JP2983003B2 - Carbon for lithium battery anode material - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a carbon material which is particularly useful as a negative electrode material for a lithium secondary battery and is obtained by specific raw materials and processes.

[0002]

2. Description of the Related Art Conventionally, various carbon materials have been used as negative electrode materials for lithium secondary batteries. Examples of the carbon material include natural graphite, glassy carbon, coke, and mesocarbon microbeads, but have the following problems.

[0003] First, natural graphite has high crystallinity and excellent conductivity, but since it is a natural product, its ash content is high, and even in the same production area, its properties vary greatly depending on the ore deposit. For this reason, simply adjusting the particle size cannot be used as it is, and requires purification by an acid and alkali washing treatment and a purification treatment at a high temperature. Due to such a purification treatment, even if the raw material is inexpensive, it is inevitable that the cost as a whole will increase.

[0004] In addition, since the shape is flat, handling is difficult, and there is still a problem in the coating property of the electrode plate (copper foil) after the paste adjustment. Furthermore, the layer of graphite particles is oriented parallel to the coated surface, becomes anisotropic, and the volume change of the battery due to charging and discharging after the battery is formed is large, which causes a problem in safety.

Next, glassy carbon is not particularly limited in electrolyte, but generally has a small capacity and a high production cost.

When coke is used for a battery, its capacity is small and only about 250 mAh / g can be expected at the maximum.

[0007] Mesocarbon microbeads are currently mainly used, are spherical, have excellent filling properties, and have high conductivity, but it is difficult to solve the high cost in the manufacturing process. For example, Japanese Unexamined Patent Publication No.
As described in 212208, a large amount of solvent is required when separating mesocarbon microbeads from a pitch matrix, and all heat treatments are performed in the form of powder, resulting in high costs.

[0008]

In order to solve the problems of the conventional materials as described above, the present inventor has a shape close to a sphere and has excellent filling properties.
It is intended to provide a carbon material suitable for a lithium secondary battery negative electrode material by an inexpensive manufacturing method, which is easy and stable in preparing a paste for a lithium battery and has a large battery capacity.

[0009]

In order to solve the above-mentioned problems, the present inventors propose that the coefficient of thermal expansion is 1 to 6 × 10 ⁻⁶ ° C.
^-1 is mixed and kneaded so that the binder pitch showing optically isotropicity becomes 20 to 50 parts by weight with respect to 100 parts by weight of a powder having an average particle diameter of 10 μm or less. A molded body was obtained by molding or cold isostatic pressing, and the artificial graphite block obtained by final graphitization was pulverized and sized to obtain an average particle diameter of 5 to 30 μm and a BET specific surface area of 8 m ^2. / G or less for a lithium battery negative electrode material.

Hereinafter, the carbon material of the present invention and a method for producing the same will be described in detail. First, as a raw material of the carbon material of the present invention, a coefficient of thermal expansion is 1 to 6 × 10 ⁻⁶ ° C. ^−1.
A powder having a coke having an average particle diameter of 10 μm or less is used. Thermal expansion coefficient of the coke raw material, when it is 1 × 10 ^-6 ℃ ^-1 or less is required in the range of the proportion of relatively needle-like portion (hereinafter referred N ratio) is high, It exceeds 50%, and after the graphitization treatment, it has a flat shape like natural graphite, and its bulk specific gravity is low.

[0011] For this reason, the filling property is deteriorated, the amount that can be filled into a limited battery capacity is reduced, and there is a disadvantage that the capacity per battery is reduced. On the other hand, if the coefficient of thermal expansion exceeds 6 × 10 ⁻⁶ ° C. ⁻¹ , the shape becomes almost spherical and there is no problem in terms of filling properties, but the capacity per gram is small, which is not preferable. In addition, the raw coke must have an amorphous portion of 20% or less of the whole when observed under a polarizing microscope, and if it exceeds 20%, there is a problem that the capacity per gram is reduced. The N of the crystalline part
It is necessary that the ratio be 50% or less, and if it exceeds 50%, undesirably, particles having a flat shape increase in pulverization after graphitization.

As a binder, 20 to 50 parts by weight of an optically isotropic binder pitch is added to 100 parts by weight of the raw coke as described above, and the mixture is blended and kneaded. In such a case, the ratio of the raw materials becomes too large, and it is difficult to obtain a good molded body. Even if it is obtained, there is a problem that it collapses during the heat treatment.
When the amount is more than the weight part, the ratio of the amorphous portion in the obtained negative electrode material is increased, and the capacity is unfavorably reduced.

After mixing and kneading the raw material coke and the binder pitch, molding is performed, and the molding method at this time needs to be die-molding or cold isostatic molding.

This is necessary in order that the ratio a / b of the major axis a to the minor axis b of the finally obtained carbon is approximately spherical with 1 ≦ a / b ≦ 3. When a method with strong anisotropy such as molding is used, only a flat and poorly filling material can be obtained. In such a case, problems may arise in the handling properties during battery production and the coating properties after the formation of the paste, and after coating, the powders are arranged anisotropically in the XY plane direction. The expansion and contraction of the battery is remarkable, and there is a safety problem.

After being molded as described above, it is subjected to an air oxidation treatment in a hot air circulating furnace, and finally to a graphitization treatment to obtain an artificial graphite block, which is crushed and sized to form a major axis a and a minor axis b.
To obtain a substantially spherical carbon material having a ratio a / b of 1 ≦ a / b ≦ 3. When the graphitization treatment temperature is a semi-graphitization treatment temperature of 2000 to 2200 ° C., the crystallinity of the finally obtained carbon is low, and there is a problem that the battery capacity is reduced. In this pulverization, for coarse pulverization and medium pulverization, a pulverization method using an impact force such as a jaw crusher, a hammer mill, or a roller mill may be used. However, if such a pulverization method is used in the final fine pulverization step, BET A material having a specific surface area of 8 m ² / g or less, that is, a material having a smooth surface cannot be obtained.

In the final step of pulverization, the particles are collided with each other in order to effectively express the ability of the binder to be added when forming a paste during battery production, and to further improve the fluidity, to provide uniform coating. It is preferable to use a pulverization method having a mechanism for scraping off the corners of the particles while moving the particles along the side wall inside the pulverizer.

[0017] Specifically, a method such as a ball mill or a jet mill, or pulverization of these in order to further reduce the specific surface area, subsequently includes metal or ceramic beads (a pulverizing medium). , Into a high peripheral speed stirrer for short-time stirring. However, the method is not limited to the above method since the pulverization method varies depending on the powder brand.

The carbon material obtained by the above-mentioned production process is smooth without fine irregularities on the surface and has a BET specific surface area of 8 m ² / g or less. The negative electrode material is kneaded with a binder (for example, polyvinylidene fluoride, polytetrafluoroethylene, polyethylene, or the like) to form a paste, and then applied to a copper foil as an electrode plate (current collector). In this case, the specific surface area is large. In addition, the amount of binder absorbed by the powder increases, and the coating property becomes difficult.

Further, even if the flowability is adjusted by increasing the amount of the binder, there is a problem in that the electric resistance of the electrode is increased and the battery performance is lowered. When the specific surface area is large in consideration of the reactivity with the electrolytic solution, if the specific surface area is large, the reaction surface becomes large and the amount of generated gas is increased, which is not preferable from the viewpoint of safety.

The shape of the material is substantially spherical, and the ratio a / b of the major axis to the minor axis of the particles is preferably 1 ≦ a / b ≦ 3, more preferably 1 ≦ a / b ≦ 2. , More preferably 1 ≦ a / b ≦ 1.2. The closer a / b is to 1, the closer to a sphere, the better the filling properties (bulk density), and the better the coating properties after forming into a paste.

[0021]

【The invention's effect】

According to the present invention, a carbon material having a nearly spherical shape and a smooth surface can be obtained. When used for a negative electrode material for a lithium battery, a material having excellent filling properties, easy preparation of a paste, and a stable state can be obtained. It is very suitable. In addition, the carbon material of the present invention can be obtained at low cost without using a large amount of solvent, and is industrially useful in this respect as well.

[0023]

【Example】

Example 1 Coal-based coke having a thermal expansion coefficient of 2.3 × 10 ^-6 ° C. ^-1 , an amorphous portion of 15% as observed under a polarizing microscope, and an N ratio of 45% was coarsely pulverized. 100 parts by weight of a powder having an average particle diameter of 10 μm performed by a roller mill and 50 parts by weight of a coal-based binder pitch having an optically isotropic (amorphous) having a softening point and 110 ° C. are mixed and heated. Kneading was performed with a kneader to sufficiently coat the binder pitch around the coke powder.

This was charged into a mold and molded by a press molding machine to obtain a molded body. This molded body is heated slowly to 1000 ° C. so that its shape does not collapse under its own atmosphere over 240 hours, and after standing to cool, it is refilled in a graphitization furnace and finally graphitized to form an artificial graphite block. did. This artificial graphite block was roughly pulverized and medium pulverized by a jaw crusher and a roller mill to have a thickness of 100 to 300 μm. This was further subjected to jet milling to obtain an average particle size of 1
It was set to 2 μm and pulverized.

Next, this powder was charged into a Henschel mixer containing alumina beads having a diameter of 2 mmφ and 20% of the effective volume, and occupied 50% of the total effective volume together with the alumina beads as a grinding medium. 1000 rpm
After stirring for 30 minutes while mixing with alumina beads, artificial graphite powder and alumina beads were separated and sized by a dry classifier to obtain a powder having an average particle diameter of 7 μm.

Example 2 100 parts by weight of coal-based coke having a coefficient of thermal expansion of 5.8 × 10 ^-6 ° C. ^-1 , an amorphous portion of 20% and an N ratio of 2% under a polarizing microscope, and a softening point of 250 ° C. After adding 20 parts by weight of the binder pitch of the above, the mixture was put into a crusher, and subjected to a mechanochemical treatment (mechanical mixing and grinding treatment) for 2 hours. And finally pulverized after graphitization, and a powder having an average particle diameter of 11 μm was obtained by a jet mill.

Example 3 Coal-based coke having a coefficient of thermal expansion of 1.3 × 10 ^-6 ° C. ^-1 , an amorphous part of 20% under a polarizing microscope and an N ratio of 50% was pulverized to an average particle diameter of 3 μm. 100 parts by weight of powder and softening point 1
Compounding 40 parts by weight of coal-based binder pitch at 25 ° C,
After the mechanochemical treatment, the mixture was put into a rubber mold and subjected to cold isostatic pressing, followed by the same method as in Example 1 to finally obtain an artificial graphite powder having an average particle diameter of 25 μm. Was.

[0028]

[Comparative example]

Comparative Example 1 An artificial graphite powder having an average particle size of 8 which is different only in that the final treatment temperature of the molded body in Example 1 is 2000 ° C.
μm of material was obtained.

Comparative Example 2 A petroleum-based coke having a coefficient of thermal expansion of 0.6 × 10 ⁻⁶ ° C. ⁻¹ , an amorphous portion of 25% under a polarizing microscope, and an N ratio of 80% was pulverized to an average particle diameter of 10 μm. Example 1 with 100 weights of powder
The mixture was heat-treated and pulverized in the same manner as in Example 1 except that the mixture was kneaded and kneaded with 50 parts by weight of the binder pitch used in Example 1 to obtain a molded product by lateral extrusion.
m of artificial graphite powder was obtained.

Comparative Example 3 Coal-based coke having a thermal expansion coefficient of 6.5 × 10 ^-6 ° C. ^-1 , an amorphous portion of 20% under a polarizing microscope and an N ratio of 0% was pulverized, and had an average particle diameter of 5 μm. A composition obtained by mixing and kneading 100 parts by weight of the powder and 100 parts by weight of the binder pitch used in Example 1 was treated in the same manner as in Example 1 to obtain an average particle size of 16 μm.
m of artificial graphite powder was obtained.

Comparative Example 4 The coke mass used in Example 3 was
Heat treatment was performed at 000 ° C. to obtain artificial graphite. Example 1
The powder was pulverized in the same manner as described above to obtain a powder having an average particle diameter of 6 μm.

Using the materials prepared in the above Examples and Comparative Examples as negative electrode materials, batteries were prepared as follows, and the characteristics were evaluated as shown in Table 1.

[Table 1] Originally, the carbon material was used as the negative electrode. However, in this test, since lithium metal was used for the counter electrode, Li metal was used as the negative electrode, and the carbon material was evaluated as the positive electrode. The electrode is manufactured by kneading 90 parts by weight of the prepared carbon material and 10 parts by weight of polyvinylidene fluoride with N-methyl-2-pyrrolidone with a three-roll mill, forming a paste, and using a coater to form a copper foil. Coated on top and dried. First, at the stage up to this point, the appearance inspection and the adhesion were confirmed for the coatability of the paste-like substance to the copper foil. Table 2 shows the results.

[Table 2]

After drying, it was peeled off from the copper foil, punched out in a circular shape so as to have an area of 3 cm ² , and pressed together with a SUS rope to form a positive electrode. 1MLi _C10 4 -EC / DEC (volume ratio as the electrolyte using Li metal as a counter electrode 1:
Using 1), a bipolar test cell was constructed, and a charge / discharge cycle test was performed at a constant current. The measurement conditions are 0 to 0
1.5 V, current density 0.1 mA / cm ² , temperature 30 ° C. The test results are summarized in Table 3.

[Table 3]

Claims (3)

(57) [Claims] 1. A binder pitch having optical isotropy of 100 parts by weight of a coke having a coefficient of thermal expansion of 1 to 6 × 10 ⁻⁶ ° C. ^-1 having an average particle diameter of 10 μm or less has a binder pitch of 2 parts.
The composition blended and kneaded so as to be 0 to 50 parts by weight,
A molded body is obtained by embedding molding or cold isostatic pressing,
An artificial graphite block obtained by performing an air oxidation treatment in a hot air circulating furnace and finally graphitizing the resulting mixture is pulverized and sized, and has an average particle diameter of 5 to 30 μm and a BET specific surface area of 8 m ² /
g or less for a lithium battery negative electrode material. 2. The ratio a / b of the major axis a to the minor axis b of the particles is 1 ≦ a.
2. The carbon for a negative electrode material of a lithium battery according to claim 1, wherein / b ≦ 3. 3. A coke having an amorphous portion of 20% or less and a crystalline portion having a needle-like portion (hereinafter referred to as N ratio) of 50% or less, as observed under a polarizing microscope. The carbon for a negative electrode material of a lithium battery according to claim 1 used.