JPH10226505A - Graphite powder for lithium secondary battery and method for producing the same - Google Patents

Abstract

(57) [Problem] To provide a graphite powder suitable for a negative electrode material of a lithium secondary battery, which can produce a lithium secondary battery having a high discharge capacity of 330 mAh / g or more. SOLUTION: The c-plane layer has a closed closed structure in which the end of a c-plane layer is loop-shaped on the surface, and the density of the gap surface between the closed structures in the graphite c-axis direction is 100 to 1500 / μm.
A graphite powder having a c-axis (002) plane lattice spacing d002 of 0.33700 nm or less obtained by a precise measurement method of lattice constants by X-ray diffraction is obtained by graphitizing a carbon material obtained by high-speed pulverization before or after carbonization or carbonization. The carbon material pulverized before or after is subjected to an oxidizing heat treatment at a temperature of 600 to 800 ° C. after graphitization, and then to a heat treatment at a temperature of 800 ° C. or more in an inert gas.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a graphite powder having a novel structure, which can form a lithium secondary battery having a high discharge capacity and is suitable as a negative electrode material of the lithium secondary battery. The present invention also relates to a negative electrode material for a lithium secondary battery comprising the graphite powder.

[0002]

2. Description of the Related Art A lithium secondary battery uses lithium as a negative electrode active material and an oxide or chalcogenide (eg, sulfide, selenide) of a transition metal as a positive electrode active material, and uses an aprotic organic solvent as an electrolyte. (eg, alkylene or dialkyl carbonate, an ether solvent, sulfolane, nitrile solvents, nitromethane, amide solvents, ester solvents, such as glycols) an inorganic or organic lithium salt (example, LiClO _4, LiBF _4, LiCl, It is a type of non-aqueous secondary battery using a solution in which LiCF ₃ SO ₃ is dissolved.

[0003] Since lithium is a very base metal, a lithium secondary battery can easily extract a large voltage. For this reason, lithium secondary batteries have attracted attention in recent years as secondary batteries with high electromotive force and energy density, and are used as dispersed or portable batteries in a wide range of fields such as electronic devices, electric devices, electric vehicles, and power storage. Are expected, and some of them have already been put into practical use.

[0004] Early lithium secondary batteries used foil-shaped lithium metal alone as a negative electrode active material. in this case,
The charge-discharge reaction proceeds by dissolution (ionization) and precipitation of Li. However, in the reaction of Li + → Li during charging, metallic Li
Tend to precipitate in the form of needles, so if charge and discharge are repeated, dendritic Li dendrites precipitate on the electrode surface. When this dendritic Li dendrite grows, it may penetrate the separator (partition wall), which causes a short circuit with the positive electrode, shortening the life of the charge / discharge cycle. was there.

To solve this problem, Li is alloyed.
(E.g., Li / Al alloying) has been considered, but the most promising solution at present is to use a material in which lithium is stored in a carbon material such as graphite as the negative electrode active material. . In this case, the negative electrode material may be substantially a carbon material only. During charging, Li ions in the electrolytic solution are taken in between the layers of the carbon material by doping, occlusion, insertion (intercalation), and the like. Released from the carbon material into the electrolyte. That is, charge and discharge (electrode reaction) occur by taking in and releasing Li ions. Therefore, this type of battery is also called a lithium ion secondary battery.

[0006] It is known to use natural graphite or artificial graphite ground as a carbon material, or optically anisotropic mesophase spheres generated during the heating process of pitch. An electrode produced by mixing such a powdery carbon material with a small amount of a binder (binder) and a solvent and molding the mixture is used as a negative electrode of a lithium secondary battery.

The theoretical capacity of a lithium secondary battery using a metal Li alone as a negative electrode active material is as high as about 3800 mAh / g. In contrast, the theoretical capacity of a lithium secondary battery composed of a negative electrode active material containing lithium in a carbon material is C ₆ Li, an intercalation compound in which lithium is regularly and densely stored between graphite layers.
Is about 372 mAh / g when is used as the negative electrode active material.

However, in the actual carbon material negative electrode, since the carbon material has surface active sites that prevent the intrusion of Li ions and a dead region for storing Li ions, the theoretical capacity of C ₆ Li is 372. It is extremely difficult to achieve Ah / kg, and various proposals have been made on methods for producing negative electrode carbon materials in order to approach this theoretical capacity as much as possible.

For example, Japanese Patent Application Laid-Open No. 7-282812 attempts to increase the capacity of a lithium secondary battery by increasing the regularity of the lamination arrangement of graphite layers in graphitized carbon fibers. According to this publication, when carbon fibers are pulverized, undesirable structural defects different from the regular arrangement of the graphite layers of the original carbon fibers are introduced, and in order to increase the capacity of the lithium secondary battery, the arrangement of the graphite layers must be increased. It is described that it is advantageous to increase the regularity of However, even if the regularity of the graphite layer stacking is increased in this way, the discharge capacity of the lithium secondary battery is at most 316 mAh / g, and the high capacity graphite-based negative electrode coal exceeds 330 mAh / g. You can't get the material.

[0010]

SUMMARY OF THE INVENTION An object of the present invention is to provide a lithium secondary battery having a high discharge capacity because there is no danger of a decrease in cycle life or short circuit due to precipitation of Li dendrite and a large amount of stored Li ions. It is an object to provide a graphite powder suitable as a negative electrode material of a lithium secondary battery and a method for producing the same. A specific goal of the present invention is at least 310 mAh / g, preferably 330 mAh / g
g or more, depending on the conditions, the theoretical capacity of a lithium secondary battery using carbon
2 mAh / g).

[0011]

Means for Solving the Problems The present inventors systematically investigated the relationship between the microscopic structure and charge / discharge characteristics of graphite powder used as a negative electrode material of a lithium secondary battery, and calculated theoretical calculations. As a result of various analyses, it has been found that the graphite powder changes into a “closed structure” in which the end of the c-plane layer is closed in a loop by an appropriate treatment. This closed structure has several layers of c-plane layers as schematically shown in FIG.
(Accurately, even-numbered layers), and a "gap surface" with an open c-plane is formed at the interface between two adjacent unit closing structures.

The closed structure of the graphite powder can be actually confirmed by observing the vicinity of the surface of the powder cross section obtained by cutting the graphite powder in a direction parallel to the c-axis with a high-resolution transmission electron microscope. One example of such an electron micrograph is shown in FIG.

As a result of further studies by the present inventors, the density of the above-mentioned interstitial plane (ie, the density of the unit closed structure) and the lattice spacing (d002) of the c-axis (002) plane in the graphite powder were found to be smaller than that of the lithium secondary battery. Greatly affect the charge and discharge characteristics of
The present inventors have found that these conditions can be controlled by pulverization before graphitization heat treatment, graphitization heat treatment conditions, and heat treatment under specific conditions thereafter, and have reached the present invention.

Here, the present invention relates to a graphite powder having a closed structure in which the end of a graphite c-plane layer is closed in a loop on the powder surface, wherein the gap surface between the closed structures in the graphite c-axis direction is formed. A graphite powder having a density of 100 to 1500 particles / μm and a c-axis (002) plane lattice spacing (d002) of 0.33700 nm or less determined by a precise measurement of lattice constants by X-ray diffraction. is there. According to the present invention, a negative electrode material for a lithium secondary battery comprising the graphite powder is also provided.

[0015] The graphite powder of the present invention comprises: "A carbon material which has been subjected to high-speed pulverization before and / or after carbonization,
A method characterized in that it is heat-treated at a temperature of 00 ° C. or more to be graphitized ”or“ after a carbon material pulverized before and / or after carbonization is heat-treated at a temperature of 2500 ° C. or more and graphitized. , Heat treatment under conditions that can scrape the surface of the obtained graphite, furthermore in an inert gas 80
A method of performing a heat treatment at a temperature of 0 ° C. or more ”.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION The graphite powder of the present invention has a closed structure in which the ends of a graphite c-plane layer are closed in a loop shape on the surface (hereinafter, also simply referred to as a closed structure). In this closed structure, the end of the graphite c-plane layer (carbon network layer), which is ideally a network structure in which six-membered carbon rings are connected in a plane, is closed by a plurality of loops resembling a fingerprint. It is a thing. The formation of such a closed structure on the surface of the graphite powder is more energetic when the ends of the two c-plane layers are combined and closed in a loop than when the ends of the c-plane layer remain cut. Is considered to be stable.

As shown in FIGS. 1 and 2, the closed structure generally has a laminated loop structure in which several layers of loops are overlapped, instead of a single-layer loop. As shown in FIG. 1, the individual laminated loop structure formed by overlapping a plurality of loops with each other is referred to as a "unit closed structure" in the present invention. For simplicity, FIG. 1 shows only a unit closed structure in which three loops (six c-plane layers) are stacked, but the number of loops varies for each unit closed structure, for example, 10 or more. The number of layers may be as described above. In FIG. 2,
The white streaks represent the graphite c-plane layer, and the arrows indicate the gap plane.

Investigation was made by molecular orbital method to determine which part of the closed structure Li ions may pass through. As shown in FIG. 1, the gap between two adjacent unit closed structures is shown in FIG.
(In the present invention, this is referred to as a “gap surface”). At the entrance of the gap, the interaction between the Li ions and the carbon atoms is weak and the barrier energy for penetration is low, so that it is considered that a large amount of Li ions can easily pass through.

If the end of the c-plane layer is not closed in a loop and remains cut, not only Li ions but also the electrolyte penetrates into the graphite and lithium grows into dendritic dendrites during charging. As a result, the charge / discharge cycle characteristics deteriorate. Further, since the cut end is chemically unstable, the cut end is liable to be powdered, and the cycle characteristics are also reduced. Accordingly, graphite having a loop-shaped closed structure that is chemically stable and hardly penetrates the electrolytic solution is more advantageous as the electrode material.

It is considered that the penetration of Li ions into the negative electrode carbon material becomes easier as the number of sites where Li ions penetrate increases. Therefore, it is considered that as the density of the gap surface between the unit clogging structures of the graphite c-plane layer described above is higher, Li ions are more likely to enter the negative electrode material, and the storage amount of Li ions in the carbon material is increased.

In consideration of such a viewpoint, in the present invention,
The density of the interstitial surface of the closed structure observed on the surface of the graphite powder was specified to be a certain value or more, and the crystallinity of the graphite powder was also specified. This is because the discharge capacity also depends on the crystallinity.

That is, the graphite powder of the present invention has a closed structure in which the end of the graphite c-plane layer is closed in a loop on the surface, and the density of the gap surface between the closed structures in the graphite c-axis direction is 100%. And the c-axis (002) plane lattice spacing (d002) determined by the precise measurement method of lattice constant by X-ray diffraction is 0.33700 nm or less.

Here, the graphite c-axis direction refers to the direction shown in FIG.
As shown in the figure, the direction is perpendicular to the c-plane layer (carbon network layer). The c-axis (002) plane lattice spacing (d002) is the distance between adjacent c-plane layers, denoted as d002 in FIG.

In general, the graphite powder is composed of several regions having different c-axis directions (corresponding to the crystal grains of the polycrystalline powder), and each region (ie, a group of regions having the same c-axis direction) is crystallized. The length of this crystallite in the c-axis direction is called the crystallite diameter.

In the graphite powder of the present invention, the end of the c-plane layer exposed on the surface of each crystallite has the above-described closed structure, but the entire surface need not have the above closed structure. It is sufficient that this closed structure is seen in at least a part of. However, it is preferable that the closed structure is formed on substantially the entire surface of the powder.

The gap surface between the loop-shaped closed structure at the end of the c-plane layer of the graphite powder and the unit-closed structure in the c-axis direction has a high resolution near the surface of the powder fragment obtained by cutting the graphite powder in a direction parallel to the c-axis. It can be seen by observing with a transmission electron microscope. From this electron micrograph, by measuring the length L (μm) in the c-axis direction and the total number N of gap surfaces appearing in this length, N The density of the gap surface (pieces / μm) can be calculated as / L (see FIG. 1).

In the graphite powder of the present invention, the reason why the density of the gap surface of the loop-shaped closed structure is set to 100 pieces / μm or more is that the distance between the gap surfaces (ie, the c-axis direction of the unit closed structure) Is about the same as the crystallite diameter, and the contribution to the increase in the number of Li ion penetration sites is reduced. In fact, with such low interstitial densities, it is not possible to achieve high discharge capacities in excess of 330 mAh / g.

The upper limit of the density of the gap plane of 1500 / μm is defined by a closed structure in which all c-plane layers form a single-layer loop between two adjacent layers (that is, each unit closed structure has two layers of c-layers).
This is the maximum gap surface density theoretically predicted from the graphite crystal structure.

The c-axis (002) plane lattice spacing (d002) is an index of crystallinity, and the smaller the spacing, the higher the crystallinity of the graphite powder. The crystallinity of the graphite powder depends on the graphitization heat treatment conditions, and the higher the heat treatment temperature and the longer the time, the more likely it is to obtain a graphite powder with high crystallinity.

The lattice spacing of a crystal can generally be determined from diffraction peaks in an X-ray diffraction diagram. The Gakushin method has been most commonly used in the past for determining the lattice spacing of carbon materials by X-ray diffraction. However, the Gakushin method measures only large diffraction peaks,
Since the midpoint of the width of 2/3 is set as the peak position, a precise value cannot be obtained due to the effect of the asymmetric component and the error of the optical system. In the present invention, as the value of d002, a more precise value obtained by a lattice constant precise measurement method using a least square method including an error of a diffractometer is employed. In the precision measurement method, all peaks are used for calculation, and when the systematic error of the measurement system is small, unlike the Gakushin method, an internal standard sample is not required.

If the value of d002 determined by the lattice constant precision measurement method is larger than 0.33700 nm, the crystallinity of the graphite powder is insufficient and a high discharge capacity of 330 mAh / g cannot be realized. The value of d002 is preferably 0.33
650 nm or less.

The closed structure at the end of the c-plane layer and d according to the present invention
A graphite powder having a 002 value can be produced by subjecting a carbon material obtained by carbonization and pulverization to a heat treatment at an appropriate temperature to be graphitized. If the pulverization is carried out under a high-speed condition only by graphitization, a graphite powder satisfying the condition of the present invention, in which the density of the gap surface of the closed structure at the end of the c-plane layer is 100 particles / μm or more, can be produced. Less than,
This manufacturing method is referred to as a first method. However, in the first method, the interstitial surface density of the obtained graphite powder is 100 particles /
It is slightly larger than μm (eg, 100 to 120 pieces / μm), and it is difficult to obtain a very high gap surface density of, for example, 200 pieces / μm or more.

According to another manufacturing method (second method), the graphite powder obtained by graphitization as described above is subjected to a heat treatment (eg, 600 to 800
(Oxidation heat treatment at a temperature of 800 ° C.) and a heat treatment at a temperature of 800 ° C. or more in an inert gas. 200 for this method
Or more, for example, a very high gap surface density of 500 to 1500 pieces / μm can be obtained.

The method for producing graphite powder according to the present invention is not limited to the first and second methods. Finally, it has a closed structure at the end of the c-plane layer,
The graphite powder according to the present invention may be manufactured by any method as long as a graphite powder having a density of the gap surface of 100 particles / μm or more can be formed.

The carbonaceous raw material used for carbonization is not particularly limited, and may be the same as that conventionally used for producing graphite powder. Specific examples of the carbonaceous raw material include coal tar pitch or petroleum pitch, and mesophase microspheres produced by heat treatment of these pitches, bulk mesophase as a matrix of these microspheres, and various organic resins or organic substances (e.g., Polyacrylonitrile, rayon) and the like. Particularly preferred carbonaceous feedstocks are mesophase microspheres and bulk mesophase.

The carbonaceous material is pulverized and carbonized to obtain a carbon material. Atomic level irregularities (layer defects) on the powder surface caused by the pulverization form the above-described closed structure during the graphitization heat treatment. Therefore, the pulverization is essential for obtaining a graphite powder having a high-density closed structure. In particular, in the first method, the pulverization conditions greatly affect the form and density of the closed structure of the graphite powder generated after the graphitization heat treatment.

If the pulverization treatment is performed after the graphitization heat treatment, a layer defect occurs in the c-plane layer of the graphite formed by the heat treatment, and the introduced closed structure may be broken by the pulverization. Is undesirable. Therefore, it is preferable that the pulverization performed before the heat treatment be performed so as to have a final particle size required for the application of the graphite powder.

For example, when used as a negative electrode material for a lithium secondary battery, if the average particle size is too large, the packing density decreases,
It is known that particles having a particle size smaller than 1 μm deteriorate the initial charge / discharge characteristics. Therefore, it is preferable that the average particle size is in the range of 5 to 50 μm and no fine particles smaller than 1 μm are present. However, light pulverization for the purpose of pulverization, classification for removing fine particles and adjusting the average particle size may be performed after the graphitization heat treatment or after the last heat treatment in the second method.

The pulverization may be performed at any time before or after the carbonization, and may be performed both before and after the carbonization. Heat treatment can be performed continuously, and there is no waste of heat energy.

The pulverization may be carried out using a conventional pulverizer such as a hammer mill, a fine mill, an attrition mill, and a ball mill. Preferred mills are those that perform impact milling, typically hammer mills. As described above, particularly in the first method, the influence of the pulverizing conditions on the crystal structure of the graphite powder is large, and high-speed pulverization is required to obtain a graphite powder having a closed structure with a gap density of 100 pieces / μm or more. It is necessary to adopt. Specific grinding conditions (eg, rotation speed, grinding time) also differ depending on the type of crusher and carbonaceous material used, so that graphite powder having a gap density of 100 pieces / μm or more after graphitization heat treatment is used. It may be determined by experiment so as to produce and obtain a powder having a desired particle size.

For example, when the bulk mesophase is pulverized by a hammer mill, the number of revolutions of the pulverizer is set to 5000 to 8000 rpm.
pm, the density of the gap surface is 100
A graphite powder having a closed structure of not less than μm can be obtained. If the rotation speed is lower than 5000 rpm, the density of the gap surface cannot be increased to 100 pieces / μm or more, and
If the rotation speed is higher than m, the specific surface area of the graphite powder after pulverization and graphitization heat treatment becomes too large, and the formation of a passivation film is likely to occur during the first charge in a Li-ion secondary battery, and a highly efficient negative electrode is obtained. Can not do. However, this is just an example, and the appropriate number of revolutions will change if the type of crusher or raw material changes.

Also in the case of the second method, such high-speed pulverization may be performed, so that the density of the gap surface is 1000 pieces / powder.
It is possible to obtain a graphite powder having a very high-density closed structure exceeding μm. However, since the gap surface density is greatly increased by the two heat treatments after the graphitization heat treatment, the pulverization by the second method does not need to be high-speed pulverization.

The carbonization conditions of the pulverized carbonaceous raw material may be selected so that the raw material is decomposed and elements other than carbon contained in the raw material are almost completely removed. The heat treatment is performed in an inert atmosphere or vacuum to prevent carbon oxidation (combustion). The heat treatment temperature for carbonization is usually 800 to 15
It is within the range of 00 ° C, and particularly preferably around 1000 ° C. The heat treatment time required for carbonization depends on the type of the raw material, the treatment amount and the temperature, but is about 30 minutes to 3 hours when the temperature is 1000 ° C.

The powdery carbon material obtained by pulverization and carbonization is heat-treated to be graphitized. The heat treatment is carried out under such conditions that graphitization (crystallization) occurs and a graphite powder having a c-axis (002) plane lattice spacing d002 of 0.33700 nm or less is finally obtained. The graphitization temperature is usually 2500 ° C. or higher, and an upper limit of about 3200 ° C. is practical with the current heating technology. However, since d002 greatly depends on the graphitization conditions, in the first method, d002 is 0.33700 nm after graphitization.
The heat treatment conditions are set as follows. The heat treatment temperature for this is usually 2500 ° C or higher, preferably 2800 ° C or higher,
More preferably, it is 2900 ° C. or higher. In the second method,
D002 by oxidation heat treatment + inert gas heat treatment after graphitization
Is somewhat smaller, so d002 after graphitization is 0.3370
It may slightly exceed 0 nm. In this case, the graphitizing heat treatment temperature is preferably 2700 ° C. or higher. However, as the heat treatment temperature increases, d002 decreases and the discharge capacity improves. The heat treatment time depends on the temperature and the treatment amount, but is generally 20 minutes or more. The heat treatment atmosphere is a non-oxidizing atmosphere, preferably an inert gas atmosphere (eg, nitrogen, helium, argon, neon, carbon dioxide, etc.) or a vacuum.

The graphite powder thus produced by the graphitization heat treatment generally has a closed structure in which the c-plane layer ends are closed in a loop on the powder surface. A graphite powder having an area density slightly exceeding 100 particles / μm is obtained. That is,
It is a graphite powder manufactured by the first method.
As described above, even when the gap surface density is a little more than 100 pieces / μm, the discharge capacity is remarkably improved as compared with the case where the density is less than 100 pieces / μm.

In the second method, the graphite powder obtained by the graphitization heat treatment is further subjected to two heat treatments, namely, an oxidation heat treatment (or a heat treatment for shaving another surface) and a heat treatment in an inert gas atmosphere, so as to block the graphite powder. Significantly increase the interstitial density of the structure. The heat treatment after graphitization in the second method will be described below.

The first oxidizing heat treatment applied to the graphite powder is performed in order to remove the powder surface by oxidation. As a result, the powder surface generated by the graphitization heat treatment (c-plane layer end)
The capped surface of the c-plane layer is cut and opened, and the end of the c-plane layer becomes flat with almost the same length, thereby obtaining a graphite powder having the c-layer end of the open structure.

The conditions of the oxidation heat treatment are not particularly limited as long as the closed structure is substantially opened by the oxidation, but the heat treatment temperature is preferably in the range of 600 to 800 ° C. This is because graphite powder having a closed structure has high oxidation resistance, so it is difficult to oxidize when the temperature of the oxidizing heat treatment is lower than 600 ° C., and at 800 ° C. or higher, oxidation proceeds rapidly, and the entire graphite powder deteriorates. The time for the oxidizing heat treatment varies depending on the temperature and the treatment amount, but is generally 1 to 10 hours. The heat treatment atmosphere is an oxygen-containing atmosphere, and may be a pure oxygen atmosphere or a mixed gas atmosphere of oxygen and an inert gas (eg, air).

The opening of the closed structure is not limited to the oxidation heat treatment. Other methods can be adopted as long as the closed structure can be opened by shaving off the surface structure of the graphite powder to obtain a flat c-plane layer end. Examples of other methods include fluorination heat treatment, hydrogenation heat treatment, and the like. The heat treatment conditions in this case may be appropriately set by experiments so that the closed structure is opened.

Thereafter, the graphite powder is further heat-treated in an inert gas atmosphere. By the heat treatment in the inert gas atmosphere, the end of the open c-plane layer is connected to the end of another c-plane layer, and a loop-shaped closed structure is formed again at the end of the c-plane layer on the surface of the graphite powder. Is done.

At the time of the loop connection of the ends of the c-plane layer, the ends of the c-plane layer on the surface of the graphite powder are flattened by the oxidizing heat treatment. A large closed structure in which a large number of connection loops are stacked as shown in FIG. 2 cannot be formed. The number of stacked loops is at most five, and most is about one to three. Therefore, the number of closed structures per c-axis length increases,
This increases the density of the gap surface. As a result, the first
In the second method, the gap surface density was slightly more than 100 pieces / μm, but in the second method, for example, 500 pieces / μm
The pitch of the gap surface can be reduced so as to have a large gap surface density exceeding m.

The inert gas atmosphere may be, for example, one or more of Ar, He, Ne and the like. The heat treatment temperature may be a temperature that causes a relatively large lattice vibration such that the ends of the c-plane layer can be connected to each other. When the closed structure is formed by connection, the energy is low and the stabilization is achieved. Therefore, when heat treatment is performed in an inert gas atmosphere to generate sufficient lattice vibration, the open ends of the c-plane layer are connected to each other. It is.
For this purpose, a heat treatment temperature of generally 800 ° C. or higher is required. Although the upper limit is not particularly limited, as described above, about 3200 ° C. is practical with the current heating technology. The heat treatment time is not particularly limited as long as the closed structure is formed, and greatly varies depending on the temperature and the treatment amount, but is generally 1 to 10 hours. For example, at 1000 ° C., about 5 hours is a standard.

The graphite powder having a closed structure according to the present invention can be used for the same applications as conventional graphite powders. The c-plane layer of the graphite powder forms an occluded structure, and the occluded structure in the c-axis direction, and therefore the density of the gap surface is as high as 100 to 1500 / μm, and at the same time, the d002 is 0.33700 nm or less and the crystallinity is low. Due to the high properties, the functions of the graphite powder such as doping, occlusion, insertion, etc., that is, the function of storing the substance between the c-plane layers are remarkably improved.

The graphite powder of the present invention is particularly suitable as a material for a negative electrode of a lithium secondary battery. Because it has many pores and voids in the closed structure, which is the site of intrusion of Li ions, it is easy for Li ions to penetrate, and more Li ions reach the material storage area of graphite powder than before. The storage amount of Li ions increases. Therefore, a lithium secondary battery with improved discharge capacity can be manufactured. Further, since the c-plane layer of the graphite powder has a closed structure, the electrolyte does not easily enter the graphite powder,
Since the precipitation of dendritic dendrites of Li during charging can be avoided, the cycle life when charging and discharging are repeated is prolonged.

When the graphite powder of the present invention is used for this purpose, an electrode of a lithium secondary battery using the same can be produced by a method similar to the conventional method. For example, if necessary, the graphite powder is classified to adjust the particle size, and then the powder is mixed with a small amount of a binder and a solvent to form a paste or slurry. Examples of binders are
Fluorinated resin powders such as polyvinylidene fluoride and polytetrafluoroethylene, and water-soluble binders such as carboxymethyl cellulose. As the solvent, a polar organic solvent,
Water can be used. Apply the obtained paste or slurry to a suitable metal current collector, dry and mold,
An electrode is obtained. The positive electrode, electrolyte, separator, and other components of the lithium secondary battery may be the same as those of the conventional product.

[0056]

This example illustrates the first and second methods in which pulverization was performed after carbonization.

The bulk mesophase pitch obtained from the petroleum pitch was coarsely pulverized, and this was heated at 1000 ° C. for 1 hour in an argon atmosphere to carbonize to obtain a carbon material. This carbon material was pulverized for 5 minutes at 4500 rpm or 7500 rpm using a hammer mill (U-Mizer manufactured by Fuji Paudal), and the pulverized carbon material was cooled from 2700 ° C. to 30 ° C. in an argon atmosphere.
Graphite powder was obtained by heat-treating at a predetermined temperature selected in steps of 100 ° C. to 00 ° C. for 30 minutes and graphitizing.

Observation of these graphite powder fragments cut in the c-axis direction with a high-resolution transmission electron microscope revealed that all of the fragments had a closed structure at the end of the surface c-plane layer.
The gap surface density of the closed structure was determined from the electron micrograph (10
Field of view). The gap surface density was 80 particles / μm for the graphite powder whose rotational speed at the time of grinding the carbon material was 4500 rpm, and all of the graphite powder was 7500 rpm for the graphite powder.
103 particles / μm, and there was almost no influence by the graphitization temperature. That is, it was found that the gap density was greatly affected by the pulverization conditions, and that when the pulverization was high-speed pulverization, a closed structure was formed at a high pitch exceeding 100 pieces / μm during the graphitization heat treatment. In the first method, in which the gap surface density is set to 100 pieces / μm or more only by pulverization, high-speed pulverization is required.

According to the second method described above, the graphite powder crushed at 4500 rpm was subjected to an oxidizing heat treatment for oxidation at 650 ° C. × 2 hours or 700 ° C. × 3 hours in an oxygen atmosphere, followed by 1000 ° C. in an argon atmosphere. Heat treatment was performed at 5 ° C. × 5 hours. The gap density of the graphite powder subjected to the oxidizing heat treatment and the heat treatment in an inert gas was determined in the same manner as described above. When the oxidizing heat treatment was performed at 650 ° C. × 2 hours, 500 particles / μm, 700 ° C. × 3 In case of time
The number was 770 / μm, which was greatly increased as compared with 103 / μm before the treatment. That is, the post-graphitization treatment by the second method was able to significantly increase the gap surface density.
In addition, as a result of observing the fragments of the graphite powder after the oxidizing heat treatment with an electron microscope in the same manner as described above, the closed structure at the end of the c-plane layer generated by the graphitizing heat treatment was cut and opened after the oxidizing heat treatment, and the c-plane layer was opened. It was observed that the ends were relatively uniformly aligned.

The c-axis (002) of each of these graphite powders
The lattice spacing (d002) and the discharge capacity of the lithium secondary battery were measured as follows.

For d002, the X-ray diffraction pattern of the graphite powder sample was measured using an X-ray diffractometer manufactured by Mac Science.
Accurate voltage: 40 kV, current: 150 mA, measurement range: 20-90 °. From this X-ray diffraction diagram, a precise measurement of the lattice constant by the least square method including the error of the diffractometer. Zu). FIG. 3A shows one example of the produced X-ray diffraction pattern. FIG. 3 (b) is an enlarged view of a part of FIG. 3 (a). To determine the lattice constant, the surface indices (002), (100), (101), (004), (110), (1)
All peak positions of 12) and (006) were used. Three times X
A line diffraction measurement is performed, a weighted average of the obtained values is taken, and d00
The value was 2.

In order to measure the discharge capacity, each of the graphite powders was sieved to 5 μm or more and 45 μm or less, and then used for producing electrodes. The average particle size of each of the graphite powders was about 15 μm. 90 parts by weight of the graphite powder and 10 parts by weight of the polyvinylidene fluoride powder were mixed in a solvent, N-methylpyrrolidone, and dried to form a paste. The obtained paste was applied to a 20 μm-thick copper foil serving as a current collector to a uniform thickness using a doctor blade, and then dried at 80 ° C. A test piece having an area of 1 cm ² cut out from this was used as a negative electrode.

The evaluation of the negative electrode characteristics was carried out by a three-electrode constant current charge / discharge test using lithium metal as a counter electrode and a reference electrode. The electrolyte used was a solution in which LiClO ₄ was dissolved at a concentration of 1 mol / l in a mixed solvent of ethylene carbonate and dimethyl carbonate at a volume ratio of 1: 1. The discharge capacity was determined by charging to 0.0 V at a current density of 0.3 mA / cm ² and discharging to 1.5 V at the same current density. Table 1 summarizes the above test results.

[0064]

[Table 1]

As can be seen from Table 1, comparing graphite powders having the same void area density of the closed structure, it can be seen that d002 decreases as the graphitization temperature increases, and therefore the crystallinity improves. When the graphitization temperature is the same, d002 tends to decrease as the gap surface density increases. That is, the crystallinity (d002) depends on both the graphitization temperature and the interplanar density. The graphite powder satisfying the conditions of the present invention, that is, the condition that the density of the gap surface is 100 to 1500 particles / μm and the d002 is 0.33700 nm or less, has a discharge capacity of 310 mA.
h / g or higher, often above 330 mAh / g, up to 364
It was possible to obtain a discharge capacity considerably close to the theoretical capacity of mAh / g.

Example 2 This example illustrates the above second method in which pulverization was performed before carbonization. The bulk mesophase pitch obtained from petroleum pitch was ground at 4500 rpm for 5 minutes using the same hammer mill as in Example 1. The pulverized bulk mesophase pitch was then carbonized by heating at 1000 ° C. for 1 hour in an argon atmosphere, and then heat treated at 2800 ° C. for 30 minutes in an argon atmosphere to graphitize, thereby obtaining a graphite powder. As a result of observing the cross section of the obtained graphite powder with an electron microscope in the same manner as in Example 1, it was found that the c-plane layer had a closed structure at the end, but the gap surface density was 80 particles / μm. Since the conditions for the carbonization and graphitization heat treatment were the same as in Example 1, the gap area density of the graphite powder obtained after graphitization was almost the same regardless of whether pulverization was performed before or after carbonization. You can see that there is.

The graphite powder was subjected to an oxidizing heat treatment at 700 ° C. in an oxygen atmosphere at intervals of 30 minutes for a maximum of 3 hours, followed by a heat treatment at 1000 ° C. for 5 hours in an argon atmosphere. Also in this case, the closed structure is once opened by the oxidizing heat treatment, and furthermore, the heat treatment in an inert gas causes
Electron microscopic observation revealed that a short-pitch closed structure was formed at the end of the surface layer.

D00 of the graphite powder after the above heat treatment
2 and the discharge capacity were examined in the same manner as in Example 1.
The results are shown together with the oxidation heat treatment time.

[0069]

[Table 2]

Even when the grinding is relatively slow at 4500 rpm,
According to the second method of the present invention, when the oxidizing heat treatment and the heat treatment in an inert gas are performed after the graphitizing heat treatment, the gap surface density is greatly increased, and accordingly, d002 is somewhat reduced (crystallinity is increased). I understand. As a result, 4500 grinding
Even at rpm, a graphite powder showing a high discharge capacity, for example, 350 mAh / g, could be obtained.

[0071]

According to the present invention, it is possible to improve the crystallinity by increasing the gap area density of the clogging structure at the end of the c-plane layer which appears on the surface of the graphite powder to 100 pieces / μm or more and d002 to 0.33700 nm or less. Thus, a graphite powder suitable for a negative electrode material of a lithium secondary battery exhibiting a high discharge capacity can be obtained. Further, as shown in the examples, in the first and second methods of the present invention, depending on the number of revolutions of the pulverizer, the conditions of the graphitization heat treatment, and the conditions of the subsequent surface treatment (oxidation heat treatment + inert gas heat treatment). Since it is possible to control the gap surface density and the value of d002, it is possible to produce a graphite powder according to the required discharge capacity level, and the performance of the produced graphite powder can be predicted to some extent from these conditions. It becomes possible.

[Brief description of the drawings]

FIG. 1 is an explanatory view showing a closed structure at the end of a c-plane layer appearing on the surface of a graphite powder according to the present invention.

FIG. 2 is an electron micrograph showing a closed structure of a graphite powder.

FIG. 3 (a) shows an example of an X-ray diffraction pattern of the graphite powder according to the present invention, and FIG. 3 (b) is an enlarged view of a part thereof.

Continued on the front page (72) Inventor Ken Abe 4-5-33 Kitahama, Chuo-ku, Osaka City Inside Sumitomo Metal Industries Co., Ltd. (72) Inventor Noriyuki Negi 4-5-33 Kitahama, Chuo-ku, Osaka Sumitomo Metal Within Industrial Co., Ltd. (72) Inventor Hideya Kaminaka 4-5-33 Kitahama, Chuo-ku, Osaka

Claims (4)

[Claims] 1. A graphite powder having a closed structure in which an end of a graphite c-plane layer is closed in a loop on the powder surface, wherein the density of gap surfaces between the closed structures in the graphite c-axis direction is 100 to 1500 particles. / Μm, and the c-axis (002) plane lattice spacing (d002) determined by the precise measurement method of lattice constant by X-ray diffraction is 0.33700 nm or less. 2. The method for producing graphite according to claim 1, wherein the carbon material that has been subjected to high-speed pulverization before and / or after carbonization is heat-treated at a temperature of 2500 ° C. or more to be graphitized. 3. A carbon material pulverized before and / or after carbonization is heat-treated at a temperature of 2500 ° C. or more to be graphitized, and then heat-treated under a condition capable of shaving the surface of the obtained graphite. And then 800 ℃ in inert gas
The method for producing graphite according to claim 1, wherein the heat treatment is performed at the above temperature. 4. A negative electrode material for a lithium secondary battery, comprising the graphite powder according to claim 1.