JP6905159B1 - Method for manufacturing graphite material - Google Patents

Abstract

The present invention provides a method for producing a graphite material that is industrially simple and inexpensive because it can obtain high electrode density, high discharge capacity, and excellent rapid charge / discharge characteristics as a negative electrode material for a lithium ion secondary battery. The method for producing a graphite material of the present invention includes a pulverization step of crushing a mesophase microsphere calcined product, a graphitization step of graphitizing the pulverized product obtained in the pulverization step in the presence of silicon element and iron element. It comprises a crushing step of crushing the graphitized product obtained in the graphitization step.

Description

The present invention relates to a method for producing a graphite material.

Lithium-ion secondary batteries have excellent characteristics of high voltage and high energy density as compared with other secondary batteries, and are therefore widely used as power sources for battery devices. In recent years, lithium-ion secondary batteries have come to be used in vehicles, and rapid charge / discharge characteristics and cycle characteristics have become more important than before.

A carbon material is usually used as the negative electrode material of the above-mentioned lithium ion secondary battery. Among them, graphite is widely used because it has excellent charge / discharge characteristics and exhibits high discharge capacity and potential flatness. As the graphite used as the negative electrode material, bulk mesophase graphite particles and mesophase small sphere graphite obtained by heat-treating mesophase pitch and mesophase small spheres made from natural graphite, graphite particles such as artificial graphite, tar, and pitch. Mesophase graphite particles and mesophase graphite fibers obtained by heat treatment after oxidative insolubilization of particles, particle-like or fibrous mesophase pitch, and further, natural graphite or artificial graphite is coated with tar, pitch, etc. and then heat-treated. Examples thereof include composite graphite particles obtained.

Among these graphite materials, mesophase microspherical graphite particles are oriented parallel to the current collector when the electrode density is improved because the crystal structure in the particles develops in random directions. It is difficult to use and has excellent cycle characteristics. On the other hand, it has lower crystallinity and smaller discharge capacity than natural graphite. Further, since the mesophase microspherical graphite particles have a spherical shape, they tend to have poor contact points between the particles and have poor rapid charge / discharge characteristics.

Therefore, attempts have been made to improve the discharge capacity and rapid charge / discharge characteristics of mesophase microspherical graphite particles.

Regarding the discharge capacity, a method of increasing the degree of graphitization by adding a metal such as iron, aluminum, nickel, cobalt, or silicon or a metal compound as a graphitization catalyst is known. For example, Patent Document 1 discloses a technique in which the discharge capacity can be particularly increased by using an iron element and a silicon element in a specific ratio as a graphitization catalyst. However, the effect on the rapid charge / discharge characteristics is not clear.

Further, as a technique for improving the rapid charge / discharge characteristics, a method of blending or compounding a conductive material such as a vapor-grown carbon fiber with a graphite material is known (Patent Document 2). However, since the discharge capacity and the initial charge / discharge efficiency of the conductive material itself are lower than those of the graphite material, these characteristics deteriorate depending on the amount of addition. That is, it is difficult to achieve both discharge capacity and rapid charge / discharge characteristics with the prior art.

Further, Patent Document 3 discloses a method of obtaining fine graphite particles having excellent rapid charge / discharge characteristics by dropping the fine uplifts generated by graphitization by mechanical energy and further separating them. However, there is a problem that the density of such fine particles is difficult to increase when the electrode is pressed, and the energy density cannot be improved. Further, this method has an extremely low yield of the separation step and is not industrially practical.

JP-A-2007-31233 Japanese Unexamined Patent Publication No. 4-237971 JP-A-2007-191369

The present invention has been made in view of the above circumstances, and can provide high electrode density, high discharge capacity, and excellent rapid charge / discharge characteristics as a negative electrode material for a lithium ion secondary battery, and is industrially simple. An object of the present invention is to provide an inexpensive method for producing a graphite material.

The present invention provides the following [1] to [ 5 ].
[1] A crushing step of crushing a mesophase small sphere fired product and
A graphitization step of graphitizing the pulverized product obtained in the pulverization step in the presence of silicon element and iron element,
A method for producing a graphite material, which comprises a crushing step of crushing the graphitized product obtained in the graphitization step.
The average particle size of the pulverized product is 10 μm or more and 15 μm or less.
A method for producing a graphite material, wherein the average particle size of the graphite material is 10 μm or more and 15 μm or less .
[ 2 ] The amount of the silicon element added is 1 part by mass or more and 5 parts by mass or less with respect to 100 parts by mass of the pulverized product.
The method for producing a graphite material according to [1], wherein the amount of the iron element added is 1 part by mass or more and 5 parts by mass or less with respect to 100 parts by mass of the pulverized product.
[ 3 ] The method for producing a graphite material according to [1] or [2] , wherein the crushing step comprises a mechanochemical treatment.
[ 4 ] The d _{002 of the} graphite material is 0.3360 nm or less, the specific surface area by the BET method is 3 m ² / g or more, and the pore volume of less than 0.1 μm measured by the mercury intrusion method is 10 μL / g or more [ 1] The method for producing a graphite material according to any one of [3].
[ 5 ] The method for producing a graphite material according to any one of [1] to [4 ], wherein the graphite material is a negative electrode material for a lithium ion secondary battery.

According to the manufacturing method of the present invention, a graphite material exhibiting high electrode density, high discharge capacity, and excellent rapid charge / discharge characteristics can be obtained industrially easily and inexpensively as a negative electrode material for a lithium ion secondary battery. It is possible to meet the demand for rapid charge / discharge characteristics of recent secondary batteries.

It is sectional drawing which shows typically the structure of the button type evaluation battery for use in the charge / discharge test in an Example.

Hereinafter, the present invention will be specifically described.

(Mesophase small sphere)
The mesophase globules, which are the starting materials of the present invention, contain 350 to 1000 petroleum-based or coal-based tar pitches containing 0.01 to 2% by mass, preferably 0.3 to 0.9% by mass of free carbon. It can be obtained by heat treatment at ° C., preferably 400 to 600 ° C., more preferably 400 to 450 ° C. Examples of the pitches include coal tar, tar light oil, tar medium oil, tar heavy oil, naphthaline oil, anthracene oil, coal tar pitch, pitch oil, oxygen-bridged oil pitch, heavy oil and the like, but coal tar pitch is preferable.

The average particle size of the mesophase globules is 20 to 70 μm, preferably 30 to 50 μm. If the particle size is smaller than 20 μm, the effect of improving the discharge capacity may be insufficient.

(Baking)
The mesophase microspheres are fired by heating at 400 to 800 ° C. for 1 to 6 hours in an inert atmosphere to obtain a mesophase microsphere calcined product. By using a mesophase small sphere calcined product, fusion during graphitization can be prevented.

(Crushing)
In the step of crushing the mesophase small sphere fired product of the present invention, the crushing method is not particularly limited, and either a dry method or a wet method can be used, but the dry method is preferable. The average particle size after pulverization is preferably 10 to 15 μm. Further, classification may be performed in order to adjust the average particle size.

(Silicon element and iron element)
The silicon element and the iron element in the present invention include not only these element elements but also silicon compounds and iron compounds. Further, as long as it evaporates in the graphitization step described later, it may contain other metal elements or may be in the form of an alloy. Preferred are silicon oxide, silicon carbide, iron oxide, iron hydroxide and ferrosilicon.
The silicon element and the iron element are preferably in the form of powder, and the average particle size thereof is preferably 5 μm or less, and more preferably 1 μm or less.

The amount of the silicon element and the iron element added is preferably 1 to 5 parts by mass in terms of elemental substances with respect to 100 parts by mass of the pulverized product 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. If it exceeds 5 parts by mass, the graphite material may be fused in the graphitization step, and the battery characteristics may be deteriorated.

It is preferable that the silicon element and the iron element are uniformly mixed with the pulverized mesophase microsphere calcined product before graphitization. The mixing method is not particularly limited, and a known mixer such as a stirring type, a rotary type, or a wind type can be used. It is also possible to mix the mesophase small sphere fired product, silicon element, and iron element before the crushing step, and crush and mix at the same time.

(Graphitization)
For graphitization in the present invention, a method of heat treatment using a known high-temperature furnace such as an Atison furnace can be adopted. As a result, the silicon element and the iron element are decomposed and evaporated, so that they do not substantially remain in the obtained graphite material. Needless to say, the heat treatment temperature is equal to or higher than the temperature at which the silicon element and the iron element evaporate, but specifically, it is 2500 ° C. or higher, preferably 3000 ° C. or higher, and more preferably 3100 ° C. or higher. The upper limit is 3300 ° C. Graphitization is preferably performed in a non-oxidizing atmosphere. The time required for graphitization cannot be unequivocally determined, but it is about 1 to 20 hours.
Whether or not silicon element or iron element remains after graphitization can be confirmed by general combustion analysis, and the ash content is preferably less than 0.03% by mass, more preferably less than 0.01% by mass. Is even more preferable.

(Crushing)
The present invention includes a step of crushing the graphitized product. This is because the silicon element reacts with the carbon material in the graphitization step and the graphite particles are fused to each other, so that it is necessary to separate the graphite particles into primary particles again. The average particle size after crushing is preferably in the range of 0.9 to 1.0 as compared with the average particle size before graphitization. If the ratio of the average particle size is less than 0.9, over-grinding may occur and the initial charge / discharge efficiency may decrease. If the ratio of the average particle size is more than 1.0, the crushing may be insufficient and the electrode density may decrease.

The crushing method is not particularly limited as long as it can achieve the above-mentioned average particle size, and a known crusher such as a hammer mill, a stirring mill, a jet mill, a ball mill, or a bead mill can be used. Preferably, a mechanochemical processing machine such as a hybridization system (Nara Kikai Seisakusho Co., Ltd.), a mechanofusion system (Hosokawa Micron Co., Ltd.), Nobilta (Hosokawa Micron Co., Ltd.), and a dry attritor (Nippon Coke Industries Co., Ltd.) ( A method using a shear compression processor) can be mentioned.

(Graphite material)
The graphite material obtained by the production method of the present invention (hereinafter, simply referred to as the graphite material of the present invention) has high crystallinity and exhibits optical anisotropy. _{The crystallinity of graphite can be indexed by the average lattice spacing d 002} of the (002) plane in X-ray wide-angle diffraction, and for the graphite material of the present invention, d ₀₀₂ is preferably 0.3360 nm or less. , 0.3358 nm or less, more preferably. If d ₀₀₂ is more than 0.3360 nm, a high discharge capacity may not be obtained.

_{Here, the average lattice spacing d 002} of the (002) plane in X-ray wide-angle diffraction is the diffraction peak of the (002) plane of the graphite material using CuKα rays as X-rays and high-purity silicon as a standard material. Measure and calculate from the position of the peak. The calculation method follows the Gakushin method (measurement method established by the 17th Committee of the Japan Society for the Promotion of Science), specifically "carbon fiber" [Sugio Otani, pp. 733-742 (March 1986)). , Modern editorial company].

Further, the graphite material of the present invention is porous and exhibits excellent rapid charge / discharge characteristics as a negative electrode material for a lithium ion secondary battery. The specific surface area of the graphite material of the present invention by the BET method ^{is preferably 3 m 2} / g or more. The upper limit is ^{preferably 5 m 2} / g.
Further, 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. If the specific surface area is ^{less than 3 m 2} / g or the volume of pores less than 0.1 μm is less than 10 μL / g, the rapid charge / discharge characteristics may deteriorate.

The average particle size 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 for a lithium ion secondary battery. The components of the battery other than the negative electrode material, that is, the positive electrode material, the electrolyte, the separator, the binder, the current collector, and the like are not particularly limited, and known techniques relating to the lithium ion secondary battery can be applied.

Hereinafter, the present invention will be specifically described with reference to Examples, but the present invention is not limited to these Examples.

Next, the present invention will be specifically described with reference to Examples, but the present invention is not limited to these Examples. Further, in the following Examples and Comparative Examples, as shown in FIG. 1, a counter electrode composed of a current collector (negative electrode) 7b to which the negative electrode mixture 2 having the negative electrode material of the present invention is attached to at least a part of the surface and a lithium foil. A button-type secondary battery for unipolar evaluation composed of (positive electrode) 4 was produced and evaluated. The actual battery can be manufactured according to a known method based on the concept of the present invention.
In the following Examples and Comparative Examples, the physical characteristics of the materials were measured by the following methods.
The average particle size is a particle size at which the cumulative frequency of the particle size distribution measured by the laser diffraction type particle size distribution diameter is 50% by volume.
The specific surface area was determined by the BET method by adsorbing nitrogen gas.
_{The average lattice plane spacing d 002} of the (002) plane in X-ray wide-angle diffraction was determined by the above-mentioned Gakushin method.
The pore volume of 0.1 μm or less was determined by the mercury intrusion method.

(Example 1)
[Preparation of graphite material]
The coal tar pitch was heat-treated at 450 ° C. in a nitrogen atmosphere to produce mesophase globules (average particle size 40 μm). Then, the pitch matrix was extracted from the coal tar pitch using the tar oil, and the mesophase globules were further separated from the tar oil and dried. The microspheres were heat-treated at 500 ° C. for 3 hours in a nitrogen atmosphere to obtain a calcined mesophase microspheres (average particle size 34 μm).
Next, the fired product was crushed with a hammer mill to have an average particle size 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 ferric oxide (2 parts by mass of iron element) were put into a screw mixer and mixed for 30 minutes. The mixture was filled in a graphite crucible and heat-treated in an Achison furnace at 3150 ° C. for 5 hours for graphitization. The ash content (combustion method) of the graphitized product was less than 0.01%.
Next, the graphitized product was put into a mechanofusion system (Hosokawa Micron Co., Ltd.) and operated at a rotor peripheral speed of 20 m / s for 30 minutes to perform crushing. Finally, the pyroclastic material was passed through a 53 μm sieve to obtain the desired graphite material.

[Preparation of negative electrode mixture paste]
Next, a negative electrode was produced using the graphite material as a negative electrode material. 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 were put into water and stirred to prepare a negative electrode mixture paste. Next, the negative electrode mixture layer coated on the copper foil was pressed at a pressure of 150 MPa. Further, the copper foil and the negative electrode mixture layer were punched into a columnar shape having a diameter of 15.5 mm to prepare a working electrode (negative electrode) having a negative electrode mixture layer in close contact with the copper foil.

[Preparation of counter electrode (positive electrode)]
Next, a button-type secondary battery for unipolar evaluation was produced using the negative electrode. For the positive electrode, a current collector made of a nickel net and a electrode plate made of a lithium metal foil in close contact with the current collector were used.

[Electrolytic solution / separator]
_{The electrolytic solution was prepared by dissolving LiPF 6} in a mixed solvent of 33% by volume of ethylene carbonate and 67% by volume of methyl ethyl carbonate at a concentration of 1 mol / L to prepare a non-aqueous electrolytic solution. The obtained non-aqueous electrolytic solution was impregnated into a polypropylene porous body having a thickness of 20 μm to prepare a separator impregnated with the electrolytic solution.

[Evaluation battery configuration]
FIG. 1 shows a button-type secondary battery as a configuration of the evaluation battery.
The outer cup 1 and the outer can 3 are sealed by caulking both peripheral portions with an insulating gasket 6 interposed at the peripheral portion thereof. From the inner surface of the outer can 3, copper with a current collector 7a made of nickel net, a cylindrical counter electrode (positive electrode) 4 made of lithium foil, a separator 5 impregnated with an electrolytic solution, and a negative electrode mixture 2 adhered to the inside thereof. It is a battery system in which a current collector 7b made of foil is laminated.
In the evaluation battery, a separator 5 impregnated with an electrolytic solution is sandwiched between a working electrode (negative electrode) composed of a current collector 7b and a negative electrode mixture 2 and a counter electrode 4 in close contact with the current collector 7a, and then laminated. The current collector 7b is housed in the outer cup 1, the counter electrode 4 is housed in the outer can 3, the outer cup 1 and the outer can 3 are combined, and an insulating gasket is provided on the peripheral edge of the outer cup 1 and the outer can 3. 6 was interposed, and both peripheral edges were crimped and hermetically sealed.
The evaluation batteries manufactured as described above were subjected to the following charge / discharge tests at a temperature of 25 ° C., and the initial charge / discharge efficiency, the fast charge rate, and the fast discharge rate were calculated.
The electrode density was calculated from the thickness and the mass of the negative electrode mixture.

[Initial charge / discharge efficiency]
After constant current charging of 0.9 mA until the circuit voltage reached 0 mV, switching to constant voltage charging was performed when the circuit voltage reached 0 mV, and charging was continued until the current value reached 20 μA. The charge capacity per mass (unit: mAh / g) was determined from the amount of electricity energized during that period. Then, it rested for 120 minutes. Next, constant current discharge was performed with a current value of 0.9 mA until the circuit voltage reached 1.5 V, and the discharge capacity per mass (unit: mAh / g) was determined from the amount of energization during this period. The initial charge / discharge efficiency was calculated by the formula (1).
Initial charge / discharge efficiency (%) = (discharge capacity / charge capacity) x 100 ... (1)
In this test, the process of occluding lithium ions in the negative electrode material was defined as charging, and the process of releasing lithium ions from the negative electrode material was defined as discharging.

[Fast charge rate]
Subsequently, quick charging was performed in the second cycle.
With the current value set to 6 mA, constant current charging was performed until the circuit voltage reached 0 mV, the charging capacity was obtained, and the quick charging rate was calculated by the formula (2).
Rapid charge rate (%) = (Rapid constant current charge capacity / initial discharge capacity) x 100 ... (2)

[Fast discharge rate]
Fast discharge was performed in the same second cycle.
After switching to constant voltage charging and fully charging in the same manner as in the first cycle, constant current discharge was performed until the circuit voltage reached 1.5 V, with the current value set to 12 mA. From the obtained discharge capacity, the rapid discharge rate was calculated by the formula (3).
Rapid discharge rate (%) = (fast discharge capacity / initial discharge capacity) x 100 ... (3)

(Example 2)
In Example 1, the pulverized particle size of the mesophase small sphere fired product was set to 10 μm. Other than that, the graphite material was prepared and the battery characteristics were evaluated in the same manner as in Example 1.

(Example 3)
In Example 1, 2.1 parts by mass of silicon dioxide (1 part by mass of silicon element) and 1.4 parts by mass of ferric oxide (1 part by mass of iron element) were mixed with a pulverized mesophase small sphere calcined product. went. Other than that, the graphite material was prepared and the battery characteristics were evaluated 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 ferric oxide (3 parts by mass of iron element) were mixed with a pulverized mesophase small sphere. went. Other than that, the graphite material was prepared and the battery characteristics were evaluated 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 ferric oxide (1 part by mass of iron element) were mixed with a pulverized mesophase small spherical body. went. Other than that, the graphite material was prepared and the battery characteristics were evaluated 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 ferric oxide was not used, and the mixture was mixed with a pulverized mesophase small sphere calcined product. Other than that, the graphite material was prepared and the battery characteristics were evaluated in the same manner as in Example 1.

(Comparative Example 2)
In Example 1, 5.7 parts by mass of ferric oxide (4 parts by mass of iron element) was used, and silicon dioxide was not used, and the mixture was mixed with the pulverized mesophase small sphere calcined product. Other than that, the graphite material was prepared and the battery characteristics were evaluated in the same manner as in Example 1.

(Comparative Example 3)
In Example 1, graphitization was carried out using only the pulverized mesophase microsphere calcined product without using silicon dioxide or ferric oxide. Other than that, the graphite material was prepared and the battery characteristics were evaluated in the same manner as in Example 1.

(Comparative Example 4)
In Example 1, silicon dioxide and ferric oxide were mixed without pulverizing the calcined mesophase microspheres. Other than that, the graphite material was prepared and the battery characteristics were evaluated in the same manner as in Example 1.

(Comparative Example 5)
In Example 1, mesophase globules having an average particle size of 15 μm were generated by changing the heating conditions. A mesophase small sphere calcined product having an average particle size of 11 μm was obtained by separating, drying, and firing the small spheres, and then mixed with silicon dioxide and ferric oxide without pulverization. Other than that, the graphite material was prepared and the battery characteristics were evaluated in the same manner as in Example 1.

(Comparative Example 6)
In Example 1, the graphite material was prepared and the battery characteristics were evaluated in the same manner as in Example 1 except that the graphitized product was not crushed.

(Comparative Example 7)
In Example 1, the heating conditions were changed so that the generated particle size of the mesophase globules was 15 μm. After the small spheres were separated, dried, and calcined, the graphitized product was crushed by a mechanofusion system in the same manner as in Example 1 without pulverization or mixing of silicon dioxide and ferric oxide. Other than that, the graphite material was prepared and the battery characteristics were evaluated in the same manner as in Example 1.

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

1 Exterior cup 2 Working electrode (negative electrode)
3 Exterior can 4 Counter electrode (positive electrode)
5 Separator 6 Insulation gasket 7a, 7b Current collector

Claims (5)

A crushing process for crushing mesophase small sphere fired products,
A graphitization step of graphitizing the pulverized product obtained in the pulverization step in the presence of silicon element and iron element,
A method for producing a graphite material, which comprises a crushing step of crushing the graphitized product obtained in the graphitization step.
The average particle size of the pulverized product is 10 μm or more and 15 μm or less.
A method for producing a graphite material, wherein the average particle size of the graphite material is 10 μm or more and 15 μm or less . The amount of the silicon element added is 1 part by mass or more and 5 parts by mass or less with respect to 100 parts by mass of the pulverized product.
The method for producing a graphite material according to claim 1 , wherein the amount of the iron element added is 1 part by mass or more and 5 parts by mass or less with respect to 100 parts by mass of the pulverized product. The method for producing a graphite material according to claim 1 or 2 , wherein the crushing step comprises a mechanochemical treatment. Claims 1 to that the d _{002 of the} graphite material is 0.3360 nm or less, the specific surface area by the BET method is 3 m ² / g or more, and the pore volume of less than 0.1 μm measured by the mercury intrusion method is 10 μL / g or more. The method for producing a graphite material according to any one of 3. The method for producing a graphite material according to any one of claims 1 to 4 , wherein the graphite material is a negative electrode material for a lithium ion secondary battery.

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