test tube is then stoppered, and the graduated flat-bottomed test tube is then dropped from a height of 5 cm 250 times, after which the density is determined from the mass and volume of the sample powder.

[0042] (Compressibility) The compressibility of the graphite particles represented by the following formula (X) may be 0.30 to 0.50, 0.32 to 0.48, or 0.35 to 0.45: Compressibility = (250 tap density - 0 tap density) / 250 tap density (X).

[0043] (Specific Surface Area) The specific surface area of ​​graphite particles determined by nitrogen adsorption measurement at 77 K is not particularly limited. For example, from the viewpoint of improving rapid charge / discharge performance and suppressing decomposition of the electrolyte, the specific surface area of ​​the graphite particles is 0.2 m ２ / g to 6.0m ２ / g, and 1.0m ２ / g to 5.0m ２ / g, more preferably 1.5m ２ / g to 4.0m ２ It is more preferable that the SiO2 content is 1 / g.

[0044] The specific surface area determined by nitrogen adsorption measurement at 77 K can be determined by the BET method from 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.

[0045] The specific surface area of ​​graphite particles can be adjusted by particle size distribution, particle structure, etc. The specific surface area of ​​graphite particles may be adjusted by coating the graphite particles with low-crystalline carbon or the like, or, when an aqueous binder is used as the graphitizable binder, by adjusting the amount of the aqueous binder. When it is desired to reduce the particle size, the specific surface area increases significantly due to unevenness caused by grinding, but by coating, the unevenness can be filled in with a coating material to smooth out the unevenness and adjust the specific surface area.

[0046] (Circularity) From the viewpoint of Li deposition resistance of a lithium ion secondary battery, the circularity of the graphite particles may be 90.0% or more, 90.5% or more, or 91.0% or more. The circularity of the graphite particles may be 95.0% or less, or 93.0% or less.

[0047] In the present disclosure, the circularity can be measured using a wet flow particle size / shape analyzer, and the circularity based on the number of particles at 50% of the cumulative total (so-called average circularity) may be taken as the circularity.

[0048] (Oil Absorption) The oil absorption of the graphite particles is preferably 40 mL / 100 g or less, more preferably 35 mL / 100 g or less, and even more preferably 32 mL / 100 g or less. The lower limit of the oil absorption of the graphite particles is not particularly limited, and may be, for example, 20 mL / 100 g or more, or 25 mL / 100 g or more. The oil absorption of the graphite particles is an indicator of the amount of pores present inside and on the surface of the particles and the amount of voids between the particles. When the oil absorption of the graphite particles is 40 mL / 100 g or less, the number of pores present inside and on the surface of the particles is small, and the contact area with the electrolyte is thought to be sufficiently small. In addition, the small interface makes it possible to reduce the amount of binder used when preparing the negative electrode, which tends to reduce electrical resistance and improve battery performance. Furthermore, the small number of pores makes it possible to reduce the amount of solvent used when drying the electrode, which is advantageous in terms of production line costs and the environment, such as reducing the equipment and power required for drying. When the oil absorption of the graphite particles is 20 mL / 100 g or more, the increase in viscosity of the slurry when kneaded with a binder or the like, which occurs when the voids between the particles are too small, tends to be suppressed. Furthermore, the binder tends to spread favorably, facilitating kneading. Furthermore, it becomes easier to secure voids between the particles for the movement of lithium ions.

[0049] In the present disclosure, the oil absorption of graphite particles is measured using the method described in JIS K6217-4:2017 "Carbon black for rubber - Fundamental properties - Part 4: Determination of oil absorption," using linseed oil (e.g., manufactured by Kanto Chemical Co., Inc.) as the reagent liquid instead of dibutyl phthalate (DBP). Specifically, linseed oil is titrated into 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 a measuring instrument, for example, an absorption measuring device (product name: S-500) manufactured by Asahi Research Institute, Ltd. can be used.

[0050] (True Density) The true density of the graphite particles is set to 2.20 g / cm from the viewpoint of being able to suppress expansion of the negative electrode when repeatedly charged and discharged. ３ Preferably, it is 2.15 g / cm or less. ３ More preferably, it is 2.10 g / cm or less. ３ The true density of the graphite particles is more preferably 1.90 g / cm or less. ３ or more, and 2.00 g / cm ３ The true density of the graphite particles can be determined by a gas substitution method (gas pycnometer) using helium gas.

[0051] The theoretical true density of graphite is 2.26 g / cm ３ The closer the graphite particles are to this theoretical value, the fewer pores there are inside and on the surface of the particles, and the fewer voids there are inside the particles. Therefore, the smaller the true density of the graphite particles, the more voids there are inside the particles, and the further away they are from this theoretical value. It is presumed that the presence of many voids inside the particles makes it possible to suppress the expansion of the negative electrode during repeated charge and discharge.

[0052] (Graphitization degree) From the viewpoint of discharge capacity when a lithium ion secondary battery is produced, the graphite particles have a graphitization degree of preferably 90.0% or more, more preferably 90.5% or more, even more preferably 91.0% or more, and particularly preferably 92.0% or more. The upper limit of the graphitization degree of the graphite particles is not particularly limited, and it is sufficient if it is 100% or less. From the viewpoint of cycle characteristics when a lithium ion secondary battery is produced, it is preferably 95.0% or less, more preferably 94.0% or less, and even more preferably 93.5% or less.

[0053] The graphite particles may or may not be treated to have a low-crystalline carbon coating on their surfaces. When the graphite particles are treated to have a low-crystalline carbon coating, the resulting lithium-ion secondary battery tends to have improved low-temperature charging characteristics. On the other hand, when the graphite particles are not treated to have a low-crystalline carbon coating on their surfaces, cracking, peeling, etc., of the graphite particles during pressing during electrode fabrication tends to be suppressed, which increases the decomposition activity of the electrolyte and reduces storage characteristics. This also has the advantage of allowing greater flexibility in manufacturing conditions. Furthermore, the negative electrode material disclosed herein can suppress lithium metal precipitation during charging, even when low-crystalline carbon is not disposed on the surface. When the graphite particles are artificial graphite particles graphitized using a graphitizable binder containing the aforementioned aqueous binder (e.g., an aqueous binder containing a compound that does not contain an aromatic ring, such as starch), the compound that does not contain an aromatic ring, such as starch, is less likely to convert to graphite. Therefore, the surface crystallinity of the graphite particles tends to be low, and good charging characteristics at low temperatures tend to be obtained even if the treatment of coating the surface of the graphite particles with low-crystalline carbon is omitted. For these reasons, by producing artificial graphite particles using the above-mentioned aqueous binder, it is possible to achieve both simplification of the process and improvement of charging characteristics.

[0054] The R value of the graphite particles may be 0.10 to 0.40, or 0.15 to 0.30. In particular, it is preferable that the surface is not subjected to a treatment for coating with low crystalline carbon, and that the R value is 0.10 to 0.40, or more preferably 0.15 to 0.30.

[0055] The R value was measured at 1580 cm in the Raman spectrum obtained by Raman spectroscopy. －１ ~1620cm －１ The intensity Ig of the maximum peak in the range of 1300 cm －１ ~1400cm －１ In Raman spectroscopy, the intensity ratio Id / Ig of the maximum peak intensity Id in the range of 1580 cm －１ ~1620cm －１ The peak intensity Ig in the range of is a peak identified as corresponding to the graphite crystal structure, for example, 1580 cm －１ This is a peak that appears around 1300 cm －１ ~1400cm －１ The peak intensity Id in the range of 1360 cm is identified as a peak corresponding to the amorphous structure of carbon, for example, －１ This is a peak that appears nearby.

[0056] The R value of the graphite particles may be determined by measuring the Raman spectrum using a Raman spectrum measuring device (for example, XploRA PLUS manufactured by Horiba, Ltd.) under the following conditions. In this case, the arithmetic mean value of 400 particles measured is taken as the R value. - Raman spectrum measurement conditions - Laser wavelength: 532 nm Laser intensity: 100 mW or more Neutral density filter: 1% Irradiation intensity: 1 mW Measurement range: 1000 cm －１ ~1800cm －１ ・Irradiation time: 30 seconds ・Irradiation area: 1μm ２ Baseline (D band): 1100 cm －１ ~1470cm －１ Baseline (G band): 1450 cm －１ ~1710cm －１ ・Number of measurements per particle: 2 ・Number of particles measured: 400 particles

[0057] In the present disclosure, the average interplanar spacing (d ００２ A carbon material having an average interplanar spacing (d ) of less than 3.40 Å is considered graphite. In the present disclosure, as will be described later, particles in which low-crystalline carbon is arranged on at least a portion of the surface of graphite particles are also referred to as "graphite particles." ００２ The theoretical value of the average interplanar spacing (d) is 3.354 Å, and the closer it is to this value, the more graphitized it is. From the viewpoint of the initial charge / discharge efficiency and energy density of lithium-ion secondary batteries, ００２ From the viewpoint of achieving excellent input / output characteristics and excellent adhesion between the negative electrode and the current collector in a lithium ion secondary battery, the average interplanar spacing (d ００２ From the above viewpoint, the average interplanar spacing (d ００２ ) is preferably 3.358 Å to 3.361 Å, more preferably 3.359 Å to 3.3605 Å, and even more preferably 3.3595 Å to 3.360 Å.

[0058] The average interplanar spacing of graphite particles (d ００２ The average interplanar spacing (d) can be calculated using the Bragg equation based on the diffraction peak corresponding to the carbon 002 plane, which appears in the vicinity of a diffraction angle 2θ of 24° to 27° in a diffraction profile obtained by irradiating a sample with X-rays (CuKα rays) and measuring the diffraction rays with a goniometer. ００２ ) can be measured under the following conditions: Radiation source: CuKα ray (wavelength = 0.15418 nm) Output: 40 kV, 20 mA Sampling width: 0.010° ​​Scanning range: 10° to 35° Scanning speed: 0.5° / min

[0059] Bragg's formula: 2d sin θ=nλ where d is the length of one period, θ is the diffraction angle, n is the reflection order, and λ is the X-ray wavelength.

[0060] The crystallite size Lc of the graphite particles may be 30 nm to 140 nm, 35 nm to 100 nm, or 40 nm to 80 nm. From the viewpoint of facilitating the production of a lithium ion secondary battery that is excellent in input / output characteristics and adhesion between the negative electrode and the current collector, the crystallite size Lc of the graphite particles is preferably 100 nm or less.

[0061] <<Method for Producing Negative Electrode Material for Lithium-ion Secondary Batteries>> The method for producing a negative electrode material for lithium-ion secondary batteries according to the present disclosure includes a step of graphitizing coke. More specifically, from the viewpoints of improving handleability during graphitization and improving the cycle characteristics of lithium-ion secondary batteries, the method for producing a negative electrode material for lithium-ion secondary batteries according to the present disclosure preferably includes the following steps (a) to (d): (a) obtaining a mixture containing a graphitizable aggregate (coke) and a graphitizable binder; (b) molding the mixture to obtain a molded product; (c) graphitizing the molded product to obtain a graphitized product; and (d) pulverizing the graphitized product to obtain a pulverized product. Note that the steps of the above method may be performed consecutively or discontinuously. The steps of the above method may be performed at the same location or at different locations.

[0062] The type of coke used in the above production method is not particularly limited, and examples include petroleum-based or coal-based cokes such as fluid coke, needle coke, mosaic coke, and semi-needle coke, which has properties intermediate between needle coke and mosaic coke.

[0063] The method for obtaining coke particles (coke particles) is not particularly limited, and can be carried out by a known method. The particle size of the coke particles is not particularly limited, and can be selected taking into consideration the desired particle size, particle structure, etc. of the graphite particles. In order to obtain particles with a lower specific surface area, it is preferable to pulverize the coke in the raw coke state. Such particles have fewer voids, cracks, etc. due to cleavage, and the compressive load of the graphite particles tends to be higher.

[0064] In the manufacturing method of the present disclosure, a mixture containing a graphitizable aggregate and a graphitizable binder is obtained, and then the mixture is molded to obtain a molded product. In the manufacturing method of the present disclosure, from the viewpoint of Li precipitation resistance of a lithium ion secondary battery, the graphitizable binder preferably contains a water-soluble or water-absorbent polymer compound, and more preferably contains an aqueous binder containing a water-soluble or water-absorbent polymer compound. The use of the aforementioned aqueous binder improves the strength of the molded product, thereby improving its handleability during subsequent graphitization treatments, etc. Generally, when the strength of a molded product is improved, the graphitized product tends to become more difficult to pulverize after the graphitization treatment. However, in the above manufacturing method, the use of the aforementioned aqueous binder improves the strength of the molded product while making it easier to pulverize the graphitized product.

[0065] The reason why the graphitized material can be easily pulverized while improving the strength of the molded product in the above-described manufacturing method is presumed to be as follows: However, the present disclosure is not limited to the following presumption.

[0066] In the above manufacturing method, an aqueous binder containing a water-soluble or water-absorbent polymer compound is used, and the aggregate is incorporated into the binder swollen with water. This makes the surface of the graphitizable aggregate slippery during molding of the mixture, promoting alignment of the mixture and reducing voids. As a result, the density and strength of the molded product tend to be improved. Furthermore, the water not incorporated into the binder fills the gaps between the aggregates, resulting in liquid crosslinking, and thus improving the strength of the molded product.

[0067] In the above-described manufacturing method, the molded product is heat-treated by heating as necessary, and then graphitized to obtain a graphitized product. By using an aqueous binder, water volatilizes during the heat treatment, graphitization, etc. Therefore, the molded product has a high density before the heat treatment, graphitization, etc., but the density of the graphitized product after graphitization is reduced. Therefore, the graphitized product can be easily pulverized.

[0068] Furthermore, the above-described manufacturing method allows for easy pulverization of the graphitized material, resulting in a pulverized material with a small specific surface area. By using a negative electrode material containing a pulverized material with a small specific surface area in the production of a lithium-ion secondary battery, the contact area of ​​the negative electrode material with the electrolyte can be reduced. This suppresses the decomposition reaction of the electrolyte, extending the battery's lifespan and, as a result, the lithium-ion secondary battery tends to have excellent cycle characteristics.

[0069] Conventionally, when raw coke is used, problems arise in that molding is difficult and the strength of the molded products is low, and it has been necessary to improve the moldability and the strength of the molded products by, for example, subjecting the raw coke to heat treatment. On the other hand, in the above-described production method, even when raw coke is used without heat treatment, molded products with excellent moldability and high strength can be obtained. Therefore, as the graphitizable aggregate, raw coke that has been heat-treated may be used to form the molded products, or raw coke that has not been heat-treated may be used to form the molded products.

[0070] In the above-described manufacturing method, since the molded product has excellent strength, there is no need to pack the mixture into a graphitization crucible or the like when graphitizing. In other words, there is no need to use a graphitization crucible or the like, and graphitization can be performed by placing only the molded product in a graphitization furnace. Therefore, compared to when a graphitization crucible or the like is used, the manufacturing efficiency of the negative electrode material for lithium-ion secondary batteries tends to be superior.

[0071] Typically, increasing the amount of graphitizable binder can increase the strength of the molded product. However, if the amount of graphitizable binder is excessive, the density of the graphitized product tends to be high and the graphitized product tends to be difficult to pulverize, which tends to decrease the battery characteristics, such as the initial charge / discharge efficiency of the negative electrode material. In this embodiment, it is possible to increase the strength of the molded product without increasing the amount of graphitizable binder, and it is also possible to prevent a decrease in battery characteristics due to excessive use of binder. Furthermore, by reducing the amount of binder, it becomes easier to apply a hard carbon-derived coating to the surface of the pulverized product, which tends to improve the input characteristics of the lithium-ion secondary battery.

[0072] In step (a), a mixture containing a graphitizable aggregate and a graphitizable binder is obtained. The mixing may be performed at a temperature at which the graphitizable binder softens. Specifically, when the graphitizable binder is pitch, tar, or the like, the temperature may be 50°C to 300°C, and when the graphitizable binder is a thermosetting resin, the temperature may be 20°C to 100°C. When the graphitizable binder is an aqueous binder containing a water-soluble or water-absorbent polymer compound, the mixing may be performed at room temperature or with heating.

[0073] The mixing method is not particularly limited. For example, a method using a planetary mixer, a kiln mixer, a huddle mixer, etc. is preferred. A kneader, which involves kneading, may also be used.

[0074] When the graphitizable aggregate is particulate, the average particle size (D50) of the graphitizable aggregate is, for example, preferably 10 μm to 30 μm, more preferably 12 μm to 28 μm, and even more preferably 15 μm to 25 μm.

[0075] The standard deviation of the particle size distribution of the graphitizable aggregate is, for example, preferably 0.35 or less, more preferably 0.20 or less, even more preferably 0.18 or less, and particularly preferably 0.16 or less. By having the standard deviation of the particle size distribution of the aggregate be 0.35 or less, the particle size variation of the aggregate can be reduced, and the particle size variation of the resulting pulverized material can also be suppressed. By using a negative electrode material containing pulverized material with small particle size variation in the production of a lithium ion secondary battery, the resistance distribution within the negative electrode can be made uniform. As a result, the rapid charging performance of the lithium ion secondary battery tends to be improved. Furthermore, by reducing the particle size variation of the aggregate, the binder function can be suitably ensured even when the content or amount of the graphitizable binder is reduced. The standard deviation of the particle size distribution is, for example, a value (volume basis) measured by laser diffraction. The lower limit of the standard deviation of the particle size distribution of the graphitizable aggregate is not particularly limited, and may be, for example, 0.05 or more, or 0.10 or more.

[0076] Methods for adjusting the average particle size of the graphitizable aggregate and the standard deviation of the particle size distribution of the graphitizable aggregate to fall within the above-mentioned ranges include sieve classification, air classification, wet classification, and the like.

[0077] The graphitizable aggregate may be spheronized before obtaining a mixture containing the graphitizable aggregate and the graphitizable binder. The spheronization time may be 5 minutes to 2 hours, or may be 10 minutes to 1 hour.

[0078] From the viewpoint of facilitating the production of graphite particles with high circularity, the circularity of the graphitizable aggregate may be 90.0% or more, or 90.5% or more, and the circularity of the graphite particles may be 97.0% or less, or 96.0% or less.

[0079] The graphitizable binder is not particularly limited as long as it can be graphitized by a graphitization treatment, and specific examples thereof include the above-mentioned aqueous binders, coal-based, petroleum-based, and artificial pitches and tars, thermoplastic resins, and thermosetting resins.

[0080] The mixture may contain only one type of graphitizable aggregate and one or more types of graphitizable binder.

[0081] The content of each material in the mixture is not particularly limited. For example, the content of the graphitizable binder may be 10 parts by mass to 30 parts by mass, 12 parts by mass to 25 parts by mass, or 14 parts by mass to 20 parts by mass, relative to 100 parts by mass of the graphitizable aggregate. When the binder is added to a solvent or dispersion medium to be used as a solution or dispersion, the parts by mass of the solvent or dispersion medium are excluded. When the content of the binder is 10 parts by mass or more, the binder tends to function favorably as a binder for the graphitizable aggregate. When the content of the binder is 30 parts by mass or less, the amount of fixed carbon in the mixture is sufficiently ensured, and the yield tends to be excellent.

[0082] The mixture may contain only one type of graphitizable aggregate and one or more types of graphitizable binder.

[0083] The mixture may contain other components in addition to the graphitizable aggregate and graphitizable binder, such as aromatic compounds, graphite, dispersants, graphitization catalysts, etc.

[0084] The aromatic compound is not particularly limited as long as it is a compound having an aromatic ring. The aromatic compound may be a compound having an aromatic ring and having a molecular weight of 500 or less, or may be a compound having a molecular weight of 300 or less. Examples of the aromatic compound include naphthalene, methylnaphthalenes such as 1-methylnaphthalene and 2-methylnaphthalene, acenaphthene, biphenyl, fluorene, benzopyrene, benzanthracene, dibenzanthracene, diphenylene oxide, quinoline, and isoquinoline. The aromatic compound contained in the mixture may be one type or two or more types.

[0085] Among these, methylnaphthalene and naphthalene are preferred as the aromatic compound from the viewpoint of moldability when forming a molded product.

[0086] When the mixture contains an aromatic compound, the amount thereof is not particularly limited. The content of the aromatic compound in the mixture is preferably 1 mass % or less, more preferably 0.5 mass % or less, and even more preferably 0 mass % based on the total amount of the mixture.

[0087] The mixture may contain graphite. Examples of graphite include natural graphite and artificial graphite. The graphite is preferably in particulate form. The mixture may contain only one type of graphite or two or more types of graphite.

[0088] From the viewpoint of facilitating dispersion of the components in the mixture in the step (b) described below, it is preferable that the mixture contains a dispersant. By including a dispersant in the mixture, it is possible to suppress variation in particle size of the pulverized material obtained by pulverizing the graphitized material, and it is easy to obtain pulverized material with a uniform particle size. As a result, the rapid charging performance of the lithium ion secondary battery tends to be improved. Furthermore, the inclusion of a dispersant in the mixture leads to a reduction in the amount of graphitizable binder, and is expected to improve battery characteristics such as the initial charge / discharge efficiency of the negative electrode material.

[0089] The type of dispersant is not particularly limited. Specific examples include hydrocarbons such as liquid paraffin, paraffin wax, and polyethylene wax; fatty acids such as stearic acid, oleic acid, erucic acid, and 12-hydroxystearic acid; fatty acid metal salts such as zinc stearate, lead stearate, aluminum stearate, calcium stearate, and magnesium stearate; fatty acid amides such as stearic acid amide, oleic acid amide, erucic acid amide, methylene bisstearic acid amide, and ethylene bisstearic acid amide; fatty acid esters such as stearic acid monoglyceride, stearyl stearate, and hydrogenated oil; and higher alcohols such as stearyl alcohol. Among these, fatty acids are preferred, and stearic acid is more preferred, because they do not affect the performance of the negative electrode material, are easy to handle because they are solid at room temperature, melt at the temperature of step (a) and therefore disperse uniformly, disappear in the process up to the graphitization treatment, and are inexpensive.

[0090] When the mixture contains a dispersant, the amount thereof is not particularly limited. For example, the content of the dispersant relative to the total mass of the mixture may be 0.1% by mass to 20% by mass, 0.5% by mass to 10% by mass, or 0.5% by mass to 5% by mass.

[0091] From the viewpoint of promoting graphitization of the graphitizable aggregate or binder, the mixture preferably contains a graphitization catalyst. The type of graphitization catalyst is not particularly limited. Specific examples include substances having graphitization catalytic activity such as silicon, iron, nickel, titanium, and boron, as well as carbides of these substances, oxides of these substances, and nitrides of these substances.

[0092] When the mixture contains a graphitization catalyst, the amount thereof is not particularly limited. For example, the content of the graphitization catalyst relative to the total mass of the mixture may be 0.1% by mass to 50% by mass, 0.5% by mass to 40% by mass, or 0.5% by mass to 30% by mass.

[0093] In step (b), the mixture obtained in step (a) is molded to obtain a molded product. Preferably, the mixture is molded into a predetermined shape using a uniaxial press or the like. By molding the mixture in this manner, it is possible to increase the amount of the mixture packed into a graphitization furnace when graphitizing the mixture, thereby improving productivity and improving the effect of the graphitization catalyst.

[0094] In step (b), the method for molding the mixture is not particularly limited, and examples thereof include a molding method in which the mixture is placed in a container such as a metal mold and pressurized in a uniaxial direction, a vibration molding method in which the mixture is placed in a container such as a metal mold, a weight is placed on the top, and vibration and impact are applied to the metal frame to mold the mixture, and an extrusion molding method in which the mixture is extruded from a nozzle or the like using a horizontal press to mold the mixture.

[0095] In step (b), the density of the molded product obtained is not particularly limited, and from the viewpoints of productivity of the negative electrode material and cycle characteristics of the lithium ion secondary battery, it is set to 0.8 g / cm ３ ~1.7g / cm ３ is preferred, and 1.0 g / cm ３ ~1.5g / cm ３ More preferably, 1.2 g / cm ３ ~1.5g / cm ３ is more preferable.

[0096] The amount of fixed carbon in the graphitizable binder in the mixture is preferably 20% by mass or less, more preferably 15% by mass or less, even more preferably 10% by mass or less, and particularly preferably 5% by mass or less, based on 100% by mass of the aggregate and binder (before graphitization) combined. The lower the amount of fixed carbon in the graphitizable binder in the mixture, the higher the coke ratio in the mixture, and the greater the tendency for the discharge capacity of the lithium-ion secondary battery to be improved. The lower limit of the amount of fixed carbon in the graphitizable binder in the mixture is not particularly limited, and may be 0.5% by mass or more, or 1.0% by mass or more, based on 100% by mass of the aggregate and binder (before graphitization) combined.

[0097] The molded product obtained in step (b) is preferably subjected to a heat treatment before being graphitized in step (c). By performing the heat treatment, organic components contained in the mixture that do not contribute to graphitization are removed, and gas generation and the like during the graphitization treatment tend to be suppressed.

[0098] The temperature of the heat treatment is not particularly limited, but is preferably lower than the temperature of the heat treatment in step (c), and may be, for example, in the range of 500°C to 1000°C.

[0099] In step (c), the molded product obtained in step (b) is graphitized. The method for graphitizing the molded product is not particularly limited as long as the conditions are such that the graphitizable component contained in the mixture can be graphitized. For example, a method of heat treating the mixture in an atmosphere in which the mixture is unlikely to be oxidized can be used. The atmosphere in which the mixture is unlikely to be oxidized is not particularly limited, and examples thereof include an inert atmosphere such as nitrogen or argon, and a vacuum.

[0100] The temperature of the heat treatment for graphitization may be, for example, 1500°C or higher, 2000°C or higher, 2500°C or higher, or 2800°C or higher. The upper limit of the heat treatment temperature is not particularly limited, but may be, for example, 3200°C or lower. When the heat treatment temperature is 1500°C or higher, crystal changes tend to occur, making graphitization more likely to proceed. When the heat treatment temperature is 2000°C or higher, the development of graphite crystals tends to be better. On the other hand, when the heat treatment temperature for graphitization is 3200°C or lower, sublimation of a portion of the graphite tends to be suppressed.

[0101] In step (d), the graphitized material obtained in step (c) is pulverized to obtain a pulverized product. The pulverization method is not particularly limited, and can be performed by a known method using a jet mill, a vibration mill, a pin mill, a hammer mill, or the like. The particle size of the pulverized product may be adjusted to a desired size. The method for adjusting the particle size is not particularly limited, and examples thereof include a method using the above-mentioned pulverizing device and a method using a sieve.

[0102] If necessary, the pulverized material obtained in step (d) may be subjected to steps such as (e) a step of disposing low-crystalline carbon on at least a portion of the surface of the pulverized material, and (f) a step of mixing the pulverized material with other negative electrode active materials.

[0103] In step (e), low-crystalline carbon can be disposed on at least a portion of the surface of the pulverized material by, for example, mixing the pulverized material with a substance (such as a resin) that can be converted to low-crystalline carbon by heat treatment, followed by heat treatment. When low-crystalline carbon is disposed on at least a portion of the surface of the pulverized material, the input / output characteristics, such as rapid charge / discharge characteristics, of a lithium-ion secondary battery using the pulverized material as a negative electrode material may be improved.

[0104] [Other Steps] The method for producing an anode material according to the present disclosure may include steps other than those described above. For example, the method for producing an anode material may include a step of adhering an organic compound to the surface of the secondary particles after graphitization and then performing a heat treatment. By adhering an organic compound to the surface of the secondary particles and performing a heat treatment, the organic compound attached to the surface is converted into low-crystalline carbon. This allows the surface of the graphite particles to be coated with low-crystalline carbon.

[0105] The method for attaching an organic compound to the surface of the secondary particles is not particularly limited, and examples thereof include a wet method in which the secondary particles are dispersed and mixed in a mixed solution in which the organic compound is dissolved or dispersed in a solvent, and then the solvent is removed to attach the secondary particles; and a dry method in which mechanical energy is applied to a mixture obtained by mixing the secondary particles with a solid organic compound to attach the secondary particles.

[0106] The organic compound is not particularly limited as long as it is a compound (carbon precursor) that changes into low-crystalline carbon upon heat treatment. Examples include petroleum pitch, naphthalene, anthracene, phenanthroline, coal tar, phenolic resin, polyvinyl alcohol, etc. One type of organic compound may be used alone, or two or more types may be used in combination.

[0107] The heat treatment temperature when the secondary particles having an organic compound attached to their surfaces are heat treated is not particularly limited as long as the organic compound attached to the surfaces of the secondary particles is converted into low-crystalline carbon, and is preferably, for example, 400° C. to 1500° C. From the viewpoint of particularly enhancing high-temperature resistance, a temperature of 1000° C. to 1500° C. is more preferable. The heat treatment is preferably carried out in an inert gas atmosphere such as a nitrogen atmosphere.

[0108] The method for mixing the pulverized material with other negative electrode active materials in step (f) is not particularly limited. By mixing the pulverized material with other negative electrode active materials, the desired characteristics of the lithium ion secondary battery may be improved compared to when only the pulverized material is used as the negative electrode active material. Examples of other negative electrode active materials include, but are not limited to, graphite particles such as natural graphite and artificial graphite, and particles containing elements capable of absorbing and desorbing lithium ions. Examples of elements capable of absorbing and desorbing lithium ions include, but are not limited to, Si, Sn, Ge, and In.

[0109] The pulverized product obtained in step (f) may contain particles in which a plurality of flat graphite particles are aggregated or bonded together, or may contain particles in which a plurality of flat graphite particles are aggregated or bonded together so that the main surfaces of the graphite particles are non-parallel to each other (hereinafter also referred to as secondary graphite particles).

[0110] When the pulverized material is in the form of graphite secondary particles, the phenomenon in which the particles of the negative electrode material are oriented along the direction of the current collector when pressed to densify the negative electrode is suppressed, and there tends to be sufficient pathways for lithium ions to enter and exit the negative electrode material.

[0111] Furthermore, by including particles in which a plurality of flat graphite particles are aggregated or bonded, the influence of pressure applied during pressing on individual graphite particles is reduced due to voids present between the plurality of flat graphite particles, and fracture, cracking, etc. of the graphite particles tend to be suppressed. As a result, the lithium ion secondary battery tends to have excellent Li precipitation resistance.

[0112] In the present disclosure, the term "flat graphite particles" refers to non-spherical graphite particles having an anisotropic shape. Examples of flat graphite particles include graphite particles having a scaly, flake, or partially lumpy shape.

[0113] The flat graphite particles preferably have an aspect ratio, expressed as A / B, where A is the length in the major axis direction and B is the length in the minor axis direction, of, for example, 1.2 to 20, and more preferably 1.3 to 10. When the aspect ratio is 1.2 or more, the contact area between particles increases, tending to further improve conductivity. When the aspect ratio is 20 or less, input / output characteristics such as rapid charge / discharge characteristics of the lithium ion secondary battery tend to further improve.

[0114] The aspect ratio is determined by observing graphite particles under a microscope, randomly selecting 100 graphite particles, measuring the A / B ratio for each, and then calculating the arithmetic mean of these measurements. In observing the aspect ratio, the length A in the major axis direction and the length B in the minor axis direction are measured as follows. That is, in a projected image of a graphite particle observed under a microscope, two parallel tangents circumscribing the periphery of the graphite particle, tangent lines a1 and a2, which have the greatest distance between them, are selected, and the distance between these tangent lines a1 and a2 is defined as the length A in the major axis direction. Two parallel tangents circumscribing the periphery of the graphite particle, tangent lines b1 and b2, which have the smallest distance between them, are selected, and the distance between these tangent lines b1 and b2 is defined as the length B in the minor axis direction.

[0115] In the present disclosure, the phrase "main surfaces are non-parallel" of graphite secondary particles means that the surfaces (main surfaces) of the plurality of flat graphite particles with the largest cross-sectional areas are not aligned in a fixed direction. Whether the main surfaces of the plurality of flat graphite particles are non-parallel to each other can be confirmed by microscopic observation. When the plurality of flat graphite particles are aggregated or bonded together with their main surfaces non-parallel to each other, the orientation of the main surfaces of the flat graphite particles in the negative electrode is suppressed, expansion of the negative electrode during charging is suppressed, and the cycle characteristics of the lithium-ion secondary battery tend to be further improved. Note that the graphite secondary particles may partially include a structure in which the plurality of flat graphite particles are aggregated or bonded together with their respective main surfaces parallel to each other.

[0116] From the viewpoint of ease of aggregation or bonding, the average particle size of the flat graphite particles is, for example, preferably 1 μm to 50 μm, more preferably 1 μm to 25 μm, and even more preferably 1 μm to 15 μm. The average particle size of the flat graphite particles can be measured by a scanning electron microscope, and the average particle size of the flat graphite particles is, for example, the arithmetic mean value of the particle sizes of 100 flat graphite particles.

[0117] <<Negative electrode material composition for lithium ion secondary batteries>> The negative electrode material composition for lithium ion secondary batteries includes the negative electrode material for lithium ion secondary batteries of the present disclosure, a binder, and a solvent. The negative electrode material composition for lithium ion secondary batteries of the present disclosure may be in the form of a slurry obtained by kneading the negative electrode material for lithium ion secondary batteries and the binder together with a solvent. Kneading can be performed using a dispersing device such as a dispersing mixer or a planetary kneader.

[0118] The binder used in preparing the negative electrode material composition for lithium ion secondary batteries is not particularly limited. Examples of binders include styrene-butadiene copolymer (SBR), homopolymers or copolymers of ethylenically unsaturated carboxylic acid esters such as methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, butyl acrylate, butyl methacrylate, hydroxyethyl acrylate, and hydroxyethyl methacrylate, and ethylenically unsaturated carboxylic acids such as acrylic acid, methacrylic acid, itaconic acid, fumaric acid, and maleic acid, and polymeric compounds with high ionic conductivity such as polyvinylidene fluoride, polyethylene oxide, polyepichlorohydrin, polyphosphazene, polyacrylonitrile, and polymethacrylonitrile. When the negative electrode material composition for lithium ion secondary batteries contains a binder, the content of the binder is not particularly limited. For example, the binder content may be 0.5 to 20 parts by mass per 100 parts by mass of the total of the negative electrode material for lithium ion secondary batteries and the binder.

[0119] The negative electrode material composition for lithium ion secondary batteries may contain a thickener. Examples of the thickener that can be used include carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose, polyvinyl alcohol, polyacrylic acid or a salt thereof, oxidized starch, phosphated starch, and casein. When the negative electrode material composition for lithium ion secondary batteries contains a thickener, the content of the thickener is not particularly limited. For example, the content may be 0.1 to 5 parts by mass per 100 parts by mass of the negative electrode material for lithium ion secondary batteries.

[0120] The negative electrode material composition for lithium ion secondary batteries may contain a conductive auxiliary material. Examples of the conductive auxiliary material include carbon materials such as carbon black, graphite, and acetylene black, and inorganic compounds such as conductive oxides and conductive nitrides. When the negative electrode material composition for lithium ion secondary batteries contains a conductive auxiliary material, the content of the conductive auxiliary material is not particularly limited. For example, the content may be 0.5 parts by mass to 15 parts by mass per 100 parts by mass of the negative electrode material for lithium ion secondary batteries.

[0121] <<Negative electrode for lithium ion secondary battery>> The negative electrode for lithium ion secondary battery of the present disclosure includes a negative electrode material layer containing the negative electrode material for lithium ion secondary batteries of the present disclosure, and a current collector. The negative electrode for lithium ion secondary battery may include other components as necessary in addition to the negative electrode material layer containing the negative electrode material for lithium ion secondary batteries of the present disclosure and the current collector.

[0122] The negative electrode for a lithium ion secondary battery can be produced, for example, by preparing the above-described negative electrode material composition for a lithium ion secondary battery according to the present disclosure and applying it onto a current collector to form a negative electrode material layer, or by forming the negative electrode material composition for a lithium ion secondary battery into a shape such as a sheet or pellet and integrating it with a current collector.

[0123] The material of the current collector is not particularly limited and can be selected from aluminum, copper, nickel, titanium, stainless steel, etc. The state of the current collector is not particularly limited and can be selected from foil, perforated foil, mesh, etc. Porous materials such as porous metal (foam metal), carbon paper, etc. can also be used as the current collector.

[0124] When the negative electrode material composition for lithium ion secondary batteries is applied to a current collector to form a negative electrode material layer, the method is not particularly limited, and known methods such as metal mask printing, electrostatic coating, dip coating, spray coating, roll coating, doctor blade coating, comma coating, gravure coating, and screen printing can be used. After the negative electrode material composition for lithium ion secondary batteries is applied to the current collector, the solvent contained in the negative electrode material composition for lithium ion secondary batteries is removed by drying. Drying can be performed using, for example, a hot air dryer, an infrared dryer, or a combination of these devices. If necessary, the negative electrode material layer may be subjected to a rolling treatment. The rolling treatment can be performed using a plate press, a calendar roll, or the like.

[0125] When the negative electrode material composition for lithium ion secondary batteries formed into a shape such as a sheet or pellet 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 using a roll, a flat press, or a combination of these means. The pressure when integrating the negative electrode material composition for lithium ion secondary batteries with the current collector is preferably, for example, about 1 MPa to 200 MPa.

[0126] The negative electrode density of the negative electrode material layer is not particularly limited and is, for example, 1.1 g / cm ３ ~1.8g / cm ３ and preferably 1.1 g / cm ３ ~1.7g / cm ３ More preferably, it is 1.1 g / cm ３ ~1.6g / cm ３ It is more preferable that the negative electrode density is 1.1 g / cm ３ By setting the density to 1.8 g / cm or more, an increase in electrical resistance is suppressed and the capacity tends to increase. ３ By setting the above, deterioration of input characteristics and cycle characteristics tends to be suppressed.

[0127] Lithium-ion secondary battery The lithium-ion secondary battery of the present disclosure includes the negative electrode for lithium-ion secondary batteries of the present disclosure, a positive electrode, and an electrolyte solution.

[0128] The positive electrode can be obtained by forming a positive electrode material layer on a current collector in the same manner as the negative electrode described above. The current collector can be made of a metal or alloy such as aluminum, titanium, or stainless steel in the form of a foil, perforated foil, mesh, or the like.

[0129] The positive electrode material used to form the positive electrode layer is not particularly limited. Examples of the positive electrode material include metal compounds (metal oxides, metal sulfides, etc.) that can dope or intercalate lithium ions, and conductive polymer materials. More specifically, lithium cobalt oxide (LiCoO ２ ), lithium nickel oxide (LiNiO ２ ), lithium manganese oxide (LiMnO ２ ), and their double oxides (LiNi ｘ Mn ｙ Co ｚ O ２ , x + y + z = 1), a composite oxide containing an additional element M' (LiNi ａ Mn ｂ Co ｃ M' ｄ O ２ , a+b+c+d=1, M′: Al, Mg, Ti, Zr or Ge), spinel-type lithium manganese oxide (LiMn ２ O ４ ), lithium vanadium compounds, V ２ O ５ , V ６ O １３ , V.O. ２ , MnO ２ , TiO ２ , MoV ２ O ８ , TiS ２ , V ２ S ５ , V.S. ２ , MoS ２ , MoS ３ , Cr ３ O ８ , Cr ２ O ５ , olivine-type LiMPO ４Examples of the positive electrode material include metal compounds such as (M: Co, Ni, Mn, Fe), conductive polymers such as polyacetylene, polyaniline, polypyrrole, polythiophene, and polyacene, and porous carbon. The positive electrode material may be one type alone or two or more types.

[0130] The electrolytic solution is not particularly limited, and for example, a solution in which a lithium salt as an electrolyte is dissolved in a non-aqueous solvent (a so-called organic electrolytic solution) can be used. As the lithium salt, LiClO ４ , LiPF ６ , LiAsF ６ , LiBF ４ , LiSO ３ CF ３ and the like. The lithium salt may be used alone or in combination of two or more. Examples of the non-aqueous solvent include ethylene carbonate, fluoroethylene carbonate, chloroethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, cyclopentanone, cyclohexylbenzene, sulfolane, propane sultone, 3-methyl sulfolane, 2,4-dimethyl sulfolane, 3-methyl-1,3-oxazolidin-2-one, γ-butyrolactone, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, butyl methyl carbonate, ethyl propyl carbonate, butyl ethyl carbonate, dipropyl carbonate, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, methyl acetate, ethyl acetate, trimethyl phosphate, and triethyl phosphate. The non-aqueous solvent may be used alone or in combination of two or more.

[0131] The state of the positive electrode and the negative electrode in the lithium ion secondary battery is not particularly limited. For example, the positive electrode, the negative electrode, and a separator disposed between the positive electrode and the negative electrode as needed may be spirally wound or may be stacked in the form of flat plates.

[0132] The separator is not particularly limited, and for example, a resin nonwoven fabric, cloth, microporous film, or a combination thereof can be used. Examples of resins include those containing polyolefins such as polyethylene and polypropylene as the main component. If the positive electrode and negative electrode do not come into contact with each other due to the structure of the lithium ion secondary battery, a separator may not be used.

[0133] The shape of the lithium ion secondary battery is not particularly limited, and examples thereof include laminated type batteries, paper type batteries, button type batteries, coin type batteries, stacked type batteries, cylindrical type batteries, and prismatic type batteries.

[0134] The lithium ion secondary battery of the present disclosure is suitable as a large-capacity lithium ion secondary battery for use in electric vehicles, power tools, power storage devices, and the like.

[0135] The present disclosure will be specifically described below using examples, but the present disclosure is not limited to these examples.

[0136] [Example 1] Petroleum-derived green coke (semi-needle coke) was used as a raw material for graphite particles. The green coke was coarsely crushed using a hammer mill. The coarsely crushed material was sieved using a sieve with 3 mm openings, and the remaining material was sieved using a sieve with 1 mm openings to collect granules with particle sizes of 1 mm to 3 mm.

[0137] The obtained coke particles were pulverized and classified using a roller mill. Next, the coke particles were subjected to a spheronization treatment at a peripheral speed of 75 m / s for 50 minutes. As a result, the coke particles shown in Table 1 were obtained. Then, the volume-based particle size distribution of the coke particles was determined using a laser diffraction particle size distribution analyzer (SALD3100, Shimadzu Corporation). The results are shown in Table 1.

[0138] 65 parts by mass of the obtained coke particles (90% by mass of fixed carbon), 7 parts by mass of starch (20% by mass of fixed carbon), and 28 parts by mass of water were kneaded using a kneader to obtain a mixture.

[0139] The resulting mixture was then uniaxially pressed at room temperature to a density of 1.3 g / cm ３The resulting molded product was molded as follows. The molded product was then heat-treated at 850°C for 8 hours in a nitrogen atmosphere. It was then graphitized at 3000°C for 30 hours. The resulting particles were then sieved through a 280-mesh screen to obtain graphite particles, which are negative electrode materials. The volume-based particle size distribution of the graphite particles was determined using a laser diffraction particle size distribution analyzer (SALD3100, Shimadzu Corporation). The D10 was 16.0 μm, the D50 was 23.6 μm, and the D90 was 34.7 μm.

[0140] [Examples 2 and 3] Coke particles were obtained in the same manner as in Example 1, except that the time for spheronization of the coke particles was changed to 35 minutes or 15 minutes. The volume-based particle size distribution of the coke particles was determined in the same manner as in Example 1. The results are shown in Table 1. Furthermore, graphite particles, which are negative electrode materials, were obtained in the same manner as in Example 1. The volume-based particle size distribution of the graphite particles was determined in the same manner as in Example 1. The results are shown in Table 2.

[0141] Comparative Example 1 Coke particles were obtained in the same manner as in Example 1, except that the coke particles were not subjected to spheronization treatment. The volume-based particle size distribution of the coke particles was determined in the same manner as in Example 1. The results are shown in Table 1. Furthermore, graphite particles, which are negative electrode materials, were obtained in the same manner as in Example 1. The volume-based particle size distribution of the graphite particles was determined in the same manner as in Example 1. The results are shown in Table 2.

[0142] The coke particles and negative electrode materials obtained in each example and comparative example were used to measure and evaluate the physical properties shown below. The results are shown in Tables 1 and 2.

[0143] (Particle size distribution) A laser diffraction particle size distribution analyzer (SALD3100, Shimadzu Corporation) was used to determine the volumetric particle size distribution of coke or graphite particles. Specifically, D10, D50, D90, and D90 / D10 were determined.

[0144] (Circularity) The circularity of the coke or negative electrode material was measured using a wet flow particle size and shape analyzer (FPIA-3000 manufactured by Malvern Instruments). The circularity based on the number of particles at 50% of the cumulative total (so-called average circularity) was taken as the circularity.

[0145] (Specific Surface Area) Coke or anode material was filled into a measurement cell, and a sample was obtained by heat pretreatment at 200°C while being degassed under vacuum. Nitrogen gas was adsorbed into the sample using a gas adsorption apparatus (ASAP2010, manufactured by Shimadzu Corporation). The obtained sample was subjected to BET analysis using a five-point method to determine the specific surface area (SSA in the table).

[0146] (Tap density) Volume 150 cm ３ 100 cm of sample powder of coke or anode material was placed in a graduated flat-bottom test tube (Kuramochi Scientific Instruments Manufacturing Co., Ltd., KRS-406). ３ The graduated flat-bottom test tube was then stoppered. The zero tap density (TAP0) was determined from the mass and volume of the sample powder before the stoppered graduated flat-bottom test tube was dropped. The 30 tap density (TAP30) and 250 tap density (TAP250) were also determined from the mass and volume of the sample powder after the graduated flat-bottom test tube was dropped 30 or 250 times from a height of 5 cm.

[0147] (Compression Ratio) The compression ratio was calculated from the TAP0 and TAP250 obtained as described above using the following formula: Compression ratio = (TAP250 - TAP0) / TAP250

[0148] (Compression Load) 3.0 g of negative electrode material was filled into a 15 mm diameter mold and compressed at a constant rate of 10 mm / min using an autograph (Shimadzu Corporation). During this compression, the distance from the bottom surface of the negative electrode material to the press surface was measured, and this was multiplied by the bottom area of ​​the mold to obtain the volume of the negative electrode material, from which the density during compression was calculated. A load cell was attached to the press hammer of the autograph, and the predetermined density of 1.7 g / cm was calculated. ３ The applied pressure (kN / cm ２ ) was used as the compressive load.

[0149] (Springback rate) A standard density of 1.7 g / cm when pressed by the above method using an Autograph (manufactured by Shimadzu Corporation) ３ The springback rate was calculated using the following formula: Springback rate = {(standard density - density after pressure release) / standard density} x 100 = {(1.7 - density after pressure release) / 1.7} x 100

[0150] (Oil Absorption) The oil absorption of coke or anode material was measured using the method described in JIS K6217-4:2017 "Carbon Black for Rubber - Fundamental Properties - Part 4: Determination of Oil Absorption," using linseed oil (e.g., manufactured by Kanto Chemical Co., Ltd.) as the reagent liquid instead of dibutyl phthalate (DBP). Specifically, linseed oil was titrated into the target powder using a constant-speed burette, and the change in viscosity characteristics was 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 (the value shown in Table 1 or Table 2) was defined as the oil absorption (mL / 100 g). An absorption measuring device (product name: S-500) manufactured by Asahi Research Institute, Ltd. was used as the measuring instrument.

[0151] (Graphitization Degree) 10 or 20 parts by weight of silicon powder (e.g., NIST SRM640f) was mixed with the negative electrode material in an agate mortar for 5 minutes, and the resulting mixture was placed in a sample holder for X-ray diffraction measurement. Using an X-ray diffraction measurement device (e.g., Rigaku Corporation X-ray Diffraction Meter MultiFlex), the diffraction angle corresponding to the graphite (002) plane and the diffraction angle corresponding to the silicon (111) plane were measured by X-ray diffraction measurement (2θ = 25° to 29°) using CuKα radiation. The theoretical diffraction angle of Si (2θ = 28.441°) was used to correct the observed diffraction angles of silicon and graphite to determine the correct diffraction angle of graphite. The interplanar spacing (Å) of the d(002) plane of the negative electrode material was calculated using Bragg's equation (2d sin θ = nλ), and the graphitization degree was calculated using the following formula: Graphitization degree = [(3.44 - interplanar spacing) / (0.086)] × 100

[0152] (Crystallite Size) In the same manner as above, an X-ray diffraction measurement apparatus (for example, an X-ray diffraction measurement apparatus manufactured by Rigaku Corporation, X-ray Diffraction Meter MultiFlex) was used to carry out X-ray diffraction measurement (75.5° to 78.5°) using CuKα radiation. Peak fitting was performed on the diffraction line corresponding to the (110) plane of the obtained graphite and the diffraction line corresponding to the (331) plane of silicon, and the diffraction lines were separated into Kα1 and Kα2 diffraction lines. Thereafter, the half width of the Kα1 diffraction line of graphite was corrected with the half width of the Kα1 diffraction line of silicon, and the crystallite size Lc (nm) was calculated using the Scherrer formula (crystallite size = K λ / β cos θ). K: shape factor (generally 0.9) λ: wavelength of X-ray β: peak half width (unit: radian) θ: diffraction angle of peak (unit: radian)

[0153] (Average Planar Spacing) 10 or 20 parts by mass of silicon powder (e.g., NIST, SRM640f) was mixed with the negative electrode material in an agate mortar for 5 minutes, and the resulting mixture was placed on a sample holder for X-ray diffraction measurement. Using an X-ray diffraction measurement device (e.g., Rigaku Corporation's X-ray Diffraction Meter MultiFlex), X-ray diffraction measurements (2θ = 25° to 29°) using CuKα radiation were performed to measure the diffraction angle corresponding to the graphite (002) plane and the diffraction angle corresponding to the silicon (111) plane. The theoretical diffraction angle of Si (2θ = 28.441°) was used to correct the observed diffraction angles of silicon and graphite to determine the correct diffraction angle of graphite. Measurements were performed under the following conditions, and the average planar spacing (d) of the negative electrode material was calculated using the Bragg equation. ００２ ) was calculated. Radiation source: CuKα ray (wavelength = 0.15418 nm) Output: 40 kV, 20 mA Sampling width: 0.010° ​​Scanning range: 10° to 35° Scanning speed: 0.5° / min Bragg's equation: 2d sin θ = nλ Here, d is the length of one period, θ is the diffraction angle, n is the reflection order, and λ is the X-ray wavelength.

[0154] (Electrode Orientation) A negative electrode for a lithium ion secondary battery was prepared as follows, and the electrode orientation was evaluated under the conditions shown below. - Preparation of a negative electrode for a lithium ion secondary battery - 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) were kneaded to prepare a negative electrode material composition. This negative electrode material composition was applied to the shiny surface of an electrolytic copper foil in an amount of 10 mg / cm. ２ The electrode was pre-dried at 90°C for 2 hours, and then adjusted by roll pressing to the electrode density shown in Table 3. The electrode was then cured by drying at 120°C for 4 hours in a vacuum atmosphere, forming a negative electrode material layer on the electrolytic copper foil to obtain a negative electrode for a lithium ion secondary battery. - Evaluation of electrode orientation - The obtained negative electrode for a lithium ion secondary battery was cut into a 5 cm wide and 600 cm area. ２ The negative electrode was formed into a rectangular sheet, and the resulting molded product was pressed with a 4 t hydraulic pressure using a roll-type press. The linear pressure at this time was 4 t / 5 cm = 0.8 t / cm. The pressed negative electrode was placed in an X-ray diffraction measurement cell, and an X-ray diffraction pattern was measured using a CuKα ray using an X-ray diffraction measurement device (X-RAY DIFFRACTION METER MultiFlex manufactured by Rigaku Corporation) under the conditions of a scan rate of 0.25° / min, a tube voltage of 40 kV, a tube current of 30 mA, a divergence slit of 1°, a scattering slit of 1°, and a receiving slit of 0.3 mm (2θ = 25.5° to 27.5°, 76.5° to 78.5°). The peak intensity (I ００２ ) and the peak intensity of the 110 diffraction line (I １１０ ) and I ００２ / I １１０ (002 / 110 in Table 3) was determined, and this value was taken as the electrode orientation.

[0155] (Adhesion Evaluation) A negative electrode material composition was prepared using the obtained negative electrode material, and the negative electrode material composition was applied to rolled copper foil to form a negative electrode material layer, and the adhesion between the rolled copper foil and the negative electrode material layer was evaluated. A negative electrode material composition was prepared using the obtained negative electrode material in the same manner as described above. Next, the negative electrode material composition was applied to a rolled copper foil with a thickness of 20 μm in an amount of 35.0 mg / cm per unit area. ２The rolled copper foil on which the negative electrode material layer was formed was punched out to a size of 1.0 cm × 5.0 cm to obtain a copper foil with a negative electrode material layer for evaluating adhesion.

[0156] Next, a double-sided tape G9000 manufactured by Dexerials Corporation was attached to a laterally movable base, and the copper foil side of the copper foil with a negative electrode material layer was attached to the side of the double-sided tape opposite to the side attached to the base. Then, an adhesive tape (18 mm wide) manufactured by 3M Company was attached to the negative electrode material layer side of the copper foil with a negative electrode material layer so that the edge of the adhesive tape was exposed, thereby preparing a sample for evaluating peeling of the negative electrode material layer.

[0157] For the prepared sample for evaluating peeling of the negative electrode material layer, the exposed end of the adhesive tape was grasped with a peel strength device (push-pull scale & digital force gauge, manufactured by Imada Co., Ltd.), and while moving the base laterally at a speed of 20 mm / min, the end of the adhesive tape was pulled upward at a speed of 20 mm / min, thereby peeling the negative electrode material layer attached to the adhesive tape and the copper foil attached to the double-sided tape, and the peel strength of the negative electrode material layer at this time was measured.

[0158] (SOC-Li (Li deposition resistance)) A lithium ion secondary battery was fabricated using the negative electrode for a lithium ion secondary battery obtained above, and the Li deposition resistance was evaluated as follows. - Fabrication of Lithium Ion Secondary Battery - A lithium ion secondary battery negative electrode obtained above, metallic lithium as a counter electrode, 1 M LiPF as an electrolyte, and ６ A lithium ion secondary battery coin cell was fabricated using a mixed solution of ethylene carbonate / ethyl methyl carbonate (3:7 volume ratio) containing 1.0% vinylene carbonate (VC), a 25 μm thick polyethylene microporous membrane as a separator, and a 250 μm thick copper plate as a spacer. -Evaluation of Li deposition resistance- The fabricated lithium ion secondary battery was placed in a thermostatic chamber set at 25°C, and a voltage of 0.005 V (V vs. Li / Li) was measured at a current value of 0.1 C from the first to third cycles. ＋), and then constant voltage charging was performed at 0.005 V until the current value reached 0.05 C. After a 30-minute pause, the battery was charged at a current value of 0.2 C until the voltage reached 1.5 V (V vs. Li / Li ＋ ) was discharged at a constant current up to 1 C. The discharge capacity of the third cycle was taken as 1 C in the Li deposition test. The fourth cycle was charged at a current density of 3 C, which was charged to the discharge capacity of the third cycle in 20 minutes, and the termination condition was 1 C capacity regulation. The first inflection point in the differential profile (dV / dQ, V is voltage, Q is capacitance) of the obtained fourth cycle charge curve was taken as the Li deposition onset point, and the capacity at this time was expressed as a percentage of the discharge capacity of the third cycle (or the charge capacity of the fourth cycle set to be the same capacity), and Li deposition resistance was evaluated. The results are shown in Table 3. The higher this percentage value, the better the Li deposition resistance.

[0159] (Measurement of DC Resistance (DCR)) The DC resistance (DCR) of the lithium ion secondary battery was measured to determine the input characteristics of the battery. Specifically, the following procedure was carried out. The negative electrode for the lithium ion secondary battery obtained above was used, and a lithium nickel cobalt manganese oxide (LiNi) was used as the positive electrode. ０．３３ Mn ０．３３ Co ０．３３ O ２ The electrode was made of a composite material containing 94 parts by mass of ethylenediamine fluoride, 3 parts by mass of carbon black, and 3 parts by mass of polyvinylidene fluoride. The electrolyte was 1M LiPF ６A single-layer laminate full cell (lithium ion secondary battery) was prepared using a mixture of ethylene carbonate / diethyl carbonate (3:7 volume ratio) and vinylene carbonate (VC) (1.0 mass%), and a 25 μm thick polyethylene microporous membrane as a separator. The lithium ion secondary battery (single-layer laminate full cell) was placed in a thermostatic chamber set at 25 ° C. and subjected to three cycles of charge / discharge under the following conditions: charge: CC / CV 0.2 C 4.2 V 0.02 C cut, discharge: CC 0.2 C 2.5 V cut. Next, a constant current charge of 0.2 C was performed to an SOC of 50%. The lithium ion secondary battery was placed in a thermostatic chamber set at 25°C and charged at constant currents of 1C, 2C, and 3C for 10 seconds each, the voltage drop (ΔV) at each constant current was measured, and the direct current resistance (DCR) was measured using the following formula: DCR [Ω] = {(2C voltage drop ΔV - 1C voltage drop ΔV) + (3C voltage drop ΔV - 2C voltage drop ΔV)} / {(3C control current value I - 2C control current value I) + (2C control current value I - 1C control current value I)}

[0160] (Void Analysis) A void analysis of graphite particles contained in a negative electrode material was performed as follows. First, a cross-section polisher manufactured by JEOL Ltd. was used to process the cross sections of the particles at room temperature, 8 kV, and 6 hours. The obtained SEM images were binarized into void portions and regions other than the void portions, with the void portions designated as 0 (black) and the regions other than the void portions (solid portions) designated as 1 (white). Eight images of graphite particles with a pixel count of 10,000 or more were selected from the images obtained by binarization. The average value of the distribution of 0s and 1s (void volume) was calculated from the distribution of 0s and 1s per pixel in the X-axis direction for each of the selected graphite particle images. The arithmetic mean value of the void volume obtained from the eight graphite particle images was taken as the average void volume, and is shown in Table 4. A larger average void volume indicates a higher proportion of solid portions. Furthermore, the number of pixels corresponding to the void portions and the number of pixels corresponding to the solid portions were calculated for each of the eight selected graphite particle images. The average number of pixels corresponding to the void portions is shown in Table 4. Next, for the image of one selected graphite particle, the number of pixels corresponding to the void portion and the number of pixels corresponding to the solid portion were each calculated. Then, the ratio of the number of pixels corresponding to the void portion to the total number of pixels corresponding to the void portion and the solid portion (number of pixels in the void portion / [number of pixels in the void portion + number of pixels in the solid portion], also referred to as the ratio of the number of pixels in the void portion) was calculated. For the remaining seven selected graphite particle images, the ratio of the number of pixels in the void portion was also calculated, and the arithmetic average value was taken as the average ratio of the number of pixels in the void portion, which is shown in Table 4. The total number of pixels for the images of the eight selected graphite particles is also shown in Table 4.

[0161] (Hue Analysis) A measurement sample for hue analysis was prepared using the negative electrode material of Example 1 or Comparative Example 1. Specifically, 3.7 g of the negative electrode material obtained as described above, 7.0 g of epoxy resin (bisphenol A-type epoxy resin), and 1.6 g of curing agent (modified aliphatic polyamine-based curing agent) were placed in a container and mixed. The resulting mixture was polished using a polishing device (product name: MetaZabe 250). The abrasives used were #600 waterproof abrasive paper, 6 μm diameter diamond, 1 μm diameter diamond, and alumina, in that order. Through the above process, a polished measurement sample was prepared. The polished measurement sample was measured using a two-dimensional birefringence evaluation system (Photonic Lattice WPA-micro). The measurement conditions were high-speed measurement accuracy, single-wavelength measurement, and a wavelength of 543 nm. The obtained measurement image was analyzed with a principal axis color display of 180° and an axial fast axis. A color image of the principal axis orientation was obtained from the two-dimensional birefringence image, and RGB (red, green, blue) and HLS (hue, brightness, saturation) data for each pixel was extracted using JMP from SAS, and graphite particles were identified. A biconical model was used to convert the RGB coordinate space to the HSL coordinate space. The hue (H) data of one graphite particle was plotted, and the standard deviation and variance of the graphite particle hue were determined. The standard deviation and variance of the hue of a total of 10 graphite particles were calculated, and their arithmetic average values ​​were used as the average standard deviation and variance of the graphite particle hue, as shown in Table 4.

[0162] The laminated lamicelle for expansion measurement was prepared under the following conditions: the negative electrode composition was graphite / conductive additive (carbon black) / CMC / SBR = 96.0 / 1.0 / 1.5 / 1.5 (mass ratio), and the coating amount was 10.0 mg / cm ２ , electrode density is 1.60 g / cm ３ The positive electrode composition was NMC811 / conductive additive (carbon black) / PVdF = 96.5 / 1.5 / 2.0 (mass ratio), and the coating amount was 14.1 mg / cm ２ , electrode density is 3.2 g / cm ３ The separator was a single layer of PP, and the electrolyte was 1 mol / L LiPF ６The battery was constructed using a 3 / 7 mixture of ethyl carbonate (EC) and diethyl carbonate (DEC) with 1.0% by mass of vinylene carbonate (VC). The capacity balance between the positive and negative electrodes was designed to be 1.07 to 1.13. Five negative electrodes (double-sided coated) and four positive electrodes (double-sided coated) were alternately stacked, and the electrolyte volume was 2100 μL. The fabricated laminated cell was pre-charged (45°C, CC = 0.03C, CV = 3.4V, 0.01C cut off → degassing 1), then aged (1st charge: 45°C CC = 0.1C, CV = 4.2V, 0.02C cut off 12h hold → degassing 2, 1st discharge: 25°C CC = 0.1C, EV = 2.8V, 2nd charge: 25°C CC = 0.2C, CV = 4.2V, 0.05C cut off, 2nd discharge: 25°C CC = 0.2C, EV = 2.8V 2cy), and then expansion was measured. For expansion measurement, a Mitutoyo LG100-0125 displacement meter was used, and the displacement sampling width was 10 seconds. A load of about 0.07 kg / cm was applied to the laminated cell. ２ The charge-discharge conditions for the expansion measurement were: discharge at 0.2 C CC 2.5 V → rest for 15 min → charge at 0.2 C CC 4.25 V → rest for 15 min → discharge at 0.2 C CC 2.5 V → rest for 15 min.

[0163] (SOC expansion) The SOC expansion was determined by subtracting the displacement in the discharged state @2.5 V from the displacement in the charged state @4.25 V at 20 cycles or 70 cycles, and dividing the result by the estimated effective mixture layer thickness (total thickness of only the negative electrode layer facing the positive electrode).

[0164] (Deterioration and Swelling) The deterioration and swelling was determined by dividing the change in discharge state @ 2.5 V after 20 cycles or 70 cycles by the estimated effective mixture layer thickness (total thickness of only the negative electrode layer facing the positive electrode).

[0165]

[0166]

[0167]

[0168]

[0169] As shown in Table 4, in Examples 1 to 3, the average void volume was within the range of 0.700 to 0.940, and SOC swelling and deterioration swelling were suppressed compared to Comparative Example 1. Furthermore, as shown in Table 3, in each Example, a lithium ion secondary battery having excellent input / output characteristics and adhesion between the negative electrode and the current collector was manufactured. Furthermore, in each Example, Li deposition resistance was superior to Comparative Example 1.

[0170] The disclosures of PCT / JP2023 / 046456, PCT / JP2023 / 046457, PCT / JP2023 / 046458, PCT / JP2023 / 046459, PCT / JP2023 / 046460, and PCT / JP2023 / 046461, filed on December 25, 2023, are incorporated herein by reference in their entireties. All publications, patent applications, and technical standards mentioned herein are incorporated herein by reference to the same extent as if each individual publication, patent application, and technical standard was specifically and individually indicated to be incorporated by reference.

Claims

1. A negative electrode material for a lithium-ion secondary battery, comprising a plurality of graphite particles. When the SEM image of the cross-section of the graphite particles is binarized into a void portion and a region other than the void portion, with the void portion being 0 and the region other than the void being 1, a plurality of images of graphite particles with 10,000 or more pixels are selected from the binarized image. For the selected images of the graphite particles, the average value of the void volume is 0.700 to 0.

940.

2. A plurality of images of graphite particles with 10,000 or more pixels are selected from the binarized image. When the number of pixels corresponding to the void portion and the number of pixels corresponding to the region other than the void portion are determined for the selected images of the graphite particles, the total number of pixels in the selected images of the graphite particles is 100,000 or more, and the average value of the number of pixels corresponding to the void portion in one image of the graphite particles is 1,000 to 20,000. The negative electrode material for a lithium-ion secondary battery according to claim 1.

3. A plurality of images of graphite particles with 10,000 or more pixels are selected from the binarized image. When the number of pixels corresponding to the void portion and the number of pixels corresponding to the region other than the void portion are determined for the selected images of the graphite particles, the total number of pixels in the selected images of the graphite particles is 100,000 or more, and the average value of the ratio of the number of pixels corresponding to the void portion in one image of the graphite particles to the total number of pixels corresponding to the void portion and the region other than the void portion in one image of the graphite particles is 0.030 to 0.

300. The negative electrode material for a lithium-ion secondary battery according to claim 1 or claim 2.

4. Regarding the standard deviation and dispersion of the hue of the graphite particles obtained from the cross-sectional birefringence image of the graphite particles, the total number of pixels in the birefringence image is 10,000 or more, the average value of the standard deviation of the hue of the graphite particles is 0.210 to 0.400, and the average value of the dispersion of the hue of the graphite particles is 0.0550 to 0.1000. The negative electrode material for a lithium-ion secondary battery according to any one of claims 1 to 3.

5. The true density of the graphite particles is 2.20 g / cm ３ The negative electrode material for a lithium ion secondary battery according to any one of claims 1 to 4, wherein the true density is 2.20 g / cm or less.

6. The graphite particles include artificial graphite particles, and the artificial graphite particles are particles obtained by graphitizing a mixture containing a graphitizable aggregate and a graphitizable binder, and the binder includes a water-soluble or water-absorbing polymer compound. The negative electrode material for a lithium ion secondary battery according to any one of claims 1 to 5.

7. The negative electrode material for a lithium ion secondary battery according to any one of claims 1 to 6, wherein the roundness of the graphite particles is 91.0% or more.

8. The negative electrode material for a lithium ion secondary battery according to any one of claims 1 to 7, wherein the oil absorption amount of the graphite particles is 35 mL / 100 g or less.

9. The negative electrode material for a lithium ion secondary battery according to any one of claims 1 to 8, wherein the graphitization degree of the graphite particles is 93.5% or less.

10. The compressive load of the graphite particles is 2.50 kN / cm ２ The negative electrode material for a lithium ion secondary battery according to any one of claims 1 to 9, wherein the above conditions are satisfied.

11. A negative electrode for a lithium ion secondary battery, including a negative electrode material layer containing the negative electrode material for a lithium ion secondary battery according to any one of claims 1 to 10, and a current collector.

12. A lithium ion secondary battery, including the negative electrode for a lithium ion secondary battery according to claim 11, a positive electrode, and an electrolytic solution.

13. A method for manufacturing a negative electrode material for a lithium ion secondary battery, which is a method for manufacturing a negative electrode material for a lithium ion secondary battery according to any one of claims 1 to 10, and includes: (a) a step of obtaining a mixture containing a graphitizable aggregate, a water-soluble or water-absorbing polymer compound, and a graphitizable binder; (b) a step of molding the mixture to obtain a molded product; (c) a step of graphitizing the molded product to obtain a graphitized product; and (d) a step of pulverizing the graphitized product to obtain a pulverized product.

14. The method for manufacturing a negative electrode material for a lithium ion secondary battery according to claim 13, including a step of spheroidizing the graphitizable aggregate before the step (a).

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