JP2002075362A - Graphite powder suitable for secondary battery negative electrode, method for manufacturing graphite powder, and its use - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a graphite powder suitable for a negative electrode material of lithium ion secondary battery, which has high charging capacity and high charge-discharge efficiency and also give an improved high-rate discharging characteristic. SOLUTION: A graphite powder produced through carbonization, crushing, and a graphitizing heat treatment is subjected to heat treatment for oxidation for shaving off the surface to expose the surface closed structure, and is subjected to another heat treatment with a rapid temperature rise in an inert gas so that a surface closed structure is formed again, and thereby graphite powder is established having a surface form in which closed regions with the end parts of graphite c-plane layers coupled together to generate closed structure are scattered on the powder surfaces in mosaic form where the length (Lc⊥) perpendicular to the graphite c-axis in each closed region is 100 nm or less.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a graphite powder having an optimum surface morphology as a negative electrode material of a lithium ion secondary battery and a method for producing the same. The present invention also relates to a negative electrode of a lithium ion secondary battery using the graphite powder and a lithium ion secondary battery including the negative electrode. When the graphite powder of the present invention is used for a negative electrode, a lithium ion secondary battery having excellent high-rate discharge characteristics (rate characteristics) can be produced.

[0002]

2. Description of the Related Art In a lithium ion secondary battery that is rapidly spreading as a small secondary battery, a negative electrode using an active material of carbon powder capable of storing Li ions is generally used.
Many of the negative electrodes are manufactured from graphite powder. This is because crystalline graphite powder has a higher discharge capacity per unit volume and higher charge / discharge efficiency than amorphous carbon powder.

[0003] As the graphite powder, a powder obtained by pulverizing natural graphite can be used. However, artificial graphite, that is, carbonaceous material is carbonized by heat-treating the carbonaceous raw material to remove organic matter from the viewpoint of quality stability. A powder of graphite obtained by further heat-treating the obtained carbon material at a high temperature to be crystallized by crystallization is more preferable.

As characteristics considered for graphite used as a negative electrode material of a lithium ion secondary battery, in addition to discharge capacity and charge / discharge efficiency (ratio of discharge capacity to charge capacity), high-rate discharge characteristics (low current Ratio of the discharge capacity at high current density to the discharge capacity at high density). High-rate discharge characteristics are important especially in the case of rapid charging.

[0005] Regarding the surface morphology of artificial graphite powder, WO 98/29335 describes that clogging formed by bonding the ends of c-plane layers of graphite crystals having a hexagonal layered crystal structure by graphitization heat treatment. It is pointed out that a structure is formed on the powder surface. In the conventional artificial graphite having this surface morphology, a gap surface, which is a gap between adjacent closed structures, serves as a main intrusion / exit site for Li ions involved in charging and discharging.

[0006]

In the above international publication, Li
It has been proposed to increase the discharge capacity by increasing the density per unit length of the gap surface, which is the main site of entry of ions, (ie, reducing the pitch of the gap surface).

[0007] The increase in the interstitial density of the graphite powder can be achieved in any of the following ways:
Introduce irregularities (layer defects) at the atomic level into the powder surface and then perform a graphitization heat treatment; or apply a heat treatment such as an oxidation heat treatment to the graphite powder obtained by the graphitization heat treatment, The closed structure on the powder surface is released, and then heat-treated again in an inert gas to re-form the closed structure.

[0008] In particular, a graphite powder having a very large gap surface density can be obtained. As described above, when the density of the gap surface, which is a site where Li ions enter, is increased, a graphite powder having a discharge capacity considerably close to the theoretical capacity of the graphite negative electrode of 372 mAh / g can be obtained. Also, the charge / discharge rate is sufficiently high.

However, as long as the invasion site of the Li ion is limited to the gap surface, the Li ion which has passed through this intrusion site diffuses into the graphite powder through the interlayer of the c-plane layer. Becomes longer. Therefore, it takes time to inject and escape Li ions, and there is room for improvement in high-rate discharge characteristics. The reason for this is that the higher the current density, the more intrusion and escape of Li ions are required.However, if the diffusion path is long and the intrusion and escape of Li ions take time, a large amount of Li ions enter at high current density. This is because, in such a case, the diffusion cannot catch up and the discharge capacity decreases.

An object of the present invention is to provide a graphite powder having improved high-rate discharge characteristics, excellent discharge capacity and charge / discharge efficiency, and suitable for a negative electrode material of a lithium ion secondary battery, and a method for producing the same. Make it an issue.

[0011]

DISCLOSURE OF THE INVENTION The present inventors have studied the effects of heat treatment on the closed structure of graphite by the international publication WO.
The closed structure was reformed by changing the heat treatment conditions in an inert gas in the above method described in 98/29335, and the surface morphology of the obtained graphite powder was examined by atomic force microscopy. . As a result, it was found that the surface morphology was different depending on the rate of temperature increase in the heat treatment.

Specifically, the surface of the graphite powder which has been subjected to a heat treatment at a temperature rising rate of less than 5 ° C./sec has a clogging structure formed by bonding the ends of the c-plane layer in the direction of the c-plane layer. That is, it is long continuously in the direction perpendicular to the c-axis (c⊥ direction), and the gap surface is also continuous in the same direction. Accordingly, a striped closed structure in which gap surfaces appear at substantially constant intervals is obtained.

On the other hand, a rapid heat treatment is performed by increasing the heating rate.
On the surface of the graphite powder that has been subjected to (rapid thermal annealing), partial aggregation of the closed structure starting from nucleation at the time of non-equilibrium becomes dominant. Therefore, the closing structure on the powder surface is not continuous in the c⊥ direction, and a mosaic (checkered) closing structure is formed. Further, the mosaic closed structure once formed is thermally stable and irreversible, so that its surface morphology is maintained even during the cooling process.

The novel graphite powder having a mosaic-shaped closed structure exhibits high discharge capacity and charge / discharge efficiency, and also has excellent high-rate discharge characteristics. This is because, as will be described in detail later, the mosaic block structure has a short diffusion path of Li ions and can easily diffuse, substitute, and release a large amount of Li ions in a short time. Can be

According to the present invention, the clogged surface of the graphite has a surface configuration in which closed end portions are connected to each other and are closed to the powder surface, and each of the closed regions is perpendicular to the graphite c axis. Characterized in that the length (Lc⊥) is 100 nm or less,
A graphite powder is provided.

The graphite powder of the present invention having the above surface morphology can be produced by a method characterized by comprising the following steps: (a) heat treating a carbon material obtained by carbonizing a carbonaceous material; Graphitizing, (b) before carbonization, between carbonization and graphitization, and / or
Or at least one grinding step performed after graphitization, (c)
Heat-treating the graphite powder obtained after the steps (a) and (b) under conditions capable of shaving the surface thereof, and (d) placing the graphite powder obtained in the step (c) in an inert gas. 5 ° C /
A process of heating at a temperature of 500 ° C. or more for heat treatment in more than a second.

The heat treatment in the step (c) is preferably an oxidation heat treatment. According to the present invention, there is also provided a negative electrode for a lithium ion secondary battery including the graphite powder having the above surface morphology, and a lithium ion secondary battery including the negative electrode.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First, the surface morphology of the graphite powder of the present invention will be described with reference to FIGS.

A conventional ordinary graphite powder obtained by pulverizing and then performing a graphitization heat treatment has a multilayer plugging structure in which several layers of plugging structures are laminated as shown in a sectional view of FIG. Although not shown, the multilayered closed structure has a surface form elongated in the c-plane layer direction (a direction perpendicular to the c-axis, that is, a c⊥ direction). The surface morphology of the graphite powder obtained after the steps (a) and (b) of the production method of the present invention is also such. Such a closed structure on the powder surface can be actually confirmed in a cross-sectional SEM or TEM photograph of the graphite powder.

In the case of the above-mentioned multilayer blocking structure, the number of gap surfaces serving as Li ion intrusion sites is, for example, one in 10 c-plane layers in the case of a multilayer blocking structure in which five layers are stacked, and the remaining 9 Since the end of the c-plane layer of the layer is closed, Li ions cannot enter. Since the number of Li ion intrusion sites is small as described above, the discharge capacity is limited, and cannot be close to the theoretical capacity.

When the graphite powder having such a multilayer laminated closing structure on the surface is subjected to an oxidizing heat treatment in the step (c) of the manufacturing method of the present invention, the surface is ground and flattened.
As shown in (b), the closed structure is opened, and the end of the c-plane layer remains disconnected without being bonded to another c-plane layer. Therefore,
All c-plane layer ends serve as gap surfaces and serve as Li ion intrusion sites. Although this form is advantageous for the intrusion of Li ions, it is chemically unstable, and the electrolyte solution easily penetrates into the inside, so that only a negative electrode having a very short cycle life can be obtained.
Poor practicality.

Thereafter, in step (d), when heat treatment is performed in an inert gas, unstable c-plane layer ends are combined for stabilization, and the closed structure is re-formed. Since the surface of the graphite powder is shaved and flattened, the number of laminations is small even if the closing structure is laminated in multiple layers at this time. For example, as shown in FIG. 1C, the surface may have a closed structure in which two layers are stacked. However, it is natural that the actual closing structure is not uniform.

However, if the rate of temperature rise in the heat treatment in this step (d) is less than 5 ° C./sec, terminal bonding will occur in a substantially equilibrium state, as shown in FIG. 1 (c). The connection between the same c-plane layers extends in one direction for a long time, and the gap surface and the closing structure form a striped pattern, which is a so-called striped closing structure. That is, although the number of stacked c-plane layers is smaller than that of the closed structure shown in FIG.
It is the same in the points that continue in the direction. However, when the number of layers decreases, the density of the gap surface increases, and the discharge capacity increases.

In the present invention, the heat treatment in the step (d) is a rapid heat treatment at a temperature rising rate of 5 ° C./sec or more. Thereby, c
The lattice vibration at the end of the face layer instantaneously increases, and the end of the c-plane layer is coupled while maintaining the non-equilibrium state. As a result, FIG.
As shown in (a) and (b), the bond is divided into left and right,
The ends are partially alternately connected between the c-plane layers.
That is, the clogging structure at the end of the c-plane layer on the graphite powder surface is represented by c
As a result, it does not extend and continue in the direction of 、 but is short and interrupted, resulting in a mosaic (checkered) blockage structure.

The mosaic closing structure appearing on the surface of the graphite powder of the present invention may be a single-layer closing structure without lamination as shown in FIG. 2 (a), or as shown in FIG. 2 (b). Alternatively, a closed structure in which two or more layers are stacked may be used, or both may coexist. The number of layers of the closed structure generally depends on the process
(a) and (b) are smaller than the graphite powder obtained later, but are not particularly limited. The mosaic occlusion structure of the graphite powder of the present invention can be seen by observing the graphite powder with an atomic force microscope, which can reproduce the fine irregularities on the surface.

If the closed structure on the surface of the graphite powder is striped as shown in FIG. 1 (a) or (c), the c-plane layer completely closed by the closed structure will be c. Even if it does, it remains a closed structure. Therefore, in the c-axis direction (c‖ direction) perpendicular to the c-plane layer, the gap plane, which is the intrusion site of Li ions, appears only between specific adjacent c-plane layers every several to several tens of layers. Li ions in the c-plane layer
The movement in the (c⊥ direction) is easy, and the movement in the c-axis direction (c‖ direction) perpendicular to the (c の direction) is more difficult because it must pass through the c-plane layer. Therefore, when the gap surface appears only in a specific c-plane layer with an interval, the diffusion distance of Li ions becomes longer and diffusion becomes more difficult.

On the other hand, when the surface closing structure of the graphite powder has a checkerboard mosaic shape as in the present invention, for example, as shown in FIG. Also, almost all of the adjacent c-plane layers have a portion that becomes a gap surface at any position in the c-plane layer direction (c⊥ direction) (in the present invention, a gap opening). All the c-plane layers have gaps, and Li ions can easily enter the c-plane through the gaps and move in the c⊥ direction in the c-plane, so that diffusion of Li ions occurs quickly. . As a result, even if a large amount of Li ions intrude in the high-rate discharge, the Li ions can easily diffuse into the inside of the powder, and the high-rate discharge characteristics are improved.

The graphite powder of the present invention has the surface morphology of the above-mentioned mosaic closed structure. The individual closed portions of the mosaic closed structure are referred to as closed regions in the present invention. Each closed region is formed by connecting and closing the ends of the graphite c-plane layer.

The length of each of the closed regions in the direction perpendicular to the graphite c-axis (Lc⊥) varies, but each is 100 nm.
It is as follows. Graphite powder having a mosaic closed structure in which the Lc⊥ value of each closed region exceeds 100 nm is practically impossible to prepare, and when such a material is to be prepared, as shown in FIG. 1 (c). The closed area is continuous in the c⊥ direction (L
c = ∞) Surface morphology.

The smaller the value of Lc⊥ of each closed area is,
The diffusion distance of Li ions is shortened, and high-rate discharge characteristics are improved. This value is preferably at most 50 nm, more preferably at most 20 nm, particularly preferably at most 10 nm. The magnitude of the Lc⊥ value of the closed region depends on the temperature rising rate and the holding temperature during the heat treatment in step (d), and the higher these are, the smaller the tendency is.

C of the individual occluded regions of the mosaic occluded structure
The axial width (Lc‖) is not particularly limited. This width depends on the number of layers of the closed structure described above, and is generally in the range of 0.3354 nm to 10 nm. 0.3354 nm is the graphite interlayer distance d0
02, which is the theoretical value of Lc‖ in a closed region of a single layer of perfect graphite crystals. In order for Lc‖ to exceed 10 nm, it is necessary to form a closed structure in which more than a dozen layers are stacked, and it is practically impossible to fabricate it. A preferred value of Lc‖ is 5 nm or less.

The c-axis direction width (Lc‖ value) of the occluded region of the mosaic occluded structure appearing on the surface of the graphite powder of the present invention can be measured from a cross-sectional SEM or TEM photograph near the surface of the graphite powder. On the other hand, the length (Lc⊥ value) of the clogged region in the c-axis perpendicular direction can be measured from an atomic force microscope photograph and a TEM photograph of the graphite powder surface.

Next, a method for producing the graphite powder of the present invention will be described. First, in the step (a), the carbonaceous material is heat-treated to carbonize (decompose organic matter), and the obtained carbon material is further heat-treated at a high temperature to be graphitized.

The carbonaceous material as the raw material is not particularly limited, and may be the same as that conventionally used for producing graphite. Specific examples include coal tar pitch or petroleum pitch,
Further, there are mesophase microspheres generated by these heat treatments, bulk mesophase which is a matrix of the microspheres, and other organic substances such as organic resins. Particularly preferred carbonaceous raw materials are mesophase microspheres and bulk mesophase, and among them, bulk mesophase is preferred from the viewpoint of cost and mass productivity.

The carbonization condition of the carbonaceous material may be selected so that the material is decomposed and elements other than carbon contained in the raw material are almost completely removed. To prevent oxidation (combustion) of carbon, the carbonization heat treatment is performed in an inert atmosphere or vacuum. The heat treatment temperature for carbonization is usually 800 ~ 1500 ℃
And particularly preferably around 1000 ° C. The heat treatment time required for carbonization depends on the type of raw material, heat treatment, and temperature, but is about 30 minutes to 3 hours when the temperature is 1000 ° C.

Next, the obtained carbon material is heat-treated to be graphitized. Graphitization usually requires temperatures above 2500 ° C. To lower the graphitization temperature, a suitable graphitization catalyst (eg,
(Boron) may be added in a small amount, in which case the graphitization catalyst may be added before carbonization. When a graphitization catalyst is added, the graphitization temperature can be lowered to about 1500 ° C. The upper limit of the graphitization heat treatment temperature is 32 with current heating technology.
It is about 00 ° C. A preferred graphitization heat treatment temperature is 2800 to 3000 ° C. when no catalyst is added.

This heat treatment is performed until the graphitization (crystallization) is completed. This time varies depending on the presence or absence of the catalyst and the treatment amount, but is generally 20 minutes to 10 hours. The heat treatment atmosphere is a non-oxidizing atmosphere (eg, an inert gas atmosphere or a vacuum). For the graphitization heat treatment, an Acheson furnace (heated by energizing the surrounding charged carbon powder) and an LWG furnace (heated by direct energizing) are industrially used. Although such an industrial firing furnace is operated in the atmosphere, the inside of the furnace has a non-oxidizing atmosphere composed of nitrogen and carbon monoxide.

Step (b) is a pulverizing step for making a powder. This pulverization may be performed on any material of the carbonaceous raw material before carbonization, the carbon material after carbonization, and the graphite after graphitization, but after the graphitization, the layered structure develops and the pulverization is performed. Since it becomes difficult, it is preferable to grind before graphitization. In addition, pulverization can be performed in two or more of these steps. However, pulverization is not performed after the next step (c).

The pulverization can be carried out using a conventional pulverizer such as a hammer mill, a fine mill, an attrition mill, a ball mill, a disk mill and the like. Regarding the particle size, when used as a negative electrode material of a lithium ion secondary battery, if the average particle size is too large, the packing density decreases, and
It is known that particles having a particle size smaller than μm deteriorate the initial charge / discharge characteristics, so that the average particle size is in the range of 5 to 50 μm, and the presence of fine particles smaller than 1 μm should be avoided. preferable.

Steps (a) and (b) are the same as the conventional method for producing graphite powder. As described above, if the thus obtained graphite powder is pulverized before graphitization, as shown in FIG. 1 (a), a multilayered closed structure usually extends in the c⊥ direction, as shown in FIG. It has a surface configuration with a closed structure. conventionally,
The graphite powder obtained from the steps (a) and (b) has been used as it is for a negative electrode material of a lithium ion secondary battery. In the present invention, the steps (c) and (d) are further performed.

In the step (c), the graphite powder obtained through the steps (a) and (b) is subjected to, for example, an oxidizing heat treatment to scrape off the surface of the c-plane layer of the graphite powder to thereby graphitize. The closed structure generated by the heat treatment is once opened to make it not bonded to another c-plane layer. By shaving off this surface, the edges of the c-plane layer on the powder surface are relatively flat.

The conditions of the oxidizing 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 about 600 to 800 ° C. Since the graphite powder having a closed structure has high oxidation resistance, it is hardly oxidized at a temperature lower than 600 ° C., and at a temperature of 800 ° C. or more, the 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).

As a result of the removal of the powder surface by the oxidation heat treatment, the weight of the graphite powder is reduced by about 2 to 5%. Also, the particle size of the powder becomes slightly smaller (eg, 1-2 μm
degree). If necessary, pulverization conditions are set in consideration of the decrease in the particle size.

The opening of the closed structure is not limited to the oxidation heat treatment. As long as the closed structure can be opened by shaving off the surface structure of the graphite powder to obtain a flat laminated structure of the c-plane layer, another method can be adopted. Other methods include, for example, fluorination heat treatment or hydrogenation heat treatment. The heat treatment conditions in this case may be appropriately set by experiments so that the closed structure is opened by scraping the surface.

The graphite powder whose surface has been shaved in this way is heat-treated in an inert gas atmosphere in step (d).
Since the ends of the c-plane layer that have been opened are bonded together for stabilization, a closed structure is formed again on the surface of the graphite powder.

In the present invention, the heat treatment conditions for reforming the closed structure are important. That is, this heat treatment is a rapid heat treatment at a temperature rising rate of 5 ° C./sec or more, and the holding temperature is 500
The temperature is at least ℃. As described above, by performing such a rapid heat treatment, the c-plane layer ends are bonded in a non-equilibrium state, so that the bonding hands are distributed to the left and right, and the closed structure to be reformed is continuous in the Lc⊥ direction. Without it, it becomes a checkered mosaic. Thereby, high rate discharge characteristics can be improved.

In the mosaic closed structure formed by this rapid heat treatment, the value of the length (Lc⊥) of each closed region of the closed structure in the c-axis ⊥ direction becomes smaller as the heating rate increases, and the The effect of improving the discharge characteristics is increased, which is advantageous. In that sense, the heating rate is preferably 10 ° C./sec or more,
It is more preferably at least 25 ° C./sec, even more preferably at least 50 ° C./sec, and can have a very high temperature rising rate of at least 100 ° C./sec. However, even if the heating rate exceeds 200 ° C., the effect is not so much improved, so that it is preferable to set the temperature to 200 ° C./sec or less.

In order to form a mosaic closed structure on the surface of the graphite powder by rapid heat treatment, it is necessary to open the closed structure by shaving the surface of the graphite powder by an oxidation heat treatment or the like. Unless the closing structure is opened in advance, a mosaic closing structure cannot be formed even by rapid heat treatment.

The holding temperature of the heat treatment must be at least 500 ° C. in order to form the above-mentioned mosaic closed structure. If the holding temperature is lower than this, it is not possible to give a lattice vibration of a magnitude necessary for bonding between the c-plane layer ends,
It becomes difficult to form a closed structure. If the heating rate is the same,
The higher the holding temperature, the smaller the value of Lc⊥ of each closed region of the formed mosaic closed structure, and the greater the effect of improving the high rate discharge characteristics, which is advantageous. In that sense,
The heat treatment holding temperature is preferably 700 ° C. or higher, more preferably 1000 ° C. or higher, and even more preferably 1500 ° C. or higher. For example, a high temperature of 2000 to 3000 ° C. or higher can be used.

The value of the width (Lc‖) in the c-axis direction of each closed region of the mosaic-shaped closed structure is not significantly affected by the heat treatment conditions (heating rate and holding temperature). Lc‖
Is mainly governed by the graphitization temperature.

The heat treatment in the step (d) is performed in an inert gas atmosphere. The inert gas atmosphere may be, for example, one or more of Ar, He, and Ne. The heat treatment time may vary depending on the temperature as long as the closed structure is formed.
Time. For example, at 1000 ° C., about 5 hours is a standard.

The graphite powder of the present invention having a surface morphology consisting of a mosaic-shaped closed structure obtained in step (d) comprises:
Suitable as a negative electrode material of a lithium ion secondary battery,
A negative electrode of a lithium ion secondary battery having excellent discharge capacity and charge / discharge efficiency and improved high-rate discharge characteristics can be manufactured.

The negative electrode of the lithium ion secondary battery can be manufactured in a conventional manner. Generally, graphite powder is formed into an electrode by molding on a current collector serving as an electrode substrate using an appropriate binder. As the current collector, any metal foil (e.g., copper foil such as electrolytic copper foil and rolled copper foil) that has good graphite powder supportability and does not elute due to decomposition when used as a negative electrode may be used. it can.

The molding can be carried out by an appropriate method which has been conventionally used when producing an electrode from a powdery active material. The negative electrode performance of the graphite powder is sufficiently brought out, and the moldability to the powder is improved. If it is high, chemically and electrochemically stable, there is no restriction. For example, a binder made of a fluororesin powder such as polytetrafluoroethylene and polyvinylidene fluoride and an organic solvent such as isopropyl alcohol are added to graphite powder, kneaded to form a paste, and this paste is screen-printed on a current collector. Method: A method in which a resin powder such as polyethylene or polyvinyl alcohol is added to graphite powder and dry-mixed, and the mixture is hot-pressed and molded using a mold and simultaneously thermocompression-bonded to a current collector; As a binder, a water-soluble binder such as the above fluororesin powder or carboxymethyl cellulose,
A method of forming a slurry using a solvent such as N-methylpyrrolidone, dimethylformamide or water or an alcohol, applying the slurry to a current collector, and drying the slurry is used.

The negative electrode prepared from the graphite powder of the present invention can be used in combination with a suitable positive electrode active material usable for a lithium ion secondary battery and a non-aqueous electrolyte in which a lithium compound is dissolved in an organic solvent. Can be produced.

[0056] As the positive electrode active material, for example, lithium-containing transition metal oxides ^{_{LiM 1 1-x M 2 x}} O 2 or LiM ¹ _2y M ² _y O ₄
(Wherein X is a numerical value in the range of 0 ≦ X ≦ 4, Y is a numerical value in the range of 0 ≦ Y ≦ 1, M ¹ and M ² represent transition metals, and Co, Ni, Mn, Cr, T
i, V, Fe, Zn, Al, In, Sn), transition metal chalcogenide, vanadium oxide (V ₂
O ₅ , V ₆ O ₁₃ , V ₂ O ₄ , V ₃ O _{8 and the} like, a lithium compound thereof, and a general formula M _x Mo ₆ S _8-y (where X is 0 ≦ X ≦ 4 and Y is 0
≦ Y ≦ 1 and M represents a metal such as a transition metal), activated carbon, activated carbon fiber and the like.

The organic solvent used for the non-aqueous electrolyte is not particularly limited, but may be, for example, propylene carbonate, ethylene carbonate, dimethyl carbonate,
Diethyl carbonate, 1,1- and 1,2-dimethoxyethane, 1,2-diethoxyethane, γ-butyrolactam,
Tetrahydrofuran, 1,3-dioxolan, 4-methyl-1,3-dioxolan, anisole, diethyl ether, sulfolane, methylsulfolane, acetonitrile,
One or more of chloronitrile, propionitrile, trimethyl borate, tetramethyl silicate, nitromethane, dimethylformamide, N-methylpyrrolidone, ethyl acetate, trimethyl orthoformate, nitrobenzene and the like can be exemplified.

As the lithium compound of the electrolyte, an organic or inorganic lithium compound soluble in the organic solvent used may be used. Examples of suitable lithium _{compounds, LiClO 4, LiBF 4, LiPF} 6, LiAsF 6, LiB (C 6 H 5), Li
One or more of Cl, LiBr, LiCF ₃ SO ₃ , LiCH ₃ SO _{3 and the} like can be mentioned.

[0059]

EXAMPLES Example 1 Bulk mesophase pitch obtained from petroleum pitch was coarsely pulverized and heated to 1000 ° C. under an argon atmosphere.
The material was carbonized by heating for an hour to obtain a carbon material. This carbon material was pulverized with an attrition mill so that about 90% by volume had a particle size of 1 to 80 μm. Next, the pulverized carbon material was heat-treated at a temperature of 3000 ° C. for 30 minutes in an argon atmosphere to perform graphitization, and further subjected to an oxidation heat treatment at 700 ° C. for 3 hours in an oxygen atmosphere, thereby shaving off the surface of the graphite powder. Thereafter, rapid heat treatment was performed in an Ar atmosphere at a heating rate of 100 ° C., a holding temperature of 2000 ° C., and a holding time of 6 hours to obtain a graphite powder of the present invention.

The graphite powder was sieved to 5 μm or more and 45 μm or less, and then provided for production of an electrode. The average particle size of the graphite powder was 12 μm. FIG. 3 shows an atomic force micrograph of the obtained graphite powder. From this photograph, it is recognized that the surface of the graphite powder has a mosaic surface block structure in which periodic block regions having a length of more than 0 nm to 20 nm are connected in a bead shape in the c-axis vertical direction. That is, Lc⊥ of the mosaic closed structure is more than 0 nm to 20 nm.

FIG. 4 shows a cross-sectional TEM photograph of the graphite powder cut in the c-axis direction. From this photograph, it can be seen that a multilayer lamination type closed structure is formed on the powder surface, and the pitch width in the c-axis direction (that is, c‖ of the closed region) is 2 to 10 nm.

FIG. 5 shows a structure in which the surface of the graphite powder is observed from directly above. According to the present invention, as shown in FIG. 5, a closed structure in which the closed area is scattered in a mosaic shape, in which the area a is the closed area and the area b is the gap, is formed on the graphite surface.

Using the graphite powder of the present invention having the mosaic closed structure obtained above, an electrode was produced by the following method. 90 parts by mass of the above graphite powder and 10 parts by mass of polyvinylidene fluoride powder were mixed in N-methylpyrrolidone as a solvent and dried to form a paste. The obtained paste was applied to a 20 μm-thick copper foil as a current collector to a uniform thickness using a doctor blade, and then dried at 80 ° C. A test piece cut out from this into a circle (area: 1.8 cm ² ) with a diameter of 15.2 mm was used as a negative electrode.

The high-rate discharge characteristics of the negative electrode were evaluated by a three-electrode constant current charge / discharge test using lithium metal as a counter electrode and a reference electrode. The electrolyte was LiPF at a concentration of 1 mol / l of a mixed solution of ethylene carbonate and dimethyl carbonate in a volume ratio of 1: 3.
What melt | dissolved ₆ was used. Charge and discharge current is 0.85, 4.
Five conditions of 0, 8.0, 12, and 20 mA were used. Converted to current density, 0.47, 2.2, 4.4, 6.6 and 11.1 respectively
mA / cm ² . The press pressure during electrode formation is 750 kg
f / cm ² . At the first time, charging is completed at a constant current and constant voltage of 0.85 mA, then discharged at the same current, and in the second cycle,
The charge / discharge capacity was measured using the current density under the above five conditions, and the discharge capacity ratio to the first time was calculated.

For comparison, the bulk mesophase pitch was pulverized by an attrition mill so that about 90% by volume became 1 to 80 μm before carbonization, and then heated at 700 ° C. for 1 hour in an Ar atmosphere to carbonize. Next, the heating temperature was raised to 3000 ° C. as it was, and heat treatment was carried out in the same manner as in the example in an Ar atmosphere to obtain a graphitized graphite powder (i.e., the steps (a) and (b) ) Alone was also examined for high-rate discharge characteristics.

FIG. 6 shows the test results of the discharge characteristics of the graphite powder according to the present invention and the conventional graphite powder at various current densities.
As can be seen from FIG. 6, in the negative electrode of the present invention using graphite powder as an active material, which has been subjected to oxidation treatment and rapid annealing treatment after graphitization, even when the current is increased to 20 mA, 95% at 0.85 mA is obtained.
%, And was excellent in high-rate discharge characteristics. The discharge capacity at 20 mA is 313 mAh / g, and the charge and discharge efficiency
(Percentage of the amount of discharged electricity to the amount of charged electricity) was 92%, which was sufficiently high in each case.

On the other hand, as shown in FIG. 6, in the negative electrode using the conventional graphite powder, a decrease in discharge capacity was observed at a current of 12 mA, and at 20 mA, the capacity decreased to 80% of that at 0.85 mA. did. The discharge capacity at this time is 272 mAh / g, and the charge / discharge efficiency is 80
%Met.

The graphite powder of the present invention is excellent in high-rate discharge characteristics because voids are expanded in a network-like manner due to formation of a mosaic-like surface closed structure by rapid heat treatment, and a path for intrusion and diffusion of Li ions can be easily secured. It is reasonable to think that there is a point.

Example 2 A graphite powder was produced in the same manner as in Example 1 except that the heating rate in the Ar atmosphere after the oxidation heat treatment and the holding temperature were changed as shown in Table 1.

The values of Lc‖ and Lc⊥ in the closed region formed on the surface of the obtained graphite powder and the high-rate discharge characteristics of the negative electrode produced therefrom (the second charge / discharge current was 20 mA (= current density of 11.1) The ratio of the discharge capacity in the case of mA / cm ² ) to the discharge capacity in the case of the initial 0.85 mA) was determined in the same manner as in Example 1, and the results are also shown in Table 1.

[0071]

[Table 1] As shown in Table 1, when the heating rate was 3 ° C./sec during the heat treatment in the Ar atmosphere after the oxidizing heat treatment, the surface of the obtained graphite powder did not have a mosaic closed structure, but had a closed region. Is c
Continuously in the 連 続 direction, the surface morphology is such that the Lc⊥ value of the closed area is 領域.

On the other hand, the heating rate of this heat treatment was 5 ° C.
/ S or more, a mosaic-shaped closed structure was generated, and the high-rate discharge characteristics were improved accordingly. From Table 1,
The higher the rate of temperature rise during heat treatment and the higher the holding temperature, the higher the Lc of each closed area of the generated mosaic-shaped closed structure.
The わ か る value tends to be small, and it can be seen that the smaller the Lc ⊥ value, the higher the high-rate discharge characteristics.

[0073]

According to the present invention, there is provided a graphite powder which is excellent as a negative electrode of a lithium ion secondary battery and has excellent high rate discharge characteristics and sufficiently high discharge capacity and charge / discharge efficiency. Thereby, the performance of the lithium ion secondary battery can be improved. Therefore, the present invention is a technology that contributes to the spread of lithium ion secondary batteries particularly in applications requiring high rate charging and discharging.

[Brief description of the drawings]

FIG. 1 is a schematic explanatory view showing a conventional surface closing structure of graphite powder and a change in the closing structure by heat treatment under conditions outside the scope of the present invention.

FIG. 2 is a schematic explanatory view showing a different aspect of the formation of a surface blocking structure in the process of producing the graphite powder of the present invention.

FIG. 3 is an atomic force micrograph showing the surface morphology of the graphite powder of the present invention.

FIG. 4 is a TEM photograph of a cross section near the surface of the graphite powder of the present invention.

FIG. 5 is a schematic view of the surface of the graphite powder of the present invention observed from directly above, where “a” indicates a closed area portion and “b” indicates a gap portion.

FIG. 6 shows the graphite powder of the present invention and a conventional graphite powder.
5 is a graph showing a change in discharge capacity with respect to charge / discharge current density.

 ────────────────────────────────────────────────── ─── Continuing from the front page (72) Inventor Shinji Soto, 1-8 Fuso-cho, Amagasaki-shi, Hyogo Sumitomo Metal Industries, Ltd. Inside Electronics Research Laboratory (72) Inventor Ken Abe 1-8 Fuso-cho, Amagasaki-shi, Hyogo No. Sumitomo Metal Industries, Ltd. Electronics Research Laboratory (72) Inventor Atsushi Komaru 2nd Hinoguchi, Motomiya-cho, Adachi-gun, Fukushima Prefecture Inside Sony Fukushima Corporation (72) Inventor Yusuke Fujishige 6-7 Kita Shinagawa, Shinagawa-ku, Tokyo No. 35 Inside Sony Corporation (72) Inventor Masayuki Nagamine No. 2 Hinoguchi, Motomiya-cho, Adachi-gun, Fukushima Prefecture F-term inside Sony Fukushima Corporation (reference) 4G046 EA02 EB02 EB04 EB13 EC02 EC06 5H029 AJ03 AJ14 AK03 AL07 AM03 AM04 AM05 AM07 BJ14 CJ02 CJ28 DJ16 DJ17 HJ04 HJ14 5H050 AA08 AA19 BA17 CA08 CA09 CB08 EA10 EA24 FA19 GA02 GA05 GA27 H A13 HA14 HA20

Claims (5)

[Claims] An end surface of a graphite c-plane layer is connected to each other to form a closed closed area scattered on a powder surface,
The length (Lc
Ii) a graphite powder having a particle size of 100 nm or less. 2. The method for producing graphite powder according to claim 1, comprising the following steps. (a) heat-treating the carbon material obtained by carbonizing the carbonaceous material to graphitize; (b) before carbonization, between carbonization and graphitization, and / or
Or at least one grinding step performed after graphitization, (c)
Heat-treating the graphite powder obtained after the steps (a) and (b) under conditions capable of shaving the surface thereof, and (d) placing the graphite powder obtained in the step (c) in an inert gas. 5 ° C /
A process of heating at a temperature of 500 ° C. or more for heat treatment in more than a second. 3. The method for producing graphite powder according to claim 2, wherein the heat treatment in the step (c) is an oxidation heat treatment. 4. A negative electrode for a lithium ion secondary battery, comprising the graphite powder according to claim 1. 5. A lithium ion secondary battery comprising the negative electrode according to claim 4.