JP2001023634A - Manufacture of graphite powder for lithium ion secondary battery negative electrode - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance discharge capacity and charge/discharge efficiency by carbonizing and graphitizing a powder of a carbon powder having a specific particle diameter mixed with a mesophase powder. SOLUTION: By mixing a carbon powder with a mesophase powder to be carbonized, the fusion during the carbonization is suppressed. Since the carbon powder does not contain volatile matter, the fusion of the mesophase powder is significantly reduced when the carbon powder is mixed to lie between the mesophase powders. The carbon powder is to have an average primary particle diameter of 1 μm or above, preferably 10-100 μm. Additionally, a ratio of the carbon powder to the total of the carbon powder and the mesophase powder is preferably 5-40 wt.%. As the carbon powder, a carbonized powder obtained by carbonizing a mesophase and a graphitized powder obtained by carbonizing and graphitizing of the mesophase are preferably used. Furthermore, the mesophase as a raw material is preferably one not subject to the surface oxidization.

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 graphite powder used for a negative electrode of a lithium ion secondary battery which is a non-aqueous secondary battery. According to the present invention, the specific surface area is small,
Accordingly, graphite powder for a negative electrode of a lithium ion secondary battery having excellent charge / discharge efficiency and good discharge capacity can be manufactured with high yield.

[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 any case, an electrode (negative electrode) is formed by molding a powdered material usually using a small amount of a binder (usually an organic resin) and pressing the material onto an electrode substrate serving 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 as a negative electrode, generally, the higher the degree of graphitization of the negative electrode (that is, the degree of crystallinity of graphite), the larger the Li ion storage capacity and the greater the discharge capacity of the negative electrode material. Are known. As an index of the degree of crystallinity of graphite, d ₀₀₂ [interlayer distance between graphite crystal planes (002 plane or c plane) of a layered structure] is usually used. The smaller this d ₀₀₂ is (that is, the ideal d ₀₀₂ value of the graphite crystal)
The higher the crystallinity of graphite (closer to 0.3354 nm).

It is known that a negative electrode material having a high crystallinity and a high discharge capacity can be obtained by carbonizing and graphitizing an optically anisotropic mesophase produced by heat treatment of tar or pitch (for example, See Japanese Unexamined Patent Publication Nos. 7-223808 and 7-226204).

For example, JP-A-7-223808 discloses that a bulk mesophase pitch which is softened and melted under heating is 3 to 25.
It is proposed to produce carbon or graphite powder by pulverizing to a size of μm, heat-treating in air at 200 to 350 ° C to oxidize the surface layer to make the surface infusible, and then heat-treating to 800 to 3000 ° C. .

Generally, it is advantageous that the specific surface area of the carbon material of the negative electrode is as small as possible. If the specific surface area is large, the reactivity with the electrolytic solution increases, and the charge / discharge efficiency and the cycle life tend to decrease.

As a method of producing carbon or graphite powder for a negative electrode of a lithium ion secondary battery having a small specific surface area from mesophase, Japanese Patent Application Laid-Open No. Hei 10-3922 discloses a method in which the amount of mesophase is 80% by weight or more and the volatile content is 25% by weight or less. After pulverizing the bulk mesophase pitch of the above to an average particle diameter of 3 to 20 μm, it is lightly oxidized so as to have an oxygen content of 2 to 8% by weight and made infusible, and then pressed and molded to obtain a molded product. Is further oxidized, if necessary, and then carbonized or graphitized, followed by grinding and sizing.

[0013]

SUMMARY OF THE INVENTION The above-mentioned JP-A-7-22
In both 3808 and 10-3922, in order to prevent the mesophase powder from fusing during carbonization, the surface of the mesophase powder must be lightly oxidized before carbonization to make the surface infusible. There is. This is because if particles are fused during carbonization, the particle size cannot be maintained, and it will be necessary to perform grinding again at the end. When the final pulverization is performed as described above, the specific surface area of the powder is significantly increased due to intragranular fracture, which has an adverse effect on charge / discharge efficiency and cycle life.

[0014] The infusibilization due to the surface oxidation described above is generally carried out for the same purpose even for mesophase spherules containing a larger amount of volatile components and more remarkably fused. It is effective for preventing fusion.
However, since the step of oxidation treatment is added separately, the number of steps is increased, and it is not always easy to control the degree of surface oxidation reliably, and the mesophase powder may be excessively oxidized. Excessive oxidation changes the crystal structure of the mesophase powder, so that the crystallinity of the finally obtained graphite powder is significantly reduced, resulting in a graphite powder having a reduced discharge capacity.

Further, the present inventors have found that even if the surface oxidation is not excessive, the oxygen introduced into the powder by the surface oxidation adversely affects the crystallinity of the graphite powder. Surface oxidation also reduces the carbonization yield.

Accordingly, it is advantageous to prevent fusing without utilizing surface oxidation. An object of the present invention is to provide a graphite powder having a high degree of crystallinity and a small specific surface area without performing an oxidation treatment for preventing fusion during carbonization, and thus having excellent discharge capacity and charge / discharge efficiency. The goal is to develop a method that can be manufactured.

[0017]

Means for Solving the Problems As a result of repeated studies to solve the above problems, the present inventors have found that a mesophase powder is mixed with a carbon powder such as a carbonized powder or a graphitized powder and carbonized and graphitized. It has been found that graphite powder having a small specific surface area can be obtained with high yield while minimizing fusion.

Here, the present invention provides a method for producing graphite powder for a negative electrode of a lithium ion secondary battery, characterized by carbonizing and graphitizing a mixed powder obtained by mixing mesophase powder with carbon powder having an average primary particle diameter of 1 μm or more. Is the way.

The mesophase powder is preferably not subjected to a surface oxidation treatment. The carbon powder has an average primary particle diameter of 10 to 100 μm, and the ratio of the carbon powder in the mixed powder is 5%.
It is preferably about 40% by weight. The carbon powder may be either carbonized powder or graphitized powder, and is preferably obtained through carbonization of mesophase or carbonization and graphitization.

In the method of the present invention, it is preferable that substantial pulverization is not performed after carbonization. "Substantial grinding" means grinding that causes intragranular fracture, and crushing is "substantial grinding".
Is not included.

[0021]

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 a mesophase powder. Mesophase is obtained by heat treating tar and / or pitch. Tar (liquid at normal temperature) and its distillation residue, pitch (solid or semi-solid at normal temperature), are classified into coal-based and petroleum-based ones, and both 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 graphitization applications.

Observation with a polarizing microscope while heating the tar and pitch shows that the pitch first melts and liquefies,
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 powder as a raw material, both mesophase microspheres and powder obtained by pulverizing 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, industrially, it is preferable to use a bulk mesophase pulverized powder as a raw material.

The heat treatment conditions for obtaining the mesophase from the tar and / or the 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 the 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, this may change the structure of the mesophase formed by the heat treatment (pattern observed by a polarizing microscope). 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 a few hours in total, and in the case of performing preliminary heating, it is often sufficient to perform any heating within one hour. 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.

When the mesophase is a bulk mesophase, it is pulverized into a mesophase powder. In the case of mesophase spheres, they can be used without crushing, but may be crushed if desired. In a preferred embodiment of the present invention, since substantial pulverization is not performed after carbonization, pulverization is performed at this stage to obtain a desired particle size for the graphite powder as a final product. The average particle size of the preferred graphite powder for the negative electrode of the lithium ion secondary battery is generally in the range of 5 to 35 μm, so that the pulverization may be performed so that the average particle size of the bulk mesophase is in this range.

According to the present invention, the fusion of the mesophase powder during carbonization can be minimized by mixing the carbon powder, so that it is possible to pulverize to the final particle size before carbonization.
The particle size varies somewhat due to the removal of volatiles during carbonization and crystallization during graphitization, but not so large. If necessary, the variation in particle size during carbonization and graphitization may be determined by experiment, and the mesophase may be pulverized in consideration of the variation.

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.

As described above, the mesophase of the raw material is
Preferably, it has not been subjected to a surface oxidation treatment. That is, not only bulk mesophase pulverized powder but also mesophase small spheres are preferably not subjected to surface oxidation treatment.

When the mesophase powder is heat-treated and carbonized and graphitized, graphite powder is obtained. 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. At the time of carbonization, a large amount of gas is generated as volatile components are removed from the mesophase powder. At this time, at least a part of the volatile component is in a liquid state, so that the mesophase powder has a very small volatile component or a surface oxidation treatment (this has a negative effect on the crystallization of graphite as described above). Except for, fusion of the mesophase powder occurs, and a fused carbonized material is obtained.

In the present invention, the carbon powder is mixed with the mesophase powder and carbonized to suppress fusion during carbonization. Since carbon powder does not contain volatile matter, it has been found that when carbon powder is mixed and carbon powder is interposed between mesophase powders, fusion of the mesophase powder is significantly reduced. Carbon powder is carbonized powder (powder made of ungraphitized carbon)
And graphitized powder, and both may be used in combination.

The carbon powder to be mixed has an average primary particle diameter of 1 μm or more. If the average primary particle size is smaller than this,
Because the powder is too fine, the specific surface area of the graphite powder obtained after the graphitization is significantly increased, and the charge / discharge efficiency is significantly reduced. In addition, the effect of suppressing fusion is small, the graphitization property is also reduced, and the discharge capacity is also significantly reduced. . The average primary particle diameter of the carbon powder to be mixed is preferably 10 to 100 μm, more preferably 25 to 70 μm.
m.

Since the carbon powder to be mixed undergoes graphitization at the end, it is preferable that the carbon powder be highly graphitizable. In this sense, carbonized powder obtained from mesophase which is easily graphitizable, that is, carbonized powder obtained by carbonization of mesophase and graphitized powder obtained by carbonization and graphitization of mesophase are preferable carbon powders. However, other carbon powders such as natural graphite, conventional ordinary artificial graphite, carbonized powder obtained by carbonizing a resin, and crushed carbon fiber can also be used. However, carbon black is not suitable because the average primary particle diameter is smaller than 1 μm.

Preferable examples of the carbon powder include the following carbon powders: pulverized powder of bulk mesophase and / or carbonized powder of carbonized mesophase spheroid, pulverized powder of bulk mesophase and / or mesophase spheroid Carbonized and graphitized graphitized powder, after forming bulk mesophase, carbonized and pulverized carbonized powder, after forming bulk mesophase, carbonized and pulverized,
After forming graphitized graphitized powder, bulk mesophase, carbonized, graphitized, pulverized graphitized powder, pulverized powder of bulk mesophase and / or on a sieve in the classification process after carbonization of mesophase spheroids or graphitization Coarse carbonized or graphitized powder.

Of these, and are particularly preferred. These carbon powders are powders generated during the production process of the graphite powder of the present invention, and there is no need to prepare a carbon powder for mixing from the outside. That is, and
It is only necessary to recycle part of the carbonized product or graphitized product obtained in the carbonization or graphitization step to the carbonization step. Means that coarse carbonized powder or graphitized powder remaining on the sieve in the classification performed after the carbonization or graphitization step is recycled to the carbonization step. These coarse powders can be recovered as a product by re-grinding, but this increases the specific surface area of the product. Therefore, especially for graphitized powder,
It is preferable to mix without pulverization, but if the amount is small, pulverization may be performed before mixing. In the case of carbon powder,
It may be ground.

The mixing ratio of the carbon powder to be mixed may be an amount necessary for preventing fusion of the mesophase powder during carbonization. This amount depends on the strength of the fusion of the mesophase powder and, therefore, the volatile content of the mesophase powder, and it is preferable to increase the proportion of the carbon powder as the volatile content increases. Normally, the ratio of the carbon powder to the total of the mesophase powder and the carbon powder is preferably in the range of 5 to 40 wt%. If the carbon powder content is less than 5 wt%, the effect of preventing fusion during carbonization is small. The effect of preventing fusion by mixing carbon powder saturates when the blending amount of carbon powder is about 40 wt%, but it may be blended in a larger amount, and there is almost no adverse effect on negative electrode characteristics. However, since the amount of carbonization at one time is reduced by the amount of the carbon powder, it is not preferable to increase the amount of the carbon powder too much.

The mesophase powder and the carbon powder are mixed as uniformly as possible. This mixing can be performed by a mixer such as a V-type mixer, a drum mixer, and an axial mixer. The obtained mixed powder is heat-treated by carbonization and graphitization to obtain a graphite powder. The heat treatment for carbonization and graphitization may be performed in the same manner as in the prior art. Generally, the temperature required for carbonization is 700-11
00 ° C., and the temperature required for graphitization is 2500 ° C. or more.
It is not impossible to perform carbonization and graphitization in a single process using the same furnace, but the graphitization temperature is extremely high,
Usually, it is performed in a separate process because a special furnace is required.

The heat treatment for carbonization and graphitization is usually 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 performed at a temperature of 700 to 1100 ° C., preferably 800 to 1000 ° C. as described above. The carbonization time is
What is necessary is just to set so that elements other than carbon may be removed almost completely, and it is usually in the range of 1 to 50 hours. During this carbonization, a gas is generated from the volatile matter, so that it is preferable to perform a heat treatment in a heating furnace equipped with a gas discharging means. Usually, an electric furnace is used as the heating furnace.

In the present invention, since carbon powder is blended with mesophase powder and carbonized, fusion of the powder during carbonization is suppressed,
There is no need to grind after carbonization. If some fusion is observed, the fusion powder remaining on the sieve may be removed by classification. This fused powder can be used as carbon powder to be blended with the mesophase powder. Since it is not pulverized after carbonization, graphite powder having a small specific surface area can be obtained, and the charge / discharge efficiency and cycle life are improved.

Graphitization is performed 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. A higher graphitization temperature is desirable because crystallization is promoted, but if the temperature is too high, the graphite powder will sublime. The preferred graphitization temperature is 2800-3200 ° C., and the graphitization heat treatment time is 0.1-10 hours.

If the mesophase is obtained from a nitrated tar and / or pitch, the nitrogen content introduced by the nitrating agent is completely removed during the graphitization. There is no adverse effect on the characteristics of the negative electrode material due to the treatment. On the other hand, if the heat treatment is stopped only by carbonization without graphitizing the mesophase, the negative electrode using the obtained carbonized material will have a remarkable decrease in charge / discharge efficiency.

When the fused powder is formed even after graphitization, it is also removed by classification, and the removed fused powder can be blended with the mesophase powder before carbonization as carbon powder for preventing fusion. . After carbonization and graphitization, pulverization by light pulverization may be performed. Such pulverization is not included in the “substantial pulverization” in the present invention.

According to the method of the present invention, since the fusion during carbonization is small, the carbonized powder which has been crushed as necessary after carbonization and classified is reduced in the proportion of coarse powder remaining on the sieve. By directly graphitizing the powder without pulverization, graphite powder can be produced with a high yield. In addition, in the present invention, the coarse powder can be recycled to the carbonization step as carbon powder for preventing fusion, thereby further improving the yield. Such coarse powder is liable to be clogged during the transportation of carbonized powder, which is generally performed by suction through a pipe, and causes a reduction in work efficiency.However, such classification is caused by classification as described above. It becomes difficult. The produced graphite powder uses mesophase as a raw material, so it has high crystallinity,
Good discharge capacity. Furthermore, since there is little fusion during carbonization and pulverization is not required after carbonization, the specific surface area is small, and charge / discharge efficiency and cycle life are excellent.

Using the graphite powder produced by the method of the present invention, an electrode can be prepared according to a conventional method and incorporated into a lithium ion secondary battery as a negative electrode. 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.

[0050]

Example: A bulk mesophase obtained by heat-treating coal tar pitch was pulverized by a hammer mill, and classified by a 325 mesh sieve (mesh size: 45 μm) to have an average particle size of about 25 μm.
Of mesophase powder was obtained. The volatile content of this mesophase powder was about 18% by weight.

An electric furnace in which a nitrogen powder is flown into this mesophase powder by mixing a carbon powder of any of the following A to E (average primary particle diameter is described in Table 1) with a drum mixer or without mixing: 1000 ℃ at a heating rate of 50 ℃ / hr
And kept at 1000 ° C. for 5 hours to carbonize.

Mixed carbon powder A: carbon powder obtained by pulverizing bulk mesophase and carbonizing it.
(Pulverized after carbonization) B: Carbon powder obtained by pulverizing bulk mesophase, carbonized and graphitized at 3000 ° C, (crushed before graphitization after carbonization) C: Filling powder obtained by pulverizing bulk mesophase into rubber rubber, 1 ton / cm ² , a powder obtained by carbonizing, graphitizing and pulverizing a molded product, D: artificial graphite powder, E: carbon black.

The obtained carbonized powder was dispersed on a preparation with alcohol or the like, and observed at a magnification of 20 to confirm the presence or absence of fusion. The small, medium and large carbonized powders were crushed by reducing the rotation speed of the hammer mill to 1/10 of the grinding time. In addition, the carbonized powder without fusion and the crushed carbonized powder other than that were classified with a 325 mesh sieve, and the ratio on the sieve was determined.

The carbonized powder under the sieve was transferred to a graphitization furnace (resistance heating furnace using a graphite heater), and was heated at 10 ° C. in an argon atmosphere.
The temperature was raised to 3000 ° C. at a rate of / min, and maintained at this temperature for 1 hour to graphitize to obtain a graphite powder. The specific surface area of the graphite powder was determined by a one-point BET measurement method using the N ₂ substitution method.

The negative electrode characteristics of the obtained graphite powder were examined as follows. After pulverizing the graphite powder in the same manner as described above, 90 parts by weight of the graphite powder and 10 parts by weight of the 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 the thickness of the electrode substrate.
It is applied to a uniform thickness on a 20μm copper foil using a doctor blade, dried and compressed by a 1 ton / cm ² cold press.
Dry at 120 ° C. in vacuo. 1cm area cut out from here
The test piece of No. ² 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, and the average value of 9 discharge capacities of 2 to 10 cycles was defined as the discharge capacity. The ratio of the amount of electricity at the time of discharge to the amount of electricity required for charging in the first charge / discharge (%)
And the charge / discharge efficiency was calculated. Based on these results, the type of carbon powder mixed, the average primary particle size and blending amount, the degree of fusion after carbonization, necessity of crushing, the ratio of carbonized powder on a 325 mesh sieve, and the ratio of graphite powder The results are shown in Table 1 together with the surface area values.

[0057]

[Table 1]

As can be seen from Table 1, when only the mesophase powder was carbonized without mixing the carbon powder as in Comparative Example 2, the fusion during carbonization was severe, and even after crushing,
As much as 25% of coarse powder remained on the 5-mesh sieve, reducing the yield.

According to the present invention, when carbon powder having no fusion property is blended with mesophase powder and carbonized, fusion of the mesophase powder during carbonization can be completely prevented or greatly suppressed, and carbonization after carbonization can be prevented. It was possible to obtain a graphite powder having a small specific surface area, a high charge / discharge efficiency and a high discharge capacity, without the necessity of crushing or without crushing after carbonization.

The average primary particle diameter of the mixed carbon powder is 10 μm
If it is as small as less than 100 μm or as large as more than 100 μm, fusion is likely to occur. Further, when a carbon powder having a small average primary particle diameter was blended, the specific surface area of the graphite powder was increased, and the charge / discharge efficiency was reduced. Even if the average primary particle diameter of the carbon powder was as large as 40 μm, for example, the effect of preventing fusion was high, but the amount of coarse powder on a 325 mesh sieve increased. However, if this is used as carbon powder mixed with mesophase powder according to the present invention, there is almost no adverse effect on the yield.

Even if the compounding amount of the carbon powder was as small as 5 wt%, some effects were obtained. Also, when the blending amount is increased to 40 wt%, the ratio on the sieve is 0% (that is, fusion is completely prevented).
It became. The effect is the same when the blending amount is further increased, but there is no effect on the negative electrode characteristics.

When the carbon powder was artificial graphite, the effect of preventing fusion during carbonization was high, but the graphitizability (crystallinity) of artificial graphite was not so high, and the specific surface area was large. The discharge capacity decreased, and the specific surface area increased and the charge / discharge efficiency decreased.

In Comparative Example 1 in which the carbon powder was carbon black having an average primary particle diameter much smaller than 1 μm, fusion was hardly suppressed, and the obtained graphite powder had a very large specific surface area, and the charge / discharge efficiency was low. It became very bad, and the discharge capacity was greatly reduced.

[0064]

According to the present invention, inexpensive tar and / or
Or, using the mesophase powder obtained from the pitch as a raw material, the crystallinity is good, the discharge capacity is high, the specific surface area is small, the charge and discharge efficiency is improved, and the graphite powder for the negative electrode of the lithium ion secondary battery is stably obtained. It can be manufactured with a high yield.

Continued on the front page (72) Inventor Toru Fujiwara 1-2-18 Ikenohata, Taito-ku, Tokyo Sumitomo Metal Industries, Ltd. Electronic Components Division (72) Inventor Masayuki Nagamine 1 Shimosugishita, Takakura, Hiwada-cho, Koriyama-shi, Fukushima Prefecture -1 Inside Sony Energy Tech Koriyama Factory (72) Inventor Atsushi Komaru 1-1 Shimosugishita Takakura, Hiwadacho, Koriyama-shi, Fukushima Prefecture Inside Sony Energy Tech Koriyama Factory (72) Inventor Yusuke Fujishige 1-1, Shimosugishita, Takakura, Hiwada-cho, Koriyama-shi, Fukushima Prefecture F-term in Sony Energy Tech Koriyama Factory 4G046 EA02 EA05 EA06 EB01 EB13 EC02 EC06 4H058 FA40 GA16 HA03 HA13 5H003 AA02 AA08 BA01 BA03 BB01 BC01 BC06 BD02 BD04 5H014 AA01 BB01 BB06 EE08 HH00 HH01 HH06

Claims (6)

[Claims] 1. An average primary particle diameter of 1 μm in a mesophase powder.
A method for producing graphite powder for a negative electrode of a lithium ion secondary battery, comprising carbonizing and graphitizing a mixed powder obtained by mixing the above carbon powder. 2. The method for producing graphite powder for a negative electrode of a lithium ion secondary battery according to claim 1, wherein the mesophase powder has not been subjected to a surface oxidation treatment. 3. The carbon powder has an average primary particle diameter of 10 to 100.
The method for producing a graphite powder for a negative electrode of a lithium ion secondary battery according to claim 1, wherein the particle size is μm. 4. The ratio of the carbon powder in the mixed powder is 5 to 40.
The method for producing graphite powder for a negative electrode of a lithium ion secondary battery according to any one of claims 1 to 3, which is wt%. 5. The lithium ion secondary battery negative electrode according to claim 1, wherein the carbon powder is a carbonized powder or a graphitized powder obtained through carbonization or carbonization and graphitization of mesophase. Of producing graphite powder for use. 6. No substantial pulverization is performed after the carbonization.
A method for producing the graphite powder for a negative electrode of a lithium ion secondary battery according to claim 1.