JPH11102702A - Manufacture of carbon for lithium ion secondary battery negative electrode - Google Patents

Abstract

PROBLEM TO BE SOLVED: To efficiently and stably manufacture a carbon material of a prescribed grain size by pulverizing the carbide in the two-stage pulverizing process, i.e., a rough-pulverizing process and the fine pulverizing process, and achieving graphitization after the grain size has been adjusted. SOLUTION: Mesophase carbon fiber is preferable for the carbide because the capacity of the secondary battery is easily increased, and the mat-like carbon fiber is adopted from the relation between the spinning and the handling property. Because pulverization is achieved in the rough-pulverizing process and the fine-pulverizing state, the grain size can be adjusted easily, and the load on a pulverizing machine is reduced. A high-speed rotary mill is optimum for each pulverizing machine used for rough pulverization and fine pulverization. The grain size of the stuff to be pulverized is adjusted by mainly controlling the peripheral speed (number of revolutions) of a rotor and the air volume. Rough pulverization is achieved at the peripheral speed of 80-140 m/sec, while fine pulverization is achieved at a peripheral speed of 105-150 m/sec in an efficient manner, and preferably the difference in the peripheral speed between the rough pulverization and fine pulverization is preferably 5-30 m/sec.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a carbon material suitable for a negative electrode material for a lithium ion secondary battery, and more particularly to a lithium ion secondary battery having excellent production efficiency and excellent quality stability. The present invention relates to a method for adjusting the particle size of a carbon material for a battery negative electrode material.

[0002]

2. Description of the Related Art In recent years, electronic devices have undergone rapid technological development with the aim of miniaturization, weight reduction, and high performance. As a result, portable electronic devices such as cellular phones, PHSs, camcorders, and personal computers have become more widespread. Advanced. With the development of these new devices, nickel-metal hydride batteries and lithium-ion secondary batteries have emerged as new secondary batteries. In particular, a lithium ion secondary battery has many advantages such as a high energy density and a high electromotive force, a wide operating temperature range due to the use of a non-aqueous electrolyte, excellent long-term storage, and light weight and small size. doing. Therefore, such a lithium ion secondary battery is expected to be put to practical use as a high-performance battery for a power source of a portable electronic device, an electric vehicle, a power storage device, and the like. The performance and safety of the lithium ion secondary battery were improved by using a carbon-based material instead of lithium metal for the negative electrode. That is, when the carbon-based material is used for the negative electrode, lithium dendrites are not formed because lithium ions are incorporated into the carbon structure, and safety is dramatically improved. For such a negative electrode material for a lithium ion secondary battery, for example, use of pitch-based milled graphite fiber is disclosed in Japanese Patent Application Laid-Open No. 5-32596.
No. 7, 6-36802, and 7-90725. The carbon-based material used for this negative electrode is
Usually, it is used after being crushed to an appropriate particle size.
JP-A-63-102166, JP-A-63-12124
8, JP-A-5-325967, JP-A-6-1687
No. 25 each publication discloses a preferred particle size range for batteries (approximately 0.1 to 200 μm, as a graphite system, preferably an average particle size of about 5 to 30 μm). The above publication does not specifically disclose under what conditions the production is preferable.

[0003]

Normally, when a carbon-based material is crushed to a particle size required as a negative electrode material of a battery by using a general-purpose crusher, the particle size distribution of the obtained carbon material tends to be wide. Is seen. For this reason, it is necessary to adjust the particle size by sieving, classification and the like after pulverization, which causes a reduction in product yield. In particular, when crushing a carbon material, such as carbon fiber, which is greatly separated from a sphere,
This tendency is remarkable. Further, depending on the operating conditions, wear of the crusher is observed, and it becomes difficult to perform stable operation for a long time without a decrease in the amount of crushing, and further, the influence on the quality of the product due to the inclusion of foreign matter is also observed. The present invention has been accomplished under the circumstances described above. That is, the present invention is capable of efficiently and stably producing a carbon material having a predetermined particle size, and further, since the abrasion of the pulverizer is small, the lithium ion secondary battery having excellent productivity and stable quality is provided. An object of the present invention is to provide a method for producing a carbon material for a negative electrode material.

[0004]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies in view of the above-mentioned problems, and have found that the pulverization process is performed in two stages, that is, coarse pulverization is performed in the first stage, fine pulverization is performed in the second stage, and It has been found that pulverization suitable for the above-mentioned purpose can be performed by controlling the operating conditions, and the present invention has been completed. That is, the present invention provides a method of pulverizing a carbide by a two-stage pulverization step including a first pulverization step of coarse pulverization and a second pulverization step of fine pulverization, adjusting the particle size, and then graphitizing. A method for producing a carbon material for a negative electrode material of a lithium ion secondary battery, wherein the crusher used in the first crushing step and the second crushing step is a high-speed rotary mill. The method according to the above or the above, wherein the first pulverizing step is performed under the pulverizing conditions of a peripheral speed of 80 to 140 m / sec. The second pulverizing step is performed at a peripheral speed of 90 to 150 m / sec. The method according to any one of the above, wherein the peripheral speed of the pulverizer used in the second pulverization step is 5 to 30 m higher than the peripheral speed of the pulverizer used in the first pulverization step. / Small second / second. The method according to any one of the above-mentioned items, wherein the carbide is a mesophase pitch-based carbon fiber carbonized at a temperature of 500 ° C. or more and 1300 ° C. or less. The method according to any one of the above items, wherein a crushing step is provided before the step of crushing, and the milled carbon fiber obtained by the second crushing step is 8
5% or less having a mean particle size of 50 μm or less and a particle size of 5 μm or less, and 4% or less having a mean particle size of 100 μm or more.
% Or less, the production method according to any one of the above-described items.

[0005]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in more detail. The carbide used in the production method of the present invention is not particularly limited as long as it is a carbon-based material having no developed graphite structure and can be graphitized by a subsequent heat treatment. Examples of such carbides include organic materials, pitches, coke, PAN-based carbon fibers, and pitch-based carbon fibers that have been heat-treated at a relatively low temperature. In particular, mesophase pitch-based carbon fibers have a large secondary battery capacity. It is preferable because it is easily formed.

[0006] Mesophase pitch-based carbon fibers can be obtained by spinning, infusibilizing, and carbonizing mesophase pitch by a conventional method. The form at this time is preferably in the form of a mat-like carbon fiber from the viewpoint of the spinning method described later and the ease of handling thereafter. Further, it is preferable that the carbon fiber is pulverized to an appropriate particle size for the negative electrode material of the secondary battery (in the case of a fiber, particularly, milled) and used. At this time, in order to facilitate the entrance and exit of lithium ions, it is preferable to shorten the fiber length, that is, to increase the fiber surface area per volume (weight) as much as possible. Further, it is preferable to perform milling so as to increase the surface area while maintaining the fiber form, in order to improve the performance of the battery.

However, if the fibers are unnecessarily pulverized, the exposure of the active graphite layer after graphitization increases more than necessary and easily reacts with the electrolytic solution, so that disadvantages such as a decrease in capacity and charge / discharge efficiency occur. Therefore, an appropriate particle size distribution and an average particle size are required. Also, particles having a large particle diameter tend to cause uneven coating of the negative electrode material and short-circuit, and it is difficult to increase the packing density. Therefore, it is preferable to reduce the amount as much as possible with particles having a small particle diameter. Therefore, in the present invention, the carbide has an average particle size of 8 to
50 μm, preferably 10 to 30 μm, and 5% or less having a particle size of 5 μm or less, preferably 3% or less, more preferably 2.5% or less, and 4% or less having a particle size of 100 μm or more; Preferably not more than 3%, more preferably
It is preferable to grind and adjust the particle size so as to be 2.5% or less. In the present invention, the value such as the average particle size is
It can be measured by a laser diffraction type particle size distribution analyzer. Hereinafter, in the present invention, the carbon fibers after pulverization (milling) are referred to as milled carbon fibers, and those further graphitized are referred to as milled graphite fibers.

In the present invention, the pulverization for adjusting the particle size is performed in two stages, that is, coarse pulverization is performed in the first step, and fine pulverization is performed in the second step to adjust the particle size. And an appropriate particle size distribution can be obtained. That is, by performing the pulverization in the particle size adjusting step in two stages, the particle size can be easily adjusted as compared with the conventional one-stage pulverization, and milled carbon fibers having a predetermined particle size can be stably manufactured. In addition, by performing the pulverization in two stages, the load on the pulverizer can be reduced, thereby reducing the wear of the pulverizer, enabling long-term operation, and further improving the quality. We can expect effect for.

In the first and second pulverizing steps of the present invention, a general-purpose jaw crusher, hammer crusher, ball mill, crusher, roller mill, jet mill, high-speed rotary mill, etc. are used. However, as a result of various studies on the relationship between the pulverized material and the particle size after pulverization, it is most preferable to use a high-speed rotating mill that rotates a rotor equipped with a blade at high speed. In this case, by appropriately adjusting the number of rotations of the rotor, the air volume, the angle of the blade, the size of the mesh of the filter attached to the periphery of the rotor, and the like, the carbide in each of the first stage pulverization and the second stage pulverization is adjusted. Can be controlled. In addition, as the hardness of the carbide increases, the wear of the crusher significantly increases.Therefore, depending on the hardness of the carbide, it is necessary to make the material such as the crushing portion hard to wear. It is effective to use a nitrided metal that is not easily worn in the collision portion, or to provide reinforcement by a coating of a hard metal such as titanium.

The two-stage pulverization in the present invention can be carried out by appropriately selecting and combining the above-mentioned pulverizers in accordance with the properties of the pulverized material and the particle size required after the pulverization, and the operating conditions thereof are also appropriately selected. However, in consideration of process control and maintainability, it is preferable to use a pulverizer having the same mode specifications, particularly a high-speed rotating mill suitable for pulverizing carbides. The high-speed rotating mill has a rotor (rotary blade) and a stator (fixed blade), and the clearance (gap) between them is 1 to 5 mm.
Further, those having a size of 2 to 4 mm can be preferably used in the method of the present invention in view of the particle size distribution of the pulverized material and the pulverization efficiency.
In such a high-speed rotating mill, the particle size of the pulverized material can be easily adjusted mainly by controlling the peripheral speed (proportional to the number of rotations) and the air volume of the rotor.

[0011] Both the first stage pulverization and the second stage pulverization
When grinding using the above-mentioned particle size range,
The operation conditions of the preferable peripheral speed and air flow are described below. First
The purpose of the pulverization process is to perform coarse pulverization.
In order not to generate much fine powder in
Grinding is required. Therefore, in the present invention, the average
Particle size is 25-60 μm, 90% cumulative diameter is 90-130
μm and a particle size of 5μm or less
It is preferable to set such grinding conditions. Therefore, 1
The pulverization of the stage is performed at a peripheral speed of 80 to 140 m / sec, preferably 10 to 140 m / sec.
It is efficient to carry out under the condition of 0 to 130 m / sec. Through
In a high-speed rotary mill, the higher the peripheral speed, the higher the grinding efficiency.
However, the wear of the equipment becomes severe. Because of this,
When the peripheral speed is less than 80 m / sec, the pulverization efficiency is reduced and the
If it is not good and it is more than 140m / sec,
The wear of the vessel is extremely increased and the fines
Is not preferred. In addition, the air volume is
0.15 ~ 0.4m / kg of carbide ^Three/ Min
Is preferred. If this value is out of the above range, pulverized material and machine
Condition becomes poor due to the collision of the vessel
).

The second-stage pulverization is performed for the purpose of pulverizing the coarsely pulverized product in the first-stage pulverization to a required final particle size. The peripheral speed can be made somewhat higher than in the first stage because the abrasion of the equipment in the second stage is alleviated by reducing the size of the pulverized material (reducing the weight). Therefore, the second
The pulverization of the stage is performed at a peripheral speed of 105 to 150 m / sec, preferably 1 to 150 m / sec.
It is efficient to perform it under the condition of 05 to 140 m / sec.
In addition, since the air volume is smaller than that of the first stage at the blower inlet since the particle size of the pulverized material is smaller than that of the first stage, 1 kg of the carbide pulverized for the same reason as the first stage may be used. It is preferably from 0.12 to 0.35 m ³ / min. Furthermore, regarding the peripheral speed difference between the first stage and the second stage crusher, the peripheral speed of the second stage crusher in the operation range of each of the crushers is set to the first stage crusher. More 5-3
It is preferable to increase the speed by 0 m / sec, and further by 8 to 25 m / sec. If the difference in peripheral speed is outside the above range, the particle size distribution of the pulverized material tends to be widened, and the product yield tends to decrease,
Further, the crushing load tends to be biased to one side of the crusher, and the equipment on the high load side becomes significantly worn, which lowers the operation efficiency, which is not preferable.

In the present invention, when crushing the mat-like carbon fiber as a carbide, it is preferable to provide a crusher prior to the two-stage crushing step. That is,
Since it is not preferable from the viewpoint of crushing efficiency and maintenance of the apparatus that the mat-shaped carbon fiber is immediately put into the crusher, the mat-shaped carbon fiber is crushed into an appropriate size by the crusher and then put into the crusher. Is preferred. As such a crusher, a knife-shaped cutter, a roll-shaped cutter or the like is preferably used. In the present invention, the carbide pulverized in the first step is transferred to the second step by, for example, a screw feeder. As such a screw feeder, any of conventionally known screw feeders can be used. In the production method of the present invention, the pulverizing step is divided into two steps, and each step is performed under the above-described conditions, whereby milled carbon fibers having a predetermined particle size as described above can be stably obtained. It is.

The outline of other main steps related to the production of pitch-based milled carbon fiber will be described below. <Raw material pitch> The starting material pitch may be any of resin-based, petroleum-based, coal-based, and synthetic pitches using a catalyst and the like, but is not limited thereto. A mesophase pitch having a mesophase content of 100% is used. The softening point of the raw material pitch is not particularly limited, but a material having a low softening point and a high infusibilization reaction rate in relation to the spinning temperature is advantageous from the viewpoint of production cost and stability. Therefore, the softening point of the raw material pitch is generally 230 ° C. or more and 350 ° C. or less.

<Spinning> The method of melt-spinning the raw material pitch is not particularly limited, and various methods such as melt spinning, melt blowing and centrifugal spinning can be used. The melt blow method is preferred from the viewpoint of the quality of the resin. Furthermore, in order to improve the performance of the battery, it is desirable to arrange the graphite layer surface of the milled graphite fiber so as to open to the fiber axis surface, and the melt blow spinning method using mesophase pitch has a low viscosity of several tens poise or less. Spinning can be performed, and the graphite layer surface can be opened to the fiber shaft surface by high-speed cooling, which is particularly preferable. The size of the spinning hole at this time is 0.1 mmΦ or more and 0.1 mmΦ.
5 mmΦ or less, preferably 0.15 mmΦ or more and 0.3 mmΦ
It is as follows. The spinning speed is 500 m / min or more, preferably 1500 m / min or more. The spinning temperature is somewhat changed depending on the raw material pitch used, but may be any temperature as long as the pitch is not changed above the softening point of the raw material pitch.
℃ to 420 ° C, preferably 300 ° C to 400 ° C
It is as follows.

<Infusibilization> Although the infusibilization method is not particularly limited, a method of performing heat treatment in an oxidizing gas atmosphere such as nitrogen dioxide or oxygen by a conventional method, or a method of treating in an oxidizing aqueous solution such as nitric acid or chromic acid is used. A method, and further, a polymerization treatment method using light, γ-ray, or the like is possible. As a simpler infusibilization method, there is a method of performing heat treatment in air at 200 to 350 ° C. for a certain period of time, and the average rate of temperature rise is 3 ° C./min or more, preferably 5 ° C./min or more. <Carburizing> The infusibilized fiber can be converted into a carbon fiber by performing a heat treatment (carbonization) in the absence of an oxidizing gas, for example, in an inert gas by a conventional method. The heating rate and the holding time at this time are not particularly limited, but the carbonization temperature is
500 ° C or higher and 1300 ° C or lower, further 600 ° C or higher and 900
It is preferable to carry out at a temperature of not more than ° C. 500 carbonization temperature
If the temperature is lower than 0 ° C., it is not preferable because the powder is easily pulverized so that the fiber shape is not maintained. On the other hand, if the carbonization temperature exceeds 1300 ° C., the abrasion of the equipment during pulverization becomes severe, and the battery capacity after graphitization tends to decrease, which is not preferable. In particular, from the viewpoint of equipment wear, it is preferable to set the carbonization temperature to 900 ° C. or less because maintenance costs such as internal lining can be reduced.

<Graphization> The milled carbon fiber obtained by the method of the present invention is then fired at a high temperature in, for example, a batch type graphitization furnace to become milled graphite fiber. Graphitization is usually carried out at a temperature of 2000 ° C. or higher, but in order to increase the capacity of the battery, it is necessary to further graphitize. For this reason, it is 2400 ° C. or more, preferably 2500
It is preferable to use those which have been graphitized at a temperature of at least ℃. Usually, in order to efficiently perform the graphitization treatment, it is preferable to increase the filling amount per volume. Therefore, graphitization after milling is also advantageous in reducing costs. A higher graphitization temperature is preferable in terms of capacity and the like, but the production cost increases sharply with an increase in the graphitization temperature, and at a graphitization temperature exceeding 3000 ° C., the durability of the furnace material for graphitization increases. Therefore, it is necessary to appropriately select the graphitization temperature according to the purpose.

The carbon material of the present invention, which has been pulverized and adjusted in particle size and then graphitized as described above, can be used as a negative electrode of a lithium ion secondary battery by a usual method. That is, a binder such as polyvinylidene fluoride or polytetrafluoroethylene styrene is added to the carbon material, a slurry is formed using an organic solvent or a water solvent, and the thickness is 10 to 10.
A method is widely used in which a coating is applied to one or both surfaces of a metal foil made of 50 μm copper, nickel, or the like, which is rolled and dried to form a sheet having a thickness of about 100 μm. After that, a method of slitting to a predetermined width and length and winding and forming the can together with the positive electrode and the separator is general.

[0019]

EXAMPLES The present invention will be described in more detail with reference to the following Examples, which should not be construed as limiting the present invention. Example 1 An optically anisotropic petroleum-based mesophase pitch having a softening point of 280 ° C. was used as a raw material, and 0.2 m in a slit having a width of 3 mm.
Using a spinneret having 1,500 mφ spinning holes in a line, heated air is blown out of the slits to pull the molten pitch, so that pitch fibers having an average fiber diameter of 13 μm are collected at a collecting portion of 2 μm.
It was collected on the belt while sucking it from the back of a belt made of a 0 mesh stainless steel wire mesh. The temperature of the collected matting pitch fibers was raised in the air at an average rate of 6 ° C./min from room temperature to 300 ° C. to perform infusibility treatment. The thus obtained mesophase pitch infusible fiber was further carbonized at 650 ° C. under a nitrogen atmosphere to obtain a mat-like carbon fiber.

The obtained mat-like carbon fiber was weighed at 70 kg /
After crushing with the crusher at the feed rate at the time, the peripheral speed is 12 with the first-stage crusher (high-speed rotating mill; rotor diameter 500 mm).
3m / s (4700rpm), air flow rate 0.19m ³ / min · k
g, and the average particle size was 32.5 μm and the cumulative particle size was 1
0%, 50%, and 90% correspond to 13.2 μm and 27.9, respectively.
μm and 112.6 μm of primary pulverized milled carbon fibers were obtained.
Also, in the primary milled milled carbon fiber, those having a size of 5 μm or less hardly exist, and those having a size of 100 μm or more are 11.9.
%Were present. Next, the primary ground milled carbon fiber is
Stage crusher (high-speed rotating mill; rotor diameter 500 mm)
With a peripheral speed of 134 m / sec (5100 rpm) and air volume of 0.17
pulverized under the condition of m ³ /min.kg, average particle size 22.4μ
m, cumulative diameter 10%, 50%, 90% are each 12.1.
μm, 19.6 μm, and 53.9 μm milled carbon fibers were obtained. Among the milled carbon fibers, those having a diameter of 5 μm or less are 0.4.
%, And 1.9% existed at 100 μm or more.
All were within the allowable range, and all of them could be used for graphitization. In addition, the blades of the pulverizer were less worn, and long-time operation was possible.

Further, the milled carbon fiber was graphitized at 3000 ° C. in an argon atmosphere to obtain a milled graphite fiber, and the battery performance was confirmed as follows. The charge and discharge capacity characteristics of the milled graphite fiber, using metallic lithium for the positive electrode and the reference electrode,
Lithium perchlorate (LiClO) was used as an electrolyte in a mixed carbonate solvent in which ethylene carbonate (EC) / dimethyl carbonate (DMC) was adjusted to a volume ratio of 1/1.
₄ ) was measured in an electrolytic solution having a concentration of 1 mol.
The measurement of the charge / discharge capacity characteristic was performed under a constant current charge / discharge of 100 mA / g, and the measured potential range was set to the reference electrode (0 to 1.5 V /
(Li / Li +) was repeated 10 times. The first discharge capacity was 312 mAh / g, the charge / discharge efficiency was 94%, the second discharge capacity was 310 mAh / g, and the charge / discharge efficiency was 99.8%, all of which were high values. In addition, from the second time to the tenth time, the discharge capacity was 310 mAh / g and the charge / discharge efficiency was 100%.

Comparative Example 1 The carbon fiber mat obtained in Example 1 was crushed by a crusher at a feed rate of 70 kg / hour in the same manner as in Example 1,
The crusher (high-speed rotating mill; rotor diameter 5) used in Example 1
00 mm) and the peripheral speed was increased so that the average particle diameter was about the same as in Example 1 [141 m / sec (5400 rpm
m), air volume 0.17 m ³ / min · kg], pulverization in one stage, average particle size 22.0 μm, cumulative diameter 10%, 50%, 90
% Obtained 10.8 μm, 19.2 μm, and 72.1 μm, respectively. 5 μm in the milled carbon fiber
The following are present in 2.9%, and those with 100 μm or more are in 5.
There was 3%. Compared with the two-stage pulverization, the particle size distribution was broadened. For this reason, sieving was required to obtain the same particle size distribution as in Example 1, and the yield was reduced to 94%. In addition, the abrasion of the blades of the pulverizer was larger than in the two-stage pulverization.

Examples 2 and 3 In Example 1, primary ground milled carbon fibers obtained by pulverization with a first stage pulverizer were used, and the second stage pulverization conditions were as shown in Table 1. The two-stage pulverization was performed in the same manner as in Example 1 except for the change. Table 1 shows the obtained results.

[0024]

[Table 1]

[0025]

As described in detail above, according to the present invention, a carbon material having a predetermined particle size can be produced efficiently and stably, and further, since the abrasion of the crusher is small,
A method for producing a carbon material for a negative electrode material of a lithium ion secondary battery having excellent productivity and stable quality can be provided.

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[Procedure amendment]

[Submission date] October 7, 1997

[Procedure amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0012

[Correction method] Change

[Correction contents]

The second-stage pulverization is performed for the purpose of pulverizing the coarsely pulverized product in the first-stage pulverization to a required final particle size. The peripheral speed can be made somewhat higher than in the first stage because the abrasion of the equipment in the second stage is alleviated by reducing the size of the pulverized material (reducing the weight). Therefore, the second
The pulverization of the stage is performed at a peripheral speed of 90 to 150 m / sec, preferably 10 to 150 m / sec.
It is efficient to carry out under the condition of 5 to 140 m / sec. In addition, since the air volume is smaller than that of the first stage at the blower inlet since the particle size of the pulverized material is smaller than that of the first stage, 1 kg of the carbide pulverized for the same reason as the first stage may be used. It is preferably from 0.12 to 0.35 m ³ / min. Further, the difference in peripheral speed between the first stage and the second stage of the crusher is the second range in the operation range of each of the crushers.
The peripheral speed of the first-stage crusher is 5 to 30 times that of the first-stage crusher.
m / sec, preferably 8 to 25 m / sec.
If the difference in peripheral speed is outside the above range, the particle size distribution of the pulverized material tends to be widened, the product yield tends to decrease, and the pulverization load tends to be biased to one of the pulverizers, and the high load side It is not preferable because wear of the equipment becomes remarkable, and operation efficiency is lowered.

Claims (8)

[Claims] 1. A method of pulverizing a carbide by a two-stage pulverization step including a first pulverization step of performing a coarse pulverization and a second pulverization step of performing a fine pulverization, adjusting the particle size, and then graphitizing. A method for producing a carbon material for a negative electrode material of a lithium ion secondary battery. 2. The method according to claim 1, wherein the crusher used in the first crushing step and the second crushing step is a high-speed rotary mill. 3. The method according to claim 1, wherein the first grinding step is performed at a peripheral speed of 80 to 140 m.
3. The method according to claim 1, wherein the pulverization is performed under a pulverization condition of / sec.
The production method described in 1. 4. The method according to claim 1, wherein the second grinding step is performed at a peripheral speed of 90 to 150 m.
The production method according to any one of claims 1 to 3, wherein the pulverization is performed under a grinding condition of / sec. 5. The peripheral speed of the crusher used in the second crushing step is 5 to 30 times lower than the peripheral speed of the crusher used in the first crushing step.
The method according to claim 1, wherein m / sec is larger. 6. The production method according to claim 1, wherein the carbide is mesophase pitch-based carbon fiber carbonized at a temperature of 500 ° C. or more and 1300 ° C. or less. 7. The method according to claim 1, wherein the carbide is in a mat form, and a crushing step is provided before the first crushing step. 8. The milled carbon fiber obtained by the second pulverizing step has an average particle size of 8 to 50 μm, and a particle size of 5 μm or less is 5% or less, and a particle size of 100 μm or more. Is not more than 4%.
8. The production method according to any one of items 7.