Method for manufacturing a negative electrode material for lithium-ion secondary batteries, negative electrode material for lithium-ion secondary batteries, negative electrode for lithium-ion secondary batteries, and lithium-ion secondary batteries

Abstract

To provide a method for manufacturing a negative electrode material for a lithium ion secondary battery, a negative electrode material for a lithium ion secondary battery, and a negative electrode for a lithium ion secondary battery that can manufacture a lithium ion secondary battery having excellent input characteristics and life characteristics.SOLUTION: In a manufacturing method of a negative electrode material for a lithium ion secondary battery, natural graphite particles having an average particle diameter (D50) of 5 μm to 12 μm and a specific surface area of 9.6 m2 / g or more as determined by nitrogen adsorption measurement at 77 K is prepared, and at least some of the surface of the natural graphite particles is coated with a carbon material.SELECTED DRAWING: None

Description

[Technical Field]

[0001] This disclosure relates to a method for manufacturing a negative electrode material for lithium-ion secondary batteries, a negative electrode material for lithium-ion secondary batteries, a negative electrode for lithium-ion secondary batteries, and a lithium-ion secondary battery. [Background technology]

[0002] Lithium-ion rechargeable batteries have long been widely used in electronic devices such as notebook PCs, mobile phones, smartphones, and tablet PCs, taking advantage of their small size, light weight, and high energy density. In recent years, against the backdrop of environmental problems such as global warming caused by CO2 emissions, clean electric vehicles (EVs) that run solely on batteries and hybrid electric vehicles (HEVs) that combine gasoline engines and batteries have become popular. More recently, they are also being used for power storage, and their applications are expanding in a wide range of fields.

[0003] In recent years, there has been an increasing demand for lithium-ion secondary batteries with superior input characteristics to improve energy utilization efficiency. Furthermore, lithium-ion secondary batteries also require excellent lifespan characteristics. However, generally, input characteristics and lifespan characteristics are in a trade-off relationship, and improving both simultaneously is required. In particular, in the automotive sector, one of the most important applications, there is a high demand for improvements in both input characteristics and lifespan characteristics.

[0004] Generally, increasing the specific surface area of ​​the negative electrode material is effective in improving input characteristics (see, for example, Patent Document 1). [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2000-340232 [Overview of the project] [Problems that the invention aims to solve]

[0006] However, when the specific surface area of the negative electrode material is increased to improve the input characteristics, the life characteristics deteriorate, and it is impossible to escape from the trade-off relationship between the input characteristics and the life characteristics as described above.

[0007] An object of one aspect of the present disclosure is to provide a method for manufacturing a negative electrode material for a lithium ion secondary battery, a negative electrode material for a lithium ion secondary battery, and a negative electrode for a lithium ion secondary battery capable of manufacturing a lithium ion secondary battery excellent in input characteristics and life characteristics.

Means for Solving the Problems

[0008] Specific means for solving the above problems include the following aspects.

[0009] <1> Prepare natural graphite particles having an average particle diameter (D50) of 5 μm to 12 μm and a specific surface area of 9.6 m 2 / g or more determined by nitrogen adsorption measurement at 77K, Coat at least a part of the surface of the natural graphite particles with a carbon material, A method for manufacturing a negative electrode material for a lithium ion secondary battery. <2> The method for manufacturing a negative electrode material for a lithium ion secondary battery according to <1>, wherein the natural graphite particles have an R value of 0.4 or less. <3> A negative electrode material for a lithium ion secondary battery containing natural graphite particles having a value (R value / SSA) obtained by dividing the R value by the specific surface area SSA determined by nitrogen adsorption measurement at 77K of 0.1 or less. <4> The natural graphite particles have a micropore volume of 0.10 cm 3 / g to 0.20 cm 3 / g, and the negative electrode material for a lithium ion secondary battery according to <3>. <5> The negative electrode material for a lithium ion secondary battery according to <3> or <4>, wherein the average particle diameter (D50) of the natural graphite particles is 5 μm to 12 μm. <6> The negative electrode material for a lithium ion secondary battery according to any one of <3> to <5>, wherein the natural graphite particles include spherical natural graphite particles. <7> At least a portion of the surface of the aforementioned natural graphite particles is coated with a carbon material. <3> ~ <6> A negative electrode material for lithium-ion secondary batteries as described in any one of the items. <8> The aforementioned natural graphite particles have a ratio B / A of porosity B, calculated from the amount of linseed oil absorbed, to porosity A, calculated from tap density, which is 0.99 or higher. <3> ~ <7> A negative electrode material for lithium-ion secondary batteries as described in any one of the items. <9> <3> ~ <8> 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 described in any one of the items, and a current collector. <10> <9> A lithium-ion secondary battery comprising a negative electrode, a positive electrode, and an electrolyte, as described in [reference]. [Effects of the Invention]

[0010] One aspect of this disclosure provides a method for manufacturing a negative electrode material for a lithium-ion secondary battery, a negative electrode material for a lithium-ion secondary battery, and a negative electrode for a lithium-ion secondary battery, which are capable of producing lithium-ion secondary batteries with excellent input characteristics and lifespan characteristics. [Modes for carrying out the invention]

[0011] The embodiments for carrying out the present invention will be described in detail below. However, the present invention is not limited to the following embodiments. In the following embodiments, the components (including elemental steps, etc.) are not essential unless otherwise specified. The same applies to numerical values ​​and their ranges, and they do not limit the present invention. In this 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, provided that the purpose of such process is achieved. In this disclosure, the numerical range indicated using "~" includes the numbers before and after "~" as the minimum and maximum values, respectively. In numerical ranges described in stages within this disclosure, the upper or lower limit of one numerical range may be replaced by the upper or lower limit of another numerical range described in stages. Furthermore, in numerical ranges described within this disclosure, the upper or lower limit of that range may be replaced by the values ​​shown in each test.

[0012] In this disclosure, each component in the negative electrode material and composition may contain multiple types of the corresponding substance. When multiple types of the corresponding substance exist in the negative electrode material and composition, the content and amount of each component refer to the total content and amount of the multiple types of substances present in the negative electrode material and composition, unless otherwise specified. In this disclosure, the negative electrode material and the composition may contain multiple types of particles corresponding to each component. When multiple types of particles corresponding to each component are present in the negative electrode material and the composition, the particle size of each component refers to the value for a mixture of such multiple types of particles present in the negative electrode material and the composition, unless otherwise specified. In this disclosure, the term "layer" includes cases where, when observing the region in which the layer exists, it is formed not only over the entire region but also over only a portion of the region. In this disclosure, the term "lamination" refers to stacking layers, and two or more layers may be bonded together or detachable.

[0013] In this disclosure, the average particle diameter (D50) is the particle diameter at which the cumulative volume distribution curve reaches 50% when plotted from the smallest diameter side in the particle diameter distribution measured by a laser diffraction particle size distribution analyzer. An example of a laser diffraction particle size distribution analyzer is the SALD-3000J manufactured by Shimadzu Corporation.

[0014] The specific surface area obtained from nitrogen adsorption measurements at 77K is determined using the BET method from the adsorption isotherm obtained from the nitrogen adsorption measurement at 77K, in accordance with JIS Z 8830:2013. A high-speed specific surface area / pore distribution analyzer (e.g., FlowSorbIII, manufactured by Shimadzu Corporation) is used to measure nitrogen adsorption at liquid nitrogen temperature (77K) using the single-point method, and the specific surface area is calculated by applying the BET method. When measuring the BET specific surface area, it is preferable to first perform a pretreatment to remove moisture by heating, as moisture adsorbed on the sample surface and within its structure is thought to affect the gas adsorption capacity. In the pretreatment, a measurement cell containing 0.05 g of the sample is subjected to a vacuum pump to reduce the pressure to 10 Pa or less, then heated to 110°C and maintained at that temperature for 3 hours or more. After this, the cell is allowed to cool naturally to room temperature (25°C) while maintaining the reduced pressure. After this pretreatment, the evaluation temperature is set to 77 K, and the evaluation pressure range is set to less than 1 in relative pressure (equilibrium pressure relative to saturated vapor pressure) for measurement.

[0015] In this disclosure, the R value is defined as 1580 cm⁻¹ in the Raman spectral spectrum obtained by Raman spectroscopy measurement. -1 The intensity Ig of the highest peak in the vicinity, and 1360cm -1 This is the intensity ratio (Id / Ig) of the intensity Id of the largest peak in the vicinity. The R-value is measured using a laser Raman spectrophotometer. The sample plate, with the sample set flat, is irradiated with argon laser light. For example, the NRS-1000 manufactured by JASCO Corporation can be used as a laser Raman spectrophotometer. The measurement conditions are as follows: Argon laser light wavelength: 532nm Wavenumber resolution: 2.56cm -1 Measurement range: 1180cm -1 ~1730cm -1 Peak Research: Background Removal

[0016] In this disclosure, the micropore volume obtained from CO2 adsorption measurement is calculated using the BET method from the adsorption isotherm obtained from carbon dioxide adsorption measurement at a measurement temperature of 273K and relative pressure P / P0 = 0.98~0.99 (P = equilibrium pressure, P0 = saturated vapor pressure). As the measuring device, for example, Belsorp II from Microtrac-Bell Co., Ltd. can be used, and as the device for pretreatment prior to measurement, Belprep II from Microtrac-Bell Co., Ltd. can also be used. Pretreatment is performed by heating to 250°C at 5°C / min under a vacuum of 1 Pa or less, holding for 10 minutes, then heating to 350°C at 3°C / min, holding for 210 minutes, then stopping the heating and cooling to room temperature (25°C).

[0017] In this disclosure, tap density is measured in accordance with JIS R 1628:1997. Tap density can be measured using a packing density measuring device (e.g., KRS-406, manufactured by Kuramochi Scientific Instruments Co., Ltd.). 100 mL of lithium-ion secondary battery negative electrode material is placed in a graduated cylinder, and the density is calculated by tapping (dropping the graduated cylinder from a predetermined height) until the density saturates.

[0018] In this disclosure, the amount of linseed oil absorbed is measured in accordance with the method described in JIS K6217-4:2008 "Carbon black for rubber - Basic properties - Part 4: Method for determining oil absorption", except that linseed oil (manufactured by Kanto Chemical Co., Ltd.) is used as the reagent liquid instead of dibutyl phthalate (DBP).

[0019] The specific method for measuring the amount of linseed oil absorbed is as follows: Linseed oil is titrated onto the sample using a constant-speed burette, and the change in viscosity characteristics is measured using a torque detector. The amount of reagent liquid added per unit mass of the sample corresponding to 70% of the maximum torque generated is defined as the amount of linseed oil absorbed (mL / 100g). An example of a measuring instrument is the absorption amount measuring device manufactured by Asahi Research Institute Co., Ltd.

[0020] In the present disclosure, the true density is determined by the pycnometer method using butanol. The true density can be measured using a true density measuring device (for example, Micromeritics AccuPyc 1330, Shimadzu Corporation).

[0021] In the present disclosure, the porosity A [%] calculated from the tapped density is a value obtained by the following formula A. (Formula A) Porosity A [%]=100 - tapped density [g / cm 3 / true density [g / cm 3 ×100

[0022] In the present disclosure, the porosity B [%] calculated from the linseed oil absorption is a value obtained by the following formula B. (Formula B) Porosity B [%]=linseed oil absorption [mL / 100g] / (linseed oil absorption [mL / 100g]+(100 / true density [g / cm 3 ))×100

[0023] <Method for manufacturing negative electrode material for lithium-ion secondary battery> A method for manufacturing a negative electrode material for a lithium-ion secondary battery (hereinafter, may be abbreviated as "negative electrode material") is to prepare natural graphite particles having an average particle diameter (D50) of 5 μm to 12 μm and a specific surface area of 9.6 m 2 / g or more obtained from nitrogen adsorption measurement at 77K (hereinafter, also referred to as "preparation step"), and at least a part of the surface of the natural graphite particles is coated with a carbon material (hereinafter, also referred to as "coating step"). Here, the natural graphite particles in the preparation step are referred to as "natural graphite particles A".

[0024] As described above, increasing the specific surface area of the negative electrode material is effective for improving the input characteristics. However, increasing the specific surface area leads to a decrease in the life characteristics, and there is a trade-off relationship between the input characteristics and the life characteristics. However, the average particle diameter (D50) is 5 μm to 12 μm and the specific surface area is 9.6 m 2It has been found that the above trade-off relationship can be overcome by preparing natural graphite particles A with a specific surface area of ​​1 / g or more, and coating at least a portion of the surface of such natural graphite particles with a carbon material. In other words, the method for manufacturing the negative electrode material of this disclosure is a method for overcoming the trade-off relationship between input characteristics and lifetime characteristics by balancing the physical properties of the natural graphite particles A before coating (the part corresponding to the core particles of the negative electrode material) and the natural graphite particles after coating (the surface part of the negative electrode material).

[0025] Furthermore, when the R value of natural graphite particles A, as determined by Raman spectroscopy, is 0.4 or less, the input characteristics tend to improve while suppressing the deterioration of lifetime characteristics.

[0026] The natural graphite particles A are preferably spherical natural graphite particles obtained by mechanical energy treatment, in which flaky, scale-like, or plate-like natural graphite particles are shaped into spheres. The spherical natural graphite particles do not have to be perfectly spherical.

[0027] The average particle size (D50) of natural graphite particles A is 12 μm or less. In order to suppress the diffusion distance of lithium from the surface to the interior of the negative electrode material and to further improve the input characteristics in lithium-ion secondary batteries, the average particle size (D50) of the natural graphite particles before coating is preferably 11.5 μm or less, more preferably 11 μm or less, and even more preferably 10.5 μm or less. Furthermore, the average particle size (D50) of the natural graphite particles A is 5 μm or larger, and may be 7 μm or larger. When the average particle size (D50) of the natural graphite particles A is 5 μm or larger, the pressing pressure required when forming the negative electrode material layer can be reduced, resulting in superior input characteristics.

[0028] The specific surface area of ​​natural graphite particle A is 9.6 m². 2 It is 1 / g or more, and 9.7m 2 It is more preferable that it be 9.8m or more per gram. 2 It is even more preferable that the amount be 1 / g or more. Furthermore, the specific surface area of ​​natural graphite particles A is 12m². 2 It is preferable that it be less than or equal to / g, and 11.6m 2It is more preferable that it be less than or equal to / g, and 11.2m 2 It is even more preferable that the density is less than or equal to / g. If the specific surface area of ​​the natural graphite particles A is within the above range, the decrease in input characteristics can be suppressed even if a part of the surface is covered with a carbon material, and a good balance between input characteristics and lifetime characteristics can be obtained.

[0029] The natural graphite particles A preferably have an R value of 0.4 or less, more preferably 0.35 or less, and even more preferably 0.3 or less, as determined by Raman spectroscopy. Furthermore, the R value of natural graphite particles A is preferably 0.1 or higher, more preferably 0.15 or higher, and even more preferably 0.2 or higher. When the R value of natural graphite particles A is 0.1 or higher, there are sufficient graphite lattice defects used for lithium ion intercalation and release, and the deterioration of input characteristics tends to be suppressed.

[0030] One way to lower the R value of natural graphite particles A is, for example, if natural graphite particles A are spherical natural graphite particles, to increase the mechanical energy imparted when the natural graphite particles are made spherical, thereby increasing the surface damage of natural graphite particles A.

[0031] In the coating process, at least a portion of the surface of the natural graphite particles A is coated with a carbon material. From the viewpoint of improving the input characteristics in lithium-ion secondary batteries, it is preferable that the carbon material used as the coating material has lower crystallinity than the natural graphite particles, for example amorphous carbon. Specifically, it is preferable that the carbon material is at least one selected from the group consisting of carbonaceous substances and carbonaceous particles obtained from organic compounds that can be converted to carbonaceous by heat treatment (hereinafter also referred to as carbon material precursors). The carbon material may be a single type or two or more types.

[0032] The precursors of the carbon material are not particularly limited and include pitch, organic polymer compounds, etc. Examples of pitch include ethylene heavy end pitch, crude oil pitch, coal tar pitch, asphalt decomposition pitch, pitch produced by thermal decomposition of polyvinyl chloride, etc., and pitch produced by polymerization of naphthalene, etc., in the presence of a superacid. Examples of organic polymer compounds include thermoplastic resins such as polyvinyl chloride, polyvinyl alcohol, polyvinyl acetate, and polyvinyl butyral, as well as natural substances such as starch and cellulose.

[0033] The carbonaceous particles used as carbon materials are not particularly limited and include particles such as acetylene black, oil furnace black, Ketjen black, channel black, thermal black, and soil graphite.

[0034] One method of coating with carbon material is to heat-treat a mixture containing natural graphite particles A and a carbon material precursor. The temperature during the heat treatment of the mixture is preferably 800°C to 1500°C, more preferably 900°C to 1300°C, and even more preferably 1050°C to 1250°C, from the viewpoint of improving the input characteristics in lithium-ion secondary batteries. The temperature during the heat treatment of the mixture may be constant from the start to the end of the heat treatment or may vary.

[0035] A method for manufacturing a negative electrode material for lithium-ion secondary batteries may include steps other than the preparation step and the coating step. Examples of other steps include a particle separation step, which may occur before the preparation step, between the preparation step and the coating step, and at least one of these steps after the coating step.

[0036] <Negative electrode material for lithium-ion secondary batteries> The negative electrode material for lithium-ion secondary batteries contains natural graphite particles in which the value obtained by dividing the specific surface area (SSA) by the R value (R value / SSA), determined from nitrogen adsorption measurements at 77K, is 0.1 or less. Here, the natural graphite particles in the negative electrode material are referred to as "natural graphite particles B".

[0037] As described above, the anode material of this disclosure overcomes the trade-off between input characteristics and lifetime characteristics by balancing the physical properties of the natural graphite particles A before coating (the portion corresponding to the core particles of the anode material) and the natural graphite particles after coating (the surface portion of the anode material). From the viewpoint of improving input characteristics, the R value is lowered by increasing the surface damage of the core natural graphite particles A. On the other hand, by coating at least a part of the surface of the natural graphite particles A with a carbon material, excessive reaction with the electrolyte, etc., is suppressed, and lifetime characteristics are improved. When coated with a carbon material, the specific surface area SSA of the coated natural graphite particles becomes lower and the R value becomes higher. Therefore, by balancing the specific surface area SSA and R value of natural graphite particles B, and setting the R value / SSA of natural graphite particles B to 0.1 or less, an anode material with excellent input characteristics and lifetime characteristics can be obtained.

[0038] The R-value / SSA of natural graphite particles B is preferably 0.1 or less, more preferably 0.09 or less, and more preferably 0.08 or less. Furthermore, the R-value / SSA of natural graphite particles B is preferably 0.03 or more, more preferably 0.04 or more, and even more preferably 0.05 or more.

[0039] The specific surface area (SSA) of natural graphite particles B is 3m², from the perspective of improving input characteristics. 2 It is preferable that the amount be 3.5m or more. 2 It is more preferable that it be 4m or more / g. 2 It is even more preferable that it be 1 / g or more. Also, the specific surface area SSA of natural graphite particles B is 6m 2 It is preferable that the amount be less than or equal to 5.5m 2 It is more preferable that it be less than or equal to / g, 5m 2 It is even more preferable that the amount be less than or equal to / g.

[0040] The R value of natural graphite particles B is preferably 0.1 to 1.0, more preferably 0.2 to 0.7, even more preferably 0.2 to 0.5, and still more preferably 0.2 to 0.4. When the R value of natural graphite particles B is 0.1 or higher, there are sufficient graphite lattice defects used for lithium ion intercalation and release, and the decrease in input characteristics tends to be suppressed. When the R value of natural graphite particles B is 1.0 or lower, the decomposition reaction of the electrolyte is sufficiently suppressed, and the decrease in initial efficiency tends to be suppressed.

[0041] Natural graphite particles B, from the perspective of improving input characteristics, have a micropore volume (hereinafter sometimes abbreviated as "micropore volume") determined by CO2 adsorption measurement, which is 0.10 cm³. 3 It is preferable that it be 0.11 cm or more. 3 It is more preferable that it be 0.12 cm or more. 3 It is even more preferable that the amount is 0.20 cm² or more. Furthermore, the micropore volume of natural graphite particles B is 0.20 cm² from the viewpoint of improving lifetime characteristics. 3 It is preferable that the value be less than or equal to 0.19 cm². 3 It is more preferable that it be less than or equal to / g, and 0.18cm 3 It is even more preferable that the amount be less than or equal to / g.

[0042] The average particle size (D50) of natural graphite particles B is preferably 12 μm or less, more preferably 11.5 μm or less, and even more preferably 11 μm or less. Furthermore, the average particle size (D50) of natural graphite particles B may be 5 μm or more, or 7 μm or more.

[0043] The natural graphite particles B are preferably spherical natural graphite particles. Furthermore, at least a portion of the surface of the natural graphite particles B may be coated with a carbon material. The presence of a carbon material on the surface of the natural graphite particles B can be confirmed by transmission electron microscopy.

[0044] It is preferable that more than half of the natural graphite particles B contained in the negative electrode material have a portion coated with carbon material, more preferably that 90% or more of the particles have a portion coated with carbon material, and even more preferably that 95% or more of the particles have a portion coated with carbon material.

[0045] The ratio B / A of the porosity B calculated from the amount of linseed oil absorbed to the porosity A calculated from the tap density is preferably 0.99 or higher, and more preferably 1.00 or higher. The ratio B / A represents the degree of spheroidization of the natural graphite; a lower ratio B / A means that the natural graphite contains many particles with a low degree of spheroidization. Increasing the degree of spheroidization can increase the electrode density, which tends to reduce the press pressure load during electrode fabrication. Reducing the press pressure load suppresses cracking and peeling of the carbon material coating during electrode fabrication, contributing to improved lifespan characteristics. Therefore, a ratio B / A of 0.99 or higher tends to improve lifespan characteristics.

[0046] The tap density of natural graphite particles B is 1.20 g / cm³. 3 Preferably, it is 1.18 g / cm³. 3 It is more preferable that the following is the case: 1.15 g / cm³ 3 It is even more preferable that the following conditions are met. Furthermore, the tap density of natural graphite particles B is 0.60 g / cm³. 3 It may be greater than or equal to 1.00 g / cm³. 3 That's fine too.

[0047] The amount of linseed oil absorbed by natural graphite particles B may be 50 mL / 100g or less, 49 mL / 100g or less, or 48 mL / 100g or less. Furthermore, the amount of linseed oil absorbed by natural graphite particles B may be 40 mL / 100g or more, or 41 mL / 100g or more.

[0048] The average interplanar spacing d of natural graphite particles B was determined by X-ray diffraction. 002 It is preferable that the average interplanar spacing d 002When the wavelength is 0.338 nm or less, lithium-ion secondary batteries tend to exhibit superior initial charge-discharge efficiency and energy density.

[0049] Average interplanar spacing d of natural graphite particles B 002 The value of tends to decrease, for example, by increasing the heat treatment temperature when manufacturing the negative electrode material. Therefore, by adjusting the heat treatment temperature when manufacturing the negative electrode material, the average interplanar spacing d of the carbon material can be controlled. 002 It can be controlled.

[0050] In this disclosure, the average interplanar spacing d 002 This is calculated using Bragg's formula from the diffraction profile obtained by irradiating the sample with X-rays (CuKα rays) and measuring the diffraction lines with a goniometer, specifically from the diffraction peak corresponding to the carbon O02 plane that appears around the diffraction angle 2θ = 24° to 27°. Specifically, the sample to be measured is filled into the recessed portion of a quartz sample holder and set on the measurement stage, and the measurement is performed using a wide-angle X-ray diffractometer (manufactured by Rigaku Corporation) under the following conditions. Radiation source: CuKα ray average spacing (wavelength = 0.15418nm) Output: 40kV, 20mA Sampling width: 0.010° Scanning range: 10°~35° Scan speed: 0.5° / min

[0051] The content of natural graphite particles B in the negative electrode material is not particularly limited, but is preferably 50% by mass or more, more preferably 80% by mass or more, even more preferably 90% by mass or more, and particularly preferably 100% by mass. The negative electrode material may contain carbon materials other than natural graphite particles B. These other carbon materials are not particularly limited and include, for example, artificial graphite, amorphous carbon, carbon black, fibrous carbon, and nanocarbon. These other carbon materials may be used individually or in combination of two or more. The negative electrode material may contain particles containing elements other than carbon materials that are capable of intercepting and releasing lithium ions. The elements capable of intercepting and releasing lithium ions are not particularly limited, but include Si, Sn, Ge, In, and the like.

[0052] <Negative electrode for lithium-ion secondary batteries> The negative electrode for a lithium-ion secondary battery according to this disclosure includes a negative electrode material layer containing the negative electrode material for a lithium-ion secondary battery according to this disclosure, and a current collector. In addition to the negative electrode material layer containing the negative electrode material and the current collector, the negative electrode for a lithium-ion secondary battery may include other components as necessary.

[0053] A negative electrode for lithium-ion secondary batteries can be manufactured, for example, by kneading a negative electrode material and a binder together with a solvent to prepare a slurry-like negative electrode material composition, which is then applied to a current collector to form a negative electrode material layer, or by molding the negative electrode material composition into a sheet, pellet, or other shape and integrating it with a current collector. Kneading can be performed using dispersion equipment such as a stirrer, ball mill, super sand mill, or pressure kneader.

[0054] The binder used in the preparation of the negative electrode material composition is not particularly limited. Examples of binders include polymers of ethylenically unsaturated carboxylic acid esters such as styrene-butadiene copolymer, methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, butyl acrylate, butyl methacrylate, acrylonitrile, methacrylonitrile, hydroxyethyl acrylate, and hydroxyethyl methacrylate; polymers of ethylenically unsaturated carboxylic acids such as acrylic acid, methacrylic acid, itaconic acid, fumaric acid, and maleic acid; and highly ionically conductive polymer compounds such as polyvinylidene fluoride, polyethylene oxide, polyepichlorohydrin, polyphosphazene, and polyacrylonitrile. If the negative electrode material composition contains a binder, the amount is not particularly limited. The binder content may be, for example, 0.5 to 20 parts by mass per 100 parts by mass of the total of the negative electrode material and the binder.

[0055] The solvent is not particularly limited as long as it is capable of dissolving or dispersing the binder. Specifically, examples include organic solvents such as N-methyl-2-pyrrolidone, N,N-dimethylacetamide, N,N-dimethylformamide, and γ-butyrolactone. The amount of solvent used is not particularly limited as long as the negative electrode material composition can be made into a desired state such as a paste. Preferably, the amount of solvent used is 60 parts by mass or more and less than 150 parts by mass per 100 parts by mass of negative electrode material.

[0056] The negative electrode material composition may contain a thickening agent. Examples of thickening agents include carboxymethylcellulose or its salts, methylcellulose, hydroxymethylcellulose, hydroxyethylcellulose, ethylcellulose, polyvinyl alcohol, polyacrylic acid or its salts, alginic acid or its salts, oxide starch, phosphorylated starch, casein, and the like. If the negative electrode material composition contains a thickening agent, the amount is not particularly limited. The content of the thickening agent may be, for example, 0.1 to 5 parts by mass per 100 parts by mass of the negative electrode material.

[0057] The negative electrode material composition may contain a conductive additive. Examples of conductive additives include artificial graphite, carbon materials such as carbon black (acetylene black, thermal black, furnace black, etc.), conductive oxides, and conductive nitrides. If the negative electrode material composition contains a conductive additive, the amount is not particularly limited. The content of the conductive additive may be, for example, 0.5 to 15 parts by mass per 100 parts by mass of the negative electrode material.

[0058] 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. In addition, porous materials such as porous metal (foamed metal) and carbon paper can also be used as current collectors.

[0059] When forming a negative electrode material layer by applying a negative electrode material composition to a current collector, the method is not particularly limited, and known methods such as metal mask printing, electrostatic coating, dip coating, spray coating, roll coating, doctor blade method, comma coating, gravure coating, and screen printing can be employed. After applying the negative electrode material composition to the current collector, the solvent contained in the negative electrode material composition is removed by drying. Drying can be carried out, for example, using a hot air dryer, an infrared dryer, or a combination of these devices. Rolling may be performed as needed. Rolling can be carried out by methods such as a flat plate press or a calender roll.

[0060] When a negative electrode material composition molded into the shape of a sheet, pellet, or the like is integrated with a current collector to form a negative electrode material layer, the method of integration is not particularly limited. For example, it can be done by rolling, flat plate pressing, or a combination of these means. The pressure during integration is preferably, for example, 1 MPa to 200 MPa.

[0061] <Lithium-ion rechargeable battery> The lithium-ion secondary battery of this disclosure includes the negative electrode for the lithium-ion secondary battery of this disclosure described above (hereinafter also simply referred to as the "negative electrode"), a positive electrode, and an electrolyte.

[0062] 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 manufacturing method described above. As the current collector, metals or alloys such as aluminum, titanium, and stainless steel can be used in the form of foil, perforated foil, mesh, etc.

[0063] The cathode material used to form the cathode layer is not particularly limited. Examples include metal compounds (metal oxides, metal sulfides, etc.) and conductive polymer materials that can be doped or intercalated with lithium ions. More specifically, lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMnO2), and their complex oxides (LiCo x Ni y Mn zO2 (x+y+z=1), complex oxide containing the additive element M' (LiCo a Ni b Mn c M' d O2, a+b+c+d=1, M':Al, Mg, Ti, Zr or Ge), spinel-type lithium manganese oxide (LiMn2O4), lithium vanadium compound, V2O5, V6O 13 Examples include lithium-containing compounds such as VO2, MnO2, TiO2, MoV2O8, TiS2, V2S5, VS2, MoS2, MoS3, Cr3O8, Cr2O5, and olivine-type LiMPO4 (M:Co, Ni, Mn, Fe), conductive polymers such as polyacetylene, polyaniline, polypyrrole, polythiophene, and polyacene, and porous carbon. The cathode material may be a single type or two or more types.

[0064] The electrolyte is not particularly limited; for example, a lithium salt dissolved in a non-aqueous solvent (a so-called organic electrolyte) can be used. Examples of lithium salts include LiClO4, LiPF6, LiAsF6, LiBF4, and LiSO3CF3. The lithium salt may be used individually or in combination of two or more types. Examples of non-aqueous solvents include ethylene carbonate, fluoroethylene carbonate, chloroethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, cyclopentanone, cyclohexylbenzene, sulfolane, propanesultone, 3-methylsulfolane, 2,4-dimethylsulfolane, 3-methyl-1,3-oxazolidine-2-one, γ-butyrolactone, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methylpropyl 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 types.

[0065] The state of the positive and negative electrodes in a lithium-ion secondary battery is not particularly limited. For example, the positive and negative electrodes, along with a separator placed between them as needed, may be wound in a spiral shape, or they may be stacked in a flat plate shape.

[0066] The separator is not particularly limited; for example, nonwoven fabrics, cloths, microporous films made of resin, or combinations thereof can be used. Examples of resins include those mainly composed of polyolefins such as polyethylene and polypropylene. If the positive and negative electrodes do not come into direct contact due to the structure of the lithium-ion secondary battery, a separator may not be used.

[0067] The shape of lithium-ion secondary batteries is not particularly limited. Examples include laminated batteries, paper batteries, button batteries, coin batteries, stacked batteries, cylindrical batteries, and prismatic batteries.

[0068] The lithium-ion secondary battery disclosed herein is suitable as a high-capacity lithium-ion secondary battery for use in electric vehicles, power tools, power storage devices, etc., due to its excellent output characteristics. In particular, it is suitable as a lithium-ion secondary battery for use in electric vehicles (EVs), hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), etc., where high-current charging and discharging are required to improve acceleration performance and brake regeneration performance. [Examples]

[0069] The present invention will be described in detail below with reference to the following examples, but the present invention is not limited to these examples.

[0070] Particle sizes (D10, D50, D90, and D99.9) were measured by the method described below. Specific surface area (SSA), R value, tap density, linseed oil absorption, true density, and micropore volume were measured by the method described above.

[0071] [Measurement of particle size] A solution prepared by dispersing a negative electrode material sample with 0.2% by mass of a surfactant (product name: Liponol T / 15, manufactured by Lion Corporation) in purified water was placed in the sample tank of a laser diffraction particle size distribution analyzer (SALD-3000J, manufactured by Shimadzu Corporation). Next, the solution was circulated by pump while applying ultrasound (pump flow rate was 65% of the maximum value), and the volume-based particle size distribution was measured by adjusting the water volume so that the absorbance was between 0.10 and 0.15. In the cumulative volume distribution (%), we determined the particle diameter D10 (μm) when the smallest particle diameters accounted for 10%, the particle diameter D50 (μm) (average particle diameter) when the smallest particle diameters accounted for 50%, the particle diameter D90 (μm) when the smallest particle diameters accounted for 90%, and the particle diameter D99.9 (μm) when the smallest particle diameters accounted for 99.9%.

[0072] [Example 1] Spherical natural graphite particles (average particle size (D50) 10.2 μm, specific surface area 10.1 m²) 2 100 parts by mass of coal tar pitch (1 / g, R value 0.26) and 3.6 parts by mass of coal tar pitch (softening point 90°C, residual carbonization rate 50% by mass) were mixed. The mixture was then heated to 1050°C at a rate of 250°C / hour under nitrogen flow, and held at 1050°C (firing temperature) for 1 hour to coat the carbon material and obtain carbon-coated carbon particles. The obtained carbon-coated carbon particles were crushed with a cutter mill, then sieved through a 350-mesh sieve, and the portion below the sieve was used as the negative electrode material. The various physical properties of the fabricated negative electrode material were measured. The physical properties are shown in Table 1.

[0073] [Comparative Example 1] The spherical natural graphite particles used as raw materials in Example 1 had an average particle size (D50) of 10.8 μm and a specific surface area of ​​9.0 m². 2 The negative electrode material was prepared in the same manner as in Example 1, except that the / g and R value were changed to 0.29. The physical properties of the prepared negative electrode material were measured. The physical properties are shown in Table 1.

[0074] [Comparative Example 2] A negative electrode material was prepared in the same manner as in Example 1, except that the amount of coal tar pitch used in Comparative Example 1 was changed to 4.8 parts by mass. The physical properties of the prepared negative electrode material were measured. The physical properties are shown in Table 1.

[0075] [Comparative Example 3] The spherical natural graphite particles used as raw materials had an average particle size (D50) of 10.9 μm and a specific surface area of ​​9.5 m². 2 The negative electrode material was prepared in the same manner as in Example 1, except that the material was changed to one with a value of 0.34 / g and R value. The physical properties of the prepared negative electrode material were measured. The physical properties are shown in Table 1.

[0076] (Manufacturing of lithium-ion secondary batteries) A lithium-ion secondary battery was fabricated using the negative electrode material prepared in each example, following the procedure described below. First, to 98 parts by mass of the negative electrode material, an aqueous solution of CMC (carboxymethylcellulose, manufactured by Daicel Finechem Co., Ltd., product number 2200) (CMC concentration: 2% by mass) was added as a thickening agent so that the solid content of CMC was 1 part by mass, and the mixture was kneaded for 10 minutes. Next, purified water was added so that the total solid content concentration of the negative electrode material and CMC was 40% to 50% by mass, and the mixture was kneaded for 10 minutes. Subsequently, an aqueous dispersion of SBR (BM400-B, Nippon Zeon Co., Ltd.), a styrene-butadiene copolymer rubber (SBR concentration: 40% by mass) was added as a binder so that the solid content of SBR was 1 part by mass, and the mixture was mixed for 10 minutes to prepare a paste-like negative electrode material composition. Next, the negative electrode material composition was applied to an electrolytic copper foil with a thickness of 11 μm at a rate of 5.9 mg / cm² per unit area. 2 The negative electrode material layer was formed by coating with a comma coater with the clearance adjusted accordingly. Then, it was applied by hand press at a rate of 1.2 g / cm³. 3 The electrode density was adjusted. A sample electrode (negative electrode) was fabricated by punching out a 16 mm diameter disc from an electrolytic copper foil on which a negative electrode material layer had been formed.

[0077] A coin-type lithium-ion secondary battery was fabricated by placing the prepared sample electrode (negative electrode), separator, and counter electrode (positive electrode) in that order into a coin-type battery container and injecting the electrolyte. The electrolyte used was a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (with a volume ratio of EC to EMC of 3:7), to which 0.5% by mass of vinylene carbonate (VC) was added to the total volume of the mixed solution, and LiPF6 was dissolved to a concentration of 1 mol / L. LiNi was used as the counter electrode (positive electrode). 0.5 Mn 0.3 Co 0.2 O2 (NMC532) was used. A 20 μm thick polyethylene microporous membrane was used as the separator.

[0078] (Measurement of DC resistance (DCR)) The DC resistance (DCR) of the fabricated lithium-ion secondary battery was measured to determine its input characteristics. Specifically, the results are as follows: The lithium-ion secondary battery described above was placed in a constant temperature bath set to 25°C and charged using constant current / constant voltage (CC / CV) charging at 0.33C, 4.2V, and a cutoff current of 0.05C. This was followed by constant current (CC) discharge at 0.33C to 2.5V, and this cycle was repeated for three cycles to complete the charge / discharge process. Subsequently, constant current charging was performed at a current of 0.2C until the State of Charge (SOC) reached 60%. Furthermore, the lithium-ion secondary battery was placed in a constant temperature bath set to -10°C, and constant current charging was performed for 10 seconds each under conditions of 0.2C, 0.5C, and 1C. The voltage drop (ΔV) for each constant current was measured, and the DC resistance (DCR) was measured using the following formula. The results are shown in Table 1. A lower DC resistance (DCR) value indicates better input characteristics. DCR[Ω] = {(0.5C voltage drop △V - 0.2C voltage drop △V) + (1C voltage drop △V - 0.5C voltage drop △V)} / 0.8

[0079] (Measurement of charge capacity retention rate) Using the lithium-ion secondary battery described above, the battery was placed in a constant temperature bath set to 25°C and charged using constant current / constant voltage (CC / CV) charging at 0.33C, 4.2V, and a cutoff current of 0.05C. This was followed by constant current (CC) discharge at 0.33C to 2.5V, and this cycle was repeated for three cycles. Next, the battery was placed in a constant temperature bath set to 25°C and charged using constant current / constant voltage (CC / CV) charging at 0.33C, 4.2V, and a cutoff current of 0.05C. After that, it was stored in a constant temperature bath set to 60°C for 10 days, and then placed in a constant temperature bath set to 25°C and discharged using constant current (CC) at 0.33C to 2.5V. The results are shown in Table 1. (Cycles 1-3) Charging conditions: Constant current charging 0.967mA, constant voltage charging 4.2V, cut-off current 0.145mA Discharge conditions: Constant current discharge 0.967mA, cut-off voltage 2.5V (4th cycle) Charging conditions: Constant current charging 0.967mA, constant voltage charging 4.2V, cut-off current 0.145mA (Store in a constant temperature bath set to 60°C for 10 days until cycles 4-5 are performed.) (5th cycle) Discharge conditions: Current charging 0.967mA, constant voltage charging 4.2V, cut-off current 0.145mA

[0080] Charge capacity maintenance rate (%)= [CC discharge capacity in the 5th cycle / CC discharge capacity in the 3rd cycle] × 100

[0081] [Table 1]

Claims

1. The average particle size (D50) is 5 μm to 12 μm, and the specific surface area determined from nitrogen adsorption measurements at 77 K is 9.6 m². ２ / g to 12m ２ To prepare natural graphite particles at a rate of / g, This includes obtaining coated natural graphite particles by coating at least a portion of the surface of the aforementioned natural graphite particles with a carbon material, The R value of the aforementioned natural graphite particles is 0.1 to 0.

3. The specific surface area SSA (m²) of the coated natural graphite particles is determined from nitrogen adsorption measurements at 77K. ２ The ratio of the R value of the coated natural graphite particles to the amount per g (R value / SSA) is 0.1 or less. A method for manufacturing negative electrode material for lithium-ion secondary batteries.

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