JP2023073103A - Method for manufacturing negative electrode material for lithium ion secondary battery, negative electrode material for lithium ion secondary battery, negative electrode for lithium ion secondary battery, and lithium ion secondary battery - Google Patents

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

The present disclosure relates to a method for producing 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.

Lithium-ion secondary batteries have been widely used in electronic devices such as notebook PCs, mobile phones, smart phones, and tablet PCs, taking advantage of their characteristics of small size, light weight, and high energy density. In recent years, against the background of environmental problems such as global warming caused by _CO2 emissions, clean electric vehicles (EV) that run solely on batteries and hybrid electric vehicles (HEV) that combine a gasoline engine and batteries are becoming popular. . Moreover, recently, it is also used for electric power storage, and its use is expanding in a wide variety of fields.

In recent years, there has been a demand for lithium ion secondary batteries with excellent input characteristics in order to improve energy utilization efficiency. In addition, lithium-ion secondary batteries are required to have excellent life characteristics. However, input characteristics and life characteristics are generally in a trade-off relationship, and it is desired to improve both of them. In particular, in the application to the automobile field, which is one of the important applications, there is a high demand for improved input characteristics and life characteristics.

In general, increasing the specific surface area of the negative electrode material is effective for improving input characteristics (see, for example, Patent Document 1).

JP-A-2000-340232

However, if the specific surface area of the negative electrode material is increased in order to improve the input characteristics, the life characteristics will deteriorate, and as described above, the input characteristics and the life characteristics cannot be escaped from the trade-off relationship.

In one aspect of the present disclosure, a method for producing a negative electrode material for a lithium ion secondary battery capable of producing a lithium ion secondary battery having excellent input characteristics and life characteristics, a negative electrode material for a lithium ion secondary battery, and a lithium ion secondary battery An object of the present invention is to provide a negative electrode for

Specific means for solving the above problems include the following aspects.

<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 as determined by nitrogen adsorption measurement at 77 K,
At least part of the surface of the natural graphite particles is coated with a carbon material;
A method for producing a negative electrode material for a lithium ion secondary battery.
<2> The method for producing 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, comprising natural graphite particles having a value (R value/SSA) of 0.1 or less obtained by dividing the R value by the specific surface area SSA determined by nitrogen adsorption measurement at 77K.
<4> The negative electrode material for a lithium ion secondary battery according to <3>, wherein the natural graphite particles have a micropore volume of 0.10 cm ³ /g to 0.20 cm ³ /g determined by CO ₂ adsorption measurement.
<5> The negative electrode material for a lithium ion secondary battery according to <3> or <4>, wherein the natural graphite particles have an average particle size (D50) of 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> The negative electrode material for a lithium ion secondary battery according to any one of <3> to <6>, wherein at least part of the surface of the natural graphite particles is coated with a carbon material.
<8> The natural graphite particles have a ratio B/A of the porosity B calculated from the linseed oil absorption to the porosity A calculated from the tap density of 0.99 or more <3> to <7>, the negative electrode material for a lithium ion secondary battery according to any one of items 7>.
<9> A negative electrode for a lithium ion secondary battery, comprising a negative electrode material layer containing the negative electrode material for a lithium ion secondary battery according to any one of <3> to <8>, and a current collector.
<10> A lithium ion secondary battery comprising the lithium ion secondary battery negative electrode according to <9>, a positive electrode, and an electrolytic solution.

In one aspect of the present disclosure, a method for producing a negative electrode material for a lithium ion secondary battery capable of producing a lithium ion secondary battery having excellent input characteristics and life characteristics, a negative electrode material for a lithium ion secondary battery, and a lithium ion secondary battery can provide a negative electrode for

DETAILED DESCRIPTION OF THE INVENTION 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 constituent elements (including element steps and the like) are not essential unless otherwise specified. The same applies to numerical values and their ranges, which do not limit the present invention.
In the present disclosure, the term "process" includes a process that is independent of other processes, and even if the purpose of the process is achieved even if it cannot be clearly distinguished from other processes. .
In the present disclosure, the numerical range indicated using "-" includes the numerical values before and after "-" as the minimum and maximum values, respectively.
In the numerical ranges described step by step in the present disclosure, the upper limit or lower limit of one numerical range may be replaced with the upper or lower limit of another numerical range described step by step. . In addition, in the numerical ranges described in this disclosure, the upper or lower limits of the numerical ranges may be replaced with the values shown in each test.

In the present disclosure, each component in the negative electrode material and in the composition may contain multiple types of applicable substances. When there are multiple types of substances corresponding to each component in the negative electrode material and composition, the content rate and content of each component are the same as those of the multiple types present in the negative electrode material and composition, unless otherwise specified. It means the total content and content of substances.
In the present disclosure, a plurality of types of particles corresponding to each component in the negative electrode material and composition may be included. When a plurality of types of particles corresponding to each component are present in the negative electrode material and composition, the particle diameter of each component is the mixture of the plurality of types of particles present in the negative electrode material and composition, unless otherwise specified. means a value for
In the present disclosure, the term "layer" includes not only the case where the layer is formed in the entire region when observing the region where the layer exists, but also the case where it is formed only in part of the region. included.
In the present disclosure, the term "laminate" indicates stacking layers, and two or more layers may be bonded, or two or more layers may be detachable.

In the present disclosure, the average particle diameter (D50) is the particle diameter when the cumulative volume distribution curve is drawn from the small diameter side to 50% in the particle diameter distribution measured by a laser diffraction particle size distribution analyzer. be. Examples of the laser diffraction particle size distribution analyzer include SALD-3000J manufactured by Shimadzu Corporation.

The specific surface area obtained from the nitrogen adsorption measurement at 77K is obtained from the adsorption isotherm obtained from the nitrogen adsorption measurement at 77K according to JIS Z 8830:2013 using the BET method. Using a high-speed specific surface area / pore distribution measuring device (e.g., FlowSorb III manufactured by Shimadzu Corporation), nitrogen adsorption at liquid nitrogen temperature (77 K) is measured by the one-point method, and the specific surface area is calculated by applying the BET method. do.
When measuring the BET specific surface area, it is considered that the water adsorbed in the sample surface and structure affects the gas adsorption capacity, so it is preferable to first perform a pretreatment to remove water by heating. .
In the pretreatment, the measurement cell containing 0.05 g of the measurement sample was evacuated to 10 Pa or less with a vacuum pump, heated at 110° C., held for 3 hours or longer, and then kept at room temperature ( Cool naturally to 25°C). After this pretreatment, the evaluation temperature is set to 77 K, and the evaluation pressure range is set to less than 1 relative pressure (equilibrium pressure with respect to saturated vapor pressure).

In the present disclosure, the R value is the intensity ratio (Id/Ig) of the maximum peak intensity Ig near 1580 cm ^-1 and the maximum peak intensity Id near 1360 cm ^-1 in the Raman spectroscopic spectrum obtained in Raman spectroscopic measurement. is.
A laser Raman spectrophotometer is used to measure the R value by irradiating an argon laser beam onto a sample plate on which the sample to be measured is set flat. As the laser Raman spectrophotometer, for example, NRS-1000 manufactured by JASCO Corporation can be used. The measurement conditions are as follows.
Argon laser light wavelength: 532 nm
Wavenumber resolution: 2.56 cm ^-1
Measurement range: 1180 cm ^-1 to 1730 cm ^-1
Peak research: background subtraction

In the present disclosure, the micropore volume determined from the CO ₂ adsorption measurement is the measurement temperature of 273 K, the relative pressure P/P ₀ = 0.98 to 0.99 (P = equilibrium pressure, P ₀ = saturated vapor pressure). It is calculated using the BET method from the adsorption isotherm obtained from the measurement. As a measuring device, for example, Belsorp II manufactured by Microtrack Bell Co., Ltd. can be used, and as a pretreatment device prior to measurement, Belprep II manufactured by Microtrack Bell Co., Ltd. can be used. Pretreatment is performed at a degree of vacuum of 1 Pa or less, raising the temperature to 250°C at a rate of 5°C/min, holding for 10 minutes, then raising the temperature to 350°C at a rate of 3°C/min, holding for 210 minutes, and then stopping heating. and cooled to room temperature (25°C).

In the present disclosure, tap density is measured according to JIS R 1628:1997. The tap density can be measured using a packing density measuring device (eg, KRS-406, manufactured by Kuramochi Scientific Instruments Manufacturing Co., Ltd.). 100 mL of the negative electrode material for lithium ion secondary batteries is placed in a graduated cylinder, and the density is calculated by tapping (dropping the graduated cylinder from a predetermined height) until the density is saturated.

In the present disclosure, the linseed oil absorption is measured according to the method described in JIS K6217-4:2008 "Carbon black for rubber - Basic properties - Part 4: Determination of oil absorption", provided that the reagent liquid is measured by using linseed oil (manufactured by Kanto Kagaku Co., Ltd.) instead of dibutyl phthalate (DBP).

A specific method for measuring the linseed oil absorption is as follows. Flaxseed oil is titrated to the measurement sample with a constant speed burette, and the change in viscosity characteristics is measured with a torque detector. The added amount of the reagent liquid per unit mass of the measurement sample corresponding to 70% of the generated maximum torque is defined as the linseed oil absorption (mL/100 g). As a measuring device, for example, an absorption measuring device manufactured by Asahi Research Institute Co., Ltd. can be used.

In the present disclosure, true density is determined by the pycnometer method using butanol. The true density can be measured using a true density meter (for example, Micromeritics Accupic 1330, Shimadzu Corporation).

In the present disclosure, the porosity A [%] calculated from the tap density is a value obtained by Formula A below.
(Formula A) Porosity A [%] = 100 - tap density [g/cm ³ ]/true density [g/cm ³ ] x 100

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/ ^cm3 ])) x 100

<Method for producing negative electrode material for lithium ion secondary battery>
The manufacturing method of the negative electrode material for lithium ion secondary batteries (hereinafter sometimes abbreviated as “negative electrode material”) has an average particle diameter (D50) of 5 μm to 12 μm and a specific surface area determined by nitrogen adsorption measurement at 77 K. Prepare natural graphite particles of 9.6 m ² /g or more (hereinafter also referred to as “preparing step”), and coat at least part of the surface of the natural graphite particles 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".

As mentioned above, it is effective to increase the specific surface area of the negative electrode material to improve the input characteristics. .
However, natural graphite particles A 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 are prepared, and at least part of the surface of such natural graphite particles having a large specific surface area is It was found that the above trade-off relationship can be overcome by coating with a carbon material. That is, the method for producing a negative electrode material of the present disclosure balances the physical properties of the natural graphite particles A before coating (the portion corresponding to the core particles of the negative electrode material) and the natural graphite particles after coating (the surface portion of the negative electrode material). This is a method of overcoming the trade-off relationship between input characteristics and life characteristics.

Furthermore, when the natural graphite particles A have an R value determined by Raman spectrometry of 0.4 or less, the input characteristics tend to be further improved while suppressing deterioration in life characteristics.

The natural graphite particles A are preferably spherical natural graphite particles obtained by spheroidizing scale-like, scale-like or plate-like natural graphite particles by mechanical energy treatment. The spherical natural graphite particles may not be perfectly spherical.

The average particle size (D50) of the natural graphite particles A is 12 µm or less. The average particle size (D50) of the natural graphite particles before coating is suppressed from increasing the diffusion distance of lithium from the surface of the negative electrode material to the inside, and the input characteristics of the lithium ion secondary battery are further improved. It is preferably 11.5 μm or less, more preferably 11 μm or less, even more preferably 10.5 μm or less.
Moreover, the average particle size (D50) of the natural graphite particles A is 5 μm or more, and may be 7 μm or more. When the average particle size (D50) of the natural graphite particles A is 5 μm or more, the pressing pressure required for forming the negative electrode layer can be reduced, resulting in better input characteristics.

The specific surface area of the natural graphite particles A is 9.6 m ² /g or more, more preferably 9.7 m ² /g or more, and even more preferably 9.8 m ² /g or more. The specific surface area of the natural graphite particles A is preferably 12 m ² /g or less, more preferably 11.6 m ² /g or less, and even more preferably 11.2 m ² /g or less. If the specific surface area of the natural graphite particles A is within the above range, even if a part of the surface is covered with the carbon material, the deterioration of the input characteristics can be suppressed, and a good balance between the input characteristics and the life characteristics can be obtained.

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

As a method for lowering the R value of the natural graphite particles A, for example, when the natural graphite particles A are spherical natural graphite particles, the mechanical energy applied when the natural graphite particles are spheroidized is increased to reduce the natural graphite particles A. There is a method of increasing the damage on the surface of the.

In the coating step, at least part of the surface of the natural graphite particles A is coated with a carbon material. From the viewpoint of improving the input characteristics of the lithium ion secondary battery, the carbon material used as the coating material preferably has a lower crystallinity than natural graphite particles, such as amorphous carbon. Specifically, 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 matter by heat treatment (hereinafter also referred to as precursors of carbonaceous materials). is preferred. The carbon material may be used singly or in combination of two or more.

Precursors of the carbon material are not particularly limited, and include pitch, organic polymer compounds, and the like. As the pitch, for example, ethylene heavy end pitch, crude oil pitch, coal tar pitch, asphalt cracked pitch, pitch produced by pyrolyzing polyvinyl chloride, etc., and naphthalene produced by polymerizing in the presence of a super strong acid. There is a pitch that Examples of organic polymer compounds include thermoplastic resins such as polyvinyl chloride, polyvinyl alcohol, polyvinyl acetate and polyvinyl butyral, and natural substances such as starch and cellulose.

The carbonaceous particles used as the carbon material are not particularly limited, and examples thereof include particles of acetylene black, oil furnace black, ketjen black, channel black, thermal black, soil graphite, and the like.

A method of coating with a carbon material includes, for example, a method of heat-treating a mixture containing natural graphite particles A and a precursor of the carbon material. The temperature at which the mixture is heat-treated is preferably 800° C. to 1500° C., more preferably 900° C. to 1300° C., more preferably 1050° C. to 1250° C., from the viewpoint of improving the input characteristics of the lithium ion secondary battery. °C is more preferred. The temperature at which the mixture is heat treated may be constant or may vary from the beginning to the end of the heat treatment.

The method for producing a negative electrode material for a lithium ion secondary battery may have steps other than the preparation step and the coating step. Other steps include a step of sizing before the preparation step, after the preparation step to before the coating step, and at least one after the coating step.

<Negative electrode material for lithium ion secondary battery>
A negative electrode material for a lithium ion secondary battery contains natural graphite particles having a value obtained by dividing the R value by the specific surface area SSA determined by nitrogen adsorption measurement at 77K (R value/SSA) of 0.1 or less. Here, the natural graphite particles in the negative electrode material are referred to as "natural graphite particles B".

As described above, the negative electrode material of the present disclosure balances the physical properties of the natural graphite particles A before coating (the portion corresponding to the core particles of the negative electrode material) and the natural graphite particles after coating (the surface portion of the negative electrode material). This overcomes the trade-off relationship between input characteristics and life characteristics. From the viewpoint of improving the input characteristics, damage to the surface of the natural graphite particles A serving as cores is increased to lower the R value. On the other hand, by coating at least part of the surface of the natural graphite particles A with a carbon material, excessive reaction with the electrolyte or the like is suppressed, and life characteristics are improved. When coated with a carbon material, the natural graphite particles after coating have a lower specific surface area SSA and a higher R value. Therefore, by balancing the specific surface area SSA and the R value in the natural graphite particles B and setting the R value/SSA of the natural graphite particles B to 0.1 or less, a negative electrode material excellent in input characteristics and life characteristics can be obtained. .

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

The specific surface area SSA of the natural graphite particles B is preferably 3 m ² /g or more, more preferably 3.5 m ² /g or more, and 4 m ² /g or more from the viewpoint of improving input characteristics. is more preferred. Further, the specific surface area SSA of the natural graphite particles B is preferably 6 m ² /g or less, more preferably 5.5 m ² /g or less, and even more preferably 5 m ² /g or less.

The R value of the 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, More preferably 0.2 to 0.4. When the R value of the natural graphite particles B is 0.1 or more, there are sufficient graphite lattice defects used for lithium ion absorption and release, and the deterioration of the input characteristics tends to be suppressed. When the R value of the natural graphite particles B is 1.0 or less, the decomposition reaction of the electrolytic solution is sufficiently suppressed, and the decrease in the initial efficiency tends to be suppressed.

From the viewpoint of improving the input characteristics, the natural graphite particles B have a micropore volume (hereinafter sometimes abbreviated as “micropore volume”) determined by CO ₂ adsorption measurement of 0.10 cm ³ /g or more. is preferred, 0.11 cm ³ /g or more is more preferred, and 0.12 cm ³ /g or more is even more preferred. In addition, the micropore volume of the natural graphite particles B is preferably 0.20 cm ³ /g or less, more preferably 0.19 cm 3 /g or less, more preferably 0.18 cm ³ /g or less, from the viewpoint of improving life characteristics. ³ /g or less is more preferable.

The average particle size (D50) of the 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. Moreover, the average particle size (D50) of the natural graphite particles B may be 5 μm or more, or may be 7 μm or more.

The natural graphite particles B are preferably spherical natural graphite particles. At least part of the surface of the natural graphite particles B may be coated with a carbon material. The presence of the carbon material on the surfaces of the natural graphite particles B can be confirmed by observation with a transmission electron microscope.

It is preferable that half or more of the natural graphite particles B contained in the negative electrode material have a portion coated with the carbon material, and 90% or more of the particles have a portion coated with the carbon material. More preferably, it has a portion in which 95% or more of the particles are coated with the carbon material.

In the natural graphite particles B, the ratio B/A of the porosity B calculated from the linseed oil absorption to the porosity A calculated from the tap density is preferably 0.99 or more, and is preferably 1.00 or more. It is more preferable to have The ratio B/A replaces the degree of spheroidization of natural graphite, and a low B/A ratio means that a large amount of natural graphite particles with a low degree of spheroidization are included. Since the electrode density can be increased by increasing the degree of sphericity, there is a tendency for the press pressure load during electrode production to decrease. Reduction of the press pressure load suppresses partial cracking and peeling of the carbon material coating during electrode fabrication, contributing to improved life characteristics. Therefore, when the ratio B/A is 0.99 or more, the life characteristics tend to be improved.

The tap density of the natural graphite particles B is preferably 1.20 g/cm ³ or less, more preferably 1.18 g/cm ³ or less, even more preferably 1.15 g/cm ³ or less. Moreover, the tap density of the natural graphite particles B may be 0.60 g/cm ³ or more, or may be 1.00 g/cm ³ or more.

The linseed oil absorption of the natural graphite particles B may be 50 mL/100 g or less, 49 mL/100 g or less, or 48 mL/100 g or less. In addition, the linseed oil absorption of the natural graphite particles B may be 40 mL/100 g or more, or 41 mL/100 g or more.

Natural graphite particles B preferably have an average interplanar spacing d ₀₀₂ of 0.334 nm to 0.338 nm as determined by an X-ray diffraction method. When the average interplanar spacing d ₀₀₂ is 0.338 nm or less, the lithium ion secondary battery tends to have excellent initial charge/discharge efficiency and energy density.

The value of the average interplanar spacing d ₀₀₂ of the natural graphite particles B tends to decrease, for example, by increasing the heat treatment temperature when producing the negative electrode material. Therefore, the average interplanar spacing _d002 of the carbon material can be controlled by adjusting the temperature of the heat treatment when producing the negative electrode material.

In the present disclosure, the average interplanar spacing d ₀₀₂ is obtained by irradiating the sample with X-rays (CuKα rays) and measuring the diffraction lines with a goniometer. It is calculated using Bragg's formula from the diffraction peak corresponding to the 002 plane.
Specifically, the measurement sample is filled in the recessed portion of a sample holder made of quartz, set on the measurement stage, and measured using a wide-angle X-ray diffractometer (manufactured by Rigaku Corporation) under the following measurement conditions.
Radiation source: CuKα ray average interplanar spacing (wavelength = 0.15418 nm)
Output: 40kV, 20mA
Sampling width: 0.010°
Scanning range: 10° to 35°
Scan speed: 0.5°/min

The content of the natural graphite particles B in the negative electrode material is not particularly limited. , 100% by weight.
The negative electrode material may contain carbon materials other than the natural graphite particles B. Other carbon materials are not particularly limited, and examples thereof include artificial graphite, amorphous carbon, carbon black, fibrous carbon, and nanocarbon. Other carbon materials may be used singly or in combination of two or more.
The negative electrode material may contain particles containing an element capable of intercalating and deintercalating lithium ions other than the carbon material. Elements capable of intercalating and deintercalating lithium ions are not particularly limited, and examples thereof include Si, Sn, Ge, and In.

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

For the negative electrode for lithium ion secondary batteries, for example, a negative electrode material and a binder are kneaded together with a solvent to prepare a slurry negative electrode material composition, which is applied onto a current collector to form a negative electrode material layer. Alternatively, the negative electrode material composition can be formed into a sheet-like or pellet-like shape and integrated with a current collector. Kneading can be performed using a dispersing device such as a stirrer, ball mill, super sand mill, pressure kneader, or the like.

The binder used for preparing the negative electrode material composition is not particularly limited. Binders include ethylenically unsaturated compounds such as styrene-butadiene copolymers, methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, butyl acrylate, butyl methacrylate, acrylonitrile, methacrylonitrile, hydroxyethyl acrylate and hydroxyethyl methacrylate. Polymers of carboxylic acid esters, polymers of ethylenically unsaturated carboxylic acids such as acrylic acid, methacrylic acid, itaconic acid, fumaric acid and maleic acid, polyvinylidene fluoride, polyethylene oxide, polyepichlorohydrin, polyphosphazene, Polymer compounds with high ion conductivity such as polyacrylonitrile can be used. When the negative electrode material composition contains a binder, the amount is not particularly limited. The content of the binder may be, for example, 0.5 parts by mass to 20 parts by mass with respect to 100 parts by mass in total of the negative electrode material and the binder.

The solvent is not particularly limited as long as it can dissolve or disperse the binder. Specific examples include organic solvents such as N-methyl-2-pyrrolidone, N,N-dimethylacetamide, N,N-dimethylformamide and γ-butyrolactone. The amount of the 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. The amount of the solvent used is preferably 60 parts by mass or more and less than 150 parts by mass with respect to 100 parts by mass of the negative electrode material, for example.

The negative electrode material composition may contain a thickener. 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, oxidized starch, phosphorylated starch, casein and the like. When the negative electrode material composition contains a thickener, the amount is not particularly limited. The content of the thickening agent may be, for example, 0.1 to 5 parts by mass with respect to 100 parts by mass of the negative electrode material.

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

The material of the current collector is not particularly limited, and can be selected from aluminum, copper, nickel, titanium, stainless steel, and the like. The state of the current collector is not particularly limited, and can be selected from foil, perforated foil, mesh, and the like. In addition, porous materials such as porous metal (foamed metal) and carbon paper can also be used as current collectors.

When the negative electrode material composition is applied to the current collector to form the negative electrode material layer, the method is not particularly limited, and metal mask printing method, electrostatic coating method, dip coating method, spray coating method, roll coating method, Known methods such as a doctor blade method, a comma coating method, a gravure coating method, and a screen printing method 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 performed using, for example, a hot air dryer, an infrared dryer, or a combination of these devices. Rolling treatment may be performed as necessary. The rolling treatment can be performed by a method such as a flat plate press or a calendar roll.

When a negative electrode material layer is formed by integrating a negative electrode material composition formed into a sheet, pellet, or the like with a current collector, the integration method is not particularly limited. For example, it can be carried out by a roll, flat plate press, or a combination of these means. The pressure during integration is preferably, for example, 1 MPa to 200 MPa.

<Lithium ion secondary battery>
The lithium ion secondary battery of the present disclosure includes the aforementioned negative electrode for lithium ion secondary battery of the present disclosure (hereinafter also simply referred to as “negative electrode”), a positive electrode, and an electrolytic solution.

A positive electrode can be obtained by forming a positive electrode material layer on a current collector in the same manner as in the method for producing the negative electrode described above. As the current collector, it is possible to use a metal or alloy such as aluminum, titanium, or stainless steel in the form of a foil, a perforated foil, or a mesh.

The positive electrode material used for forming the positive electrode layer is not particularly limited. Examples include metal compounds (metal oxides, metal sulfides, etc.) capable of doping or intercalating lithium ions, and conductive polymer materials. More specifically, lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ), lithium manganate (LiMnO ₂ ), their composite oxides (LiCo _x Ni _{y Mnz} O ₂ , x + y + z = 1), Multiple oxide containing additive element M' (LiCo _a Ni _b Mn _c M' _d O ₂ , a + b + c + d = 1, M': Al, Mg, Ti, Zr or Ge), spinel type lithium manganese oxide (LiMn ₂ O ₄ ), lithium vanadium compounds _{, V2O5} , _V6O13 , _{VO2 , MnO2} , _{TiO2 , MoV2O8} , _TiS2 , _V2S5 , _VS2 , _{MoS2 , MoS3} , _Cr3 Lithium-containing compounds such as O ₈ , Cr ₂ O ₅ and olivine-type LiMPO ₄ (M: Co, Ni, Mn, Fe), conductive polymers such as polyacetylene, polyaniline, polypyrrole, polythiophene and polyacene, and porous carbon. be done. The positive electrode material may be used singly or in combination of two or more.

The electrolytic solution is not particularly limited, and for example, a solution obtained by dissolving a lithium salt as an electrolyte in a non-aqueous solvent (so-called organic electrolytic solution) can be used.
Lithium salts include LiClO ₄ , LiPF ₆ , LiAsF ₆ , LiBF ₄ , LiSO ₃ CF ₃ and the like. Lithium salts may be used singly or in combination of two or more.
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-oxazolidin-2-one, γ-butyrolactone, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, butyl methyl carbonate, ethyl propyl carbonate, butyl ethyl carbonate, dipropyl carbonate, 1, 2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, methyl acetate, ethyl acetate, trimethyl phosphate, triethyl phosphate and the like. The non-aqueous solvent may be used alone or in combination of two or more.

The states of the positive electrode and the negative electrode in the lithium ion secondary battery are not particularly limited. For example, the positive electrode, the negative electrode, and, if necessary, the separator disposed between the positive electrode and the negative electrode may be spirally wound, or may be stacked in a plate shape.

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

The shape of the lithium ion secondary battery is not particularly limited. For example, laminate-type batteries, paper-type batteries, button-type batteries, coin-type batteries, laminated-type batteries, cylindrical-type batteries, and square-type batteries can be used.

Since the lithium ion secondary battery of the present disclosure has excellent output characteristics, it is suitable as a large-capacity lithium ion secondary battery used in electric vehicles, power tools, power storage devices, and the like. In particular, it is used in electric vehicles (EV), hybrid electric vehicles (HEV), plug-in hybrid electric vehicles (PHEV), etc., which require high-current charging and discharging in order to improve acceleration performance and brake regeneration performance. It is suitable as a lithium ion secondary battery.

EXAMPLES The present invention will be specifically described below with reference to Examples, but the present invention is not limited to these Examples.

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

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

[Example 1]
Spherical natural graphite particles (average particle diameter (D50) 10.2 μm, specific surface area 10.1 m ² /g, R value 0.26) 100 parts by mass and coal tar pitch (softening point 90 ° C., carbonization rate (carbonization rate) 50% by mass) 3.6 parts by mass were mixed. Next, the temperature was raised to 1,050° C. at a rate of 250° C./hour under nitrogen flow, and the particles were coated with the carbon material by holding at 1,050° C. (firing treatment temperature) for 1 hour to obtain carbon-coated carbon particles. After crushing the obtained carbon-coated carbon particles with a cutter mill, they were sieved with a 350-mesh sieve, and the under-sieved portion was used as a negative electrode material.
Each physical property value was measured about the produced negative electrode material. Table 1 shows each physical property value.

[Comparative Example 1]
The same as in Example 1 except that the spherical natural graphite particles used as raw materials in Example 1 were changed to those having an average particle diameter (D50) of 10.8 μm, a specific surface area of 9.0 m ² /g, and an R value of 0.29. Then, a negative electrode material was produced. Each physical property value was measured about the produced negative electrode material. Table 1 shows each physical property value.

[Comparative Example 2]
A negative electrode material was produced 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. Each physical property value was measured about the produced negative electrode material. Table 1 shows each physical property value.

[Comparative Example 3]
A negative electrode material was prepared in the same manner as in Example 1, except that the spherical natural graphite particles used as the raw material had an average particle diameter (D50) of 10.9 μm, a specific surface area of 9.5 m ² /g, and an R value of 0.34. was made. Each physical property value was measured about the produced negative electrode material. Table 1 shows each physical property value.

(Production of lithium ion secondary battery)
Using the negative electrode material produced in each example, a lithium ion secondary battery was produced in the following procedure.
First, for 98 parts by mass of the negative electrode material, an aqueous solution (CMC concentration: 2% by mass) of CMC (carboxymethyl cellulose, manufactured by Daicel Finechem Co., Ltd., product number 2200) as a thickener is added so that the solid content of CMC becomes 1 part by mass. and kneaded for 10 minutes. Then, purified water was added so that the total solid content concentration of the negative electrode material and CMC was 40% by mass to 50% by mass, and the mixture was kneaded for 10 minutes. Subsequently, an aqueous dispersion (SBR concentration: 40% by mass) of SBR (BM400-B, Nippon Zeon Co., Ltd.), which is a styrene-butadiene copolymer rubber, is added as a binder so that the solid content of SBR becomes 1 part by mass. and mixed for 10 minutes to prepare a pasty negative electrode material composition. Next, the negative electrode material composition is applied to an electrolytic copper foil having a thickness of 11 μm with a comma coater whose clearance is adjusted so that the coating amount per unit area is 5.9 mg/cm ² to form a negative electrode material layer. bottom. After that, the electrode density was adjusted to 1.2 g/cm ³ with a hand press. The electrolytic copper foil on which the negative electrode material layer was formed was punched into a disk shape with a diameter of 16 mm to prepare a sample electrode (negative electrode).

The prepared sample electrode (negative electrode), separator, and counter electrode (positive electrode) were placed in a coin-shaped battery container in this order, and an electrolytic solution was injected to produce a coin-shaped lithium ion secondary battery. As the electrolytic solution, a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (the volume ratio of EC and EMC is 3:7) was mixed with 0.5 of vinylene carbonate (VC) with respect to the total amount of the mixed solution. % by mass and LiPF ₆ dissolved to a concentration of 1 mol/L was used. LiNi _0.5 Mn _0.3 Co _0.2 O ₂ (NMC532) was used as the counter electrode (positive electrode). A polyethylene microporous membrane having a thickness of 20 μm was used as the separator.

(Measurement of DC resistance (DCR))
The direct current resistance (DCR) of the produced lithium ion secondary battery was measured to obtain the input characteristics of this battery. Specifically, it is as follows.
Put the lithium ion secondary battery in a constant temperature bath set at 25 ° C., perform constant current constant voltage (CC / CV) charging at 0.33 C, 4.2 V, final current 0.05 C, then 0.33 C 3 cycles of constant current (CC) discharge to 2.5 V were performed to charge and discharge. Then, constant current charging was performed at a current value of 0.2C to an SOC of 60%.
In addition, the lithium ion secondary battery is placed in a constant temperature bath set at -10 ° C., and constant current charging is performed for 10 seconds each under the conditions of 0.2 C, 0.5 C, and 1 C, and the voltage drop at each constant current. (ΔV) was measured and the direct current resistance (DCR) was measured using the following equation. Table 1 shows the results. A lower DC resistance (DCR) value indicates better input characteristics.
DCR [Ω] = {(0.5C voltage drop ΔV - 0.2C voltage drop ΔV) + (1C voltage drop ΔV - 0.5C voltage drop ΔV)}/0.8

(Measurement of charge capacity retention rate)
Using the above lithium ion secondary battery, put it in a constant temperature bath set at 25 ° C., and perform constant current constant voltage (CC / CV) charging at 0.33 C, 4.2 V, final current 0.05 C, Three cycles of constant current (CC) discharge to 2.5 V at 0.33 C were performed for charging and discharging. Next, put it in a constant temperature bath set at 25 ° C. After performing constant current constant voltage (CC / CV) charging at 0.33 C, 4.2 V, final current 0.05 C, in a constant temperature bath set at 60 ° C. After being stored at room temperature for 10 days, it was placed in a constant temperature bath set at 25° C., and constant current (CC) discharge was performed at 0.33 C to 2.5 V. Table 1 shows the results.
(1st to 3rd cycles)
Charging conditions: constant current charge 0.967mA, constant voltage charge 4.2V, cut current 0.145mA
Discharge conditions: constant current discharge 0.967mA, cut voltage 2.5V
(4th cycle)
Charging conditions: constant current charge 0.967mA, constant voltage charge 4.2V, cut current 0.145mA
(Store in a constant temperature bath set at 60°C for 10 days from the 4th cycle to the 5th cycle)
(5th cycle)
Discharge conditions: current charge 0.967mA, constant voltage charge 4.2V, cut current 0.145mA

Charge capacity retention rate (%) =
[5th cycle CC discharge capacity/3rd cycle CC discharge capacity] × 100

Claims (10)

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 as determined by nitrogen adsorption measurement at 77 K,
At least part of the surface of the natural graphite particles is coated with a carbon material;
A method for producing a negative electrode material for a lithium ion secondary battery. The method for producing a negative electrode material for a lithium ion secondary battery according to claim 1, wherein the natural graphite particles have an R value of 0.4 or less. A negative electrode material for a lithium ion secondary battery, comprising natural graphite particles having a value (R value/SSA) of 0.1 or less obtained by dividing the R value by the specific surface area SSA determined by nitrogen adsorption measurement at 77K. 4. The negative electrode material for a lithium ion secondary battery according to claim 3, wherein said natural graphite particles have a micropore volume of 0.10 cm ³ /g to 0.20 ^{cm 3} /g determined by CO ₂ adsorption measurement. The negative electrode material for a lithium ion secondary battery according to claim 3 or 4, wherein the average particle size (D50) of the natural graphite particles is 5 µm to 12 µm. The negative electrode material for a lithium ion secondary battery according to any one of claims 3 to 5, wherein the natural graphite particles include spherical natural graphite particles. The negative electrode material for a lithium ion secondary battery according to any one of claims 3 to 6, wherein at least part of the surface of said natural graphite particles is coated with a carbon material. The ratio B/A of the porosity B calculated from the linseed oil absorption amount to the porosity A calculated from the tap density of the natural graphite particles is 0.99 or more, according to claims 3 to 7. The negative electrode material for a lithium ion secondary battery according to any one of items 1 and 2. A negative electrode for a lithium ion secondary battery, comprising a negative electrode material layer containing the negative electrode material for a lithium ion secondary battery according to any one of claims 3 to 8, and a current collector. A lithium ion secondary battery comprising the negative electrode for a lithium ion secondary battery according to claim 9, a positive electrode, and an electrolytic solution.