JPH1012217A - Negative electrode for lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a negative electrode for a lithium ion secondary battery, which is easy to manufacture an electrode plate, has a high charge / discharge capacity, and has good cycle stability. SOLUTION: A graphite crystal powder and an organic binder are mixed, a high shear force is applied to cause a mechanochemical reaction to be dispersed and compounded, and the mixture is extruded so that the graphite crystal is highly oriented. Lithium used for the negative electrode is obtained by pulverizing a `` graphite, carbon '' composite carbon material obtained by firing in an active atmosphere or a non-oxidizing atmosphere to carbonize the contained organic matter into an amorphous or turbostratic carbon state. In the negative electrode for an ion secondary battery, the graphite / carbon weight ratio was 81
/ 19 or more and 95/5 or less.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a negative electrode for a lithium ion secondary battery containing a graphite / carbon composite carbon material prepared by orienting graphite crystal powder with a binder which is a carbon material having low crystallinity.

[0002]

2. Description of the Related Art All batteries using metallic lithium as a negative electrode active material are called lithium batteries. To be precise, it is called "manganese dioxide lithium battery" in combination with the positive electrode active material. Lithium batteries are batteries that have been put into practical use in recent years,
Since the voltage is twice as high as that of a normal dry battery, the capacity is large, and the storage life is 5 years or more, the battery is widely used although it is expensive. Especially in recent years, as a result of the rapid development of miniaturization technology in electronic devices,
The battery used as the power source also needs to be downsized,
Improvements in high energy density, large capacity, and high electromotive force have become essential, and research and development of new lithium ion secondary batteries has been activated.

However, conventional lithium secondary batteries often use metallic lithium foil or the like as a negative electrode active material, and therefore have the following problems to be solved. That is, metallic lithium is eluted into the electrolytic solution with discharging, so that lithium is reprecipitated during charging. At this time, lithium precipitates in a dendritic (dendritic) state or becomes fine particles. Dendrites cause short-circuits or drop off, resulting in a reduction in capacity, leading to a decrease in cycle characteristics and safety. Since dendrites are easily generated at high currents, rapid charging deteriorates cycle life.

Therefore, a method has been proposed in which a material capable of absorbing lithium, such as a lithium / aluminum alloy or a wood alloy, is used for the negative electrode. The solution has not been reached. Among the substances capable of storing lithium, the most probable negative electrode material is carbon. In recent years, research on supporting carbon on various carbon materials such as graphite has been actively conducted. JP 57
According to Japanese Patent Publication No. -2008079, when charging is performed using graphite as a negative electrode, lithium in the positive electrode is electrochemically intercalated (inserted) between the layers of the negative electrode graphite, and lithium is discharged from the graphite layer into the electrolytic solution during discharging. Thus, a negative electrode is disclosed in which graphite powder is formed into a paste together with a binder and applied to a current collector of a metal foil because the ions can be deintercalated as ions into the positive electrode.

[0005] However, the theoretical value of the discharge capacity of metallic lithium is 3860 mAh / g. However, assuming that graphite can occlude up to C ₆ Li, the theoretical value of the discharge capacity per gram of graphite is 372 mAh. 1/10 or less of lithium. Therefore, even if a negative electrode material in which lithium is carried on a conventionally proposed graphite material is used, a high capacity cannot always be expected, and graphite alone has a poor lithium storage capacity,
There is a problem that the charge / discharge capacity as a lithium ion battery is small.

It has been found that when graphite is used alone as a negative electrode, the decomposition reaction of propylene carbonate (PC) as an electrolytic solution proceeds with a Coulomb efficiency of almost 100%, and it becomes difficult to occlude lithium. When graphite is used alone as a negative electrode, when lithium is intercalated and deintercalated between graphite layers by charge and discharge, graphite crystals repeatedly expand and contract in the C-axis direction. The crystal structure does not recover to the state before the intercalation, but rather expands, the adhesion between the negative electrode current collector and the graphite powder decreases, This has caused a phenomenon that the current collection efficiency at the negative electrode decreases and the charge / discharge characteristics of the battery decrease. Further, the voltage of the graphite alone changes rapidly at the end of discharge, so that it is difficult to indicate the remaining battery level.

[0007] Instead of graphite, a low-crystalline carbonaceous material obtained by carbonizing an organic material is used as a negative electrode.
For example, it is described in JP-A-62-12266. However, the low crystalline carbonaceous material alone has a small lithium storage amount, which is less than the theoretical value. JP-A-7-32
No. 6,343 describes that a carbon material composed of highly crystalline graphite and low crystalline carbon is used as a negative electrode material of a lithium ion secondary battery. However, it is presumed that graphite is not sufficiently covered with low-crystallinity carbon because the mixture of graphite and the organic material before carbonization is a mere mixture of powders and is incomplete. Just as if they were mixed together, they simply obey the compound rule of the two. In other words, the characteristics of graphite, in which the discharge voltage is stable but changes abruptly at the end of discharge, and the characteristics of low crystalline carbon, in which the voltage gradually changes from the initial stage to the end of the discharge, are intermediate between the two. Presented according to the ratio.

On the other hand, the applicant of the present application has filed Japanese Patent Application No.
No. 409, a composition obtained by mixing a graphite crystal fine powder and an organic binder and performing a mechanochemical reaction by applying a high shearing force to disperse and composite the mixture so that the graphite crystals are highly oriented. After extrusion molding to a graphite / carbon composite carbon material obtained by carbonizing an organic binder,
It was proposed to use it as a negative electrode material for lithium ion secondary batteries. This Japanese Patent Application No. 7-32409 describes that the weight ratio of graphite to low-crystalline carbon in the graphite / carbon composite carbon material is 20/80 or more and 80/20 or less.

[0009]

However, when a graphite / carbon composite carbon material within the above weight ratio is used as a negative electrode material of a lithium ion secondary battery, several cycles are required until the charge / discharge capacity is stabilized. There is a problem that the charge / discharge operation is required. Therefore, the object of the present invention, when used as a negative electrode material of a lithium ion secondary battery,
An object of the present invention is to provide a graphite / carbon composite carbon material in which the charge / discharge capacity of the secondary battery is stabilized early.

[0010]

According to the present invention, there is provided a graphite / carbon composite carbon material comprising graphite bound by low-crystalline carbon having lower crystallinity than graphite, wherein the graphite / carbon composite material has a low crystallinity. , Wherein the weight ratio of the negative electrode is not less than 81/19 and not more than 95/5.

When the weight ratio of graphite to low-crystalline carbon in the graphite / carbon composite carbon material is less than 81/19, the charge / discharge capacity does not stabilize early because of the formation at the crystal end face of graphite. This is because several cycles are required to break the low crystalline carbon in the direction of expansion of the graphite. Therefore, early stabilization of the charge / discharge capacity is achieved by reducing the ratio of low crystalline carbon to a weight ratio of 81/19 or more. Among them, the case where the weight ratio is 87/13 or more is preferable because the voltage rise at the end of discharging is gradual and the charging / discharging capacity is stabilized early. Also,
If the weight ratio is 95/5 or more, the characteristics become close to those of graphite alone, and the above-described problem of graphite alone occurs.

The graphite crystal has a natural lattice constant d = (002) by X-ray wide-angle diffraction of 0.338 nm or less, and a crystallite size Lc = (002) in the C-axis direction of 40 nm or more. It is preferably at least one selected from the group consisting of graphite, quiche graphite, pyrolytic graphite, vapor-grown graphite, and artificial graphite. Low crystalline carbon has an average lattice constant d = (002) by X-ray wide-angle diffraction of 0.350 nm or more and a crystallite size Lc = (002) in the C-axis direction of 2
Preferably it is 5 nm or less.

The composite carbon material is dispersed and compounded by mixing a graphite crystal fine powder in which crystals are highly developed and an organic binder, and applying a high shearing force to perform a mechanochemical reaction. The composition is extruded so that the graphite crystals are highly oriented, and then fired in an inert atmosphere or a non-oxidizing atmosphere to reduce the organic binder contained to an amorphous or turbostratic carbon state. And obtained by carbonization.

An organic binder is an organic substance which leaves a carbide having an irregular or turbostratic structure when fired in an inert atmosphere or a non-oxidizing atmosphere, and is a natural or synthetic organic polymer substance, monomer / oligomer. And at least one selected from the group consisting of tars, pitches, carbonized pitches, thermoplastic resins, and thermosetting resin prepolymers.

Here, the starting material of the binder carbon will be described. Examples of natural and synthetic organic polymer substances include substances other than the thermoplastic resins and thermosetting resins described below, such as lignin, cellulose, tragacanth gum, gum arabic, natural gums and derivatives thereof, saccharides, chitin, chitosan, and the like. Compounds having a ring aromatic in the basic structure of the molecule, and indanthrene-based vat dyes derived from formalin condensate of naphthalene sulfonic acid, dinitronaphthalene, pyrene, pyranthrone, biolanthrone, benzanthrone, and intermediates thereof is there.

Examples of the thermoplastic resin include polyvinyl chloride, polyacrylonitrile, polyvinylidene chloride, post-chlorinated polyvinyl chloride, polyvinyl acetate, polyvinyl alcohol, polyvinylpyrrolidone, ethyl cellulose, carboxymethyl cellulose, and polyvinyl chloride / vinyl acetate. As a carbon precursor treatment using a normal thermoplastic resin such as a polymer, and a heat-resistant thermoplastic resin such as polyphenylene oxide, polyparaxylene, polysulfone, polyimide, polyamideimide, polybenzimidazole, and polyoxadiazole. Oxidatively crosslinked.

As the thermosetting resin, a phenol resin,
Furan resin, epoxy resin, xylene resin, copna resin, etc. are used, and when heated, they flow and generate intermolecular cross-links, become three-dimensional and harden, and have a high carbon residue without special carbon precursor treatment. It shows the rate.
As pitches, petroleum pitch, coal tar pitch,
Asphalt and carbonized dry pitches of hydrocarbon compounds such as pitches and synthetic resins (processed at 400 ° C. or less; carbon residue yield is 75% to 95%) are subjected to non-graphitizing treatment such as oxidation treatment. It was done.

Next, in the present invention, the graphite fine powder used in combination with the organic material as a starting material of the binder carbon will be described. In order to make the battery reaction satisfactorily performed, it is important to prepare a composite carbon material in which the crystal end face (edge face) of highly developed graphite is tissue-oriented so as to be vertically aligned with the reaction face of the electrode. Therefore, graphite whiskers, highly oriented gas phase pyrolytic graphite (HOPG), vapor grown graphite (VGCF), quiche graphite, and crystalline natural graphite are preferably used. Although the particle size of the graphite fine powder varies depending on the intended configuration of the battery, the maximum particle size is preferably about 10 μm.

Next, a method for producing the negative electrode carbon material of the present invention will be described. As the organic substance leaving the amorphous carbon constituting the binder carbon material, one or two or more of the above natural polymer substances, synthetic polymer substances, thermosetting resins, thermoplastic resins, pitches, etc. are appropriately selected. The starting material is mixed with the crystalline graphite fine powder according to the purpose, and the powder is sufficiently dispersed with a Henschel mixer or the like.

If necessary, for example, to increase the density or impart capacity, a dry distillation pitch having a high carbon residue yield is blended with the above composition, and a plasticizer, a solvent, etc. are added thereto, and a pressure kneader or a secondary kneader is added. Using a kneader that can apply a high shearing force such as this roll, a mechanochemical reaction is performed, and the mixture is sufficiently mixed and dispersed, granulated with a pelletizer, and then extruded with a screw or plunger type extruder. Extrusion molding is performed at a high speed to a diameter of, and orientation operation is performed so that the composite graphite crystals are well arranged in the extrusion direction to obtain a formed product.

Next, this preform is treated for 10 hours in an air oven heated to 180 ° C. while applying a stretching operation to obtain a precursor (carbon precursor) material.
Further, the temperature is controlled at 1,100 ° C.
The carbonization is completed by gradually heating to the following predetermined temperature to obtain a target carbon material for a negative electrode. The final firing temperature is usually 500 to 1,100 ° C.,
A temperature of 0 ° C. is preferred.

The negative electrode is formed by subjecting the obtained carbon material for negative electrode to impact pulverization to have an average particle size of about 5 to 50 μm. About 5% by weight of polytetrafluoroethylene (PTF)
E) and subjected to powder compression molding to form a sheet, which was punched out into a circular shape to obtain a negative electrode. The prepared negative electrode was heated in a vacuum state to make it a completely dry state. ADVANTAGE OF THE INVENTION According to this invention, a high performance carbon material for negative electrodes can be manufactured very easily. By using this negative electrode material, a lithium ion secondary battery with high charge / discharge capacity and good cycle stability can be manufactured. .

[0023]

FIG. 1 shows the configuration of a test cell for a lithium ion battery. An excessive amount of metallic lithium was used for the counter electrode 10, and filter paper was used three times for the separator 12. The counter electrode 10 and the working electrode 14 were pressed by a stainless steel bar 16 urged by a spring 15. The stainless steel rod 16 was placed in a glass tube 18 and the cell was sealed by an O-ring 20. Reference numeral 22 denotes a malloined film, and 24 denotes a conductor. As the electrolytic solution 26, ethylene carbonate (EC) and diethyl carbonate (D
EC) to which lithium perchlorate was added. Example 1 A chlorinated vinyl chloride resin (T-74 manufactured by Nippon Carbide Co., Ltd.) was used as a binder carbon raw material for a negative electrode.
2) 20 parts by weight of diallyl phthalate monomer as a plasticizer was added to 48 parts by weight of 100 parts by weight of a composition obtained by combining 52 parts by weight of natural graphite fine powder (CSSP-B manufactured by Nippon Graphite Co., Ltd., average particle size: 1 μm) with 48 parts by weight. Henschel
After dispersing using a mixer, kneading was sufficiently repeated using two mixing rolls having a surface temperature kept at 120 ° C. to obtain a sheet-like composition, which was pelletized with a pelletizer to obtain a molding composition. The pellets were extruded with a screw type extruder at a speed of 3 m / sec at 130 ° C. using a die having a diameter of 0.7 mm, and treated in an air oven heated to 180 ° C. for 10 hours while applying a stretching operation. This was used as a pre-cursor (carbon precursor) wire.
Next, this is heated up to 500 ° C. in nitrogen gas at 10 ° C./hour,
The temperature was raised from 500 ° C. to 1000 ° C. at a rate of 50 ° C./hour, kept at 1000 ° C. for 3 hours, and then cooled naturally to complete firing.

The weight ratio of “graphite / carbon” of the obtained carbon material for negative electrode was “81/19”, the diameter was 0.5 mm, and the open porosity of the structure was 20%. Next, the carbon material for a negative electrode is subjected to impact pulverization to have an average particle size of about 50 μm. This was mixed with about 5% by weight of PTFE and subjected to powder compression molding to form a sheet, which was punched out into a 5.3 mm diameter circle to obtain a negative electrode. The prepared electrode has an actual weight of 1-2.
It was around mg. It was dried in a vacuum at 110 ° C. for 1 day to make it completely dry. The produced negative electrode was subjected to a charge / discharge test using the test cell shown in FIG. An excessive amount of metallic lithium was used for the counter electrode, and filter paper was used in triplicate for the separator. The electrodes were pressed together with a stainless steel rod having a diameter of 8 mm. The stainless steel rod was placed in a glass tube having an outer diameter of 10 mm and an inner diameter of 8 mm, and the cell was sealed with an O-ring. As the electrolyte, ethylene carbonate (EC) and diethyl carbonate (DE) in a weight ratio of 1: 1 were used.
C) to which lithium perchlorate (LiClO ₄ ) was added.

The test cell was subjected to a charge / discharge test by the following method. The first charge (the direction of the current in which lithium ions enter the carbon electrode is referred to as charge) is performed at a constant current of 0.1 mA / cm ² until the potential reaches 3 V from the potential difference of lithium, ie, 3V. On the other hand, the discharge was performed up to 1.5 V at the same current density. The charge / discharge after the second time is 0
The current density was also measured between V and 1.5 V. The charge / discharge curve obtained as a result is shown in FIG.

In FIG. 2 and subsequent drawings, the curve on the lower right as a whole is a charge curve, and the curve on the upper right as a whole is a discharge curve. Preferred characteristics as a secondary battery are obtained, in which the voltage at the time of discharging is stable and the voltage change at the end of discharging is not abrupt. The value on the horizontal axis at the intersection of the charge curve and the line with a potential difference of 0 V is the charge capacity [mAh / g], and the value on the horizontal axis at the intersection of the discharge curve and the line with a voltage difference of 1.5 V is the discharge capacity [mAh / g]. Is shown. The value of the charge / discharge capacity in each charge / discharge cycle is shown in the upper right of the figure. In the case where the weight ratio of graphite to low crystalline carbon in FIG. 2 is 81/19, the charge / discharge capacity is almost stable after the second time. Example 2 A negative electrode carbon material having a graphite / carbon weight ratio of 90/10 was obtained by the same process as in Example 1 except that the weight ratio between the binder carbon material and the graphite powder was changed. FIG. 3 shows a charge / discharge curve. 5.7% by weight of PTFE is used as a binder. If the graphite / carbon weight ratio is 90/10,
The charge capacity is almost stable from the second time, and the discharge capacity is almost stable from the first time. (Example 3) A negative electrode carbon material having a graphite / carbon weight ratio of 95/5 was obtained by the same process as in Example 1 except that the weight ratio of the binder carbon raw material and the graphite powder was changed. Figure 4 shows the charge / discharge curve.
Shown in 5.0% by weight of PTFE is used as a binder. When the weight ratio of graphite / carbon is 95/5, the charge capacity is almost stable from the second time and the discharge capacity is almost stable from the first time. In addition, the discharge curve at the end of discharge is nearly vertical, and it is somewhat difficult to indicate the remaining battery level. Example 4 A negative electrode carbon material having a graphite / carbon weight ratio of 87/13 was obtained by the same process as in Example 1 except that the weight ratio between the binder carbon material and the graphite powder was changed. FIG. 5 shows a charge / discharge curve. 5.7% by weight of PTFE is used as a binder. If the graphite / carbon weight ratio is 87/13,
The charge capacity is almost stable from the second time and the discharge capacity is almost stable from the first time, and the voltage rise at the end of discharge is gradual. Comparative Example 1 A negative electrode carbon material having a graphite / carbon weight ratio of 75/25 was obtained by the same process as in Example 1 except that the weight ratio between the binder carbon material and the graphite powder was changed. FIG. 6 shows a charge / discharge curve. 4.9% by weight of PTFE is used as a binder. If the graphite / carbon weight ratio is 75/25,
The charge / discharge capacity is not stable even at the third time.

[0027]

According to the present invention, a high-performance carbon material for a negative electrode can be produced very easily. By using this negative electrode material, a lithium ion secondary battery having a high charge / discharge capacity and good cycle stability can be obtained. Secondary batteries can be manufactured.

[Brief description of the drawings]

FIG. 1 is a structural explanatory view of a test cell used for a charge / discharge test.

FIG. 2 is a charge / discharge curve diagram of Example 1.

FIG. 3 is a charge / discharge curve diagram of Example 2.

FIG. 4 is a charge / discharge curve diagram of Example 3.

FIG. 5 is a charge / discharge curve diagram of Example 4.

FIG. 6 is a charge / discharge curve diagram of Comparative Example 1.

Claims (7)

[Claims] 1. A graphite / carbon composite carbon material comprising graphite bound by low-crystalline carbon having lower crystallinity than graphite, wherein the weight ratio of graphite to low-crystalline carbon is 81 /
Negative electrodes for lithium ion secondary batteries, including those having a ratio of 19 to 95/5. 2. The graphite has an average lattice constant d = (002) by X-ray wide-angle diffraction of 0.338 nm or less, and a crystallite size Lc = (002) in the C-axis direction of 40 nm or more. Natural graphite, quiche graphite, pyrolytic graphite, vapor-grown graphite,
The negative electrode for a lithium ion secondary battery according to claim 1, wherein the negative electrode is at least one selected from the group consisting of artificial graphite. 3. The low-crystalline carbon has an average lattice constant d = (002) by X-ray wide-angle diffraction of 0.350 nm or more and a crystallite size Lc = (002) in the C-axis direction of 25.
The negative electrode for a lithium ion secondary battery according to claim 1, which has a thickness of at most nm. 4. The negative electrode for a lithium ion secondary battery according to claim 1, wherein the composite carbon material is in a powder form, and further comprises a binder for binding the powdery composite carbon material. 5. The composite carbon material is obtained by mixing a graphite crystal fine powder and an organic binder, and performing a mechanochemical reaction by applying a shearing force to disperse and composite the composition. 2. The lithium ion secondary battery according to claim 1, which is obtained by extruding so as to orient and then firing in an inert atmosphere or a non-oxidizing atmosphere to carbonize an organic binder contained therein. Negative electrode for secondary battery. 6. The organic binder is a natural or synthetic organic polymer, a monomer / oligomer, a tar /
The negative electrode for a lithium ion secondary battery according to claim 5, wherein the negative electrode is at least one selected from the group consisting of pitches, carbonized pitches, thermoplastic resins, and thermosetting resin prepolymers. 7. The lithium ion secondary battery according to claim 5, wherein the calcination and the carbonization are performed by heat treatment in an inert atmosphere or a non-oxidizing atmosphere in a temperature range of 500 to 1,100 ° C. For negative electrode.