TWI739534B - Method for manufacturing graphite material - Google Patents

Abstract

The present invention provides a method for manufacturing a graphite material, which can obtain a graphite material that has high electrode density, high discharge capacity, and excellent rapid charge and discharge characteristics as a negative electrode material for lithium ion secondary batteries, and is also simple and inexpensive in industry . The method for producing the graphite material of the present invention includes a pulverization step of pulverizing the fired product of Mesophase microspheres, and graphitizing the pulverized product obtained in the foregoing pulverization step in the presence of silicon and iron. The graphitization step and the crushing step of crushing the graphitized compound obtained in the foregoing graphitization step.

Description

Manufacturing method of graphite material

The present invention relates to a manufacturing method of graphite materials.

Lithium-ion secondary batteries have excellent characteristics of so-called high voltage and high energy density compared with other secondary batteries, so they are widely and popularly used as power sources for battery devices. In recent years, lithium ion secondary batteries have become used in vehicles, and rapid charge and discharge characteristics or cycle characteristics are more important than in the past.

A general carbon material is used for the negative electrode material of the above-mentioned lithium ion secondary battery. Among them, graphite is also widely used because of its excellent charge and discharge characteristics, high discharge capacity and potential flatness. Examples of graphite used as a negative electrode material include graphite particles such as natural graphite and artificial graphite, mesophase pitch or mesophase small spheres obtained by heat treatment using tar and pitch as raw materials. Bulk mesophase graphitic particles particles) or mesophase microspherical graphitic particles (Mesophase microspherical graphitic particles), mesophase graphite particles or mesophase graphitic fibers obtained by oxidizing the particulate or fibrous mesophase pitch without melting and then undergoing heat treatment. Examples include composite graphite particles obtained by coating natural graphite or artificial graphite with tar, pitch, etc. and then heat-treating it.

Among these graphite materials, in particular, mesophase small spherical graphite particles are also used. Because the crystalline structure in the particles develops in a random direction, it is difficult to align the current collector in parallel when the electrode density is increased, and the cycle characteristics are excellent. feature. On the other hand, compared with natural graphite, its crystallinity is low and its discharge capacity is small. In addition, since the shape of the mesophase small spherical graphite particles is spherical, the contact points between the particles are relatively lacking, and the rapid charge-discharge characteristics tend to be deteriorated.

Moreover, no attempt has been made to improve the discharge capacity and rapid charge-discharge characteristics of the mesophase small spherical graphite particles.

Regarding the capacitance, a method of increasing the degree of graphitization by adding metals or metal compounds such as iron, aluminum, nickel, cobalt, and silicon as a graphitization catalyst is known. For example, Patent Document 1 discloses that by using a specific ratio of iron and silicon as a graphitization catalyst, the discharge capacity can be particularly improved. However, the effect on the rapid charge-discharge characteristics is also unclear.

In addition, as a technique for improving the rapid charge-discharge characteristics, a method of adding conductive materials such as vapor-grown carbon fibers to a graphite material or compounding them is known (Patent Document 2). However, since the discharge capacity or initial charge-discharge efficiency of the conductive material itself is lower than that of the graphite material, these characteristics are lowered according to the added amount. In other words, it has been technically difficult in the past to have both discharge capacity and rapid charge and discharge characteristics.

In addition, Patent Document 3 discloses a method for obtaining fine graphite particles with excellent rapid charge-discharge characteristics by separating the minute bumps generated by graphitization by mechanical energy. However, the density of such fine particles is difficult to increase when the electrode is pressed, and there is a problem that the energy density cannot be increased. In addition, when this method is used, the yield of the separation step is extremely low, and it is not practical in industry. [Prior Technical Literature] [Patent Literature]

[Patent Document 1] JP 2007-31233 A [Patent Document 2] Japanese Patent Application Laid-Open No. 4-237971 [Patent Document 3] JP 2007-191369 A

[Problem Solved by Invention]

The present invention is obtained in view of the above situation, and aims to improve the manufacturing method of industrially simple and inexpensive graphite material that is used as a negative electrode material for lithium ion secondary batteries to obtain high electrode density, high discharge capacity, and excellent rapid charge and discharge characteristics. . [Means to solve the problem]

The present invention provides the following inventions [1] to [6]. [1] Including a pulverization step of pulverizing the fired product of mesophase small spheres, a graphitization step of graphitizing the pulverized product obtained in the foregoing pulverization step in the presence of silicon and iron, and making the aforementioned graphite A manufacturing method of graphite material obtained by crushing the graphitized compound obtained in the chemical step through the crushing step. [2] The method for producing the graphite material described in [1] in which the average particle diameter of the pulverized product is 10 μm or more and 15 μm or less. [3] The addition amount of the aforementioned silicon element is 1 part by mass or more and 5 parts by mass or less per 100 parts by mass of the pulverized product, and the addition amount of the iron element is 1 part by mass or more per 100 parts by mass of the pulverized product, and 5 parts by mass or less of the method for producing the graphite material described in [1] or [2]. [4] The aforementioned crushing step includes the method for producing the graphite material described in any one of [1] to [3] obtained by mechanochemical treatment. [5] The average particle diameter of the aforementioned graphite material is 10 μm or more and 15 μm or less, d ₀₀₂ is 0.3360 nm or less, the specific surface area by the BET method is 3 m ² /g or more, and the mercury intrusion method is less than 0.1 μm. The method for producing the graphite material described in any one of [1] to [4] with a pore volume of 10 μL/g or more. [6] The aforementioned graphite material is the method for producing the graphite material described in any one of [1] to [5] in the negative electrode material of a lithium ion secondary battery. [Effects of Invention]

According to the manufacturing method of the present invention, a graphite material exhibiting high electrode density, high discharge capacity, and excellent rapid charge and discharge characteristics as a negative electrode material for lithium ion secondary batteries can be obtained, and it can also be easily and cheaply obtained industrially. Meet the requirements for rapid charge and discharge characteristics of secondary batteries in recent years.

[Types of Implementation of Invention]

Hereinafter, the present invention will be described in detail.

(Mesophase small sphere) The mesophase pellet system of the starting material of the present invention consists of petroleum-based or petroleum-based tar pitches containing 0.01-2% by mass of free carbon, preferably 0.3-0.9% by mass, at 350-1000°C, which is higher It is preferably obtained by heat treatment at 400 to 600°C, more preferably 400 to 450°C. Examples of the pitches include coal tar, tar light oil, medium tar oil, heavy tar oil, naphthalene oil, anthracene oil, coal tar pitch, pitch oil, oxygen-crosslinked petroleum pitch, heavy oil, etc., but coal tar pitch is preferred .

The average particle size of the mesophase spheres is 20 to 70 μm, preferably 30 to 50 μm. If the particle size is smaller than 20 μm, the discharge capacity improvement effect becomes insufficient.

(Fired) The mesophase spheres are heated at 400-800°C for 1 to 6 hours in an inert environment, and fired to obtain a fired product of mesophase spheres. It can prevent melting during graphitization by forming a fired product of mesophase spheres.

(Crush) Regarding the step of pulverizing the burned material of mesophase pellets in the present invention, the pulverization method is not particularly limited, and any one of a dry method and a wet method can be used, but the dry method is preferred. The average particle size after pulverization is preferably 10-15 μm. In addition, there is no effect even if the classification to adjust the average particle size is performed.

(Silicon and Iron) The silicon element and iron element in the present invention not only contain these element monomers but also silicon compounds and iron compounds. In addition, in the graphitization step described later, if it is vaporized, it may be in the form of an alloy even if it contains other metal elements. Preferably, it is silicon oxide, silicon carbide, iron oxide, iron hydroxide, and ferrosilicon. The silicon and iron are preferably in powder form, and the average particle size is preferably 5 μm or less, and more preferably 1 μm or less.

The addition amount of silicon element and iron element is preferably 1 to 5 parts by mass of each of the element monomers in terms of 100 parts by mass of the pulverized material of the mesophase small sphere fired product. If it is less than 1 part by mass, the effect of the present invention may not be sufficiently obtained in some cases. When it exceeds 5 parts by mass, the graphite material will melt during the graphitization step, thereby degrading battery characteristics.

It is better that silicon and iron are uniformly mixed with the crushed material of the mesophase spheroid before graphitization. The mixing method is not particularly limited, and known mixers such as a stirring type, a rotating type, and a wind type can be used. In addition, before the pulverization step, it may be made to contain the fired product of mesophase pellets, silicon element, and iron element, and pulverization and mixing may also be performed at the same time.

(Graphitization) Graphitization in the present invention can be carried out using, for example, a heat treatment method using a well-known high-temperature furnace such as an Acheson furnace. Because of this, silicon and iron will decompose and evaporate, so they will not remain in the graphite material. Although the heat treatment temperature is undoubtedly the temperature at which silicon and iron can be evaporated, it is specifically 2500°C or higher, preferably 3000°C or higher, and preferably 3100°C or higher. The upper limit is 3300°C. Graphitization is preferably performed in a non-oxidizing environment. Although the time required for graphitization cannot be summarized, it is about 1 to 20 hours. And whether silicon or iron remains after graphitization can be determined by general combustion analysis. As ash content, it is better to have less than 0.03% by mass, and more preferably less than 0.01% by mass.

(crush) The present invention contains the step of crushing the graphite compound. This is because the silicon element reacts with the carbon material in the graphitization step to fuse the graphite particles with each other, so it must be separated to become primary particles again. The average particle size after crushing is preferably in the range of 0.9 to 1.0 compared with the average particle size before graphitization. If the ratio of the average particle size is less than 0.9, the initial charge and discharge efficiency may be reduced due to excessive crushing. If the ratio of the average particle size exceeds 1.0, the crushing will become insufficient, and the electrode density may decrease.

The method of crushing is not particularly limited as long as it can achieve the above-mentioned average particle size, and known crushers such as hammer mills, stirring mills, jet mills, ball mills, and bead mills can be used. Preferably, the use of a hybridization system ((stock) Nara Machinery Manufacturing Co., Ltd.), a mechanical fusion (Mechanofusion) system (Hosokawa Micron (stock)), Nobilta (Hosokawa Micron (stock)), a dry mill (Nippon Coke) Industries (stock)) and other mechanochemical processing machines (shear compression processing machine).

(Graphite material) The graphite material obtained by the manufacturing method of the present invention (hereinafter only described as the graphite material of the present invention) is highly crystalline and exhibits optical anisotropy. _{The crystallinity of graphite can be based on the average lattice spacing d 002} of the (002) plane in X-ray wide-angle diffraction. For the graphite material of the present invention, d ₀₀₂ is preferably 0.3360 nm or less, and 0.3358 nm or less For better. If d ₀₀₂ exceeds 0.3360 nm, high discharge capacity cannot be obtained.

_{Among them, the so-called average lattice plane spacing d 002} of the (002) plane in X-ray wide-angle diffraction means that CuKα rays are used as X-rays, and high-purity silicon is used as a standard material to measure the diffraction absorption peak of the (002) plane of graphite materials. , Calculated from the position of the absorption peak. The calculation method is based on the Gakushin method (measurement method determined by the 17th Committee of the Japan Society for the Promotion of Science), specifically based on the "carbon fiber" [Otani Sugirou, 733-742 (March 1986), Modern Compilation Company] The value determined by the method.

In addition, the graphite material of the present invention is porous and exhibits excellent rapid charge and discharge characteristics as a negative electrode material for lithium ion secondary batteries. The specific surface area obtained by the BET method of the graphite material of the present invention ^{is preferably 3 m 2} /g or more. The upper limit is preferably 5m ² /g. In addition, the volume of pores less than 0.1 μm measured by the mercury intrusion method is preferably 10 μL/g or more. The upper limit is preferably 20 μL/g. Although the specific surface area is less than 3m ² /g, but the pore volume of less than 0.1 μm is less than 10 μL/g, the rapid charge and discharge characteristics may decrease.

In addition, the average particle diameter of the graphite material of the present invention is preferably 10 to 15 μm.

(Lithium ion secondary battery) The graphite material of the present invention can be used as a negative electrode material of a lithium ion secondary battery. The constituent elements of the battery other than the negative electrode material, that is, the positive electrode material, electrolyte, separator, binder, current collector, etc., are not particularly limited, and known technologies related to lithium ion secondary batteries can be used. [Example]

The following examples illustrate the present invention in more detail, but the present invention is not limited to these examples.

Next, the present invention will be described in more detail with examples, but the present invention is not limited to these examples. In addition, in the following examples and comparative examples, as shown in FIG. 1, the current collector (negative electrode) 7b of the negative electrode mixture 2 having the negative electrode material of the present invention adhered to at least a part of the surface, and the current collector (negative electrode) 7b made of lithium foil The single-pole evaluation of the counter electrode (positive electrode) 4 constitutes a key-type secondary battery for evaluation. The actual battery is based on the concept of the present invention and can be manufactured according to known methods. In addition, for the following Examples and Comparative Examples, the physical properties of the materials can be measured by the following methods. The average particle diameter is the particle diameter at which the cumulative degree of the particle size distribution measured by the laser diffraction particle size distribution diameter becomes 50% by volume. The specific surface area is obtained by the BET method by nitrogen adsorption. _{The average lattice plane interval d 002} of the (002) plane in X-ray wide-angle diffraction can be obtained by the aforementioned Gakushin method. The pore volume of 0.1μm or less can be obtained by mercury intrusion method.

(Example 1) [Production of Graphite Material] The coal tar pitch is heat-treated at 450° C. in a nitrogen environment to generate mesophase small spheres (average particle size 40 μm). Secondly, the tar oil is used to extract the pitch matrix from the coal tar pitch, and the mesophase spheres are further separated from the tar oil and dried. After the small spheres were heat-treated at 500° C. for 3 hours in a nitrogen atmosphere, a fired product of mesophase small spheres (average particle size: 34 μm) was obtained. Next, the fired product was pulverized with a hammer mill to have an average particle diameter of 15 μm. 100 parts by mass of the pulverized product, 4.3 parts by mass of silicon dioxide (2 parts by mass of silicon element), and 2.9 parts by mass of second iron oxide (2 parts by mass of iron element) were put into a screw mixer and mixed for 30 minutes. This mixture was filled in a graphite crucible, and heat-treated in an Acheson furnace at 3150°C for 5 hours to perform graphitization. The ash content of the graphitized compound (combustion method) is less than 0.01%. Next, the aforementioned graphitized compound was put into a mechanical fusion system (Hosokawa Micron (strand)), and it was operated for 30 minutes at a roller speed of 20 m/s and crushed. Finally, the crushed material is passed through a 53μm sieve to obtain the target graphite material.

[Making of negative electrode mixture paste] Next, use the graphite material as a negative electrode material to produce a negative electrode as desired. First, 96 parts by mass of the negative electrode material, 2 parts by mass of carboxymethyl cellulose as a binder, and 2 parts by mass of styrene-butadiene rubber are put into water, and the negative electrode mixture paste is adjusted by stirring. Next, the negative electrode mixture layer coated on the copper foil was pressurized at a pressure of 150 MPa. Further, the copper foil and the negative electrode mixture layer were punched into a column with a diameter of 15.5 mm to produce a working electrode (negative electrode) having a negative electrode mixture layer adhered to the copper foil.

[Production of counter electrode (positive electrode)] Secondly, the aforementioned negative electrode was used to make a key type secondary battery for unipolar evaluation. The positive electrode is an electrode plate made of a current collector made of nickel mesh and a lithium metal foil adhered to the current collector.

[Electrolyte･Separator] The electrolyte is prepared by dissolving it in a mixed solvent of 33% by volume of ethylene carbonate and 67% by volume of methyl ethyl carbonate until _{the concentration of LiPF 6} becomes 1 mol/L. Water electrolyte. The obtained non-aqueous electrolyte was impregnated into a polypropylene porous body with a thickness of 20 μm to form a separator impregnated with the electrolyte.

[Assess the composition of the battery] Figure 1 shows a button type secondary battery as a component of the evaluation battery. The outer packaging cup 1 and the outer packaging can 3 are formed by caulking and sealing both edges with an insulating gasket 6 interposed at the edge. In this interior, from the inner surface of the exterior tank 3, a current collector 7a made of nickel mesh, a cylindrical counter electrode (positive electrode) 4 made of lithium foil, and a separator impregnated with electrolyte ( Separator) 5. A battery system in which a current collector 7b made of copper foil attached to the negative electrode mixture 2 is laminated in order. The aforementioned evaluation battery was fabricated as follows. A separator 5 impregnated with an electrolyte was sandwiched between a working electrode (negative electrode) formed by a current collector 7b and a negative electrode mixture 2, and a working electrode (negative electrode) closely adhered to the current collector 7a Laminate between the opposing poles 4, place the current collector 7b in the outer cup 1, place the opposing pole 4 in the outer cup 3, combine the outer cup 1 and the outer cup 3, and then place the outer cup 1 and the outer cup 3 together. 1 The insulating gasket 6 is interposed between the edge part of 1 and the outer can 3, and the two edge parts are caulked and sealed to produce. For the evaluation battery produced as described above, the charge-discharge test shown below was performed at a temperature of 25°C, and the initial charge-discharge efficiency, rapid charge rate, and rapid discharge rate were calculated. In addition, the electrode density is calculated from the thickness and the mass of the negative electrode mixture.

[Initial charge and discharge efficiency] After the 0.9mA constant current charging reaches the loop voltage to 0mV, when the loop voltage reaches 0mV, switch to constant voltage charging, and then continue charging until the current value reaches 20μA. The charge capacity (unit: mAh/g) in a mass unit is obtained from the energization amount during the period. Then it stopped for 120 minutes. Secondly, perform constant current discharge with a current value of 0.9 mA to reach a loop voltage of 1.5V. At this time, the discharge capacity in mass units (unit: mAh/g) is obtained by the amount of energization in the meantime. Calculate the initial charge and discharge efficiency by formula (1). Initial charge and discharge efficiency (%)=(discharge capacity/charge capacity)×100 ･･･(1) In this test, the process of storing lithium ions in the negative electrode material is regarded as charging, and the process of disassociating from the negative electrode material is regarded as discharging.

[Rapid charging rate] The rapid charging is continued in the second cycle. Set the current value to 6mA, perform constant current charging to reach a loop voltage of 0mV, obtain the charging capacity, and calculate the rapid charging rate by formula (2). Rapid charging rate (%)=(rapid constant current charging capacity/initial discharge capacity)×100 ･･･(2)

[Rapid discharge rate] In the same way, rapid discharge is performed in the second cycle. As in the first cycle, after switching to constant voltage charging and fully charging, the current value is set to 12 mA, and constant current discharge is performed to reach a circuit voltage of 1.5V. From the obtained discharge capacity, the rapid discharge rate is calculated by formula (3). Rapid discharge rate (%)=(rapid discharge capacity/initial discharge capacity)×100 ･･･(3)

(Example 2) In Example 1, the pulverized particle diameter of the fired product of mesophase small spheres was set to 10 μm. Otherwise, in the same manner as in Example 1, the preparation of the graphite material and the evaluation of the battery characteristics were performed.

(Example 3) For Example 1, 2.1 parts by mass of silicon dioxide (1 part by mass of silicon element) and 1.4 parts by mass of second iron oxide (1 part by mass of iron element) were used to mix with the pulverized material of the mesophase pellet fired body. Otherwise, the preparation of the graphite material and the evaluation of battery characteristics were performed in the same manner as in Example 1.

(Example 4) In Example 1, 2.1 parts by mass of silicon dioxide (1 part by mass of silicon element) and 4.3 parts by mass of second iron oxide (3 parts by mass of iron element) were used to mix with the pulverized material of the mesophase pellet fired body. Otherwise, the preparation of the graphite material and the evaluation of battery characteristics were performed in the same manner as in Example 1.

(Example 5) In Example 1, 6.4 parts by mass of silicon dioxide (3 parts by mass of silicon element) and 1.4 parts by mass of second iron oxide (1 part by mass of iron element) were used to mix with the pulverized material of the mesophase pellet fired body. Otherwise, the preparation of the graphite material and the evaluation of battery characteristics were performed in the same manner as in Example 1.

(Comparative example 1) In Example 1, 8.6 parts by mass of silicon dioxide (4 parts by mass of silicon element) was used, and the second iron oxide was not used, and mixing with the pulverized product of the mesophase pellet fired body was performed. Otherwise, the preparation of the graphite material and the evaluation of battery characteristics were performed in the same manner as in Example 1.

(Comparative example 2) In Example 1, 5.7 parts by mass of the second oxide iron (4 parts by mass of iron element) was used, and no silicon dioxide was used, and the mixture with the pulverized product of the mesophase pellet fired body was carried out. Otherwise, the preparation of the graphite material and the evaluation of battery characteristics were performed in the same manner as in Example 1.

(Comparative example 3) For Example 1, silicon dioxide and second iron oxide were not used, and only the pulverized product of the mesophase pellet fired was used for graphitization. Otherwise, the preparation of the graphite material and the evaluation of battery characteristics were performed in the same manner as in Example 1.

(Comparative Example 4) For Example 1, mixing with silicon dioxide and second iron oxide was performed without crushing the fired material of mesophase pellets. Otherwise, the preparation of the graphite material and the evaluation of battery characteristics were performed in the same manner as in Example 1.

(Comparative Example 5) For Example 1, the heating conditions were changed to generate mesophase spheres with an average particle diameter of 15 μm. After the small spheres are separated, dried, and fired to obtain a fired product of mesophase small spheres with an average particle diameter of 11 μm, it is mixed with silicon dioxide and second iron oxide without being pulverized. Otherwise, the preparation of the graphite material and the evaluation of battery characteristics were performed in the same manner as in Example 1.

(Comparative Example 6) Regarding Example 1, the preparation of the graphite material and the evaluation of the battery characteristics were performed in the same manner as in Example 1, except that the graphitized material was not crushed.

(Comparative Example 7) For Example 1, the heating conditions were changed, and the particle size of the mesophase spheres was set to 15 μm. After the small spheres were separated, dried, and fired, without pulverization and mixing of silicon dioxide and second iron oxide, the graphite compound was crushed under the mechanical fusion system in the same manner as in Example 1. Otherwise, the preparation of the graphite material and the evaluation of battery characteristics were performed in the same manner as in Example 1.

The above evaluation results are shown in Table 1. The graphite material of the present invention, as a negative electrode material for lithium ion secondary batteries, exhibits high electrode density, high discharge capacity, and excellent rapid charge and discharge characteristics.

1: Outer cup 2: Working electrode (negative electrode) 3: External cans 4: opposite pole (positive) 5: Separator 6: Insulating gasket 7a, 7b: current collector

[Fig. 1] A cross-sectional view showing the structure of a button type evaluation battery used in a charge-discharge test in an embodiment in a model manner.

Claims (4)

A method for manufacturing graphite material, which includes a pulverization step of pulverizing the burned material of mesophase small spheres. The pulverized product obtained in the foregoing pulverization step is processed at 2500~3300°C in the presence of silicon and iron. Heat treatment, decomposing and evaporating the aforementioned silicon and iron elements, the graphitization step of graphitization, and the crushing step of crushing the graphitized product obtained in the aforementioned graphitization step; the average particle diameter of the aforementioned pulverized product is 10μm or more and 15μm or less, for 100 parts by mass of the pulverized product, the addition amount of the silicon element is 1 part by mass or more and 5 parts by mass or less, and for 100 parts by mass of the pulverized product, the addition amount of the iron element is 1 part by mass or more and 5 parts by mass or less. The graphite material manufacturing method of claim 1, wherein the aforementioned crushing step includes mechanochemical treatment. The graphite material manufacturing method of claim 1 or 2, wherein the average particle diameter of the graphite material is 10 μm or more and 15 μm or less, d ₀₀₂ is 0.3360 nm or less, and the specific surface area by the BET method is 3 m ² /g or more, The pore volume of less than 0.1 μm as measured by the mercury intrusion method is 10 μL/g or more. According to claim 1 or 2, the graphite material manufacturing method, wherein the aforementioned graphite material is a negative electrode material of a lithium ion secondary battery.

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