JP2004099438A - Graphite powder, negative pole for lithium secondary battery and lithium secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide graphite powder suitable for a lithium secondary battery having high capacity and small irreversible capacity in a 1st cycle and an excellent quick charge and discharge characteristic, a negative pole for the lithium secondary battery having small irreversible capacity in a 1st cycle and the excellent quick charge and discharge characteristic, and the lithium secondary battery having the small irreversible capacity in the 1st cycle and the excellent quick charge and discharge characteristic. <P>SOLUTION: The graphite powder has 10-50 μm average particle diameter, ≤8 m<SP>2</SP>/g specific surface area, an aspect ratio of ≤5 and ≥500 Å crystallite size Lc(002) in the (c) axial direction. The negative pole for the lithium secondary battery contains the graphite powder and the lithium secondary battery has the negative pole and a positive pole. <P>COPYRIGHT: (C)2004,JPO

Description

The present invention relates to a graphite powder, a method for producing the same, a negative electrode for a lithium secondary battery, and a lithium secondary battery. More specifically, a lithium secondary battery with a small irreversible capacity, excellent rapid charge / discharge characteristics, etc., and a high capacity lithium secondary battery suitable for use in portable equipment, electric vehicles, power storage, etc. The present invention relates to a negative electrode for a battery and the graphite powder for a negative electrode for a lithium secondary battery.

Conventional graphite particles include, for example, natural graphite particles, artificial graphite particles obtained by graphitizing coke, artificial graphite particles obtained by graphitizing an organic polymer material or pitch, graphite particles obtained by crushing them, and a high-density graphite molded product obtained by crushing. There are graphite particles and the like. These graphite particles are mixed with an organic binder and an organic solvent to form a graphite paste, the graphite paste is applied to the surface of a copper foil, and the solvent is dried to be used as a negative electrode for a lithium secondary battery. . For example, as shown in JP-B-62-23433, the problem of internal short circuit due to lithium dendrite is solved by using graphite for the negative electrode, and the cycle characteristics are improved.

However, graphite materials such as natural graphite in which graphite crystals have been developed and artificial graphite obtained by graphitizing coke have a weaker bonding force between layers of crystals in the c-axis direction than bonding in the crystal plane direction. The pulverization breaks the bond between the graphite layers, resulting in so-called scale-like graphite particles having a large aspect ratio. Since the scale-like graphite has a large aspect ratio, when the electrode is produced by kneading with a binder and applying to a current collector, the scale-like graphite particles are oriented in the plane direction of the current collector, and as a result, the graphite crystal is formed. There is a problem that the inside of the electrode is broken by expansion and contraction in the C-axis direction caused by repetition of occlusion and desorption of lithium, thereby deteriorating cycle characteristics. In addition, these graphites have a problem that the surface state of the graphite particles changes due to the impact due to the pulverization, resulting in an increase in the specific surface area and an increase in the irreversible capacity in the first cycle of the manufactured lithium secondary battery. Therefore, there is a demand for a graphite powder that has a small irreversible capacity in the first cycle of the lithium secondary battery and can improve the cycle characteristics of high-capacity rapid charge / discharge characteristics.

The inventions of claims 1 and 2 provide a method for producing a graphite powder suitable for a lithium secondary battery having a high capacity, a small irreversible capacity in the first cycle, and an excellent rapid charge / discharge characteristic. The third aspect of the present invention provides a graphite powder suitable for a lithium secondary battery having a high capacity, a small irreversible capacity in the first cycle, and excellent rapid charge / discharge characteristics. The fourth aspect of the present invention is to provide a negative electrode for a lithium secondary battery having a small irreversible capacity in the first cycle and excellent in rapid charge / discharge characteristics. The invention described in claim 5 provides a lithium secondary battery having a small irreversible capacity in the first cycle and excellent in rapid charge / discharge characteristics.

The present invention relates to a method for producing graphite powder, comprising a step of pulverizing a graphite molded body having a bulk density of 1.6 g / cm ³ or less. Further, the present invention provides a graphite molded article having a bulk density of 1.6 g / cm ³ or less, which is obtained by graphitizing a graphite precursor containing at least one component that volatilizes in a range of 400 to 3200 ° C. And a method for producing graphite powder. The present invention also relates to a graphite powder having an average particle size of 10 to 50 μm, a specific surface area of 8 m ² / g or less, and an aspect ratio of 5 or less. The present invention also relates to a negative electrode for a lithium secondary battery containing the graphite powder or the graphite powder obtained by the production method. Further, the present invention relates to a lithium secondary battery having the above-described negative electrode and positive electrode.

According to the production method of claims 1 and 2, a graphite powder suitable for a lithium secondary battery having high capacity, small irreversible capacity in the first cycle, and excellent in rapid charge / discharge characteristics can be obtained. The graphite powder according to claim 3 is suitable for a lithium secondary battery having a high capacity, a small irreversible capacity in the first cycle, and excellent in rapid charge / discharge characteristics. The negative electrode for a lithium secondary battery according to claim 4 has a small irreversible capacity in the first cycle and is excellent in rapid charge / discharge characteristics. The lithium secondary battery according to claim 5 has a small irreversible capacity in the first cycle and is excellent in rapid charge / discharge characteristics.

The graphite powder of the present invention is characterized by including a step of pulverizing a graphite molded body having a bulk density of 1.6 g / cm ³ or less. The bulk density of the graphite molded grinding is preferably in the range of 1.0~1.6g / cm ^3, more preferably be in the range of 1.2~1.5g / cm ^3, 1.2~1.45g / A range of cm ³ is particularly preferable. If the bulk density of the graphite compact to be crushed exceeds 1.6 g / cm ³ , a large crushing force is required in crushing, and the state of the surface of the graphite powder to be produced is changed by the crushing force, and as a result, the specific surface area increases. However, the irreversible capacity of the manufactured lithium secondary battery is reduced. Further, the crystallinity of the surface of the graphite powder to be produced also decreases, and the discharge capacity of the obtained lithium secondary battery decreases. On the other hand, when the density of the graphite molded body to be ground is less than 1.0 g / cm ³ , the handleability of the graphite molded body tends to decrease. Further, the furnace filling weight at the time of graphitization tends to decrease, and the graphitization treatment efficiency tends to deteriorate. The bulk density can be calculated from the measured values of the weight and volume of the graphite molded body.

There is no particular limitation on the method for producing a graphite molded body having a bulk density of 1.6 g / cm ³ or less. For example, it can be obtained by mixing a material containing a graphitizable aggregate or graphite and a graphitizable binder, and then heat-treating the graphite precursor formed into a predetermined shape in a non-oxidizing atmosphere.

骨 The graphitizable aggregate is not particularly limited as long as it is a powder material that can be graphitized. For example, coke powder, carbide of resin, and the like can be used. As the graphite, for example, natural graphite powder, artificial graphite powder and the like can be used, but there is no particular limitation as long as the powder is in the form of powder. The particle size of the graphitizable aggregate or graphite is preferably smaller than the particle size of the graphite powder produced in the present invention. Examples of the binder which can be graphitized include tar, pitch, and organic materials such as thermosetting resins and thermoplastic resins. The amount of the binder is preferably 30 to 50% by weight based on the total amount of the graphitizable aggregate or graphite from the viewpoint of the density of the graphite compact to be produced and the aspect ratio and the specific surface area of the graphite powder to be produced. The method of mixing the graphitizable aggregate or graphite with the graphitizable binder is not particularly limited, but it is preferable to mix at a temperature equal to or higher than the softening temperature of the binder, and the temperature varies depending on the type of the binder used. The range of -350 ° C is preferred.

成形 The shape of the graphite precursor is not particularly limited, but is preferably, for example, a block, a cylinder, or a lump in terms of workability and packing efficiency in a furnace during graphitization. There is no particular limitation on the size of the molded body, but a molded article having a length of 5 mm or more is preferable. Here, in order to obtain the above bulk density, a method of including a component which is volatilized in a temperature range of 400 to 3200 ° C. in the graphite precursor, a method of lowering the pressing pressure when press-forming the graphite precursor, and reducing the bulk density And the like, but it is preferable to include a component that volatilizes in a temperature range of 400 to 3200 ° C. in the graphite precursor. In this method, if the volatilization temperature of the contained component is lower than 400 ° C., the bulk density of the graphite molded body tends to increase, and if it exceeds 3200 ° C., there is a problem that the volatile component tends to remain in the graphite molded body. Here, the volatilization temperature refers to a temperature at which the components volatilize at normal pressure due to sublimation, decomposition, melt gasification, and the like, causing a weight loss. In general, graphite precursors tend to shrink and become denser when carbonized at a firing temperature of 400 to 900 ° C. Therefore, as a volatile component, even after shrinkage during carbonization at 400 to 1000 ° C., it is preferable in terms of the density of a graphite molded body that easily remains, and the volatilization temperature is preferably in a range of 800 to 3000 ° C., The range of 1000-3000 degreeC is more preferable.

The addition of a component that volatilizes in a temperature range of 400 to 3200 ° C. may be added before molding the graphite precursor, but in terms of uniform mixing, a graphitizable aggregate or graphite and a graphitizable binder are added. It is preferable to add them at the time of mixing and mix them simultaneously. A graphite precursor containing a component that volatilizes in a temperature range of 400 to 3200 ° C. is volatilized when performing a heat treatment for firing or graphitization in a non-oxidizing atmosphere, so that the bulk density is 1.6 g / cm ^3. The following low-density graphite molded body can be obtained. There are no particular restrictions on the types of components that evaporate in the temperature range of 400 to 3200 ° C., for example, thermoplastic resins such as polyvinyl alcohol (evaporation temperature of 400 to 700 ° C.) and resins obtained from plants such as rosin (evaporation temperature of 400). To 1000 ° C.), metals such as silicon, boron, iron, titanium and nickel, oxides and carbides thereof, and the like (volatile temperature of 2200 to 3200 ° C.). Among these, the use of a metal, an oxide or a carbide thereof also functions as a graphitization catalyst, and is more preferable in view of the crystallinity of the graphite powder to be produced.

焼 成 The firing of the graphite precursor is preferably performed under conditions that are difficult to oxidize, and examples include a method of firing in a vacuum in a nitrogen atmosphere, an argon atmosphere, or a vacuum. The firing temperature is preferably 2000 ° C. or higher, more preferably 2500 ° C. or higher, even more preferably 2800 ° C. or higher. If the firing temperature is low, the development of graphite crystals is poor, and the discharge capacity tends to be low. When the temperature for graphitization is low, the added volatile component tends to remain in the graphite molded body, and the density of the graphite molded body tends to increase. Although there is no upper limit on the firing temperature, it is generally 3200 ° C. or lower. When the volatile components remain in the graphite compact to be produced, not only the specific surface area of the obtained graphite powder increases, but also the discharge capacity tends to decrease. Is preferably increased. The temperature for firing may be increased stepwise. For example, it is also possible to temporarily fire at about 500 to 1200 ° C. and then fire at 2000 ° C. or more.

Thus, a graphite molded body having a bulk density of 1.6 g / cm ³ or less is obtained, and then pulverized. The method of pulverization is not particularly limited, and for example, impact pulverization such as a jet mill, a hammer mill, and a pin mill is preferable in terms of specific surface area, irreversible capacity, and discharge capacity. The average particle size of the pulverized graphite powder is preferably 10 to 50 μm. In the present invention, the average particle size can be measured by a laser diffraction particle size distribution meter.

According to the method as described above, the obtained graphite powder can have a specific surface area of 8 m ² / g or less. The specific surface area is more preferably 6 m ² / g or less, further preferably 4 m ² / g or less. The specific surface area can be measured by a BET method using nitrogen gas adsorption. The obtained graphite powder preferably has an aspect ratio of 5 or less, more preferably 3 or less. If the aspect ratio is too large, the rapid charge / discharge characteristics tend to decrease. The aspect ratio is represented by A / B, where A is the length of the graphite particle in the major axis direction and B is the length of the graphite particle in the minor axis direction. The aspect ratio of the present invention is obtained by magnifying graphite particles with a microscope, arbitrarily selecting 10 or more graphite particles, measuring A / B, and taking the average value.

{Also, the interlayer distance d (002) of the obtained graphite powder in X-ray wide angle diffraction is preferably 3.40 ° or less, more preferably 3.38 ° or less, and particularly preferably 3.37 ° or less. The crystallite size Lc (002) in the c-axis direction is preferably at least 500 °, more preferably at least 1000 °. As the interlayer distance d (002) of the crystal decreases or the crystallite size Lc (002) in the c-axis direction increases, the discharge capacity increases.

(4) The graphite powder of the present invention is used for a negative electrode for a lithium secondary battery, and is kneaded with an organic binder and a solvent, and is formed into a sheet shape, a pellet shape, or the like. As the organic binder, for example, polyethylene, polypropylene, ethylene propylene terpolymer, butadiene rubber, styrene butadiene rubber, butyl rubber, a polymer compound having a high ionic conductivity, and the like can be used. In the present invention, as the polymer compound having a large ionic conductivity, polyvinylidene fluoride, polyethylene oxide, polyepichlorohydrin, polyphosphazene, polyacrylonitrile, and the like can be used. Among these, a polymer compound having a high ionic conductivity is preferable, and polyvinylidene fluoride is particularly preferable.

混合 The mixing ratio of the graphite powder and the organic binder is preferably 20 parts by weight or less, more preferably 3 to 10 parts by weight, with respect to 100 parts by weight of the graphite powder. The solvent is not particularly limited, and N-methyl 2-pyrrolidone, dimethylformamide, isopropanol and the like are used. The amount of the solvent is not particularly limited as long as it can be adjusted to a desired viscosity, but is preferably used in an amount of 30 to 70% by weight based on the mixture. The graphite powder is kneaded with an organic binder and a solvent, and after adjusting the viscosity, is applied to a current collector and integrated with the current collector to form a negative electrode. As the current collector, for example, a metal current collector such as a foil of nickel, copper or the like, or a mesh can be used. In addition, the integration can be performed by a molding method such as a roll, a press, or the like, and may be integrated by combining these.

The negative electrode thus obtained is used as a negative electrode of a lithium ion secondary battery, a lithium polymer secondary battery, or the like. For example, in a lithium ion secondary battery, a positive electrode is arranged to face a separator, and an electrolyte is injected. By using the negative electrode for a lithium secondary battery of the present invention, compared with a lithium secondary battery using a conventional carbon material for the negative electrode, the irreversible capacity is small, high capacity, rapid charge / discharge characteristics, and excellent cycle characteristics. A lithium secondary battery can be manufactured.

The material used for the positive electrode of the lithium secondary battery in the present invention is not particularly limited, and for example, LiNiO ₂ , LiCoO ₂ , LiMn ₂ O ₄ , or the like can be used alone or as a mixture. Examples of the electrolyte include lithium salts such as LiClO ₄ , LiPF ₆ , LiAsF ₆ , LiBF ₄ , and LiSO ₃ CF _3. A so-called organic electrolytic solution dissolved or contained in a solid polymer electrolyte such as vinylidene fluoride and polyaniline can be used. As a separator used when a liquid electrolyte is used, for example, a nonwoven fabric, a cloth, a microporous film, or a combination thereof, containing a polyolefin such as polyethylene or polypropylene as a main component can be used.

FIG. 1 shows a partial cross-sectional front view of an example of a cylindrical lithium ion secondary battery. The cylindrical lithium ion secondary battery shown in FIG. 1 is obtained by winding a positive electrode 1 processed into a thin plate and a negative electrode 2 processed in the same manner via a separator 3 such as a polyethylene microporous membrane. It is turned and inserted into a battery can 7 made of metal or the like to be sealed. The positive electrode 1 is joined to a positive electrode lid 6 via a positive electrode tab 4, and the negative electrode 2 is joined to a battery bottom via a negative electrode tab 5. The positive electrode lid 6 is fixed to the battery can 7 with a gasket 8.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

Example 1
50 parts by weight of coke powder having an average particle size of 10 μm, 15 parts by weight of pitch, 10 parts by weight of silicon carbide (volatilization temperature 2500 to 3000 ° C.), and 10 parts by weight of coal tar were mixed and stirred at 200 ° C. for 1 hour. Next, this mixture was pulverized to an average particle size of 20 μm, and the pulverized product was put into a mold and press-formed to obtain a rectangular graphite precursor molded body having a size of 15 mm × 25 cm × 6 cm. This graphite precursor compact was heat-treated at 1000 ° C. in a nitrogen atmosphere, and further heat-treated at 3000 ° C. in a nitrogen atmosphere to obtain a graphite compact. The bulk density of the obtained graphite molded body was calculated from the measured values of the weight and volume of the graphite molded body. Further, the graphite compact was pulverized to obtain graphite powder. The average particle size of the obtained graphite powder was determined by a laser diffraction particle size analyzer, and the specific surface area was determined by a BET five-point method using nitrogen gas adsorption. The aspect ratio was measured by enlarging the obtained graphite powder with an electron microscope, arbitrarily selecting ten pieces, and measuring the average value of the aspect ratio. Table 1 shows the measurement results of the density of the graphite compact before pulverization, the average particle size of the graphite powder, the specific surface area, and the aspect ratio.

Next, to 90% by weight of the obtained graphite particles, 10% by weight of solid content of polyvinylidene fluoride (PVDF) dissolved in N-methyl-2-pyrrolidone was added and kneaded to prepare a graphite paste. This graphite paste was applied to a rolled copper foil having a thickness of 10 μm, further dried, and compression-molded under a pressure of 490 MPa (0.5 ton / cm ² ) to obtain a sample electrode. The thickness of the graphite particle layer was 90 μm, and the density was 1.5 g / cm ³ . The prepared sample electrode was charged and discharged at a constant current by a three-terminal method, and evaluated as a negative electrode for a lithium secondary battery.

FIG. 2 is a schematic diagram of the prepared lithium secondary battery. The evaluation of the sample electrode was performed by using LiPF ₆ as an electrolyte 10 in ethylene carbonate (EC) and dimethyl carbonate (DMC) (FIG. 2). A solution prepared by dissolving EC and DMC at a volume ratio of 1 mol / L in a mixed solvent of 1: 1) is added, and the sample electrode 11, the separator 12, and the counter electrode 13 are stacked and arranged. 14 was suspended from above to produce a lithium secondary battery. Note that metallic lithium was used for the counter electrode 13 and the reference electrode 14, and a polyethylene microporous membrane was used for the separator 4. The obtained lithium secondary battery was charged between the sample electrode 11 and the counter electrode 13 at a constant current of 0.2 mA / cm ² to 5 mV (Vvs. Li / Li ⁺ ) with respect to the area of the sample electrode. The test of discharging to 1 V (Vvs. Li ^/ Li ⁺ ) was repeated. The charge / discharge was repeated while replacing the counter electrode metal lithium with a new one every 30 cycles. Table 1 shows the discharge capacity at the first cycle, the irreversible capacity, and the discharge capacity at the 100th cycle.

Example 2
A method similar to that of Example 1 except that 50 parts by weight of coke powder having an average particle size of 10 μm, 15 parts by weight of pitch, 5 parts by weight of silicon carbide, and 10 parts by weight of coal tar were mixed and stirred at 200 ° C. for 1 hour. A graphite molded body and a graphite powder were produced. Table 1 shows the measurement results of the density of the graphite compact before pulverization, the average particle size of the graphite powder, the specific surface area, and the aspect ratio. A charge / discharge test was performed in the same manner as in Example 1. Table 1 shows the discharge capacity at the first cycle, the irreversible capacity, and the discharge capacity at the 100th cycle.

Example 3
A method similar to that of Example 1 except that 50 parts by weight of coke powder having an average particle diameter of 10 μm, 15 parts by weight of pitch, 2 parts by weight of silicon carbide, and 10 parts by weight of coal tar were mixed and stirred at 200 ° C. for 1 hour. A graphite molded body and a graphite powder were produced. Table 1 shows the measurement results of the density of the graphite compact before pulverization, the average particle size of the graphite powder, the specific surface area, and the aspect ratio. A charge / discharge test was performed in the same manner as in Example 1. Table 1 shows the discharge capacity at the first cycle, the irreversible capacity, and the discharge capacity at the 100th cycle.

Example 4
A method similar to that of Example 1 except that 50 parts by weight of coke powder having an average particle size of 10 μm, 15 parts by weight of pitch, 1 part by weight of silicon carbide, and 10 parts by weight of coal tar were mixed and stirred at 200 ° C. for 1 hour. A graphite molded body and a graphite powder were produced. Table 1 shows the measurement results of the density of the graphite compact before pulverization, the average particle size of the graphite powder, the specific surface area, and the aspect ratio. A charge / discharge test was performed in the same manner as in Example 1. Table 1 shows the discharge capacity at the first cycle, the irreversible capacity, and the discharge capacity at the 100th cycle.

Comparative Example 1
50 parts by weight of coke powder having an average particle size of 10 μm, 15 parts by weight of pitch, and 10 parts by weight of coal tar were mixed and stirred at 200 ° C. for 1 hour. Next, this mixture was pulverized to an average particle size of 20 μm, and the pulverized product was put into a mold and formed into a rectangular parallelepiped by a cold isostatic press (CIP press). Next, after heat-treating this compact at 1000 ° C. in a nitrogen atmosphere, pitch impregnation was performed, and this operation was repeated twice to obtain a graphite precursor. This graphite precursor was heat-treated at 1000 ° C. in a nitrogen atmosphere and further heat-treated at 3000 ° C. in a nitrogen atmosphere to obtain a high-density graphite molded body. Further, the graphite compact was pulverized to obtain graphite powder. Table 1 shows the measurement results of the density of the graphite compact before pulverization, the average particle size of the graphite powder, the specific surface area, and the aspect ratio. A charge / discharge test was performed in the same manner as in Example 1. Table 1 shows the discharge capacity at the first cycle, the irreversible capacity, and the discharge capacity at the 100th cycle.

　表１に示されるように、本発明になる黒鉛粉末は、比表面積及びアスペクト比が小さく、第一サイクル目の不可逆容量が小さく、さらにサイクル特性に優れたリチウム二次電池用負極として好適であることが示された。 Comparative Example 2
50 parts by weight of natural graphite powder having an average particle size of 15 μm, 15 parts by weight of pitch, and 20 parts by weight of coal tar were mixed and stirred at 200 ° C. for 1 hour, and the same procedure as in Example 1 was repeated to form a graphite compact and graphite powder. Was prepared. Table 1 shows the measurement results of the density of the graphite compact before pulverization, the average particle size of the graphite powder, the specific surface area, and the aspect ratio. A charge / discharge test was performed in the same manner as in Example 1. Table 1 shows the discharge capacity at the first cycle, the irreversible capacity, and the discharge capacity at the 100th cycle.

As shown in Table 1, the graphite powder according to the present invention has a small specific surface area and an aspect ratio, a small irreversible capacity in the first cycle, and is suitable as a negative electrode for a lithium secondary battery having excellent cycle characteristics. It was shown.

It is a partial cross-sectional front view of a cylindrical lithium secondary battery. FIG. 2 is a schematic view of a lithium secondary battery used for measuring charge / discharge characteristics and irreversible capacity in an example of the present invention.

Explanation of reference numerals

DESCRIPTION OF SYMBOLS 1 Positive electrode 2 Negative electrode 3 Separator 4 Positive tab 5 Negative tab 6 Positive lid 7 Battery can 8 Gasket 9 Glass cell 10 Electrolyte 11 Sample electrode (negative electrode)
12 Separator 13 Counter electrode (positive electrode)
14 Reference electrode

Claims (3)

Graphite powder having an average particle size of 10 μm to 50 μm, a specific surface area of 8 m ² / g or less, an aspect ratio of 5 or less, and a crystallite size Lc (002) in the c-axis direction of 500 ° or more. A negative electrode for a lithium secondary battery, comprising the graphite powder of claim 1. A lithium secondary battery comprising the negative electrode according to claim 2 and a positive electrode.

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