JP2001023633A - Manufacture of graphite powder for lithium ion secondary battery negative electrode - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance the crystallinity of graphite and increase the discharge capacity of a negative electrode by conducting a crushing process before and/or after a carbonization process under a non-oxidizing atmosphere. SOLUTION: A graphite powder is produced through processes of carbonization, graphitization and crushing using a mesophase as a raw material. At this time, the crushing is conducted before and/or after the carbonization process under a non-oxidizing atmosphere and is not conducted after the graphitization. When the crushing is conducted after the graphitization, the graphite powder having a significantly large specific surface area is obtained, thus the charge/ discharge efficiency and the cycle lifetime of a negative electrode is shortened. Additionally, the mesophase of the raw material is preferably not subject to the surface oxidation. The time for crushing may be determined according to the fusion during the carbonization in the mesophase of the raw material. That is, when the fusion during the carbonization is great, the mesophase is crushed after the carbonization. When no or a little fusion is observed, the mesophase is better to be crushed before the carbonization.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for producing a graphite powder having a high crystallinity and capable of giving an excellent discharge capacity, which is used for a negative electrode of a lithium ion secondary battery which is a non-aqueous secondary battery.

[0002]

2. Description of the Related Art A lithium ion secondary battery is a carbon material capable of reversibly occluding and releasing lithium ions in a negative electrode, such as a lithium compound (eg, a composite oxide of lithium and a transition metal such as Ni or Co). And a non-aqueous secondary battery using a solution in which a lithium compound is dissolved in an organic solvent in an electrolyte.

[0003] A lithium secondary battery using a lithium metal or lithium alloy for the negative electrode has a very high battery capacity, but has a poor cycle life and safety due to precipitation and pulverization of lithium in a dendritic state during charging. Cause problems. On the other hand, in a lithium ion secondary battery in which the negative electrode is made of a carbon material, lithium is always present in the form of ions in the battery and is prevented from being precipitated as a metal. Solvable.

[0004] Lithium-ion secondary batteries have high safety, long cycle life, high operating voltage and energy density, can be charged in a short time, and are non-aqueous electrolytes. It is well known that it is rapidly spreading as a small secondary battery. Furthermore, research is also being conducted on the use of large batteries such as batteries for electric vehicles.

[0005] Carbon materials used for the negative electrode of lithium ion secondary batteries include crystalline graphite, graphitizable carbon (soft carbon) which is a precursor of graphite, and non-graphitizable carbon which does not become graphite even at high temperature treatment. There is carbon (hard carbon). Soft carbon and hard carbon can be obtained by heat-treating organic substances such as pitch and resin in an inert atmosphere at about 1000 ° C until the volatile components disappear, but hard carbon has low crystallinity and an amorphous structure. Is a material with on the other hand,
Graphite can be obtained by heat-treating soft carbon at a temperature of about 2500 ° C. or higher. In each case, an electrode (negative electrode) is formed by molding a powdered carbon material using a small amount of a binder (usually an organic resin) and then pressing it on an electrode substrate that serves as a current collector. .

[0006] In a negative electrode made of graphite, lithium ions are absorbed (intercalated) from an electrolyte between layers of graphite crystals having a layered structure during charging, and released (deintercalated) into the electrolyte during discharging. Occur. The amount of lithium ions that can be inserted between the layers is a maximum amount corresponding to C ₆ Li, and the capacity in that case is 372 mAh / g. Therefore, this capacity becomes the theoretical maximum capacity.

On the other hand, it has also been reported that when a carbon material having lower crystallinity is used for the negative electrode, the capacity greatly changes, and in some cases, a capacity exceeding the theoretical maximum capacity (372 mAh / g) of the graphite-based negative electrode material is obtained. Have been. It is considered that since the carbon material has not developed crystals, in addition to the intercalation of lithium ions between layers, lithium ions are intercalated not only between layers but also in parts such as lattice defects of crystals. However, since the carbon material has a lower density than graphite, even if the capacity is higher than graphite, the capacity per unit volume is low, which is disadvantageous in battery applications where the volume is fixed. From the above, it is considered that graphite is more advantageous as a negative electrode material of a lithium ion secondary battery.

In a lithium ion secondary battery using graphite powder as a negative electrode material, generally, the higher the degree of graphitization (ie, the degree of graphite crystallization) of the graphite powder, the greater the Li ion storage capacity and the higher the discharge capacity of the negative electrode material. Increase. As an index of the degree of crystallinity of graphite, usually d ₀₀₂ [graphite crystal plane having a layered structure (002 plane or c plane)
Plane) interlayer distance is used. As this d ₀₀₂ becomes smaller (that is, 3.35 which is the d ₀₀₂ value of an ideal graphite crystal)
The crystallinity of graphite increases as the temperature approaches 4Å.

A negative electrode material having a high degree of crystallinity and a high discharge capacity can be obtained by carbonizing and graphitizing an optically anisotropic mesophase (mesophase small sphere or bulk mesophase) generated by heat treatment of tar or pitch. It has been known. It is considered that the mesophase is because the layered structure is developed, and the layered crystal structure of graphite is easily developed.

Japanese Patent Application Laid-Open No. Hei 7-223808 discloses that a bulk mesophase pitch which is softened and melted under heating is pulverized to 3 to 25 μm, and then heat-treated at 200 to 350 ° C. in air to oxidize the surface layer to make the surface non-conductive. It has been proposed that after melting, heat treatment at 800-3000 ° C. to produce spherical carbon or graphite powder. The reason why the surface layer of the mesophase powder is oxidized and made infusible is to prevent the powder from being fused during the next heat treatment at 800 to 3000 ° C. It is described that the obtained graphite powder has high crystallinity and is spherical, and thus has excellent filling properties.

[0011]

SUMMARY OF THE INVENTION The above-mentioned Japanese Patent Application Laid-Open No. Hei 7-2238.
Graphite powder obtained by graphitizing the mesophase powder according to the method described in JP 08 Publication has a d ₀₀₂ value as an index of crystallinity.
The crystallinity is not sufficiently high. Although this publication does not show the value of the discharge capacity, it is not considered that a high discharge capacity can be obtained from this d ₀₀₂ value.

An object of the present invention is to establish a method for producing a graphite powder capable of forming a negative electrode of a lithium ion secondary battery having a small d ₀₀₂ value and an increased discharge capacity.

[0013]

The present inventors have studied the crystallization of graphite powder used for the negative electrode of a lithium ion secondary battery. As a result, the crystallization of graphite powder, which greatly affects the discharge capacity, is inhibited by oxygen. I decided to be.

That is, the reason that the graphite powder produced by the method described in the above-mentioned Japanese Patent Application Laid-Open No. 7-223808 does not have a sufficiently small d ₀₀₂ value is that the mesophase powder is preliminarily coated on the surface to prevent fusing during carbonization. It is believed that there is at least part of the cause of the oxidation. Oxygen introduced into the powder by this surface oxidation inhibits crystallization during graphitization, so that graphite does not sufficiently crystallize.

Further investigation revealed that surface oxidation during grinding also significantly inhibited crystallization of graphite. If grinding was performed in a non-oxidizing atmosphere to prevent surface oxidation during grinding, the d ₀₀₂ value was small, and It has been found that graphite powder with increased capacity can be obtained with good yield.

Unlike easily oxidizable materials such as hydrogen storage alloys, mesophases and carbonized materials are hardly oxidized at around room temperature.
Grinding was performed in the atmosphere (that is, in an oxidizing atmosphere). However, the surface was oxidized by the pulverization in the atmosphere, and this oxygen inhibited the crystallization of graphite.

Here, the present invention is a method for producing graphite powder for a negative electrode of a lithium ion secondary battery from a mesophase through carbonization, graphitization, and pulverization steps, wherein the pulverization step is performed before and after the carbonization step. And / or after the carbonization step, in a non-oxidizing atmosphere.

Since the present invention prevents the inhibition of graphite crystallization by oxygen, it is preferable that the mesophase of the raw material has not been subjected to surface oxidation treatment for preventing fusion.

[0019]

DETAILED DESCRIPTION OF THE INVENTION The raw material used in the method for producing graphite powder for a negative electrode of a lithium ion secondary battery according to the present invention is mesophase. Mesophase is obtained by heat treating tar and / or pitch. Tar (liquid at room temperature) and its distillation residue, pitch (solid or semi-solid at room temperature), are classified into coal-based and petroleum-based ones, all of which can be used in the present invention. Are preferred. Tar and pitch are remarkably inexpensive compared to resins and are more easily graphitizable than resins, so they are suitable for graphitizing raw materials.

When the tar and the pitch are observed with a polarizing microscope while heating, the pitch is first melted and liquefied,
When the temperature exceeds 400 ° C., optically anisotropic spherical particles appear in the liquid phase. These particles are mesophase microspheres. With continued heating, the amount of mesophase spherules increases and eventually coalesce to form an optically anisotropic matrix, which ultimately becomes entirely optically anisotropic. This matrix material having optical anisotropy or a material having optical anisotropy as a whole is called bulk mesophase.

As the mesophase, both mesophase microspheres and bulk mesophase can be used. However, the mesophase microspheres need to be separated from the optically isotropic matrix by solvent extraction, which requires a large amount of organic solvent for separation, and the matrix is discarded, so that the raw material utilization is low. Therefore, it is industrially preferable to use bulk mesophase as a raw material.

The heat treatment conditions for obtaining the mesophase from the tar and / or pitch are not particularly limited, but are usually at 400 to 600 ° C, preferably 450 to 550 ° C. Since the oil evaporates during this heat treatment, the heat treatment is preferably performed under a reduced pressure of about 10 to 100 Torr in order to promote the volatilization. When heat treatment is performed at atmospheric pressure, it is preferable to perform heat treatment under a flow of an inert gas such as nitrogen gas in order to promote removal of oil and prevent oxidation of the material during the heat treatment.

The heat treatment time is selected so that the desired mesophase formation (formation of mesophase microspheres or bulk mesophase) takes place. Of course, if other conditions are the same, the bulk mesophase generation requires a longer heat treatment time than the generation of mesophase microspheres. However, if sufficient pressure reduction and an appropriate temperature are selected, a bulk mesophase can be obtained in a heat treatment time of several hours.

Prior to this mesophase heat treatment, the starting tar and / or pitch may be pretreated by heating in the presence of a nitrating agent. By this pretreatment, a nitro group is introduced into the aromatic ring of the raw material molecule, and a nitro group condensation reaction occurs (with elimination of the nitro group).
As a result, the raw material is dimerized and further multimerized. That is, the raw material undergoes polycondensation to increase the molecular weight. As a result, the amount of volatiles in the mesophase formation and subsequent carbonization decreases, and the yield of the graphite powder finally obtained increases.

It is conceivable that the nitrating agent is added during the heat treatment for mesophase formation. However, the structure of the mesophase formed by the heat treatment (pattern observed by a polarizing microscope) may be changed. The effect of increasing the yield by increasing the molecular weight can hardly be obtained. Therefore, the treatment with the nitrating agent is preferably performed as a preliminary treatment before the mesophase heat treatment.

Examples of the nitrating agent used in this pretreatment include, but are not limited to, nitric acid, ammonium nitrate, acetyl nitrate, nitrobenzene, nitrotoluene,
Nitronaphthalene and the like can be mentioned. The amount of nitrating agent added generally depends on the starting material (tar and / or pitch)
Is in the range of 0.1 to 15% by weight. If the amount is less than 0.1 wt%, the increase in molecular weight hardly progresses, and if the amount of nitrating agent exceeds 15 wt%, the graphitization will be adversely affected and the crystallinity of the final graphite powder will decrease. Therefore, the discharge capacity decreases. The preferred range of the addition amount of the nitrating agent is
0.5 to 10% by weight, more preferably 1 to 5% by weight.

The heating in the presence of the nitrating agent is performed at a temperature of 300 ° C. or more and less than 400 ° C. In this temperature range, the high molecular weight of the starting material by nitration and polycondensation proceeds in a short time. If the heating temperature is lower than 300 ° C., polycondensation does not proceed sufficiently, and if the heating temperature is higher than 400 ° C., mesophase formation proceeds. Prior to the addition of the nitrating agent, it may be preheated at a temperature in the range of 100-300 ° C. This preheating further promotes nitration of the raw material. The heating time is preferably within several hours as a whole, and in the case of performing preheating, it is often sufficient that any heating be performed within two hours. In order to avoid surface oxidation during heating, the heating is preferably performed under a flow of an inert gas. The introduced nitro group or nitrogen content is completely removed when it is finally graphitized.

From the raw material mesophase, carbonization, graphitization,
Through each of the steps of pulverization and pulverization, a graphite powder can be produced. In the present invention, the grinding is performed in a non-oxidizing atmosphere before and / or after the carbonization step. Grinding is not performed after graphitization. This is because, when pulverized after graphitization, graphite powder having an extremely large specific surface area can be obtained. The specific surface area of the graphite powder affects the charge and discharge efficiency and cycle life of the negative electrode,
The larger the specific surface area, the lower the charge / discharge efficiency and cycle life tend to be.

As described above, the mesophase of the raw material is
Preferably, it has not been subjected to a surface oxidation treatment. However, even in the case of a mesophase subjected to a surface oxidation treatment, when pulverization is performed in a non-oxidizing atmosphere according to the present invention, the pulverization is obtained by starting the same surface oxidation treatment in the mesophase and performing the pulverization in the atmosphere. It is possible to obtain a graphite powder having a higher crystallinity than graphite powder. Therefore, a mesophase subjected to a surface oxidation treatment can also be used as a raw material.

The timing of pulverization may be determined according to the fusibility during carbonization of the raw mesophase. In other words, in the case where the fusion occurs severely during carbonization, it is necessary to pulverize at some point after carbonization, but as described above, pulverization after graphitization is not preferable, and pulverization is performed after the carbonization step. Will be. On the other hand, when no or little fusion occurs during carbonization, it is preferable to grind before carbonization. In this case, it is also possible to pulverize after carbonization.However, pulverization before carbonization facilitates carbonization and reduces the specific surface area during carbonization. A powder is obtained. Of course, pulverization may be performed both before and after carbonization. 2 grinding before graphitization
When the pulverization is performed more than once, it is preferable that all pulverization is performed in a non-oxidizing atmosphere in order to sufficiently achieve the effects of the present invention. However, it is also within the scope of the present invention if at least one milling is performed in a non-oxidizing atmosphere and the other milling is performed in air.

For example, when the raw material is mesophase spherules, it is not necessary to pulverize it before carbonization because it is a powder from the beginning. However, the mesophase spheres, which contain more volatile components than the bulk mesophase, are very likely to fuse during carbonization. (Therefore, commercially available mesophase spheres have been subjected to surface oxidation treatment to prevent fusion. ). Therefore,
For mesophase microspheres that have not been surface oxidized,
It is preferable to pulverize the carbonized material fused after carbonization. Mesophase microspheres that have been subjected to surface oxidation are not preferable as raw materials in the method of the present invention because the degree of surface oxidation is remarkable.

When the raw material is a bulk mesophase, if the volatile content is 15 wt% or less, there is almost no severe fusion during carbonization. Therefore, it is possible to pulverize the graphite powder to the final particle size required before carbonization, and to prevent subsequent pulverization (pulverization in which the average particle diameter substantially changes). Thus, a graphite powder having a very small specific surface area is obtained.

If the bulk mesophase has a large amount of volatiles and considerable fusion during carbonization is unavoidable, it is usually necessary to pulverize the powder by lightly pulverizing it before carbonization, and then pulverize the fused powder again after carbonization. Is preferred. In this case, it is preferable that both of the pulverizations are performed in a non-oxidizing atmosphere as described above, but one of the pulverizations may be performed in the air. In the latter case, grinding before carbonization in the air and grinding after carbonization in a non-oxidizing atmosphere has less adverse effect on the crystallization of graphite.

The average particle size of the graphite powder suitable for the negative electrode material is generally preferably in the range of 5 to 35 μm. Therefore, the pulverization may be performed so that the average particle size falls within this range. The particle size decreases somewhat during carbonization and graphitization, but not significantly. If necessary, a reduction in the particle size during carbonization and graphitization may be determined by experiment, and pulverization may be performed in anticipation of the reduction.

The pulverization can be performed using a suitable pulverizer. For example, a crusher mainly acting on impact or impact / milling such as a hammer mill, a ball mill, a rod mill, or a crusher mainly acting on shear such as a disk crusher can be used. Two or more pulverizers may be used in combination.

In the present invention, the pulverization is performed in a non-oxidizing atmosphere to prevent surface oxidation of the pulverized powder. Grinding in a non-oxidizing atmosphere is performed, for example, by using an inert gas (eg, nitrogen,
Or a rare gas such as argon), pulverize in a chamber with an inert gas flow, pulverize under a slight positive pressure of an inert gas, pulverize in vacuum or reduced-pressure air. Such means can be realized. The pressure of the decompressed air is preferably 100 torr or less.

The carbonization and graphitization may be performed according to a conventional method. Carbonization is a step of almost completely thermally decomposing and removing elements other than carbon, and graphitization is a step of developing a layered crystal structure of graphite. Generally, the temperature required for carbonization is 700
11100 ° C., and the temperature required for graphitization is 2500 ° C. or more. Although it is not impossible to carry out carbonization and graphitization in a single step using the same furnace, the carbonization and graphitization are usually performed in separate steps because the graphitization temperature is extremely high and a special furnace is required.

Both the heat treatments for carbonization and graphitization are generally performed in a non-oxidizing atmosphere. The heat treatment atmosphere is an inert gas (eg, a rare gas such as nitrogen or argon) and a reducing gas.
(Eg, a mixed gas of hydrogen and an inert gas). Since oxidation of carbon causes a decrease in crystallinity and an increase in specific surface area after graphitization, it is preferable to reduce the concentration of oxidizing gas such as oxygen, water vapor, and carbon dioxide in the atmosphere as much as possible. At the graphitization temperature, since a reducing gas such as hydrogen and possibly nitrogen may also react with carbon, the atmosphere for the heat treatment at the time of graphitization is preferably a rare gas such as argon.

The carbonization is generally performed at 700 to 1100 as described above.
C., preferably at a temperature of 800-1000.degree. The carbonization time may be set so that organic substances are substantially completely removed, and is usually in the range of 1 to 50 hours. At the time of this carbonization, decomposition of organic matter occurs and gas is generated. Therefore, it is preferable to perform heat treatment in a heating furnace provided with a gas discharging means such as a fan. Usually, an electric furnace is used as the heating furnace. If fusion occurs during carbonization, grinding is performed after carbonization. If the amount of cohesion is small, classify without grinding
The fused material may be removed by (sieving).

Graphitization is carried out in a high-frequency heating furnace or an Acheson-type resistance heating furnace in which resistance is heated to a high temperature by direct energization of carbon. When a carbon material is heated to 2500 ° C. or more, the carbon is crystallized into graphite. The higher the graphitization temperature, the more the crystallization is promoted, which is desirable. However, if the temperature is too high, the graphite powder will sublime. A preferred graphitization temperature is 2800-3200 ° C,
Graphitization heat treatment time is 0.1 to 10 hours. Since the powder surface is not oxidized by the pulverization performed up to the graphitization step, crystallization is not hindered by oxygen, and a graphite powder having a high crystallinity can be obtained in a relatively short graphitization time.

After carbonization or graphitization, crushing may be performed if necessary. Crushing need not be performed in a non-oxidizing atmosphere. After carbonization and / or graphitization, classification and / or sizing can also be performed for the purpose of removing coarse particles or fine particles or adjusting the particle size.

The method of the present invention allows graphitization with little surface oxidation, and thus without interfering with the crystallization of graphite by oxygen. Therefore, d
₀₀₂ is as small as about 3.360 ° or less, and a graphite powder having a large discharge capacity can be produced. In addition, since surface oxidation also reduces the yield of heat treatment (carbonization or graphitization) performed thereafter, the present invention achieves an improvement in yield by minimizing surface oxidation.

Using the graphite powder produced by the method of the present invention, an electrode can be prepared according to a conventional method, and can be incorporated as a negative electrode in a lithium ion secondary battery. A common method of manufacturing electrodes is to wet or dry graphite powder with a small amount of a suitable binder (e.g., polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, hexafluropolypropylene, polyvinyl alcohol, carboxymethyl cellulose, etc.). An electrode substrate that is formed in a dry process and becomes a current collector (eg,
(Metal foil such as copper foil). In the case of wet molding, a method of screen-printing or applying a slurry on an electrode substrate, and pressing the roll to consolidate the slurry is common. In the case of dry molding, a method of separately molding by hot pressing or the like and then thermocompression bonding to the electrode substrate can be adopted. An electrode can also be produced by using the powder produced by the method of the present invention together with another graphite powder.

[0044]

EXAMPLES (Example 1) Coal tar was placed in a vacuum distillation apparatus, and concentrated nitric acid in an amount shown in Table 1 was added as a nitrating agent.
The mixture was heated at 350 ° C. for 1 hour under a reduced pressure of 80 Torr with stirring to increase the molecular weight by polycondensation. While stirring the obtained pitch-like raw material in the same distillation apparatus without cooling,
Perform a mesophase heat treatment at 500 ° C under reduced pressure of rr,
A bulk mesophase with a volatile content of 10 wt% was obtained.

The obtained bulk mesophase was pulverized in a non-oxidizing atmosphere at 8000 rpm using a hammer mill placed in an appropriate chamber so that the average particle size became about 15 μm. The non-oxidizing atmosphere was achieved by closing the chamber and introducing nitrogen gas, or by flowing nitrogen gas or argon gas at a flow rate of 1 l / min. For comparison, pulverization was performed in the atmosphere.

Each of the bulk mesophase powders obtained by these pulverizations was placed in an electric furnace through which nitrogen gas was passed, and heated at 50 ° C.
The mixture was heated to 1000 ° C. at a heating rate of / hr, and kept at that temperature for 5 hours to carbonize. The weight of the obtained carbonized material was measured, and the carbonization yield was calculated as a ratio to the charged amount of the mesophase.

Since this carbonized material was not fused, it was transferred to a graphitization furnace (a resistance heating furnace using a graphite heater) and heated to 3000 ° C. at a rate of 10 ° C./min in an argon atmosphere. This temperature was maintained for 1 hour to graphitize. The d ₀₀₂ of the resulting graphite powder was obtained in the same manner as International Publication No. WO98 / twenty-nine thousand three hundred thirty-five from powder method X-ray diffraction pattern of the powder.

This graphite powder was used for producing an electrode by the following method. 90 parts by weight of graphite powder and 10 parts by weight of polyvinylidene fluoride powder were mixed in N-methylpyrrolidone as a solvent to form a paste. The obtained paste-like negative electrode material is applied to a uniform thickness using a doctor blade on a 20-μm-thick copper foil of an electrode substrate, dried, compressed by a cold press of 1 ton / cm ² , and then vacuumed. Dry at 120 ° C. A test piece having an area of 1 cm ² cut out from this was used as an electrode (negative electrode).

The evaluation of the negative electrode characteristics was performed by a three-electrode constant current charge / discharge test using lithium metal as a counter electrode and a reference electrode.
As the electrolyte, a non-aqueous solution in which LiClO ₄ was dissolved at a concentration of 1 M in a mixed solvent of ethylene carbonate and dimethyl carbonate at a volume ratio of 1: 1 was used. Li this cell, the negative electrode during charging to 0.0 V vs. Li reference electrode at a current density of 0.3 mA / cm ²
Is stored, and 1.50 with respect to the Li reference electrode at the same current density.
Charge / discharge cycle to discharge (discharge Li ions) to V
10 cycles were performed. The average value of 9 discharge capacities in 2 to 10 cycles was taken as the discharge capacity. The results obtained are shown in Table 1 together with the carbonization yield and the value of d ₀₀₂ of the graphite powder.

[0050]

[Table 1]

As can be seen from Table 1, according to the present invention,
When the mesophase is crushed in a non-oxidizing atmosphere, d
₀₀₂ was as small as about 3.360 ° or less (ie, high in crystallinity), and a graphite powder having a high discharge capacity of 340 mAh / g or more was obtained. The carbonization yield was as high as 91.7 to 91.9%.

On the other hand, in the comparative example in which the mesophase was pulverized in the air, d ₀₀₂ was as large as 3.3634 nm, and the discharge capacity was as low as 315 mAh / g. Furthermore, the weight loss during carbonization is large due to surface oxidation, and the carbonization yield is 87.6%.
This is about 4% lower than in the case of the present invention.

(Example 2) Bulk mesophase obtained by heat treatment of coal tar pitch at 500 ° C without nitration treatment
(Volatile content 23 wt%) was pulverized by a hammer mill at 5000 rpm in a nitrogen stream, and carbonized and graphitized in the same manner as in Example 1. However, since the powder was fused after carbonization,
Using a hammer mill of 8000 rpm, pulverization was performed in a nitrogen stream, an argon stream, or the atmosphere to an average particle size of about 15 μm, and then graphitized. Table 2 shows the test results of the obtained graphite powder.

[0054]

[Table 2]

When pulverization is performed after carbonization, similarly to Example 1 in which pulverization is performed before carbonization, if the pulverization atmosphere is a non-oxidizing atmosphere, the carbonization yield is high, d ₀₀₂ is small, and the discharge capacity is high. Was.

[0056]

According to the present invention, inexpensive tar and / or
Or, using the mesophase obtained from the pitch as a raw material,
Graphite powder for a negative electrode of a lithium ion secondary battery having a high crystallinity and thus providing a high discharge capacity can be produced with a high yield.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Takeshi Shiro 1-2-2, Ikenoba, Taito-ku, Tokyo Sumitomo Metal Industries, Ltd. Electronic Components Division (72) Inventor Masayuki Nagamine Character of Takakura, Hiwadacho, Koriyama-shi, Fukushima 1-1 Shimosugishita, Sony Energy Tech Koriyama Factory (72) Inventor Atsushi Komaru 1-1, Takakura, Hiwada-cho, Koriyama-shi, Fukushima Prefecture Sony Energy Tech Koriyama Factory (72) Invention Person Yusuke Fujishige 1-1 Shimosugishita, Takakura, Hiwada-cho, Koriyama-shi, Fukushima Prefecture F-term in Sony Energy Tech Koriyama Factory 4G046 EA02 EB01 EB04 EC02 EC06 5H003 AA02 BA01 BA04 BB01 BC01 5H014 AA01 BB01 EE08

Claims (2)

[Claims] 1. A method for producing graphite powder for a negative electrode of a lithium ion secondary battery from a mesophase through carbonization, graphitization, and pulverization steps, wherein the pulverization step is performed before and / or after the carbonization step. A method characterized in that the method is performed in a non-oxidizing atmosphere. 2. The method according to claim 1, wherein the mesophase is a mesophase that has not been subjected to a surface oxidation treatment.