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

Abstract

To provide a negative electrode material for a lithium ion secondary battery that can improve battery characteristics even without being coated with amorphous carbon, a method for manufacturing the negative electrode material for the lithium ion secondary battery, a negative electrode material slurry for the lithium ion secondary battery, a negative electrode for the lithium ion secondary battery, and the lithium ion secondary battery.SOLUTION: A negative electrode material for a lithium ion secondary battery includes carbon particles with an oxygen content of 0.15 mass% or less.SELECTED DRAWING: Figure 1

Description

[Technical field]

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

[0002] Lithium-ion secondary batteries have a higher energy density than other secondary batteries such as nickel-cadmium batteries, nickel-metal hydride batteries, and lead-acid batteries, and are therefore widely used as power sources for portable electrical appliances such as laptops and mobile phones. In recent years, the use of lithium-ion secondary batteries has expanded significantly not only in relatively small electrical appliances, but also in electric vehicles, power storage devices, and other applications.

[0003] As a negative electrode material for lithium-ion secondary batteries, there are known graphite particles whose surfaces are coated with amorphous carbon in order to provide properties such as improved charge / discharge speed and suppression of side reactions with the electrolyte while taking advantage of the high capacity property of graphite (see, for example, Patent Documents 1 and 2). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2004-196609 A [Patent Document 2] Japanese Patent Application Publication No. 11-354122 Summary of the Invention [Problem to be solved by the invention]

[0005] In anode materials in which the surface of graphite particles is coated with amorphous carbon, which is harder than graphite, cracks may occur in the amorphous carbon coating when pressed under high pressure during electrode production, which may cause a reaction with the electrolyte and lead to expansion of the electrode. Therefore, there is a need to develop an anode material that can improve battery characteristics without relying on an amorphous carbon coating.

[0006] In view of the above circumstances, an object of one aspect of the present invention is to provide a negative electrode material for a lithium ion secondary battery that can improve battery characteristics even without being coated with amorphous carbon, a manufacturing method for the negative electrode material for a lithium ion secondary battery, a negative electrode material slurry for a lithium ion secondary battery, a negative electrode for a lithium ion secondary battery, and a lithium ion secondary battery. [Means for solving the problem]

[0007] Specific means for solving the above problems include the following embodiments. <1> A negative electrode material for lithium-ion secondary batteries, comprising carbon particles having an oxygen content of 0.15 mass % or less. <2> The R value of the carbon particles is 0.45 or less. <1> The negative electrode material for lithium ion secondary batteries according to claim 1. <3> The rhombohedral content of the carbon particles is greater than 0.20; <1> or <2> The negative electrode material for lithium ion secondary batteries according to claim 1. <4> The carbon particles have a circularity of greater than 0.8; <1> ~ <3> 13. The negative electrode material for a lithium ion secondary battery according to claim 12. <5> In the carbon particles, no DTA exothermic peak is detected in the range of 500°C to 650°C in a differential thermal analysis. <1> ~ <4> 13. The negative electrode material for a lithium ion secondary battery according to claim 12. <6> The specific surface area of ​​the carbon particles measured by nitrogen gas adsorption is 4.0 m 2 / g or more <1> ~ <5> 13. The negative electrode material for a lithium ion secondary battery according to claim 12. <7> <1> ~ <6> 13. A negative electrode material slurry for a lithium ion secondary battery, comprising the negative electrode material for a lithium ion secondary battery according to any one of claims 1 to 12, an organic binder, and a solvent. <8> A current collector and a conductive film formed on the current collector. <1> ~ <6> and a negative electrode material layer comprising the negative electrode material for lithium ion secondary batteries according to any one of claims 1 to 5. <9> A positive electrode, an electrolyte, <8> and a lithium ion secondary battery comprising the negative electrode for lithium ion secondary batteries according to claim 1. <10> A method for producing a negative electrode material for lithium ion secondary batteries, comprising: a step of heating carbon particles to a temperature of 400°C to 1300°C; and a step of cooling the carbon particles after the heating until the temperature of the carbon particles is less than 400°C, wherein the heating and cooling are each carried out in a non-oxidizing atmosphere. <11> <1> ~ <6> For producing the negative electrode material for a lithium ion secondary battery according to any one of claims 1 to 4, <10> A method for producing the negative electrode material for lithium ion secondary batteries according to claim 1. Effect of the Invention

[0008] According to one aspect of the present invention, there are provided a negative electrode material for a lithium ion secondary battery that can improve battery characteristics even without being coated with amorphous carbon, a manufacturing method for the negative electrode material for a lithium ion secondary battery, a negative electrode material slurry for a lithium ion secondary battery, a negative electrode for a lithium ion secondary battery, and a lithium ion secondary battery. [Brief description of the drawings]

[0009] [Figure 1] FIG. 2 is a schematic cross-sectional view showing the configuration of an apparatus used in evaluating pressability. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] Hereinafter, the 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, the components (including element steps, etc.) are not essential unless otherwise specified or considered to be obviously essential in principle. The same applies to the numerical values ​​and their ranges, and they do not limit the present invention. In the present disclosure, the term "step" includes not only a step that is independent of other steps, but also a step that cannot be clearly distinguished from other steps as long as the purpose of the step is achieved. In the present disclosure, a numerical range indicated using "~" includes the numerical values ​​before and after "~" as the minimum and maximum values, respectively. In the numerical ranges described in the present disclosure in stages, the upper or lower limit value described in one numerical range may be replaced with the upper or lower limit value of another numerical range described in stages. In addition, in the numerical ranges described in the present disclosure, the upper or lower limit value of the numerical range may be replaced with a value shown in the examples. In the present disclosure, each component may contain multiple types of the corresponding substance. When multiple substances corresponding to each component are present in the composition, the content or amount of each component means the total content or amount of the multiple substances present in the composition, unless otherwise specified. In the present disclosure, the particles corresponding to each component may include multiple types. 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 terms "layer" and "film" include cases where the layer or film is formed over the entire area when the area in which the layer or film is present is observed, as well as cases where the layer or film is formed over only a portion of the area.

[0011] <Anode material for lithium-ion secondary batteries> The negative electrode material for lithium ion secondary batteries (hereinafter also simply referred to as the negative electrode material) of the present disclosure contains carbon particles having an oxygen content of 0.15 mass % or less. The negative electrode material having the above-mentioned structure maintains the performance (particularly, storage characteristics) of a lithium ion secondary battery using the negative electrode material, even if the carbon particles are not coated with amorphous carbon. The reason for this is not necessarily clear, but is presumed to be as follows.

[0012] When the carbon particles are graphite particles, the edges of the graphite that constitutes them contain oxygen-containing functional groups such as -OH (hydroxy group), >C=O (carbonyl group), -COOH (carboxy group), etc., and it is believed that these oxygen-containing functional groups react with the electrolyte. Therefore, by keeping the amount of oxygen-containing functional groups present at the edges of the graphite below a certain range, it is believed that the reaction with the electrolyte is suppressed, and the storage characteristics of the battery are maintained well.

[0013] Furthermore, lithium ion secondary batteries using carbon particles with an oxygen content of 0.15% by mass or less have improved charge / discharge efficiency compared to batteries using carbon particles with an oxygen content of more than 0.15% by mass. This is believed to be because oxygen-containing functional groups present at the edge of graphite function as a barrier to the insertion and desorption of lithium ions from the edge surface, and the reduction in the amount of oxygen-containing functional groups facilitates the insertion and desorption of lithium ions.

[0014] In the present disclosure, the oxygen content of the carbon particles is a value measured by an infrared absorption method, which is performed by the method described in the Examples section below. The oxygen content of the carbon particles can be reduced to 0.15% by mass or less by heating the carbon particles at a temperature at which the oxygen-containing functional group decomposes, as described in the manufacturing method of the negative electrode material described later. The oxygen content of the carbon particles is preferably 0.12% by mass or less, more preferably 0.10% by mass or less, more preferably 0.07% by mass or less, and even more preferably 0.04% by mass or less.

[0015] The lower limit of the oxygen content of the carbon particles is not particularly limited, but from the viewpoint of easily achieving both high-temperature storage characteristics and input / output characteristics, it is preferably 0.005 mass% or more, more preferably 0.007 mass% or more, more preferably 0.01 mass% or more, more preferably 0.015 mass% or more, and even more preferably 0.02 mass% or more.

[0016] As the carbon particles, graphite particles such as natural graphite particles and artificial graphite particles are preferred. Among the graphite particles, scaly particles of natural graphite are more preferred from the viewpoint of large crystallites and high capacity. Natural graphite tends to contain oxygen-containing functional groups during the process of production from ore and processing. For this reason, it is advantageous to set the oxygen content of the natural graphite particles to 0.15 mass% or less from the viewpoint of suppressing reaction with the electrolyte and improving charge / discharge efficiency.

[0017] Examples of carbon particles include carbon particles having a shape such as scale, flake, flake, and block, and spherical particles such as spherical graphite obtained by spheronizing flat graphite particles, and spherical particles are preferred. Spherical particles are less likely to be oriented in one direction by pressing when making an electrode than flat carbon particles, and are suitable for rapid charging and discharging when the electrode is densified. On the other hand, spherical particles tend to contain oxygen-containing functional groups because new edge surfaces are generated in the process of spheronizing flat carbon particles. For this reason, it is advantageous to set the oxygen content of the spherical particles to 0.15 mass% or less from the viewpoint of suppressing reaction with the electrolyte and improving charge and discharge efficiency. In the present disclosure, particles with a circularity of more than 0.8 are considered to be spherical particles.

[0018] In the present disclosure, the circularity of carbon particles is measured by a wet flow particle size and shape analyzer. As a measuring instrument, FPIA-3000 (Malvern Instruments) can be used. As a pretreatment for this measurement, 0.06 g of carbon particles and purified water containing 0.2% by mass of a surfactant (product name: Liponol T / 15, Lion Corporation) may be placed in a test tube (12 mm x 120 mm, Maruemu Corporation), stirred for 20 seconds with a test tube mixer (Pasolina NS-80, AS ONE Corporation), and then stirred with ultrasonic waves for 1 minute. As an ultrasonic cleaner, US102 (high frequency output 100 W, oscillation frequency 38 kHz) from SND Corporation can be used.

[0019] The particle size of the carbon particles is not particularly limited. For example, the volume average particle size is preferably 1 μm to 50 μm, more preferably 2 μm to 45 μm, more preferably 3 μm to 35 μm, more preferably 5 μm to 25 μm, and even more preferably 7 μm to 20 μm. The volume average particle size of the carbon particles can be measured by a laser diffraction particle size distribution measuring device, and is the particle size (D50) at which the cumulative total from the small diameter side in the volume-based particle size distribution is 50%.

[0020] (R value) The carbon particles preferably have an R value of 0.45 or less. The R value is an index showing the degree of crystallinity of the surface of the carbon particles, and the smaller the R value, the higher the crystallinity. In addition, when the carbon particle surface is damaged by processing such as spheroidization, the R value tends to increase. The carbon particles preferably have an R value of 0.42 or less, more preferably 0.40 or less, more preferably 0.32 or less, and even more preferably 0.30 or less. The lower limit of the R value is not particularly limited, but from the viewpoint of the balance of the battery characteristics, it is preferably 0.05 or more, more preferably 0.10 or more, more preferably 0.15 or more, more preferably 0.20 or more, and even more preferably 0.22 or more. When high charging performance is desired, the R value is preferably 0.15 or more.

[0021] In the present disclosure, the R value of the carbon particles is determined by the Raman spectrum obtained in the Raman measurement described below, which is calculated as 1580 cm -1 The intensity of the maximum peak IA near 1360 cm -1 The intensity ratio (IB / IA) is the intensity ratio of the maximum peak IB in the vicinity of the peak IB.

[0022] The Raman measurements are performed using a Raman spectrometer, a "laser Raman spectrophotometer (model number: NRS-1000, manufactured by JASCO Corporation)" by irradiating a laser beam onto a sample plate on which a negative electrode material for lithium ion secondary batteries or an electrode obtained by applying a negative electrode material for lithium ion secondary batteries to a current collector and applying pressure to the electrode is set flat. The measurement conditions are as follows. Laser light wavelength: 532nm Wavenumber resolution: 2.56cm -1 Measurement range: 1180cm -1 ~1730cm -1 Peak Research: Background Removal

[0023] (rhombohedral crystal content) When the carbon particles contain graphite, the rhombohedral content is preferably greater than 0.20, more preferably 0.21 or more, more preferably 0.22 or more, more preferably 0.23 or more, and even more preferably 0.26 or more. When the rhombohedral content of the carbon particles is within the above range, the rapid charge / discharge characteristics, high-temperature storage characteristics, etc. tend to be excellent. In general graphite particles, when the rhombohedral content increases (for example, exceeds 0.33), pulverization tends to proceed rather than spheroidization, and the oxygen content rises rapidly and high-temperature storage properties begin to deteriorate rapidly. However, the inventors' studies have found that carbon particles with an oxygen content of 0.15 mass% or less can improve input and output while suppressing deterioration of high-temperature storage properties. The upper limit of the rhombohedral crystal content is not particularly limited, but may be, for example, less than 0.50, 0.45 or less, 0.40 or less, 0.35 or less, or 0.30 or less.

[0024] The rhombohedral content of carbon particles can be calculated from the peak intensity (P1) of the (101) plane of the rhombohedral crystal structure and the peak intensity (P2) of the (101) plane of the hexagonal crystal structure measured by X-ray diffraction using the following formula. Rhombohedral crystal amount = P1 / (P1+P2) P1: Peak intensity of the (101) plane of the rhombohedral crystal structure (diffraction angle 2θ is around 43°) P2: Peak intensity of the (101) plane of the hexagonal crystal structure (diffraction angle 2θ is around 44°)

[0025] The intensities of the peaks can be measured as follows. The diffraction peaks are measured by filling the concave portion of a quartz sample holder with a sample powder and setting it on the measurement stage, and using a wide-angle X-ray diffractometer (Rigaku Corporation) under the following measurement conditions. Then, the Kα2 peak and background are removed, and the peaks are separated by a profile shape function (Pseudo-Voigt). Radiation source: CuKα ray (wavelength=0.15418nm) Divergence slit DS: 1° Receiving slit RS: 0.3 mm Scattering slit SS: 1°

[0026] The carbon particles do not need to be coated with amorphous carbon. As described above, carbon particles having an oxygen content of 0.15 mass% or less tend to suppress reaction with the electrolyte even if they are not coated with amorphous carbon, and the storage characteristics of the battery tend to be maintained well. Whether or not carbon particles are coated with amorphous carbon, and the degree of coating, can be determined, for example, by differential thermal analysis (DTA). If carbon particles are coated with amorphous carbon, a DTA exothermic peak tends to appear in the range of 500°C to 650°C. When the carbon particles are coated with amorphous carbon, the amount is preferably so small that there is no DTA exothermic peak in the range of 500° C. to 650° C. For example, the amount of amorphous carbon may be less than 1 mass % of the entire carbon particles (the total of the core particles and amorphous carbon).

[0027] In the present disclosure, differential thermal analysis (DTA) is performed using a thermogravimetry and differential thermal analyzer (for example, EXSTAR TG / DTA6200 manufactured by Seiko Instruments Inc.). Specifically, using α-alumina as a reference, measurements are performed under a flow of dry air of 300 mL / min and a temperature increase rate of 2.5°C / min, and the presence or absence of a DTA exothermic peak at 400°C to 1000°C is confirmed. In addition, when the negative electrode material contains a conductive assistant described later in addition to carbon particles, there is a risk that an exothermic peak derived from the conductive assistant may also be detected, so it is preferable to perform centrifugation before performing differential thermal analysis on the negative electrode material to extract only the precipitate and then perform the measurement.

[0028] (Oil absorption amount) The oil absorption of the carbon particles is, for example, preferably 25 mL / 100 g or more, more preferably 30 mL / 100 g or more, more preferably 35 mL / 100 g or more, more preferably 40 mL / 100 g or more, and even more preferably 45 mL / 100 g or more. The oil absorption of carbon particles is an index showing the ratio of voids in and between carbon particles. In the process of spheroidization, flat particles are folded or granulated to become high-density spherical particles, and the oil absorption tends to decrease. As mentioned above, from the viewpoint of particle orientation when pressing the electrode, it is considered that spherical carbon particles (i.e., carbon particles with low oil absorption) are preferable. On the other hand, from the viewpoint of securing the amount of electrolyte required for lithium ion migration and suppressing the deterioration of charge / discharge characteristics, it is preferable that the oil absorption of carbon particles is not too small. For example, when the oil absorption of carbon particles is 25 mL / 100 g or more, there is a sufficient amount of electrolyte required for lithium ions to migrate at high speed, and a high electrode density (e.g., 1.7 g / cm) is obtained. 3 Even when the density of the electrode is set to 100 or more, various characteristics tend to be well maintained when the electrode is made into a battery. When high capacity is not required or when input / output characteristics are important, there is no need to make the electrode dense, so there is no restriction on this.

[0029] The oil absorption of the carbon particles is, for example, more preferably 100 mL / 100 g or less, more preferably 90 mL / 100 g or less, more preferably 80 mL / 100 g or less, more preferably 70 mL / 100 g or less, and even more preferably 60 mL / 100 g or less. When the oil absorption of the carbon particles is 100 mL / 100 g or less, the amount of water required to adjust the slurry viscosity to the target can be reduced, and energy consumption during drying of the electrodes can be saved.

[0030] In the present disclosure, the oil absorption of carbon particles can be measured by using linseed oil (e.g., manufactured by Kanto Chemical Co., Ltd.) instead of dibutyl phthalate (DBP) as the reagent liquid described in JIS K6217-4:2008 "Carbon black for rubber - Basic properties - Part 4: Determination of oil absorption". 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 that corresponds to 70% of the maximum torque generated is defined as the oil absorption (ml / 100g). For example, an absorption measurement device manufactured by Asahi Research Institute Co., Ltd. can be used as the measurement device.

[0031] (Tap density) The carbon particles have a tap density of 0.70 g / cm 3 or more, and may be 0.75 g / cm 3 or more, 0.80 g / cm 3 or more, and may be 0.85 g / cm 3 or more, 0.90 g / cm 3 The tap density of the carbon particles may be 0.70 g / cm or more. 3 If the amount is more than this, the binder required for forming the electrode into a plate shape will adhere in a larger amount to the surfaces of the carbon particles, and problems such as peeling at the current collector interface will tend to be less likely to occur. The carbon particles have a tap density of 1.30 g / cm 3 It may be less than or equal to 1.25 g / cm 3 or less, 1.20 g / cm 3 or less, 1.15 g / cm 3 or less, 1.10 g / cm 3 The tap density of the carbon particles may be 1.30 g / cm or less. 3 If the thickness is less than this, the amount of voids between the carbon particles increases, tending to increase flexibility during pressing.

[0032] In the present disclosure, the tap density of carbon particles is defined as the density measured by placing 100 mL of carbon particles in a measuring cylinder (manufactured by Takahashi Rika Riki Kogyo Co., Ltd., inner diameter φ31 mm) using a packing density measuring device (KRS-406, manufactured by Kuramochi Scientific Instruments Manufacturing Co., Ltd.) and dropping the measuring cylinder repeatedly a specified number of times (250 times) from a height at which the bottom of the measuring cylinder is 6 cm above the surface.

[0033] (specific surface area) The specific surface area of ​​the carbon particles is an index showing the area of ​​the interface between the carbon particles and the electrolyte. The smaller the value of the specific surface area, the less the area of ​​the interface between the carbon particles and the electrolyte, and the more the increase in the reaction field of the decomposition reaction of the electrolyte is suppressed, and the more gas generation is suppressed, and the more the initial charge / discharge efficiency tends to be good. In addition, the larger the value of the specific surface area, the less the current density per unit area is likely to rise suddenly, and the load is reduced, so that the charge / discharge efficiency, charge acceptance, rapid charge / discharge characteristics, etc. tend to be good. In the present disclosure, since the oxygen content that causes gas generation is 0.15 mass % or less, the amount of gas generation can be suppressed without reducing the specific surface area. The specific surface area of ​​the carbon particles is not particularly limited. For example, 2 / g or more, and 1.0m 2 / g or more is more preferable, and 2.0m 2 / g or more is more preferable, and 3.0m 2 / g or more is more preferable, and 4.0m 2 It is more preferable that the molecular weight is 1 / g or more. The specific surface area of ​​the carbon particles is 20.0 m 2 / g or less, and 15.0m 2 / g or less is more preferable, and 12.0m 2 / g or less is more preferable, and 10.0m 2 / g or less is more preferable, and 8.0m 2 It is more preferable that the molecular weight is not more than 1 / g.

[0034] The specific surface area of ​​carbon particles can be measured by the BET method (nitrogen gas adsorption method). Specifically, carbon particles are filled into a measurement cell, and a sample is obtained by performing a heat pretreatment at 200°C for 120 minutes or more while vacuum degassing, and nitrogen gas is adsorbed into the sample using a gas adsorption device (ASAP2010, manufactured by Shimadzu Corporation). The obtained sample is subjected to a BET analysis using a five-point method to calculate the specific surface area. The specific surface area of ​​the carbon particles can be adjusted to a desired range, for example, by adjusting the average particle size (a smaller average particle size tends to increase the specific surface area, and a larger average particle size tends to decrease the specific surface area).

[0035] The negative electrode material may contain a plurality of types of carbon particles having different shapes, particle sizes, etc. as carbon particles having an oxygen content of 0.15 mass % or less. The negative electrode material may contain a negative electrode material other than carbon particles having an oxygen content of 0.15 mass % or less, for example, a negative electrode material containing an element capable of absorbing and releasing lithium ions, such as Si, Sn, Ge, or In.

[0036] When the negative electrode material contains carbon particles having an oxygen content of 0.15 mass% or less and other negative electrode materials, the proportion of the carbon particles having an oxygen content of 0.15 mass% or less in the entire negative electrode material is preferably 50 mass% or more, more preferably 70 mass% or more, and even more preferably 80 mass% or more. When the negative electrode material contains carbon particles having an oxygen content of 0.15 mass % or less and other negative electrode materials, it is preferable that the oxygen content of the entire negative electrode material is 0.15 mass % or less.

[0037] <Method of manufacturing 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 heating carbon particles to a temperature of 400°C to 1300°C, and a step of cooling the carbon particles after the heating until the temperature of the carbon particles is less than 400°C, and the heating and cooling are each performed in a non-oxidizing atmosphere.

[0038] According to the above method, carbon particles (negative electrode material) having an oxygen content of 0.15 mass % or less can be produced for the following reasons.

[0039] The oxygen-containing functional groups contained in the carbon particles start to decompose at a temperature of 400°C or higher, decompose more at a temperature of 600°C or higher, and decompose mainly at a temperature of 750°C or higher. On the other hand, when the temperature of the carbon particles exceeds 1300°C, the graphite crystals develop, the lattice defects of the carbon particles decrease, the amount of rhombohedral crystals decreases, and the input / output characteristics may decrease. Therefore, by heating the carbon particles to a temperature of 400°C to 1300°C, the oxygen-containing functional groups can be reduced or eliminated until the oxygen content becomes 0.15 mass% or less without deteriorating the input / output characteristics.

[0040] On the other hand, carbon particles tend to oxidize at temperatures of 400° C. or higher. Therefore, by cooling the heated carbon particles in a non-oxidizing atmosphere until the temperature falls below 400° C., it is possible to prevent the carbon particles, whose oxygen content has been reduced by heating, from being oxidized and causing the oxygen content to increase again.

[0041] In the above method, there is no particular limitation on the time for heating the carbon particles to a temperature of 400° C. to 1300° C. For example, the temperature of the carbon particles may be maintained within the above range for 30 minutes to 2 hours.

[0042] In the above method, the carbon particles may be cooled in a non-oxidizing atmosphere until the temperature of the carbon particles becomes 200° C. or lower, or may be cooled in a non-oxidizing atmosphere until the temperature of the carbon particles becomes room temperature (25° C.) or lower.

[0043] The non-oxidizing atmosphere used in the above method is not particularly limited, and may be a reducing atmosphere such as hydrogen, helium, argon, nitrogen, or the like, an inert atmosphere, or a vacuum atmosphere. The non-oxidizing atmosphere preferably does not contain oxygen, but may contain oxygen to the extent that oxidation of the carbon particles does not proceed. For example, the oxygen content may be 0.3 vol.% or less, preferably 0.1 vol.% or less, and more preferably 0.05 vol.% or less. The higher the treatment temperature, the lower the oxygen content is preferably.

[0044] The method for heating and cooling the carbon particles is not particularly limited, and can be performed using a general device such as an electric furnace.

[0045] The details and preferred embodiments of the carbon particles produced by the above method are the same as the details and preferred embodiments of the carbon particles described above.

[0046] <Slurry for negative electrode material for lithium-ion secondary batteries> The negative electrode material slurry for lithium ion secondary batteries (hereinafter also referred to as negative electrode material slurry) of the present disclosure contains the above-mentioned negative electrode material, an organic binder, and a solvent.

[0047] There is no particular limitation on the organic binder. For example, styrene-butadiene rubber, polymer compounds having ethylenically unsaturated carboxylic acid esters (methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, hydroxyethyl (meth)acrylate, etc.) as polymerization components, polymer compounds having ethylenically unsaturated carboxylic acids (acrylic acid, methacrylic acid, itaconic acid, fumaric acid, maleic acid, etc.) as polymerization components, polyvinylidene fluoride, polyethylene oxide, polyepichlorohydrin, polyphosphazene, polyacrylonitrile, polyimide, polyamideimide, etc. can be mentioned. In the present disclosure, (meth)acrylate means either or both of methacrylate and acrylate.

[0048] The solvent is not particularly limited. For example, water, an organic solvent, or a mixture thereof may be used. Examples of the organic solvent include N-methylpyrrolidone, dimethylacetamide, dimethylformamide, and γ-butyrolactone.

[0049] The negative electrode material slurry may contain a thickener for adjusting the viscosity, if necessary. Examples of the thickener include carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose, polyvinyl alcohol, polyacrylic acid and its salts, oxidized starch, phosphorylated starch, and casein.

[0050] The negative electrode material slurry may contain a conductive assistant as necessary. Examples of the conductive assistant include carbon black, graphite, graphene, acetylene black, carbon nanotubes, and conductive nitrides.

[0051] The proportion of the negative electrode material contained in the negative electrode material slurry is not particularly limited, and may be, for example, within the range of 90% by mass to 99% by mass of the non-volatile content (components excluding the solvent) of the negative electrode material slurry.

[0052] <Negative electrodes for lithium-ion secondary batteries> The negative electrode for a lithium ion secondary battery (hereinafter also referred to as the negative electrode) of the present disclosure has a current collector and a negative electrode material layer that contains the above-mentioned negative electrode material formed on the current collector.

[0053] The material and shape of the current collector are not particularly limited. For example, a material such as a strip-shaped foil, a strip-shaped perforated foil, or a strip-shaped mesh made of a metal or alloy such as aluminum, copper, nickel, titanium, or stainless steel can be used. In addition, porous materials such as porous metal (foamed metal) and carbon paper can also be used.

[0054] The method of forming the negative electrode material layer containing the negative electrode material on the current collector is not particularly limited. For example, the negative electrode material slurry may be used by known methods such as metal mask printing, electrostatic painting, dip coating, spray coating, roll coating, doctor blade, gravure coating, and screen printing. When the negative electrode material layer and the current collector are integrated, they may be integrated by known methods such as rolling, pressing, and combinations thereof.

[0055] The negative electrode obtained by forming the negative electrode material layer on the current collector may be subjected to a heat treatment. By performing heat treatment with a binder type, the solvent contained in the negative electrode material layer is removed, the strength is increased by hardening the binder, and the adhesion between the particles and between the particles and the current collector can be improved. The heat treatment may be performed in an inert atmosphere such as helium, argon, nitrogen, etc., or in a vacuum atmosphere to prevent oxidation of the current collector during the treatment.

[0056] The electrode density of the negative electrode is not particularly limited and can be adjusted, for example, by applying pressure to a negative electrode material layer formed on a current collector. The electrode density of the negative electrode is, for example, 1.5 g / cm 3 ~1.9g / cm 3 1.6 g / cm 3 ~1.8g / cm 3 The higher the electrode density, the higher the volumetric capacity and the better the adhesion of the negative electrode layer to the current collector. In applications where current density is more important than volume density (e.g., for HEVs), the electrode density is 1.5 g / cm 3 It may be the following.

[0057] <Lithium-ion secondary battery> The lithium ion secondary battery of the present disclosure has a positive electrode, an electrolyte, and the above-mentioned negative electrode. The lithium ion secondary battery may have other members as necessary. For example, the lithium ion secondary battery may have a configuration in which at least a negative electrode and a positive electrode are arranged to face each other via a separator, and an electrolyte solution containing an electrolyte is injected.

[0058] The positive electrode can be obtained by forming a positive electrode layer on the surface of a current collector in the same manner as the negative electrode. The current collector can be a strip-shaped foil, a strip-shaped perforated foil, a strip-shaped mesh, or the like, made of a metal or alloy such as aluminum, titanium, or stainless steel.

[0059] The positive electrode material used in the positive electrode layer is not particularly limited. For example, metal compounds, metal oxides, metal sulfides, and conductive polymer materials capable of doping or intercalating lithium ions can be used. Furthermore, lithium cobalt oxide (LiCoO 2 ), lithium nickel oxide (LiNiO 2 ), lithium manganese oxide (LiMnO 2 ), and their double oxides (LiCo x Ni y Mn z O 2, x + y + z = 1, 0 < x, 0 < y; LiNi 2-x Mn x O 4 , 0 < x ≦ 2), lithium manganese spinel (LiMn 2 O 4 ), lithium vanadium compound, V 2 O 5 , V 6 O 13 , VO 2 , MnO 2 , TiO 2 , MoV 2 O 8 , TiS 2 , V 2 S 5 , VS 2 , MoS 2 , MoS 3 , Cr 3 O 8 , Cr 2 O 5 , olivine-type LiMPO 4 (M: Co, Ni, Mn, Fe), polyacetylene, polyaniline, polypyrrole, polythiophene, polyacene and other conductive polymers, porous carbon, etc. can be used alone or in combination of two or more. Among them, lithium nickelate (LiNiO 2 ) and its complex oxides (LiCo x Ni y Mn z O 2 , x + y + z = 1, 0 < x, 0 < y; LiNi 2-x Mn x O 4 , 0 < x ≦ 2) is suitable as a cathode material because of its high capacity. From the viewpoint of further increasing the capacity, nickel-cobalt-aluminum (NCA) cathode materials can also be preferably used.

[0060] Examples of the separator include non-woven fabrics, cloths, microporous films and combinations thereof mainly composed of polyolefins such as polyethylene and polypropylene. In addition, when the lithium ion secondary battery has a structure in which the positive electrode and the negative electrode do not contact each other, it is not necessary to use a separator.

[0061] The electrolyte is LiClO 4 , LiPF 6 , LiAsF 6 , LiBF 4 , LiSO 3 CF 3 A so-called organic electrolyte solution can be used in which a lithium salt such as ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, fluoroethylene carbonate, cyclopentanone, sulfolane, 3-methylsulfolane, 2,4-dimethylsulfolane, 3-methyl-1,3-oxazolidin-2-one, γ-butyrolactone, dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate, methylpropyl carbonate, butylmethyl carbonate, ethylpropyl carbonate, butylethyl carbonate, dipropyl carbonate, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, methyl acetate, ethyl acetate, etc. is dissolved in a non-aqueous solvent of a single component or a mixture of two or more components. Among them, an electrolyte solution containing fluoroethylene carbonate is preferred because it tends to form a stable SEI (solid electrolyte interface) on the surface of the negative electrode material and significantly improves cycle characteristics.

[0062] The form of the lithium ion secondary battery is not particularly limited, and examples thereof include paper type batteries, button type batteries, coin type batteries, laminated type batteries, cylindrical type batteries, square type batteries, etc. In addition to lithium ion secondary batteries, the negative electrode material for lithium ion secondary batteries can be applied to all electrochemical devices such as hybrid capacitors, which use insertion and desorption of lithium ions as a charging and discharging mechanism.

[0063] The lithium ion secondary battery of the present disclosure has excellent storage characteristics (especially high-temperature storage characteristics), and is therefore particularly suitable for applications expected to be used in high-temperature environments, such as electric vehicles and power storage systems. EXAMPLES

[0064] The above embodiment will be described in more detail below based on examples, but the above embodiment is not limited to the following examples.

[0065] (1) Preparation of anode material The carbon particles of the raw material shown below were heated to the heating temperature (maximum temperature) shown in Table 1, maintained for 30 minutes, and then cooled to room temperature (25° C.) to produce the negative electrode material of the example. Heating and cooling were performed continuously in a nitrogen atmosphere (oxygen content 0.005 to 0.2% by volume) using an electric furnace.

[0066] Carbon particles 1: Spherical natural graphite with a volume average particle size of 9.9 μm and a circularity of 0.95 Carbon particles 2: Spherical natural graphite with a volume average particle size of 12.2 μm and a circularity of 0.92 Carbon particles 3: Spherical natural graphite with a volume average particle size of 15.8 μm and a circularity of 0.85 Carbon particles 4: Spherical natural graphite with a volume average particle size of 18.1 μm and a circularity of 0.88 Carbon particles 5: Spherical natural graphite with a volume average particle size of 21.6 μm and a circularity of 0.90

[0067] As the negative electrode material in Comparative Examples 1 to 5, the raw carbon particles were used as they were (without heat treatment). As the negative electrode material of Comparative Example 6, carbon particles coated with amorphous carbon (3% by mass) (carbon particles 6) were used, which were obtained by mixing 100 parts by mass of carbon particles 1 and 6 parts by mass of coal tar pitch (softening point 90°C, residual carbon rate (carbonization rate) 50%), heating the mixture to 1000°C at a heating rate of 20°C / hour under a nitrogen flow, and holding the mixture at 1000°C (calcination treatment temperature) for 1 hour.

[0068] (Measurement of specific surface area, oil absorption, tap density, volume average particle size, R value, and rhombohedral crystal amount) For each negative electrode material, the specific surface area, oil absorption, tap density, volume average particle diameter (D50) R value, and rhombohedral crystal content were measured by the methods described above. The results are shown in Table 1.

[0069] (DTA analysis) DTA analysis was performed on each negative electrode material by the method described above to check whether it had a peak in the range of 500° C. to 650° C. As a result, a peak was observed in the range of 500° C. to 650° C. for carbon particles 6 coated with amorphous carbon (Comparative Example 6), while no peak was observed in the range of 500° C. to 650° C. for the examples and comparative examples using carbon particles 1 to 5.

[0070] (Pressability) 3.0 g of negative electrode material was filled into a 15 mm diameter mold as shown in Figure 1 and compressed at a constant speed of 10 mm / min using an autograph (Shimadzu Corporation). During this compression, the distance from the bottom surface of the negative electrode material to the press surface was measured, and this was multiplied by the bottom area of ​​the mold (1.767 cm). 2 The density during compression was calculated from the volume of the negative electrode material obtained by multiplying the density by 1.7 g / cm. The Autograph press hammer was equipped with a load cell and the density was adjusted to the specified density of 1.7 g / cm. 3 The applied pressure (kN / cm 2 ) was defined as pressability.

[0071] (Oxygen content measurement) The oxygen content (mass%) of the negative electrode material was measured by infrared absorption method under the conditions shown below. The measurement device used was "TCH-600" manufactured by LECO Japan LLC. The results are shown in Table 1.

[0072] -Analysis parameters- Outgas cycle: 2 times Analysis Delay 30 seconds Analysis Delay Comparator 1.0% Analysis type: Automatic analysis

[0073] -Furnace parameters- Furnace control mode power (W) -Purge time 15 seconds Outgassing time: 30 seconds Outgassing Cooldown 5 seconds Low outgassing (start) power 5400W Outgas High (End) Power 5400W ·Low (starting) power during analysis 5000W Power consumption during analysis (end) 5000W Analysis heating rate: 0℃ / sec

[0074] -Element parameters- Element Oxygen -Minimum integration time: 55 seconds Integration Delay 10 seconds Comparator Level 0.50%

[0075] -others- Crucible High temperature graphite crucible Sample mass: approx. 1.0g

[0076] (2) Evaluation of battery and plate characteristics A slurry was prepared by adding water to 98 parts by mass of the negative electrode material, 1 part by mass of styrene butadiene rubber (BM-400B, Zeon Corporation), and 1 part by mass of carboxymethyl cellulose (CMC2200, Daicel Corporation). This slurry was applied to a current collector (copper foil with a thickness of 10 μm) to form a negative electrode material layer, and then dried in the air at 110° C. for 1 hour. The negative electrode material layer was formed by applying a coating amount of the slurry per unit area of ​​10.0 mg / cm. 2 Next, the coating layer (negative electrode material layer) was roll-pressed to a predetermined electrode density (1.70 g / cm 3 ) to prepare a negative electrode.

[0077] The negative electrode and positive electrode were made of metallic lithium, and the electrolyte was 1.0M LiPF 6 A 2016-type coin cell for evaluation was produced using a mixture of ethylene carbonate / ethyl methyl carbonate (3 / 7 volume ratio) containing ethylene carbonate and vinylene carbonate (0.5 mass%), a 25 μm-thick polyethylene microporous membrane as a separator, and a 230 μm-thick copper plate as a spacer. The battery characteristics and electrode plate characteristics (electrode expansion rate) shown below were evaluated using this evaluation cell. The results are shown in Table 2.

[0078] (irreversible capacity) The fabricated lithium-ion secondary battery was placed in a thermostatic chamber set at 25°C, and the current was 0.1C and the voltage was 0V (V vs. Li / Li + ), and then constant voltage charging was performed at 0 V until the current value reached 0.02 C. The capacity at this time was defined as the initial charging capacity. After a 30-minute break, the current was 0.1 C and the voltage was 1.5 V (V vs. Li / Li + The capacity at this time was defined as the initial discharge capacity. In addition, the irreversible capacity was calculated by subtracting the initial discharge capacity from the initial charge capacity, and was used as an index of the initial charge / discharge characteristics. The unit of current value, "C," means "current value (A) / battery capacity (Ah)."

[0079] (electrode expansion rate) Electrode density 1.7g / cm 3 The ratio of the thickness of the negative electrode layer at full charge to the thickness of the negative electrode layer before the initial charge was defined as the charge expansion coefficient, and the ratio of the thickness of the negative electrode layer after discharge was defined as the discharge expansion coefficient.

[0080] (Initial efficiency) The initial efficiency was defined as the ratio of the initial discharge capacity to the initial charge capacity measured first.

[0081] (Discharge load characteristics) The ratio of the discharge capacity in the third cycle at 2.0 C to the discharge capacity in the second cycle at 0.2 C (rate (2.0 C / 0.2 C)) was used as an index of the discharge load characteristics. Other conditions (charging current, cutoff voltage, rest time, etc.) were the same as those for the initial charge / discharge.

[0082] (Retention rate after storage) After measuring the initial efficiency, the evaluation cell was placed in a thermostatic chamber set at 25°C and the current was set at 0.2C and the voltage was set at 0V (V vs. Li / Li + ), then constant voltage charging was performed at 0 V until the current value reached 0.02 C. After a 30-minute break, the battery was charged at a current value of 0.2 C and a voltage of 1.5 V (V vs. Li / Li+ ) at which the discharge capacity (second discharge capacity at 25°C) was measured. Then, the current is 0.2C and the voltage is 0V (V vs. Li / Li + ), and then constant voltage charging was performed at 0 V until the current value reached 0.02 C. The evaluation cell in this state was placed in a thermostatic chamber set at 60° C. and stored for 5 days. The evaluation cell was then placed in a thermostatic chamber set at 25°C and left for 60 minutes. The current was 0.2C and the voltage was 1.5V (V vs. Li / Li + The discharge capacity at this time (first discharge capacity at 25°C after storage at 60°C for 5 days) was measured, and the post-storage retention rate was calculated from the following formula.

[0083] Retention rate after storage (%) = (first discharge capacity at 25°C after storage for 5 days at 60°C) / (second discharge capacity at 25°C) x 100

[0084] (DCR after storage (25℃)) The evaluation cell was placed in a thermostatic chamber set at 25°C and charged and discharged for one cycle under the following conditions: Charge: CC / CV 0.2C 0V 0.02C cut, Discharge: CC 0.2C 1.5V cut. Then, constant current charging was performed at a current value of 0.2C up to SOC 50%. The evaluation cell was then placed in a thermostatic chamber set at 25°C and charged at constant currents of 1C, 3C, and 5C for 10 seconds each. The voltage drop (ΔV) at each constant current was measured, and the direct current resistance (DCR) was calculated using the following formula, which was defined as the DCR after storage at 25°C.

[0085] DCR[Ω]={(3C voltage drop ΔV-1C voltage drop ΔV)+(5C voltage drop ΔV-3C voltage drop ΔV)} / 4

[0086] (DCR after storage (-30℃)) The evaluation cell was placed in a thermostatic chamber set at 25°C and charged and discharged for one cycle under the following conditions: Charge: CC / CV 0.2C 0V 0.02C cut, Discharge: CC 0.2C 1.5V cut. Then, constant current charging was performed at a current value of 0.2C up to SOC 50%. The evaluation cell was then placed in a thermostatic chamber set at -30°C and charged at constant currents of 0.1C, 0.3C, and 0.5C for 10 seconds each. The voltage drop (ΔV) at each constant current was measured, and the direct current resistance (DCR) was calculated using the following formula, which was defined as the DCR after storage at -30°C.

[0087] DCR[Ω]={(0.3C voltage drop ΔV-0.1C voltage drop ΔV)+(0.5C voltage drop ΔV-0.3C voltage drop ΔV)} / 0.4

[0088] [Table 1]

[0089] [Table 2]

[0090] The "irreversible capacity per unit area" in Table 2 is the value obtained by dividing the measured irreversible capacity by the specific surface area.

[0091] As shown in Tables 1 and 2, the negative electrode materials of Examples 1 to 8 and Comparative Examples 1 to 5, which are not coated with amorphous carbon, have a smaller electrode expansion coefficient than the negative electrode material of Comparative Example 6, which is coated with amorphous carbon. Therefore, it is considered that the electrode expansion coefficient can be suppressed by not coating with amorphous carbon. In addition, when comparing the pressing properties of the negative electrode material produced in Example 5, which uses the same carbon particles as the raw material, and the negative electrode material produced in Comparative Example 2, it can be confirmed that the pressing property of Example 5 was pressed at a lower pressure and the expansion rate was also reduced. Since electrode expansion suggests that the strain energy stored by pressing is released as energy as expansion, peeling due to insufficient adhesive strength, and Li deposition and growth on the active material surface, etc., a negative electrode in which electrode expansion can be suppressed tends to have excellent charge and discharge characteristics.

[0092] When comparing the anode materials of the Examples, which are made of the same carbon particles and have an oxygen content of 0.15 mass% or less, with the anode materials of the Comparative Examples, which have an oxygen content of more than 0.15 mass% (for example, Examples 1 to 4 and Comparative Example 1, which use carbon particles 1 as the raw material), the anode materials of the Examples tend to have smaller irreversible capacity. From this result, it is believed that when the oxygen content of the carbon particles is 0.15 mass% or less, the reaction activity of the anode material with the electrolyte is kept low.

[0093] When the negative electrode material of the Example and the negative electrode material of the Comparative Example, which are made of the same carbon particles as the raw material, are compared, the negative electrode material of the Example tends to have a higher initial efficiency. From this result, it is considered that when the oxygen content of the negative electrode material is 0.15 mass%, the charge and discharge characteristics of the battery are improved. In addition, even in the DCR after high-temperature storage, the resistance value tends to be smaller for the negative electrode material of the Example. From this result, it can be seen that decomposition of the electrolyte and growth of the SEI film are suppressed even when the material is left in a highly active state during high-temperature storage for a long period of time.

[0094] When comparing the negative electrode materials of the Examples and Comparative Examples, which are made of the same carbon particles as the raw material, the negative electrode materials of the Examples tend to have a higher retention rate after high-temperature storage and a better balance with input / output characteristics. From this result, it is believed that when the oxygen content of the carbon particles is 0.15 mass% or less, it is possible to achieve both high-temperature durability and the maintenance of the charge / discharge performance of the battery, which is generally considered to be in a trade-off relationship.

Claims

1. A negative electrode material for a lithium ion secondary battery, comprising carbon particles having an oxygen content of 0.15 mass % or less.

2. 2. The negative electrode material for a lithium ion secondary battery according to claim 1, wherein the R value of the carbon particles is 0.45 or less.

3. The negative electrode material for a lithium ion secondary battery according to claim 1 or 2, wherein the rhombohedral crystal content of the carbon particles is greater than 0.

20.

4. The negative electrode material for lithium ion secondary batteries according to any one of claims 1 to 3, wherein the carbon particles have a circularity of more than 0.

8.

5. The negative electrode material for lithium ion secondary batteries according to any one of claims 1 to 4, wherein the carbon particles have no DTA exothermic peak detected in a range of 500 ° C. to 650 ° C. in differential thermal analysis.

6. The specific surface area of ​​the carbon particles measured by nitrogen gas adsorption method is 4.0 m ２ The negative electrode material for lithium ion secondary batteries according to any one of claims 1 to 5, wherein the molecular weight of the negative electrode material is 1 / g or more.

7. A negative electrode material slurry for lithium ion secondary batteries comprising the negative electrode material for lithium ion secondary batteries according to any one of claims 1 to 6, an organic binder, and a solvent.

8. A negative electrode for a lithium ion secondary battery comprising: a current collector; and a negative electrode material layer formed on the current collector, the negative electrode material comprising the negative electrode material for a lithium ion secondary battery according to any one of claims 1 to 6.

9. A lithium ion secondary battery comprising a positive electrode, an electrolyte, and the negative electrode for a lithium ion secondary battery according to claim 8.

10. A method for producing a negative electrode material for a lithium ion secondary battery, comprising: a step of heating carbon particles to a temperature of 400°C to 1300°C; and a step of cooling the carbon particles after the heating until the temperature of the carbon particles is less than 400°C, wherein the heating and cooling are each performed in a non-oxidizing atmosphere.

11. The method for producing the negative electrode material for lithium ion secondary batteries according to claim 10, for producing the negative electrode material for lithium ion secondary batteries according to any one of claims 1 to 6.

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