Negative electrode material for lithium ion secondary battery, negative electrode for lithium ion secondary battery, and lithium ion secondary batterυ

Abstract

A negative electrode material for a lithium ion secondary battery, said negative electrode material including graphite particles, wherein the graphite particles satisfy conditions in which, with regard to an R value which is the intensity ratio Id / Ig of the intensity Ig of the maximum peak in the range 1580 cm–1 to 1620 cm–1 and the intensity Id of the maximum peak in the range 1300 cm–1 to 1400 cm–1 in a Raman spectrum obtained through Raman spectroscopy, the percentage of particles for which R≥0.2 is greater than or equal to 10% by number, and the average value of the half-value width of Id in the R-value top 10 spectrum is less than or equal to 60 cm–1.

Description

Negative electrode material for lithium ion secondary battery, negative electrode for lithium ion secondary battery, and 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, and 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 has led to an increasing demand for anode materials for lithium-ion secondary batteries. To improve battery performance, it is particularly desirable to reduce the irreversible capacity during the initial charge / discharge cycle and increase initial efficiency. To enhance convenience, there is a strong need for rapid charging, and high-rate charging is required for anode active materials.

[0007] The problem with such high-rate charging is that, due to IR drop when a large current is applied, some of the lithium ions cannot be inserted between the graphite layers, and lithium metal is likely to precipitate on the surface of the graphite particles, etc. If lithium metal precipitates in the form of needles, it may break through the separator, causing an internal short circuit and leading to thermal runaway.

[0008] From the above viewpoints, a negative electrode material for lithium ion secondary batteries is desirable that can suppress the deposition of lithium metal and enable the production of lithium ion secondary batteries that are excellent in resistance to lithium (Li) deposition.

[0009] An object of the present disclosure is to provide a negative electrode material for lithium ion secondary batteries that can produce lithium ion secondary batteries that have excellent initial efficiency, can suppress lithium metal precipitation, and has excellent Li precipitation resistance, as well as a negative electrode for lithium ion secondary batteries that includes the same, and a lithium ion secondary battery.

[0010] The means for solving the above problems include the following aspects: <1> The graphite particles include a graphite particle having a peak at 1580 cm in a 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 The ratio of particles with R≧0.2 is 10% by number or more, and the average half-width of Id in the top 10 spectra with R values ​​is 60 cm －１ <2> A negative electrode material for a lithium ion secondary battery that satisfies the following: <2> The negative electrode material contains graphite particles, and the graphite particles have a Raman spectrum obtained by Raman spectroscopy measurement. －１ ~1620cm －１ The intensity Ig of the maximum peak in the range of 1300 cm －１ ~1400cm －１The negative electrode material for a lithium ion secondary battery is characterized in that, with respect to an R value, which is the intensity ratio Id / Ig of the maximum peak intensity Id in the range of 10 to 100, the proportion of particles satisfying R≧0.2 is 10% by number or more, and the graphite particles have an oxidation mass loss reaction onset temperature T of 700°C or higher, as determined by TG-DTA measurement in a dry air atmosphere. <3> The negative electrode material for a lithium ion secondary battery according to <1> or <2>, wherein the graphite particles contain artificial graphite particles. <4> The negative electrode material for a lithium ion secondary battery according to <3>, wherein the artificial graphite particles are obtained by graphitizing a mixture containing a graphitizable aggregate and a graphitizable binder, and the binder contains a water-soluble or water-absorbent polymer compound. <5> The negative electrode material for a lithium ion secondary battery according to any one of <1> to <4>, wherein the graphite particles have a circularity of 93% or less. <6> The negative electrode material for a lithium ion secondary battery according to any one of <1> to <5>, wherein the graphite particles have an oil absorption of 40 mL / 100 g to 70 mL / 100 g. <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 ratio of particles satisfying R≧0.2 of 80% or less by number. <8> The negative electrode material for a lithium ion secondary battery according to any one of <1> to <7>, wherein the surfaces of the graphite particles are not subjected to a treatment for coating with low-crystalline carbon. <9> 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 <8>; and a current collector. <10> A lithium ion secondary battery comprising: the negative electrode for a lithium ion secondary battery according to <9>, a positive electrode, and an electrolyte.

[0011] The present disclosure can provide a negative electrode material for lithium ion secondary batteries that can produce lithium ion secondary batteries that have excellent initial efficiency, can suppress lithium metal precipitation, and has excellent Li precipitation resistance, as well as a negative electrode for lithium ion secondary batteries that includes the same, and a lithium ion secondary battery.

[0012] Hereinafter, embodiments for carrying out the present invention will be described in detail. However, the present invention is not limited to the following embodiments. In the following embodiments, components (including element steps, etc.) are not essential unless otherwise specified. The same applies to numerical values ​​and their ranges, and do not limit the present invention.

[0013] 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.

[0014] 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%.

[0015] <<Negative electrode material for lithium ion secondary batteries>> Hereinafter, with regard to the negative electrode material for lithium ion secondary batteries of the present disclosure, a negative electrode material for lithium ion secondary batteries of a first embodiment and a negative electrode material for lithium ion secondary batteries of a second embodiment will be described in order. Note that the present invention is not limited to these embodiments.

[0016] [First embodiment] A negative electrode material for a lithium ion secondary battery according to a first embodiment of the present disclosure (hereinafter also simply referred to as a negative electrode material) contains graphite particles, and the graphite particles have a peak at 1580 cm in a 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 The ratio of particles satisfying R≧0.2 (hereinafter also referred to as “the ratio of R≧0.2”) is 10% by number or more, and the average half-width of Id in the top 10 spectra of R values ​​is 60 cm －１ Satisfy the following:

[0017] By using the negative electrode material of the first embodiment, it is possible to manufacture a lithium ion secondary battery that has excellent initial efficiency, can suppress lithium metal precipitation, and is excellent in resistance to lithium (Li) precipitation. The reason for this is presumed to be, for example, as follows.

[0018] In Raman spectroscopy, 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.

[0019] The graphite particles contained in the negative electrode material satisfy a relatively high proportion of R values, which are the intensity ratio Id / Ig, of 0.2 or more, i.e., the graphite particles have a relatively low surface crystallinity. The low surface crystallinity enables a lithium ion secondary battery using the negative electrode material of this embodiment to increase resistance to Li precipitation without impairing discharge capacity.

[0020] The graphite particles have an average half-width of Id of 60 cm in the top 10 R-value spectra. －１ The following is satisfied. In other words, this means that for graphite particles with low surface crystallinity contained in the negative electrode material, the half-width of the peak identified as corresponding to the amorphous structure of carbon is relatively narrow. It is presumed that the relatively narrow half-width suppresses the generation of carbon defects and suppresses the increase in irreversible capacity during initial charging due to the presence of various functional groups in the carbon defects. As a result, lithium-ion secondary batteries using the negative electrode material of this embodiment tend to have excellent initial efficiency.

[0021] The graphite particles contained in the negative electrode material have an R value (Id / Ig) of R≧0.2, which is 10% by number or more, and from the viewpoint of Li deposition resistance of the lithium ion secondary battery, the R value is preferably 15% by number or more, and more preferably 20% by number or more. By using graphite particles having an R value of R≧0.2 of 10% by number or more, the input characteristics of the lithium ion secondary battery tend to be improved.

[0022] With respect to the R value, which is the intensity ratio Id / Ig, of the graphite particles, from the viewpoint of the initial efficiency of the lithium ion secondary battery, the proportion of R≧0.2 may be 80% by number or less, or may be 70% by number or less.

[0023] The graphite particles contained in the negative electrode material have an average half-width of Id in the top 10 R-value spectra of 60 cm －１The following is satisfied, and from the viewpoint of the initial efficiency of the lithium ion secondary battery, 55 cm －１ It is preferable that the following is satisfied, and 50 cm －１ It is more preferable that the following is satisfied: the average half-width of Id in the top 10 spectra with the R value is 60 cm －１ By using graphite particles that satisfy the following conditions, the storage characteristics of the lithium ion secondary battery tend to be improved.

[0024] The graphite particles have an average half-width of Id in the top 10 R-value spectra of 20 cm －１ It may be 30cm or more. －１ The above may be satisfied.

[0025] In the present disclosure, the R value, which is Id / Ig, the ratio of R≧0.2, and the average value of the half-width of Id can be measured by the following method.

[0026] 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

[0027] By dividing the R values ​​of the 400 particles measured as described above, the proportion (number %) of particles with R≧0.2 and the proportion (number %) of particles with R<0.2 can be determined.

[0028] Of the 400 particles whose R values ​​were measured as described above, the half-widths of Id in the top 10 spectra with the highest R values ​​were determined, and their arithmetic mean values ​​were used as the average half-widths of Id in the top 10 spectra with the highest R values.

[0029] The graphite particles contained in the negative electrode material of the present disclosure preferably include artificial graphite particles from the viewpoint of Li precipitation resistance in lithium ion secondary batteries. The artificial graphite particles are preferably particles obtained by graphitizing a mixture containing a graphitizable aggregate and a graphitizable binder. The proportion of R≧0.2 is likely to satisfy 10% by number or more, and 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.

[0030] 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.

[0031] By using artificial graphite particles as the graphite particles, by using a binder containing a water-soluble or water-absorbent polymer compound when producing the artificial graphite particles by graphitizing a mixture containing a graphitizable aggregate and a graphitizable binder, and by not applying a treatment to coat the surfaces of the graphite particles with low-crystalline carbon, it becomes easier to obtain a negative electrode material that satisfies the numerical conditions for the average value of the half-width of the R value and Id described above.

[0032] (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 5.0 μm to 20.0 μm.

[0033] From the viewpoint of further improving Li precipitation resistance, the average particle size of the graphite particles may be 18.0 μm or less or 17.0 μm or less, and may be 8.0 μm or more or 10.0 μm or more.

[0034] 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).

[0035] 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.

[0036] (D10) The D10 of the graphite particles may be 0.1 μm to 10.0 μm, or 0.5 μm to 8.0 μm, more preferably 1.0 μm to 7.0 μm, and particularly preferably 3.0 μm to 7.0 μm.

[0037] The D10 of the graphite particles can be measured using a laser diffraction particle size distribution analyzer (for example, SALD3100, Shimadzu Corporation).

[0038] (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 7.0, or 2.5 to 6.0. The particle size distribution D90 / D10 can be measured using a laser diffraction particle size distribution analyzer (for example, SALD3100, Shimadzu Corporation).

[0039] Methods for measuring the particle size distribution D90 / D10 of graphite particles contained in a negative electrode include preparing a sample electrode, embedding the electrode in epoxy resin, mirror-polishing the electrode, and observing the electrode cross section with a scanning electron microscope (e.g., Keyence Corporation, "VE-7800"); and preparing an electrode cross section using an ion milling device (e.g., Hitachi High-Technologies Corporation, "E-3500") and measuring the electrode cross section 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. (1) 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. (2) Assuming that the particle is an ideally shaped perfect sphere, the circle-equivalent diameter Ln = √Sn / π is determined from the area Sn. (3) From the circle-equivalent diameter Ln, the sphere volume Vn = (4 / 3)π(Ln) ３ (4) Repeat steps (1) to (3) for the selected 100 particles. (5) D90 / D10 can be calculated by taking the particle size at the point where the distribution curve, with the vertical axis representing the cumulative percentage of the volume of 100 particles and the horizontal axis representing the particle size, intersects with the horizontal axis at 10% as the 10% diameter (D10) and the particle size at the point where it intersects with the horizontal axis at 90% as the 90% diameter (D90).

[0040] (Standard Deviation of Particle Size Distribution) The standard deviation of the particle size distribution of the graphite particles may be 0.18 to 0.40, or 0.25 to 0.35. The standard deviation of the particle size distribution can be measured using a laser diffraction particle size distribution analyzer (e.g., SALD3100, Shimadzu Corporation) based on a frequency distribution graph in which the horizontal axis represents the logarithmic scale of particle size and the vertical axis represents the particle amount (%).

[0041] (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 １１０may be 700 or less, 600 or less, or 500 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.

[0042] (250 tap density) The 250 tap density of the graphite particles is 0.80 g / cm from the viewpoint of easily improving the input / output characteristics and energy density of the lithium ion secondary battery. ３ ~1.20 g / cm ３ It is preferable that the density is 0.90 g / cm ３ ~1.10 g / cm ３ It is more preferable that:

[0043] The 250 tap density of graphite particles can be determined, for example, as follows: 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 graduated flat-bottom test tube is then stopped up, and the graduated flat-bottom test tube is then dropped 250 times from a height of 5 cm. The density value calculated from the mass and volume of the sample powder is the 250 tap density.

[0044] (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 1.5m ２ / g to 5.0m ２ / g, and more preferably 2.0m ２ / g to 4.0m ２ It is more preferable that the SiO2 content is 1 / g.

[0045] 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.

[0046] 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.

[0047] (Circularity) From the viewpoint of Li precipitation resistance of a lithium ion secondary battery, the circularity of the graphite particles is preferably 95% or less, more preferably 93% or less, and even more preferably 92% or less. The circularity of the graphite particles may be 85% or more, or may be 88% or more.

[0048] The circularity of the graphite particles can be measured by 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.

[0049] (Oil Absorption) The oil absorption of graphite particles is preferably 40 mL / 100 g to 70 mL / 100 g, and more preferably 45 mL / 100 g to 60 mL / 100 g. The oil absorption of 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 graphite particles is 60 mL / 100 g or less, the number of pores present inside and on the surface of the particles is small, and it is believed that the contact area with the electrolyte is sufficiently small. In addition, the small interface makes it possible to reduce the amount of binder used when preparing a 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 has advantages in terms of cost and environment of the production line, such as reducing the equipment and power required for drying. When the oil absorption of graphite particles is 40 mL / 100 g or more, the increase in viscosity of the slurry when kneaded with a binder, etc., which occurs when the voids between the particles are too small, tends to be suppressed. In addition, the binder tends to spread easily and kneading tends to be easier, and it also becomes easier to ensure voids between particles for the movement of lithium ions.

[0050] 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.

[0051] 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.

[0052] 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 of the graphite particles is not subjected to a treatment for coating with low crystalline carbon, and the R value is 0.10 to 0.40, and more preferably 0.15 to 0.30.

[0053] In the present disclosure, the average interplanar spacing (d ００２ A carbon material having an average interplanar spacing (d ) of less than 0.340 nm is considered to be 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." ００２The theoretical value of the average interplanar spacing (d) is 0.3354 nm, 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 above viewpoint, the average interplanar spacing (d ００２ ) is preferably 0.3354 nm to 0.33600 nm, more preferably 0.3354 nm to 0.33596 nm, and even more preferably 0.3354 nm to 0.33592 nm.

[0054] 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

[0055] 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.

[0056] <Particle Structure of Graphite Particles> The graphite particles may include composite particles (composite particles) in which multiple graphite particles are aggregated or bonded, or composite particles (hereinafter also referred to as specific composite particles) having a structure in which multiple flat graphite particles are stacked. The specific composite particles, for example, are thought to be able to reduce the contact area with the electrolyte inside the particles and effectively suppress decomposition of the electrolyte, compared to composite particles having a structure in which the main surfaces of multiple flat graphite particles are randomly oriented. Furthermore, even when pressure is applied during the preparation of the negative electrode, the particles are less likely to deform or break, and electrolyte paths between the particles are more likely to be secured. Furthermore, the electrolyte located inside composite particles having a structure in which the main surfaces of multiple flat graphite particles are randomly oriented has a complex and slow diffusion path. Therefore, even active materials with a larger number of particle interfaces are thought to be unable to exhibit high input / output performance due to diffusion obstructions. This tendency is particularly pronounced under input / output conditions with a high C rate.

[0057] The flat graphite particles contained in the specific composite particles refer to non-spherical graphite particles having anisotropic shapes. Examples of flat graphite particles include graphite particles having shapes such as scales, flakes, and partial blocks. The composite particles refer to particles in a state in which primary particles are aggregated or bonded together. In other words, the specific composite particles have a structure in which multiple flat graphite particles are stacked and aggregated or bonded together with their main surfaces facing each other. Therefore, multiple flat graphite particles are stacked in a substantially parallel state to form the composite particles. Whether the flat graphite particles are stacked can be confirmed by microscopic observation.

[0058] The state in which a plurality of flat graphite particles are aggregated or bonded refers to a state in which two or more flat graphite particles are aggregated or bonded. Bonding refers to a state in which the particles are chemically bonded to each other directly or via a carbon substance. Aggregation refers to a state in which the particles are not chemically bonded to each other but maintain the shape of an aggregate due to their shape or the like. The flat graphite particles may be aggregated or bonded via a carbon substance. Examples of carbon substances include graphite and carbides obtained by graphitizing a graphitizable binder. From the viewpoint of mechanical strength, a state in which two or more flat graphite particles are bonded via a carbon substance is preferred. Whether the flat graphite particles are aggregated or bonded can be confirmed, for example, by observation using a scanning electron microscope.

[0059] In order to reduce the proportion of composite particles having a structure in which the main faces of multiple flaky graphite particles are oriented in random directions, it is best to use as little binder as possible. Composite particles are not produced without a binder. On the other hand, excessive binder causes more primary particles to form around the binder clumps, i.e., aggregate intermediates in which multiple primary particles are coordinated, and the primary particles coordinated in these intermediates are likely to be in a non-equilibrium facing state.

[0060] A dispersant may be used to reduce clumps due to uneven distribution of binders such as tar and pitch. The use of a dispersant tends to make it easier for the binder to be dispersed uniformly, reducing binder clumps. This is thought to have the effect of reducing the proportion of composite particles having a structure in which the main faces of multiple flat graphite particles are oriented in random directions. In addition, the functionality of the binder is enhanced, block moldability is improved, and the binder blending ratio can be reduced. Examples of dispersants include those that can be used in step (b) described below.

[0061] The flaky graphite particles and their raw materials are not particularly limited, and examples thereof include artificial graphite, scaly natural graphite, scaly natural graphite, coke, resin, etc. Among these, artificial graphite obtained by graphitizing coke is preferred from the viewpoints of resistance to deformation and a small specific surface area.

[0062] When the graphite particles contain specific composite particles, all of the graphite particles may be specific composite particles, or a portion of the graphite particles may be graphite particles other than the specific composite particles.

[0063] [Second embodiment] A negative electrode material for a lithium ion secondary battery according to a second embodiment of the present disclosure (hereinafter also simply referred to as a negative electrode material) contains graphite particles, and the graphite particles have a peak at 1580 cm in a 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 With respect to the R value, which is the intensity ratio Id / Ig of the maximum peak intensity Id in the range of 100 to 1000, the proportion of particles satisfying R≧0.2 is 10 or more, and the starting temperature T of the mass reduction reaction due to oxidation of the graphite particles, obtained by TG-DTA measurement in a dry air atmosphere, is 700°C or more.

[0064] By using the negative electrode material of the second embodiment, it is possible to manufacture a lithium ion secondary battery that has excellent initial efficiency, can suppress lithium metal precipitation, and is excellent in resistance to lithium (Li) precipitation. The reason for this is presumed to be, for example, as follows.

[0065] In the negative electrode material of this embodiment, as in the negative electrode material of the first embodiment, the ratio of graphite particles having an R value, which is the intensity ratio Id / Ig, of 0.2 or more is relatively high, that is, the surface crystallinity of the graphite particles is relatively low. The low surface crystallinity enables a lithium ion secondary battery using the negative electrode material of this embodiment to increase resistance to Li precipitation without impairing the discharge capacity.

[0066] The graphite particles have an onset temperature T of a mass loss reaction due to oxidation of the graphite particles, obtained by TG-DTA measurement (simultaneous differential thermal-thermogravimetry) in a dry air atmosphere, of 700°C or higher. Specifically, using a TG-DTA measurement device, the onset temperature T of a mass loss reaction due to oxidation of the graphite particles (e.g., the temperature at which DTA > 0) is found from the TG curve and DTA curve is 700°C or higher. As a result, lithium ion secondary batteries using this negative electrode material tend to have excellent initial efficiency and be able to suppress lithium metal precipitation.

[0067] The starting temperature of the mass reduction reaction may be 700°C to 800°C, or 700°C to 750°C.

[0068] The onset temperature of the mass reduction reaction tends to be higher when the graphite particles contain artificial graphite particles, whereas the onset temperature of the mass reduction reaction tends to be lower when the graphite particles are natural graphite particles or when the surfaces of the graphite particles are treated to be coated with low-crystalline carbon.

[0069] The configuration described in the section on the negative electrode material of the first embodiment and the configuration described in the section on the negative electrode material for lithium ion secondary batteries of the second embodiment may be combined as appropriate.

[0070] <<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.

[0071] 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.

[0072] 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.

[0073] 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.

[0074] 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.

[0075] 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.

[0076] 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.

[0077] 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.

[0078] 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.

[0079] 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.

[0080] 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.

[0081] 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.

[0082] 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.

[0083] When the graphitizable aggregate is particulate, the average particle size (D50) of the graphitizable aggregate is, for example, preferably 5 μm to 40 μm, more preferably 10 μm to 30 μm, and even more preferably 10 μm to 25 μm.

[0084] 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.

[0085] 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.

[0086] 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.

[0087] The mixture may contain only one type of graphitizable aggregate and one or more types of graphitizable binder.

[0088] 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.

[0089] The mixture may contain only one type of graphitizable aggregate and one or more types of graphitizable binder.

[0090] The mixture may contain other components in addition to the graphitizable aggregate and graphitizable binder, such as aromatic compounds, graphite, dispersants, graphitization catalysts, etc.

[0091] 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.

[0092] Among these, methylnaphthalene and naphthalene are preferred as the aromatic compound from the viewpoint of moldability when forming a molded product.

[0093] 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.

[0094] 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.

[0095] 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.

[0096] 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.

[0097] 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.

[0098] 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.

[0099] 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.

[0100] 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.

[0101] 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.

[0102] 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.

[0103] 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.

[0104] 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.

[0105] 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.

[0106] 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.

[0107] 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.

[0108] 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.

[0109] 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.

[0110] 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.

[0111] [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.

[0112] 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.

[0113] 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.

[0114] 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.

[0115] 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.

[0116] 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).

[0117] 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.

[0118] 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.

[0119] In the present disclosure, the term "flat graphite particles" refers to non-spherical graphite particles having anisotropic shapes. Examples of flat graphite particles include graphite particles having a scaly, flake, or partially lumpy shape.

[0120] 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.

[0121] 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.

[0122] 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.

[0123] 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.

[0124] The flat graphite particles and the raw material thereof are not particularly limited, and examples thereof include artificial graphite, scaly natural graphite, scaly natural graphite, coke, resin, tar, pitch, etc. Among these, graphite obtained from artificial graphite, natural graphite, or coke tends to be soft particles with high crystallinity, which makes it easier to increase the density of the negative electrode.

[0125] The negative electrode material may contain spherical graphite particles. When the negative electrode material contains spherical graphite particles, the spherical graphite particles themselves have a high density, so that the pressing pressure required to obtain a desired electrode density tends to be reduced.

[0126] Examples of spherical graphite particles include spherical artificial graphite and spherical natural graphite. From the viewpoint of increasing the density of the negative electrode, the spherical graphite particles are preferably high-density graphite particles. Specifically, spherical natural graphite that has been subjected to a particle spheroidization treatment to enable a high tap density is preferred. Furthermore, a negative electrode material layer containing spherical natural graphite has excellent peel strength and tends to be less likely to peel off from the current collector even when pressed with a strong force.

[0127] When the negative electrode material contains spherical graphite particles, it may contain both the above-mentioned flat graphite particles and spherical graphite particles. When the negative electrode material contains the above-mentioned flat graphite particles and spherical graphite particles, the ratio between the two is not particularly limited and can be set depending on the desired electrode density, pressure conditions during pressing, desired battery characteristics, etc.

[0128] When the negative electrode material contains flat graphite particles and spherical graphite particles, the negative electrode material may be in a state where flat graphite particles and spherical graphite particles are mixed, or in a state where flat graphite particles and spherical graphite particles are bonded together (hereinafter also referred to as composite particles), etc. Examples of composite particles include particles in which flat graphite particles and spherical graphite particles are bonded together via an organic carbide.

[0129] The composite particles can be produced, for example, by using a mixture containing flat graphite particles or raw materials thereof and spherical graphite particles in the step (a).

[0130] <<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.

[0131] 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.

[0132] 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.

[0133] 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.

[0134] <<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.

[0135] 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.

[0136] 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.

[0137] 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.

[0138] 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.

[0139] 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.

[0140] 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.

[0141] 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.

[0142] 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 Mnｙ 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 ４ Examples 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.

[0143] 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 ３and 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.

[0144] 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.

[0145] 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.

[0146] 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.

[0147] 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.

[0148] The present disclosure will be specifically described below using examples, but the present disclosure is not limited to these examples.

[0149] [Example 1] Petroleum-derived raw coke (needle coke) was used as a raw material for graphite particles. The raw coke was crushed using a hammer mill. The 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.

[0150] The obtained coke particles were pulverized and classified using a roller mill to obtain flat coke particles having a D50 of 10.1 μm. The standard deviation (σ) of the particle size distribution of the coke particles was 0.22.

[0151] 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. The binder amount in Table 1 is calculated as the ratio (%) of the fixed carbon amount of the binder to the total fixed carbon amount of the aggregate and the binder.

[0152] The resulting mixture was then uniaxially pressed at room temperature to a density of 1.3 g / cm ３ The 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 6.2 μm, the D50 was 12.2 μm, and the D90 was 18.9 μm.

[0153] Example 2 A mixture was obtained in the same manner as in Example 1, except that coke particles having a D50 of 10.5 μm and a standard deviation (σ) of particle size distribution of 0.33 were used as flat coke particles. Graphite particles serving as a negative electrode material were obtained using the obtained mixture 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 1.

[0154] Example 3 A mixture was obtained in the same manner as in Example 1, except that 57 parts by mass of the coke particles (90% by mass of fixed carbon) obtained in Example 1, 15 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. Graphite particles, which are a negative electrode material, were then obtained using the obtained mixture 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 1.

[0155] [Example 4] Flat coke particles were obtained in the same manner as in Example 1, except that petroleum-derived raw coke (mosaic coke) was used as the raw material for the graphite particles. The coke particles had a D50 of 9.1 μm and a standard deviation (σ) of the particle size distribution of 0.24. A mixture was obtained in the same manner as in Example 1, except that the flat coke particles obtained in Example 4 were used. Then, graphite particles serving as a negative electrode material were obtained using the obtained mixture 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 1.

[0156] [Comparative Example 1] The coke particles obtained in Example 1 were packed into a graphitization case without being mixed with a binder, and baked at 850°C to evaporate impurities in the mixture, followed by graphitization at 3000°C. The resulting particles were then sieved through a 280-mesh screen to obtain graphite particles as a negative electrode material. 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 1.

[0157] [Comparative Examples 2 to 4] Artificial graphite particles were prepared having the volumetric particle size distributions shown in Table 1. The artificial graphite particles prepared in Comparative Examples 2 to 4 were formed by graphitizing a mixture of coke particles and coal tar pitch. In the artificial graphite particles of Comparative Examples 3 and 4, the surfaces of the graphite particles were treated to be coated with low-crystalline carbon.

[0158]

[0159] The negative electrode materials obtained in each of the examples and comparative examples were used to measure and evaluate the physical properties shown below. The results are shown in Table 2.

[0160] (Specific Surface Area) The negative electrode material was filled into a measurement cell, and a sample was obtained by heat pretreatment at 200°C while being degassed under vacuum, and 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.

[0161] (Tap density) Volume 150 cm ３ A 100 cm sample of graphite powder was placed in a graduated flat-bottom test tube (Kuramochi Scientific Instruments Manufacturing Co., Ltd., KRS-406). ３ The graduated flat-bottomed test tube was dropped 250 times from a height of 5 cm, and the 250 tap density was calculated from the mass and volume of the sample powder.

[0162] (R Value) The R value of the negative electrode material was determined by measuring the Raman spectrum using a Raman spectrum measuring device (XploRA PLUS, manufactured by HORIBA, Ltd.) under the following conditions. The arithmetic mean value of 400 measured particles was 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

[0163] (Proportion of particles with R≧0.2 and proportion of particles with R<0.2) The R values ​​of the 400 particles measured as described above were divided, and the proportion (number %) of particles with R≧0.2 and the proportion (number %) of particles with R<0.2 were determined.

[0164] (Average Half-Width of Id) Of the 400 particles whose R values ​​were measured as described above, the half-widths of Id in the top 10 spectra with the highest R values ​​were determined, and then their arithmetic mean values ​​were calculated.

[0165] (Measurement of the initiation temperature T of the mass loss reaction due to oxidation of graphite particles) The initiation temperature T of the mass loss reaction due to oxidation of graphite particles contained in the negative electrode material (oxidation reaction initiation temperature) was measured by TG-DTA measurement in a dry air atmosphere. Specifically, the weight loss and differential heat were measured in the range from room temperature to 900°C (heating rate 20°C / min) using approximately 20 mg of sample, a Pt pan as the container, and a dry air flow rate of 300 ml / min. The temperature at which DTA > 0 was defined as the oxidation reaction initiation temperature T.

[0166] (Oil absorption) The oil absorption of the negative electrode 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 (for example, 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 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 device.

[0167] (Circularity) The circularity of the 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.

[0168] (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 slurry. This slurry was applied to the shiny surface of an electrolytic copper foil in an amount of 10 g / cm. ２ After pre-drying at 90°C for 2 hours, the electrode density was adjusted to 1.65 g / cm by roll pressing. ３Then, a curing treatment was performed by drying at 120°C for 4 hours in a vacuum atmosphere, and a negative electrode material layer was formed 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 size of 5 cm wide and 600 cm in area. ２ The negative electrode was formed into a rectangular sheet, and the resulting molded product was pressed with a hydraulic pressure of 4 t 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 scanning speed 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 2) was determined, and this value was taken as the electrode orientation.

[0169] (Discharge Capacity) The lithium ion secondary battery prepared as described above was placed in a thermostatic chamber set at 25° C., and discharged at a current value of 0.2 C and a voltage of 0.005 V (V vs. Li / Li ＋ ), and then constant voltage charging was performed at 0.005 V until the current value reached 0.02 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 ＋ The battery was discharged at a constant current until the discharge capacity reached 100 kJ / s.

[0170] (Initial Efficiency) The initial efficiency of the lithium ion secondary battery prepared as described above was determined according to the following steps (1) to (3). (1) The battery was charged to 0.005 V (vs. Li / Li+) at a constant current of 0.2 C, and then constant voltage charged at 0.005 V until the current value reached 0.02 C. The capacity at this time was designated as the initial charge capacity. (2) After a 30-minute rest period, the battery was discharged to 1.5 V (vs. Li / Li+) at a constant current of 0.2 C. The capacity at this time was designated as the initial discharge capacity. (3) The initial efficiency was determined from the charge / discharge capacities determined in steps (1) and (2) above using the following formula (1): Initial efficiency (%) = (initial discharge capacity (mAh) / initial charge capacity (mAh)) × 100 (formula 1)

[0171] (SOC-Li (Li deposition resistance)) A lithium ion secondary battery was fabricated using the negative electrode obtained above, and the Li deposition resistance was evaluated as follows. - Fabrication of Lithium Ion Secondary Battery - A lithium ion secondary battery was fabricated using the negative electrode obtained above, metallic lithium as a counter electrode, and 1 M LiPF as an electrolyte. 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. The discharge capacity at the third cycle was taken as 1C in the Li deposition test. The fourth cycle was charged at a current density of 3C, which was charged to the discharge capacity at the third cycle in 20 minutes, and the termination condition was 1C 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 at the third cycle (or the charge capacity at the fourth cycle set to be the same capacity), and Li deposition resistance was evaluated. The results are shown in Table 2. The higher this percentage value, the better the Li deposition resistance.

[0172] (Measurement of DC Resistance (DCR)) The DC resistance (DCR) of the lithium ion secondary batteries fabricated in Examples 1 and 4 was measured to determine the input characteristics of the batteries. Specifically, the following was done. The negative electrode 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 constant current charging was performed at 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)} In Example 1, the value was 1.79 Ω, and in Example 4, it was 1.68 Ω. A lower value of direct current resistance (DCR) indicates better input characteristics.

[0173] (Measurement of High-Temperature Storage Retention Rate) The high-temperature storage retention rates of the lithium ion secondary batteries fabricated in Examples 1 and 4 were determined. Specifically, the procedure is as follows. The lithium ion secondary battery (single-layer laminate full cell) after the DCR measurement was placed in a thermostatic chamber set at 25°C, and was charged at a constant current of 0.2 C to a voltage of 4.2 V. After a 30-minute pause, the battery was discharged at a constant current of 0.2 C to a voltage of 2.5 V. Thereafter, the battery was charged at a constant current of 0.2 C to a voltage of 4.2 V, and placed in a thermostatic chamber set at 60°C and stored for 10 days. Thereafter, the battery was placed in a thermostatic chamber set at 25°C and left for 60 minutes, and discharged at a constant current of 0.2 C to a voltage of 2.5 V (V vs. Li / Li ＋) and constant current discharge was performed. Thereafter, the battery was charged at a constant current of 0.2 C to a voltage of 4.2 V, and then, after a 30-minute break, was discharged at a constant current of 0.2 C to a voltage of 2.5 V. The high-temperature storage retention rate was calculated using the following formula: High-temperature storage retention rate (%) = [(Second discharge capacity at 25°C after storage at 60°C for 10 days) / (First discharge capacity at 25°C)] × 100 In Example 1, the value was 99.0%, and in Example 4, it was 98.7%. A higher value of the high-temperature storage retention rate indicates better high-temperature storage characteristics.

[0174]

[0175] In Table 2, it was confirmed that the lithium ion secondary batteries in each Example tended to be excellent in initial efficiency and resistance to Li deposition.

[0176] All publications, patent applications, and technical standards mentioned in this specification are herein incorporated by reference to the same extent as if each individual publication, patent application, or technical standard was specifically and individually indicated to be incorporated by reference.

Claims

1. Contains graphite particles, The graphite particles have a Raman spectrum obtained by Raman spectroscopy at 1580 cm －１ ~1620cm －１ and the maximum peak intensity Ig in the range of 1300 cm －１ ~1400cm －１ The ratio of particles with R≧0.2 is 10% by number or more, and the average half-width of Id in the top 10 spectra with R values ​​is 60 cm －１ A negative electrode material for lithium-ion secondary batteries that satisfies the following requirements.

2. Contains graphite particles, The graphite particles have a Raman spectrum obtained by Raman spectroscopy at 1580 cm －１ ~1620cm －１ and the maximum peak intensity Ig in the range of 1300 cm －１ ~1400cm －１ a ratio of particles satisfying R≧0.2 for an R value that is an intensity ratio Id / Ig of a maximum peak intensity Id in a range of 10% by number or more, and an initiation temperature T of a mass reduction reaction due to oxidation of the graphite particles, which is obtained by TG-DTA measurement in a dry air atmosphere, is 700° C. or more.

3. The negative electrode material for a lithium ion secondary battery according to claim 1 or 2, wherein the graphite particles include artificial graphite particles.

4. 4. The negative electrode material for lithium ion secondary batteries according to claim 3, wherein the artificial graphite particles are particles obtained by graphitizing a mixture containing a graphitizable aggregate and a graphitizable binder, and the binder contains a water-soluble or water-absorbent polymer compound.

5. 3. The negative electrode material for lithium ion secondary batteries according to claim 1, wherein the graphite particles have a circularity of 93% or less.

6. The negative electrode material for lithium ion secondary batteries according to claim 1 or 2, wherein the graphite particles have an oil absorption of 40 mL / 100 g to 70 mL / 100 g.

7. 3. The negative electrode material for lithium ion secondary batteries according to claim 1, wherein the graphite particles have a ratio of particles with R≧0.2 of 80% or less by number.

8. 3. The negative electrode material for a lithium ion secondary battery according to claim 1, wherein the surface of the graphite particles is not subjected to a treatment for coating with low crystalline carbon.

9. A negative electrode for a lithium ion secondary battery, comprising: a negative electrode material layer containing the negative electrode material for a lithium ion secondary battery according to claim 1 or 2; and a current collector.

10. A lithium ion secondary battery comprising the negative electrode for lithium ion secondary batteries according to claim 9, a positive electrode, and an electrolyte solution.