KR20210153693A - Spheroidal graphite, coated spheroidized graphite, negative electrode for lithium ion secondary battery and lithium secondary battery - Google Patents

Abstract

Provided is spheroidized graphite excellent in output characteristics when used as a negative electrode material for a lithium ion secondary battery. In the particle size distribution of primary particles obtained using X-ray CT, the spheroidized graphite has a volume ratio of primary particles having a spherical equivalent diameter of 0.8 μm or less of 40.0% or less, and a spherical equivalent diameter of 1.5 μm or more and 3.0 The volume ratio of the primary particles of ㎛ or less is 13.0% or more.

Description

Spheroidal graphite, coated spheroidized graphite, negative electrode for lithium ion secondary battery and lithium secondary battery

The present invention relates to spheroidized graphite, coated spheroidized graphite, a negative electrode for a lithium ion secondary battery, and a lithium secondary battery.

A lithium ion secondary battery has a negative electrode, a positive electrode, and a nonaqueous electrolyte as main components. Lithium ions act as a secondary battery by moving between the negative electrode and the positive electrode in the discharging process and the charging process.

Conventionally, as a negative electrode material of a lithium ion secondary battery, spheroidized graphite (spheroidized graphite) may be used (patent document 1).

(Patent Document 1) Japanese Patent Application Laid-Open No. 2014-146607

The negative electrode material of a lithium ion secondary battery may be requested|required that it is excellent in output characteristics (output resistance is small).

In particular, it is expected that many lithium ion secondary batteries will be mounted in automobiles (hybrid automobiles, electric vehicles, etc.) in the future. For example, when an automobile accelerates rapidly, more excellent output characteristics are required.

Then, an object of this invention is to provide the spheroidized graphite excellent in output characteristics, when it uses as a negative electrode material of a lithium ion secondary battery.

MEANS TO SOLVE THE PROBLEM As a result of earnest examination, the present inventors discovered that the said objective was achieved by employ|adopting the following structure, and completed this invention.

That is, the present invention provides the following [1] to [9].

[1] In the particle size distribution of primary particles obtained using X-ray CT, the volume ratio of primary particles having a sphere equivalent diameter of 0.8 μm or less is 40.0% or less, and the sphere equivalent diameter is Spheroidized graphite, wherein the volume ratio of the primary particles of 1.5 μm or more and 3.0 μm or less is 13.0% or more.

[2] In the particle shape distribution of secondary particles obtained using X-ray CT, the volume ratio of spherical secondary particles is 14.0% or more, and the volume ratio of rod-shaped secondary particles is 34.0 % or less, the spheroidized graphite according to the above [1].

[3] The spheroidized graphite according to the above [1] or [2], wherein the average secondary particle diameter is 5.0 µm or more and 15.0 µm or less, and the specific surface area is 5.0 m 2 /g or more and 15.0 m 2 /g or less.

[4] The spheroidized graphite according to any one of the above [1] to [3], wherein the natural graphite is spheroidized.

[5] Covered spheroidized graphite comprising the spheroidal graphite according to any one of [1] to [4], and carbonaceous material for coating the spheroidal graphite.

[6] The coated spheroidized graphite according to the above [5], wherein the average secondary particle diameter is 5.0 µm or more and 50.0 µm or less, and the specific surface area is 0.5 m 2 /g or more and 10.0 m 2 /g or less.

[7] The coated spheroidized graphite according to [5] or [6], wherein the pore volume corresponding to pores having a pore diameter of 7.8 nm or more and 36.0 nm or less is 0.015 cm 3 /g or more and 0.028 cm 3 /g or less.

[8] A negative electrode for a lithium ion secondary battery comprising the coated spheroidized graphite according to any one of [5] to [7].

[9] A lithium ion secondary battery having the negative electrode according to [8] above.

ADVANTAGE OF THE INVENTION According to this invention, when it uses as a negative electrode material of a lithium ion secondary battery, the spheroidized graphite excellent in output characteristics can be provided.

1A is a three-dimensional image of a spherical particle.
FIG. 1B is a three-dimensional image of a spherical particle observed from a different angle than FIG. 1A.
1C is a three-dimensional image of a spherical particle observed from an angle different from FIGS. 1A-B.
1D is a three-dimensional image of a spherical particle observed from an angle different from FIGS. 1A-C.
2A is a three-dimensional image of rod-shaped particles.
Fig. 2b is a three-dimensional image of rod-shaped particles observed from a different angle than Fig. 2a.
Fig. 2c is a three-dimensional image of rod-shaped particles observed from an angle different from Figs. 2a-b.
FIG. 2D is a three-dimensional image of rod-shaped particles observed from an angle different from FIGS. 2A to 2C.
3A is a three-dimensional image of other secondary particles.
Fig. 3b is a three-dimensional image of the other secondary particles observed from a different angle than Fig. 3a.
Fig. 3c is a three-dimensional image of other secondary particles observed from an angle different from Figs. 3a-b.
Fig. 3D is a three-dimensional image of other secondary particles observed from an angle different from Figs. 3A-C.
4 is a cross-sectional view of an evaluation battery prepared to evaluate battery characteristics in Examples and Comparative Examples.

(Form for implementing the invention)

[Spheroidized Graphite]

In the particle size distribution of primary particles obtained using X-ray CT, the spheroidized graphite of the present invention has a volume ratio of primary particles having a sphere equivalent diameter of 0.8 μm or less (hereinafter also referred to as “fine particles”) of 40.0 % or less, and the volume ratio of primary particles (hereinafter also referred to as “granulation”) having a sphere equivalent diameter of 1.5 µm or more and 3.0 µm or less is 13.0% or more.

By using the spheroidal graphite of this invention as a negative electrode material of a lithium ion secondary battery, it is excellent in an output characteristic. It is estimated that this is because lithium ions are easy to enter and exit when the fine particles and granules are in the above ratio.

<Particle and Assembly>

As described above, in the spheroidal graphite of the present invention, the volume ratio of fine particles is 40.0% or less, and the volume ratio of granules is 13.0% or more.

From the reason that the output characteristic is more excellent, 39.0 % or less is preferable, as for the volume ratio of fine particle, 34.0 % or less is more preferable, 28.0 % or less is still more preferable, 23.0 % or less is especially preferable.

On the other hand, the lower limit is not particularly limited, and the volume ratio of the fine particles is, for example, 5.0% or more, preferably 10.0% or more, more preferably 13.0% or more, and still more preferably 15.0% or more.

From the reason that output characteristics are more excellent, 14.0 % or more is preferable, as for the volume ratio of granulation, 18.0 % or more is more preferable, 21.0 % or more is still more preferable, 25.0 % or more is especially preferable.

On the other hand, as for the volume ratio of granulation, an upper limit is not specifically limited, For example, 60.0 % or less, 50.0 % or less is preferable, 40.0 % or less is more preferable, 35.0 % or less is still more preferable, 32.0 % or less. is particularly preferred.

《Particle Size Distribution of Primary Particles》

A method of obtaining the particle size distribution of the primary particles constituting the spheroidized graphite will be described.

In order to grasp the size of the primary particles, it is necessary to visualize the spheroidized graphite with non-destructive and high resolution. Then, spheroidized graphite is observed by X-ray CT (computed tomography) using a radiation source. In more detail, in beam line BL24XU of SPring-8, imaging-type X-ray CT is implemented on condition of the following.

·X-ray energy: 8keV

・Image resolution: 1248 (H) × 2048 (W) pixels

・Execution pixel size: 68nm/pixel

・Exposure time: 0.5 seconds

・The number of shots of the projected image: 1200

Defocus: 0.3mm

The spheroidized graphite as a sample is filled in a quartz glass capillary (inner diameter: about 0.1 mm), and it uses it for X-ray CT.

After photographing a projection image of spheroidized graphite, it is reconstructed into a cross-sectional slice image. Next, using the Watershed Analysis function of ExFact VR (manufactured by Nippon Visual Science), which is a commercially available image analysis software, adjacent primary particles are divided and recognized individually, and the volume of each primary particle is calculated. Moreover, about each primary particle, a sphere equivalent diameter is calculated|required from the obtained volume. The particle size distribution of the primary particles is obtained by plotting the data of each primary particle on a graph (horizontal axis: equivalent sphere diameter, ordinate: volume ratio to the total volume of each primary particle).

<Invention and stick figure>

From the reason that the spheroidal graphite of the present invention has more excellent output characteristics, in the particle shape distribution of secondary particles obtained using X-ray CT, the volume of spherical secondary particles (hereinafter also referred to as “spherical particles”) It is preferable that the ratio is 14.0% or more, and the volume ratio of rod-shaped secondary particles (hereinafter also referred to as "rod-shaped particles") is 34.0% or less.

From the reason that an output characteristic is further excellent, 16.0 % or more is preferable, as for the volume ratio of a spherical particle, 18.0 % or more is more preferable, 20.0 % or more is still more preferable.

On the other hand, as for the volume ratio of a spherical particle, an upper limit is not specifically limited, For example, 50.0 % or less, 40.0 % or less is preferable, 30.0 % or less is more preferable, 25.0 % or less is still more preferable, 23.0 % The following are particularly preferred.

30.0% or less is preferable, as for the volume ratio of rod-shaped particle|grains from the reason that it is further excellent in an output characteristic, 28.0 % or less is more preferable, and its 25.0 % or less is still more preferable.

On the other hand, as for the volume ratio of rod-shaped particle|grains, a lower limit is not specifically limited, For example, it is 5.0 % or more, 10.0 % or more is preferable, 15.0 % or more is more preferable, 20.0 % or more is still more preferable.

<<Particle Shape Distribution of Secondary Particles>>

The method of calculating|requiring the particle shape distribution of the secondary particle which comprises spheroidized graphite is demonstrated.

In order to grasp the shape of secondary particles, it is required to visualize spheroidized graphite with non-destructive and high resolution. Then, spheroidized graphite is observed by X-ray CT using a radiation light source. In more detail, in the beam line BL24XU of SPring-8, projection type X-ray CT is implemented on condition of the following.

·X-ray energy: 20keV

・Image resolution: 2048 (H) × 2048 (W) pixels

・Execution pixel size: 325nm/pixel

・Exposure time: 0.1 second

・The number of shots of the projected image: 1800

The distance between the sample and the detector: 10 mm

The spheroidized graphite as a sample is filled in a borosilicate glass capillary (inner diameter: about 0.6 mm), and it uses it for X-ray CT.

After photographing a projection image of spheroidized graphite, it is reconstructed into a cross-sectional slice image. Next, using the Watershed Analysis function of ExFact VR (manufactured by Nippon Visual Science), which is a commercially available image analysis software, adjacent secondary particles are divided and recognized individually, and the volume of each secondary particle is calculated.

Next, for each secondary particle, three axes of inertia orthogonal to each other are set, and the respective central moments are obtained. Among the three central moments, the largest is L, the smallest is S, and the middle is M. According to the following definition, the particle shape of each secondary particle is classified into spherical shape, rod shape, and others.

Spherical: S/L≥0.5, also M/L≥0.5

Rod shape: S/L < 0.5, M/L < 0.5

The volume ratio of secondary particles (spherical particles) classified as spherical and the volume ratio of secondary particles (rod-shaped particles) classified as rods with respect to the total volume of each secondary particle are obtained. In this way, the shape distribution of the secondary particles is obtained.

Here, the example of the three-dimensional image obtained by performing image analysis of X-ray CT data of a secondary particle is shown.

1A to 1D are three-dimensional images of spherical particles (S/L=0.79, M/L=0.91).

2A to 2D are three-dimensional images of rod-shaped particles (S/L=0.11, M/L=0.19).

3A to 3D are three-dimensional images of other secondary particles (ellipsoid-shaped particles) (S/L=0.22, M/L=0.88).

1A to 1D, the same single secondary particle is observed, and the observation angles are different. This is the same also in FIGS. 2A-D and 3A-D.

<Average secondary particle size>

5.0 micrometers or more are preferable, as for the average secondary particle diameter (simply referred to as "average particle diameter") of the spheroidized graphite of this invention, 6.5 micrometers or more are more preferable, and 7.0 micrometers or more are still more preferable.

On the other hand, the average particle diameter of the spheroidized graphite of the present invention is preferably 15.0 µm or less, more preferably 14.0 µm or less, still more preferably 12.0 µm or less, particularly preferably 10.0 µm or less, and most preferably 9.8 µm or less. .

An average particle diameter is a particle diameter from which the number of cumulative degrees of particle size distribution calculated|required using a laser diffraction type particle size distribution analyzer (made by Seishin Industrial Co., Ltd., LMS2000e) becomes 50% by volume percentage.

<Specific surface area>

5.0 m 2 /g or more is preferable, as for the specific surface area of the spheroidizing graphite of this invention, 7.0 m 2 /g or more is more preferable, 9.5 m 2 /g or more is still more preferable.

On the other hand, the specific surface area of the spheroidized graphite of the present invention is preferably 15.0 m 2 /g or less, more preferably 13.0 m 2 /g or less, still more preferably 11.0 m 2 /g or less, and particularly preferably 10.0 m 2 /g or less. .

The specific surface area is a BET specific surface area measured based on JIS Z 8830:2013 "Method for Measuring Specific Surface Area of Powder (Solid) by Gas Adsorption". Specifically, the sample was pre-dried at 50° C., followed by flowing nitrogen gas for 30 minutes, and then using MONOSORB (manufactured by Quantachrome Instruments Japan Kodo Co., Ltd.), by the BET one-point method by nitrogen gas adsorption. save

[Method for producing spheroidized graphite]

Although it does not specifically limit as a method of manufacturing the spheroidal graphite of this invention, For example, the method of processing a raw material into a spherical shape is mentioned.

Here, the raw material is graphite having a shape other than spherical (including ellipsoidal shape), for example, scaly graphite. Although either natural graphite or artificial graphite may be sufficient as graphite, natural graphite is preferable from reasons, such as high crystallinity.

More specifically, For example, the method of mixing a raw material in the coexistence of granulation aids, such as an adhesive agent and resin; A method of applying a mechanical external force to a raw material without using a granulation aid; a method of combining both; and the like.

Among these, the method of applying a mechanical external force to a raw material without using a granulation aid is preferable. Hereinafter, this method is demonstrated in more detail.

More specifically, the raw material (eg, scale-like graphite) is pulverized and granulated by applying a mechanical external force using a pulverizing device. In this way, the raw material is spheroidized and spheroidized graphite is obtained.

Examples of the grinding device include a rotary ball mill, a counter jet mill (manufactured by Hosokawa Micron), a current jet (manufactured by Nissei Engineering, Inc.), a hybridization system (manufactured by Naragi Kaisei Co., Ltd.), a CF mill ( Ubekyosan Co., Ltd.), Mechanofusion System (manufactured by Hosokawa Micron, Inc.), Theta Composer (manufactured by Toku Chugosakusho Co., Ltd.), etc. are mentioned. Among them, a hybridization system (manufactured by Naragi Kaisei Seisakusho Co., Ltd.) is preferable. do.

In the present invention, after arranging a plurality of pulverizing devices in series, it is preferable that the raw materials continuously pass through these pulverizing devices. That is, it is preferable to arrange a plurality of pulverizing devices in series so that, immediately after the raw material passes through one pulverizing device, pulverization and granulation are performed in the next pulverizing device.

At this time, the number of the crushing apparatus is, for example, 2 or more, 3 or more are preferable, 4 or more are more preferable, 5 or more are still more preferable, 6 or more are especially preferable.

On the other hand, 10 or less are preferable, as for the number of crushing apparatus, 8 or less are more preferable, and 7 or less are still more preferable.

In one pulverizing apparatus, the time for pulverizing and granulating the raw material (hereinafter also referred to as "pulverization time") is preferably 8 minutes or longer, more preferably 13 minutes or longer, and still more preferably 18 minutes or longer.

On the other hand, 60 minutes or less are preferable, as for the grinding|pulverization time in one grinding|pulverization apparatus, 50 minutes or less are more preferable, and its 40 minutes or less are still more preferable.

The product of the number of pulverizers and the pulverization time in one pulverizer (hereinafter also referred to as "total pulverization time") is preferably 30 minutes or longer, more preferably 50 minutes or longer, and 90 minutes or longer more preferably.

On the other hand, 180 minutes or less are preferable and, as for total grinding time, 160 minutes or less are more preferable.

The crushing device usually incorporates a rotor.

30 m/sec or more is preferable, as for the circumferential speed of the rotor in each grinding device, 40 m/sec or more is more preferable, and its 60 m/sec or more is still more preferable.

On the other hand, 100 m/sec or less is preferable and, as for the circumferential speed of the rotor in each crushing apparatus, 80 m/sec or less is more preferable.

In order to make it easy to apply a shearing force and a compressive force to a raw material, it is preferable that the quantity of the raw material filled in each grinding device is small.

[Coated spheroidized graphite]

The coating spheroidal graphite of this invention contains spheroidization graphite and the carbonaceous material which coat|covers this spheroidization graphite. And spheroidizing graphite is the spheroidizing graphite of this invention mentioned above.

<Carbon content>

1.0 mass % or more is preferable, as for carbonaceous content in the covering spheroidizing graphite of this invention, 3.0 mass % or more is more preferable, 8.0 mass % or more is still more preferable, 10.0 mass % or more is especially preferable.

If the carbonaceous content is within this range, the active edge surface of the spheroidized graphite is easily covered, and the initial charge/discharge efficiency is excellent.

On the other hand, as for carbonaceous content in the coated spheroidal graphite of this invention, 30.0 mass % or less is preferable, 25.0 mass % or less is more preferable, 20.0 mass % or less is still more preferable, 15.0 mass % or less is especially preferable. .

When the carbonaceous content is within this range, the relatively low carbonaceous discharge capacity decreases, and the discharge capacity is excellent.

In addition, when the carbonaceous content is within this range, since the amount of the carbonaceous precursor to be described later is reduced, fusion is less likely to occur during mixing and firing described later, and cracking and peeling of the finally obtained carbonaceous material is suppressed, Excellent initial charging and discharging efficiency.

As for carbonaceous content, the average value of the whole of covered spheroidizing graphite should just exist in the said range. It is not necessary for all of the individual covering spheroidization graphite to exist within the said range, You may contain the covering spheroidization graphite outside the said range in a part.

Carbonaceous content is the same as the conditions at the time of calcining the mixture of spheroidized graphite and a carbonaceous precursor, only a carbonaceous precursor is calcined, and it calculates|requires from the residual carbon amount.

<Average secondary particle diameter>

5.0 micrometers or more are preferable and, as for the average secondary particle diameter (average particle diameter) of the coated spheroidizing graphite of this invention, 7.0 micrometers or more are more preferable.

On the other hand, 50.0 micrometers or less are preferable, as for the average particle diameter of the coating spheroidizing graphite of this invention, 30.0 micrometers or less are more preferable, and its 20.0 micrometers or less are still more preferable.

<Specific surface area>

The specific surface area of the coated spheroidized graphite of the present invention is preferably 0.5 m 2 /g or more, more preferably 1.5 m 2 /g or more, still more preferably 3.0 m 2 /g or more, and particularly preferably 4.0 m 2 /g or more.

On the other hand, the specific surface area of the coated spheroidized graphite of the present invention is preferably 10.0 m 2 /g or less, more preferably 8.0 m 2 /g or less, still more preferably 7.0 m 2 /g or less, and particularly preferably 5.5 m 2 /g or less. do.

<pore volume>

The present inventors paid attention to the pore volume calculated by the DFT (Density Functional Theory) method from the nitrogen adsorption isotherm as an index correlated with the resistance accompanying the occlusion and release of lithium in the coated spheroidal graphite.

After that, the present inventors discovered that the pore volume corresponding to a pore with a pore diameter of less than 7.8 nm originates in amorphous carbon, and it is difficult to contribute to the resistance accompanying lithium occlusion and release. Furthermore, the present inventors have discovered that the pore volume corresponding to the pore whose pore diameter is 7.8 nm or more and 36.0 nm or less is a good parameter|index correlated with resistance.

Specifically, from the reason that the output characteristics are more excellent, in the coated spheroidized graphite of the present invention, the pore volume corresponding to the pores having a pore diameter of 7.8 nm or more and 36.0 nm or less (hereinafter also referred to as "pore volume V" for convenience) 0.015 cm<3>/g or more is preferable and, as for silver, 0.016 cm<3>/g or more is more preferable.

From the same reason, 0.028 cm<3>/g or less is preferable, as for the pore volume V of the coated spheroidal graphite of this invention, 0.026 cm<3>/g or less is more preferable, and its 0.023 cm<3>/g or less is still more preferable.

The measurement of the pore volume by the DFT method is calculated|required based on JIS Z 8831-2 (method of measuring mesopores and macropores by gas adsorption) and JIS Z 8831-3 (method of measuring micropores by gas adsorption). . At this time, the measurement of the pore volume is started from the ^{relative pressure of 5x10 -2 Pa.}

[Method for producing coated spheroidized graphite]

Although it does not specifically limit as a method of manufacturing the coated spheroidal graphite of this invention, For example, the method of adding and mixing a carbonaceous precursor to the spheroidizing graphite of this invention which is a core material, and baking after that is suitable can be heard According to this method, a carbonaceous precursor becomes carbonaceous which coat|covers a core material (spheroidized graphite) through mixing and baking. That is, coated spheroidized graphite is obtained.

Hereinafter, this method will be described in detail.

<Carbonaceous precursor>

Examples of the carbonaceous precursor include tar pitches and/or resins, which are carbon materials that have low crystallinity compared to graphite and cannot form graphite crystals even if subjected to a high-temperature treatment required for graphitization.

As tar pitches, for example, coal tar, tar light oil, tar heavy oil, tar heavy oil, naphthaline oil, anthracene oil, coal tar pitch, pitch oil, mesophase pitch, oxygen crosslinked petroleum pitch, heavy oil and the like.

As resins, For example, thermoplastic resins, such as polyvinyl alcohol and polyacrylic acid; thermosetting resins such as phenol resins and furan resins; and the like.

From a viewpoint of cost, it is preferable that a carbonaceous precursor consists only of tar pitches, without containing resin. As such a carbonaceous precursor, a carbonaceous precursor with a coal tar pitch of 80 mass % or more is mentioned suitably, for example.

<mix>

A core material (spheroidized graphite) and a carbonaceous precursor are mixed. As for a mixing ratio, the mixing ratio from which carbonaceous content becomes the above-mentioned content in the finally obtained covered spheroidized graphite is preferable.

The method of mixing will not be specifically limited if it can mix homogeneously, A well-known mixing method is used. For example, the method of heating and mixing using the twin-screw type kneader etc. which has heating mechanisms, such as a heater and a heat medium, is mentioned.

The atmosphere at the time of mixing is not specifically limited, For example, it is an air atmosphere.

5 degreeC or more is preferable, as for the temperature (mixing temperature) at the time of mixing, 10 degreeC or more is more preferable, and 25 degreeC or more is still more preferable. On the other hand, 150 degrees C or less is preferable, as for mixing temperature, 100 degrees C or less is more preferable, and its 60 degrees C or less is still more preferable.

<Firing>

The mixture obtained by the above-mentioned mixing is baked.

Although the method of baking is not specifically limited, In order to prevent oxidation at the time of baking, it is preferable to bake in an inert atmosphere. At this time, it is preferable to use a tubular furnace (管狀爐).

The atmosphere at the time of baking is an argon atmosphere, a helium atmosphere, nitrogen atmosphere etc. can be illustrated as a non-oxidizing atmosphere.

700 degreeC or more is preferable and, as for the temperature (calcination temperature) at the time of baking, 900 degreeC or more is more preferable. On the other hand, 2000 degrees C or less is preferable, as for a calcination temperature, 1300 degrees C or less is more preferable, and 1200 degrees C or less is still more preferable.

Specifically, for example, it is preferable to bake at 700°C or higher and 2000°C or lower in a nitrogen stream.

The firing time is preferably 5 minutes or longer. On the other hand, the firing time is preferably 30 hours or less.

As a form of raising the temperature to the firing temperature, various forms such as a linear temperature increase and a stepwise temperature increase in which the temperature is held at regular intervals can be adopted.

In the present invention, it is preferable not to pulverize after firing.

In addition, before baking, you may adhere, embed|buried, or compound the graphite material of a different type to a core material (spheroidized graphite). As a different type of graphite material, For example, carbonaceous or graphite fiber; carbonaceous precursor materials such as amorphous hard carbon; organic materials; inorganic materials; and the like.

The above-mentioned "spheroidal graphite of this invention" and "coated spheroidized graphite of this invention" may be collectively called "negative electrode material of this invention" hereafter.

[Negative electrode for lithium ion secondary battery (negative electrode)]

The negative electrode for lithium ion secondary batteries of this invention is a negative electrode for lithium ion secondary batteries containing the negative electrode material of this invention. The negative electrode for lithium ion secondary batteries is also simply called "negative electrode."

The negative electrode of the present invention is produced according to a normal negative electrode.

At the time of preparation of a negative electrode, it is preferable to use the negative electrode mixture previously prepared by adding a binder to the negative electrode material of this invention. The negative electrode mixture may contain an active material or a conductive material other than the negative electrode material of the present invention.

The binder is preferably chemically and electrochemically stable with respect to the electrolyte, and includes, for example, fluorine-based resins such as polytetrafluoroethylene and polyvinylidene fluoride; resins such as polyethylene, polyvinyl alcohol, and styrene-butadiene rubber; carboxymethyl cellulose; etc. are used, and these can also be used 2 or more types together.

A binder is normally used in the ratio of about 1-20 mass % with respect to the total amount of negative mix.

More specifically, first, optionally, the negative electrode material of the present invention is adjusted to a desired particle size by classification or the like. Thereafter, the negative electrode material of the present invention is mixed with a binder, and the resulting mixture is dispersed in a solvent to prepare a paste-like negative electrode mixture. As a solvent, water, isopropyl alcohol, N-methylpyrrolidone, dimethylformamide, etc. are mentioned. A well-known stirrer, a mixer, a kneader, a kneader, etc. are used for mixing and dispersion|distribution.

The prepared paste is applied to one or both surfaces of the current collector and dried. In this way, a negative electrode mixture layer (negative electrode) uniformly and strongly adhered to the current collector is obtained. 10-200 micrometers is preferable and, as for the thickness of a negative mix layer, 20-100 micrometers is more preferable.

After forming the negative electrode mixture layer, by performing crimping such as press pressurization, the adhesion strength between the negative electrode mixture layer (negative electrode) and the current collector can be further increased.

Although the shape of an electrical power collector is not specifically limited, For example, it is thin, mesh, networks, such as an expanded metal, etc. As a material of an electrical power collector, copper, stainless steel, nickel, etc. are preferable. The thickness of the current collector is preferably about 5 to 20 µm in the case of a thin film.

<Orientation diagram>

It is preferable that the orientation of graphite is suppressed even if the negative electrode of this invention has a high density. The orientation degree of the negative electrode can be quantitatively evaluated by X-ray diffraction. The method thereof will be described below.

First, a negative electrode (density: 1.20 g/cm 3 ) punched out into a disk shape of 2 cm 2 is adhered onto a glass plate so that the negative electrode is directed upward. When the sample prepared in this way is irradiated with X-rays and diffracted, a plurality of diffraction peaks corresponding to the crystal plane of graphite appear. Among the plurality of diffraction peaks, the ratio (I ₀₀₄ /I _{) of the peak intensity I 004 in the} vicinity of 2θ = 54.6° originating from the (004) plane to _{the peak intensity I 110} in the vicinity of 2θ = 77.4° originating in the (110) plane ₁₁₀ ) is the orientation degree of the negative electrode.

The lower the degree of orientation of the negative electrode, the smaller the expansion rate of the negative electrode during charging, and the better the nonaqueous electrolyte permeability and fluidity. As a result, battery characteristics, such as rapid charging property, rapid discharge property, and cycling characteristics, of a lithium ion secondary battery become favorable.

Specifically, when the density of the negative electrode of the present invention is 1.20 g/cm 3 , the degree of orientation (I ₀₀₄ /I ₁₁₀ ) is preferably 5.0 or less, more preferably 4.0 or less, and still more preferably 3.5 or less.

[Lithium Ion Secondary Battery]

The lithium ion secondary battery of this invention is a lithium ion secondary battery which has the negative electrode of this invention.

The lithium ion secondary battery of this invention has a positive electrode, a nonaqueous electrolyte, etc. in addition to the negative electrode of this invention. The lithium ion secondary battery of the present invention is constituted by, for example, stacking a negative electrode, a nonaqueous electrolyte, and a positive electrode in this order and accommodating it in a battery packaging material.

The lithium ion secondary battery of the present invention can be arbitrarily selected from among cylindrical shape, square shape, coin shape, button shape, etc. according to the use, mounted device, required charge/discharge capacity, and the like.

<Positive pole>

It is preferable to select the material (positive electrode active material) of the positive electrode that can occlude/desorb a sufficient amount of lithium. As a positive electrode active material, In addition to lithium, For example, lithium containing compounds, such as a lithium containing transition metal oxide, a transition metal chalcogenide, vanadium oxide, and its lithium compound; Chevrel phase represented by the general formula M _X Mo ₆ S _8-Y (where M is at least one transition metal element, X is 0≤X≤4, Y is a value in the range of 0≤Y≤1) compound; activated carbon; activated carbon fiber; and the like. Vanadium oxide is represented by V ₂ O ₅ , V ₆ O ₁₃ , V ₂ O ₄ , V ₃ O ₈ .

The lithium-containing transition metal oxide is a composite oxide of lithium and a transition metal, and lithium and two or more transition metals may be dissolved in a solid solution. A composite oxide may be used individually or may be used in combination of 2 or more types.

Lithium-containing transition metal oxide is, specifically, LiM ¹ _1-X M ² _X O ₂ (wherein M ¹ and M ² are at least one type of transition metal element, and X is a numerical value in the range of 0≤X≤1 ), or LiM ¹ _1-Y M ² _Y O ₄ (wherein M ¹ and M ² are at least one transition metal element, and Y is a value in the range of 0≤Y≤1).

Transition metal elements represented by M ¹ , M ² are Co, Ni, Mn, Cr, Ti, V, Fe, Zn, Al, In, Sn, etc., preferably Co, Fe, Mn, Ti, Cr, V, Al etc. Preferred specific examples are LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiNi _0.9 Co _0.1 O ₂ , LiNi _0.5 Co _0.5 O _{2 ,} and the like.

Lithium-containing transition metal oxide is, for example, lithium, transition metal oxides, hydroxides, salts, etc. as starting materials, these starting materials are mixed according to the composition of the desired metal oxide, in an oxygen atmosphere at 600 to 1000 ° C. It can be obtained by firing at a temperature of

The positive electrode active material may be used individually or in combination of two or more types of the above-mentioned compounds. For example, a carbon salt such as lithium carbonate can be added to the positive electrode. In forming a positive electrode, various additives, such as a conventionally well-known electrically conductive agent and a binder, can be used suitably.

The positive electrode is produced by, for example, applying a positive electrode mixture comprising a positive electrode active material, a binder, and a conductive agent for imparting conductivity to the positive electrode on both surfaces of a current collector to form a positive mixture layer.

As a binder, the binder used for preparation of a negative electrode can be used.

As the conductive agent, a known conductive agent such as graphitized material or carbon black is used.

Although the shape of an electrical power collector is not specifically limited, Foil shape, a network shape, etc. are mentioned. The material of the current collector is aluminum, stainless steel, nickel or the like. As for the thickness of an electrical power collector, 10-40 micrometers is preferable.

Similarly to the positive electrode and the negative electrode, the paste-like positive electrode mixture may be applied to the current collector and dried, and thereafter, pressure bonding such as press pressure may be performed.

<Non-aqueous electrolyte>

The nonaqueous electrolyte may be a liquid nonaqueous electrolyte (nonaqueous electrolyte solution) or a polymer electrolyte such as a solid electrolyte or a gel electrolyte.

In the former case, the nonaqueous electrolyte battery is configured as a so-called lithium ion secondary battery. In the latter case, the nonaqueous electrolyte battery is configured as a polymer electrolyte battery such as a polymer solid electrolyte and a polymer gel electrolyte battery.

_{Examples of the non-aqueous electrolyte include LiPF 6} , LiBF ₄ , LiAsF ₆ , LiClO ₄ , LiB(C ₆ H ₅ ), LiCl, LiBr, LiCF ₃ SO ₃ , LiCH ₃ SO ₃ , which are electrolyte salts used in ordinary non-aqueous electrolyte solutions. LiN(CF ₃ SO ₂ ) ₂ , LiC(CF ₃ SO2 ₂ ) ₃ , LiN(CF ₃ CH ₂ OSO ₂ ) ₂ , LiN(CF ₃ CF ₂ OSO ₂ ) ₂ , LiN(HCF ₂ CF ₂ CH ₂ OSO _{2 )} ) ₂ , LiN((CF ₃ ) ₂ CHOSO ₂ ) ₂ , LiB[{C ₆ H ₃ (CF ₃ ) ₂ }] ₄ , LiAlCl ₄ , LiSiF ₆ or the like is used. From the viewpoint of oxidation stability, LiPF ₆ and LiBF ₄ are preferable.

0.1-5.0 mol/L is preferable and, as for the density|concentration of the electrolyte salt in a nonaqueous electrolyte solution, 0.5-3.0 mol/L is more preferable.

As a solvent for preparing a nonaqueous electrolyte solution, For example, carbonates, such as ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate; 1,1- or 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, γ-butyrolactone, 1,3-dioxolane, 4-methyl- ethers such as 1,3-dioxolane, anisole and diethyl ether; thioethers such as sulfolane and methylsulfolane; nitriles such as acetonitrile, chloronitrile and propionitrile; Trimethyl borate, tetramethyl silicate, nitromethane, dimethylformamide, N-methylpyrrolidone, ethyl acetate, trimethylorthohormate, nitrobenzene, benzoyl chloride, benzoyl bromide, tetrahydrothiophene, dimethyl sulfoxide, 3-methyl aprotic organic solvents such as -2-oxazolidone, ethylene glycol, and dimethyl sulfite; and the like.

When the non-aqueous electrolyte is a polymer electrolyte such as a solid electrolyte or a gel electrolyte, it is preferable to use a polymer gelled with a plasticizer (non-aqueous electrolyte solution) as a matrix.

Examples of the polymer constituting the matrix include ether-based polymer compounds such as polyethylene oxide and crosslinked products thereof; poly(meth)acrylate-based high molecular compounds; fluorine-based high molecular compounds such as polyvinylidene fluoride and vinylidene fluoride-hexafluoropropylene copolymer; etc. are suitably used.

0.1-5.0 mol/L is preferable and, as for the density|concentration of the electrolyte salt in the nonaqueous electrolyte solution which is a plasticizer, 0.5-2.0 mol/L is more preferable.

Polymer electrolyte WHEREIN: 10-90 mass % is preferable and, as for the ratio of a plasticizer, 30-80 mass % is more preferable.

<Separator>

In the lithium ion secondary battery of the present invention, a separator can also be used.

Although the material of the separator is not specifically limited, For example, a woven fabric, a nonwoven fabric, synthetic resin microporous film etc. are used. Among these, synthetic resin microporous membranes are preferable, and among these, polyolefin microporous membranes are more preferable in terms of thickness, membrane strength, and membrane resistance. Suitable examples of the polyolefin microporous membrane include a polyethylene microporous membrane, a polypropylene microporous membrane, a composite microporous membrane, and the like.

Example

Hereinafter, an Example is given and this invention is demonstrated concretely. However, this invention is not limited to the Example demonstrated below.

<Example 1>

《Preparation of spheroidized graphite》

Scale-like natural graphite (average particle diameter: 8 µm) as a raw material was successively passed through 5 pulverizers arranged in series (manufactured by Naragi Kai Seisakusho, Hybridization System). In each pulverization apparatus, the pulverization time was 30 minutes, and the circumferential speed of the rotor was 50 m/sec. In this way, spheroidized graphite was obtained by grind|pulverizing and granulating a raw material.

Each physical property (volume ratio of fine particles and granules, etc.) of the obtained spheroidized graphite was calculated|required by the method mentioned above. The results are shown in Table 1 below.

<<Preparation of coated spheroidized graphite>>

Coal tar pitch which is a carbonaceous precursor was added to the obtained spheroidized graphite, and it heated at 50 degreeC using a twin-screw kneader, and mixed for 30 minutes. The carbonaceous precursor was added in an amount such that the finally obtained carbonaceous content is shown in Table 1 below. Then, it baked at 1100 degreeC for 10 hours under nitrogen 5 L/min flow (in a non-oxidizing atmosphere) using a tube furnace. In this way, the coated spheroidized graphite in which the spheroidal graphite was coat|covered with carbonaceous material was obtained.

Each physical property (average secondary particle diameter, etc.) of the obtained coated spheroidized graphite was calculated|required by the method mentioned above. The results are shown in Table 1 below.

《Production of Negative Drama》

98 parts by mass of coated spheroidal graphite (negative electrode material), 1 part by mass of carboxymethylcellulose (binder), and 1 part by mass of styrene-butadiene rubber (binder) were placed in water and stirred to prepare a negative electrode mixture paste.

The prepared negative electrode mixture paste was applied to a uniform thickness on copper foil (thickness: 16 µm), and further dried at 90°C in a vacuum to form a negative electrode mixture layer. Next, the negative electrode mixture layer was pressurized at a pressure of 120 MPa with a hand press. Thereafter, the copper foil and the negative electrode mixture layer were punched out in a circular shape with a diameter of 15.5 mm. In this way, a negative electrode (thickness: 60 µm, density: 1.20 g/cm 3 ) adhered to the current collector made of copper foil was produced.

In addition, in parallel with the preparation of the negative electrode, the orientation degree of the negative electrode was determined according to the method described above. The results are shown in Table 1 below.

《Production of positive pole》

The lithium metal foil was pressed against a nickel net and punched out in a circular shape with a diameter of 15.5 mm. Thereby, the positive electrode which consists of lithium metal foil (thickness: 0.5 mm) closely_contact|adhered to the electrical power collector which consists of a nickel net was produced.

<< production of evaluation battery >>

As an evaluation battery, the button-type secondary battery shown in FIG. 4 was produced.

4 is a cross-sectional view showing a button-type secondary battery. In the button-type secondary battery shown in FIG. 4 , the peripheral portions of the outer cup 1 and the outer can 3 are caulked through an insulating gasket 6 to form a sealed structure. Inside the sealed structure, the current collector 7a, the positive electrode 4, the separator 5, the negative electrode 2, and the current collector sequentially from the inner surface of the outer can 3 toward the inner surface of the outer cup 1 (7b) is laminated.

The button-type secondary battery shown in FIG. 4 was produced as follows.

First, a non-aqueous electrolyte solution was prepared by dissolving _{LiPF 6} in a mixed solvent of ethylene carbonate (33 vol%) and methylethyl carbonate (67 vol%) at a concentration of 1 mol/L. A separator 5 impregnated with the nonaqueous electrolyte was produced by impregnating the obtained nonaqueous electrolyte into a polypropylene porous body (thickness: 20 µm).

Next, the produced separator 5 was sandwiched between the negative electrode 2 in close contact with the current collector 7b made of copper foil and the positive electrode 4 in close contact with the current collector 7a made of nickel net, and laminated. . Thereafter, the current collector 7b and the negative electrode 2 are accommodated in the exterior cup 1, and the collector 7a and the positive electrode 4 are accommodated in the exterior can 3, and the exterior cup ( 1) and the outer can (3) were combined. Further, the peripheral portions of the outer cup 1 and the outer can 3 were sealed by caulking with an insulating gasket 6 interposed therebetween. In this way, a button-type secondary battery was produced.

Using the produced button-type secondary battery (evaluation battery), the battery characteristics were evaluated by the charge-discharge test described below. The results are shown in Table 1 below.

In the following charge/discharge tests, the process of occluding lithium ions in the negative electrode material was defined as charging, and the process of lithium ions desorbing from the negative electrode material was defined as discharge.

《Charge and Discharge Test: Discharge Capacity and Initial Charge and Discharge Efficiency》

First, with a current value of 0.9 mA, constant current charging was performed until the circuit voltage reached 0 mV. When the circuit voltage reached 0 mV, constant voltage charging was switched to, and charging was continued until the current value reached 20 µA. The charge capacity (unit: mAh) was calculated|required from the electricity supply amount during this time. After that, it was rested for 120 minutes. Next, with a current value of 0.9 mA, constant current discharge was performed until the circuit voltage reached 1.5 V. The discharge capacity (unit: mAh) was calculated|required from the electricity supply amount during this time. This was referred to as the first cycle.

From the charge capacity and the discharge capacity in the first cycle, the initial charge/discharge efficiency (unit: %) was calculated based on the following formula. The results are shown in Table 1 below.

Initial charge/discharge efficiency [%] = (discharge capacity/charge capacity) x 100

《Charging and Discharging Test: 25°C Output Resistivity》

In a temperature atmosphere of 25°C, constant current charging was performed at 1.0C until the circuit voltage reached 3.82V. Then, it adjusted to the temperature atmosphere of 0 degreeC, and rested for 3 hours.

Next, after discharging at 0.5C for 10 seconds, resting for 10 minutes, charging at 0.5C for 10 seconds, and resting for 10 minutes.

Next, after discharging at 1.0C for 10 seconds, resting for 10 minutes, charging at 0.5C for 20 seconds at 50% SOC (State of Charge: charge rate), and resting for 10 minutes.

Next, after discharging at 1.5C for 10 seconds, resting for 10 minutes, charging at 0.5C for 30 seconds at 50% SOC, and resting for 10 minutes.

Next, after discharging at 2.0C for 10 seconds, it was paused for 10 minutes, charged at 0.5C for 40 seconds at 50% SOC, and paused for 10 minutes.

After the test, the discharge capacity (unit: mAh) calculated above was multiplied by each C rate (0.5C, 1.0C, 1.5C, 2.0C) to calculate the current value. In addition, the voltages (10 second values) at the time of discharging at the C rate were respectively determined.

The results at each C rate were plotted with the current value as the x-coordinate and the voltage as the y-coordinate, and the slope of their linear approximation straight line was calculated from the least squares method. This slope was made into output resistance (unit: Ω). It can be evaluated that it is excellent in an output characteristic, so that this value is small.

In addition, the 25 degreeC output resistivity (unit: %) of each example (Example and a comparative example) was calculated|required from the following formula. The results are shown in Table 1 below.

25°C output resistivity [%] = (output resistance of each example/output resistance of Example 1) x 100

<Example 2>

The number of pulverizers for passing the raw materials was set to seven, and in each pulverizer, the pulverization time was set to 15 minutes and the circumferential speed of the rotor was set to 80 m/sec. Other than that, it carried out similarly to Example 1. The results are shown in Table 1 below.

<Example 3>

The number of pulverizers for passing the raw materials was set to four, and in each pulverizer, the pulverization time was set to 10 minutes and the circumferential speed of the rotor was set to 60 m/sec. Other than that, it carried out similarly to Example 1. The results are shown in Table 1 below.

<Example 4>

The number of pulverizers for passing the raw materials was set to four, and in each pulverizer, the pulverization time was set to 20 minutes and the circumferential speed of the rotor was set to 60 m/sec. Other than that, it carried out similarly to Example 1. The results are shown in Table 1 below.

<Example 5>

The number of pulverizers for passing the raw materials was set to four, and in each pulverizer, the pulverization time was 25 minutes and the circumferential speed of the rotor was 60 m/sec. Other than that, it carried out similarly to Example 1. The results are shown in Table 1 below.

<Example 6>

The number of pulverizers through which the raw materials were passed was set to six, and in each pulverizer, the pulverization time was set to 20 minutes and the circumferential speed of the rotor was set to 60 m/sec. Other than that, it carried out similarly to Example 1. The results are shown in Table 1 below.

<Comparative Example 1>

The number of pulverizing apparatuses for passing the raw materials was set to four, and in each pulverizing apparatus, the pulverization time was 5 minutes and the circumferential speed of the rotor was 30 m/sec. Other than that, it carried out similarly to Example 1. The results are shown in Table 1 below.

<Comparative Example 2>

The number of pulverizers through which the raw materials were passed was set to one, and in each pulverizer, the pulverization time was set to 10 minutes and the circumferential speed of the rotor was set to 30 m/sec. Other than that, it carried out similarly to Example 1. The results are shown in Table 1 below.

<Comparative example 3>

The number of pulverizing apparatuses for passing the raw materials was set to four, and in each pulverizing apparatus, the pulverization time was 5 minutes and the circumferential speed of the rotor was 50 m/sec. Other than that, it carried out similarly to Example 1. The results are shown in Table 1 below.

<Comparative Example 4>

The number of pulverizers for passing the raw materials was set to nine, and in each pulverizer, the pulverization time was set to 10 minutes, and the circumferential speed of the rotor was set to 90 m/sec. Other than that, it carried out similarly to Example 1. The results are shown in Table 1 below.

<Comparative Example 5>

The number of pulverizers through which the raw materials were passed was set to 10, and in each pulverizer, the pulverization time was set to 20 minutes and the circumferential speed of the rotor was set to 40 m/sec. Other than that, it carried out similarly to Example 1. The results are shown in Table 1 below.

<Summary of evaluation results>

As shown in Table 1 above, Examples 1 to 6 in which the volume ratio of the particles is 40% or less and the volume ratio of the granules is 13% or more, than Comparative Examples 1 to 5, which do not satisfy at least any of these, The output characteristics were good.

1: Outer cup
2: negative electrode
3: External can
4: positive pole
5: separator
6: Insulation gasket
7a: current collector
7b: current collector

Claims (9)

In the particle size distribution of primary particles obtained using X-ray CT, the volume ratio of primary particles having a sphere equivalent diameter of 0.8 μm or less is 40.0% or less, and a sphere equivalent diameter of 1.5 μm or more Spheroidized graphite, wherein the volume ratio of the primary particles of 3.0 μm or less is 13.0% or more. According to claim 1,
In the particle shape distribution of secondary particles obtained using X-ray CT, the volume ratio of spherical secondary particles is 14.0% or more, and the volume ratio of rod-shaped secondary particles is 34.0% or less, Nodular graphite. 3. The method of claim 1 or 2,
Spheroidized graphite having an average secondary particle diameter of 5.0 µm or more and 15.0 µm or less, and a specific surface area of 5.0 m 2 /g or more and 15.0 m 2 /g or less. 4. The method according to any one of claims 1 to 3,
Spheroidized graphite formed by spheroidizing natural graphite. The coated spheroidized graphite containing the spheroidizing graphite in any one of Claims 1-4, and the carbonaceous material which coat|covers the said spheroidal graphite. 6. The method of claim 5,
Covered spheroidized graphite whose average secondary particle diameter is 5.0 micrometers or more and 50.0 micrometers or less, and the specific surface areas are 0.5 m<2>/g or more and 10.0 m<2>/g. 7. The method according to claim 5 or 6,
Covered spheroidized graphite whose pore volumes corresponding to pores having a pore diameter of 7.8 nm or more and 36.0 nm or less are 0.015 cm 3 /g or more and 0.028 cm 3 /g or less. The negative electrode for lithium ion secondary batteries containing the covering spheroidizing graphite in any one of Claims 5-7. The lithium ion secondary battery which has the negative electrode of Claim 8.

Images