JPH09330703A - Manufacture of negative electrode for lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To easily manufacture a negative electrode carbon material with high performance to manufacture a lithium ion secondary battery with high charge and discharge capacity by dissolving graphite crystal fine powder and a furan resin to an excessive quantity of THF, and carbonizing the solution after evaporating the solvent to form a negative electrode material. SOLUTION: In the manufacture of a negative electrode carbon material for secondary battery, graphite crystal fine powder and an organic binder are dissolved in an excessive solvent, and the solvent is then evaporated to form a composition formed of graphite and the organic binder. This composition is baked in inert atmosphere or in non-oxygen atmosphere, whereby the contained organic binder is carbonized to provide a graphite/carbon composite carbon material. A secondary battery using this graphite/carbon composite carbon material as negative electrode has characteristics preferred as secondary battery of stable voltage at discharge and gentle voltage change in discharge end.

Description

Detailed Description of the Invention

[0001]

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

[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 a "manganese dioxide-lithium battery" in combination with the positive electrode active material. The lithium battery is a battery that has been put to practical use in recent years,
The voltage is about twice as high as that of a normal dry battery, the capacity is large, and the storage life is 5 years or more. Especially in recent years, as a result of rapid development of miniaturization technology for electronic devices,
The batteries used as the power source are also required to be downsized,
Improvements in high energy density, large capacity, and high electromotive force are essential, and research and development of new lithium-ion secondary batteries are becoming active.

However, in the conventional lithium secondary battery, since a metal lithium foil or the like is often used as the negative electrode active material, it has the following problems to be solved. That is, since metallic lithium is eluted into the electrolytic solution as it is discharged, lithium is redeposited during charging. At this time, lithium deposits in the form of dendrites (dendritic) or becomes fine particles. The dendride causes a short circuit or drops off to bring about a decrease in capacity, which leads to deterioration of cycle characteristics and safety. Since dendrites are easily generated at high current, 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 supporting lithium in a graphite material that has been conventionally proposed cannot be expected to have a high capacity, 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. Also, when graphite is used alone as the negative electrode, when lithium is intercalated / deintercalated between the graphite layers by charge / discharge, the graphite crystals repeatedly expand and contract in the C-axis direction, but the charge / discharge cycle Repeating the above, the crystal structure remains expanded, and the adhesion between the negative electrode current collector and the graphite powder decreases, or the graphite powder falls off from the current collector and the current collecting efficiency at the negative electrode decreases. Or the charging / discharging characteristics of the battery are deteriorated. Furthermore, with graphite alone, it is difficult to indicate the remaining battery level because the voltage changes rapidly at the end of discharge.

[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.

[0009]

SUMMARY OF THE INVENTION An object of the present invention is to further improve the method for manufacturing a negative electrode for a lithium ion secondary battery proposed in Japanese Patent Application No. 7-32409, simplifying the manufacturing process, and , To further improve the characteristics of the battery.

[0010]

According to the present invention, a graphite crystal fine powder and an organic binder are dissolved in excess solvent and then the solvent is evaporated to form a composition comprising graphite and the organic binder, By firing the composition in an inert atmosphere or a non-oxygen atmosphere, the organic binder contained therein is carbonized to obtain a graphite / carbon composite carbon material. Provided is a method for producing a negative electrode for a lithium-ion secondary battery, which comprises the step of using it as a material for a negative electrode for a secondary battery.

It is preferable that the weight ratio of the graphite crystal fine powder to the organic binder is such that the composition becomes powdery after the solvent is evaporated. By adopting a step of dissolving with an excess solvent instead of kneading under the action of a high shearing force (high shear kneading), the manufacturing process is simplified. Further, if the weight ratio of the fine graphite powder to the organic binder is within a predetermined range, it becomes powdery after evaporating the solvent and is powdery even after carbonization, so a pulverization step is not required. Therefore, the process is further simplified.

Further, the battery using the composite carbon material obtained in the above process as a negative electrode material has a higher charge / discharge capacity and higher rate characteristics (charge / discharge property at a current value higher than the rated value) as compared with the case of the high shear kneading process. Has been improved. According to the observation with an electron microscope, this is because the surface of the composite carbon material that has been subjected to the high shear kneading and powdering process has a scratch that is considered to have occurred in the high shear kneading or crushing process, whereas the solvent is used. The surface of the product was in a very clean state, and it is considered that the charge and discharge capacity and high rate characteristics of the battery were improved for that reason.

The organic binder is an organic substance that leaves an amorphous or turbostratic carbide when fired in an inert atmosphere or a non-oxidizing atmosphere, and includes natural and synthetic organic polymeric substances, monomers and oligomers. It is preferable that it is at least one selected from the group consisting of the following compounds: tars, pitches, dry-distilled pitches, thermoplastic resins, and prepolymers of thermosetting resins.

The starting material for the binder carbon will now 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.

The thermoplastic resins include polyvinyl chloride, polyacrylonitrile, polyvinylidene chloride, post-chlorinated polyvinyl chloride, polyvinyl acetate, polyvinyl alcohol, polyvinylpyrrolidone, ethyl cellulose, carboxymethyl cellulose, polyvinyl chloride / vinyl acetate. As a carbon precursor treatment using an ordinary thermoplastic resin such as polymer and polyphenylene oxide, polyparaxylene, polysulfone, polyimide, polyamideimide, polybenzimidazole, polyoxadiazole, etc. , 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, oil 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, a description will be given of the graphite fine powder used in combination with the organic material which is the starting material of the binder carbon. 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. The particle size of the graphite fine powder varies depending on the intended configuration of the battery, but the maximum particle size is preferably several μm or less.

Next, a method for producing the negative electrode carbon material of the present invention will be described. As the organic material that leaves the amorphous carbon that constitutes 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. As a starting material, the crystalline graphite fine powder is mixed with the starting material according to the purpose, and these are dissolved in an excess solvent such as tetrahydrofuran, and then the solvent is evaporated to obtain a composition composed of graphite and an organic substance.

Dry distillation pitch having a high carbon residue yield is blended with the above composition, if necessary, such as for increasing the density and imparting capacity. Next, the composition was heated with air heated to 180 ° C.
It is treated in an oven for 10 hours to obtain a pre-cursor (carbon precursor) material. Further, while controlling the temperature rising rate in nitrogen gas, the carbon material is gradually heated to a predetermined temperature of 1,100 ° C. or lower to complete the carbonization and obtain the intended carbon material for the negative electrode.

The final firing temperature is usually 500 to 1,
Although it is 100 ° C, a temperature of 700 to 900 ° C is preferable. For molding the negative electrode, the carbon material for a negative electrode obtained was approximately 5
The mixture was mixed with polytetrafluoroethylene (PTFE) in an amount of wt%, powder compression molding was performed to form a sheet, which was punched out into a circle 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. .

[0021]

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 in triplicate for the separator 12. The counter electrode 10 and the working electrode 14 were pressed against each other by the stainless rod 16 biased by the spring 15. The stainless 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) As a binder carbon raw material for the negative electrode, 50 parts by weight of a furan resin (VF-302 manufactured by Hitachi Chemical Co., Ltd.) and 50 parts by weight of natural graphite fine powder (manufactured by Nippon Graphite Co., Ltd. CSSP-B average particle size 1 μm) were compounded. With respect to 100 parts by weight of the above composition, an excess amount of tetrahydrofuran (T
HF) was added and dissolved, and the solvent was evaporated,
It was treated for 10 hours in an air oven heated to 0 ° C. to obtain a pre-cursor (carbon precursor). Next, this is heated in nitrogen gas up to 500 ° C. at 10 ° C./hour, 500 ° C. to 10 ° C.
Heat up to 00 ℃ at a heating rate of 50 ℃ / hour to 1000 ℃
After being held for 3 hours, it was naturally cooled to complete firing.

The "graphite / carbon" weight ratio of the obtained carbon material for negative electrode was "75/25". Next, the powdery carbonaceous material for negative electrode was mixed with about 5% by weight of PTFE and powder-molded to form a sheet having a diameter of 5.
It was punched out into a circle of 3 mm to obtain a negative electrode. The actual weight of the manufactured electrode was about 1 to 2 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. For the counter electrode, use an excessive amount of metallic lithium,
As the separator, three filter papers were used. The electrodes were pressed together by a stainless rod having a diameter of 8 mm. Put the stainless rod in a glass tube with an outer diameter of 10 mm and an inner diameter of 8 mm.
The cell was sealed with an O-ring. As the electrolytic solution, a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) to which lithium perchlorate (LiClO ₄ ) was added at a weight ratio of 1: 1 was used.

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 the subsequent drawings, the curve to the right is generally the charging curve, and the curve to the upper right is the discharging curve as a whole. 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.

For comparison, the charge / discharge characteristics of the composite carbon material obtained through the steps of high shear kneading and crushing with the same composition ratio are shown in FIG. 4.9 wt% PTF as a binder
E is used. Comparing the two, it can be seen that the charge-discharge capacity is improved by adopting the solvent method. 0.
Charge and discharge characteristics, that is, high rate characteristics at a current density of 5 mA / cm ² are shown in FIGS. 4 and 5. FIG. 4 shows the result by the solvent method, and FIG. 5 shows the result by the kneading method. It can be seen that the adoption of the solvent method significantly improves the charge / discharge capacity at high rates.

If the graphite / resin composition after evaporation of the solvent is in powder form, it is in powder form even after firing, and the subsequent pulverization step can be omitted, which is also preferable in terms of battery characteristics. Whether the graphite / resin composition after evaporating the solvent is powdery or lumpy depends on the composition ratio of graphite and resin and the type of resin. Table 1 shows the experimental results regarding the state of the composition at various resin / solvent combinations and graphite / resin weight ratios. The last column of Table 1 shows the results when graphite and resin were mixed without using a solvent as a reference.

[0027]

[Table 1]

According to the results shown in Table 1, in the combination of furan resin and THF as a solvent, powdery powder was obtained when the weight ratio of graphite to resin was 50/50 and 80/20. When the ratio of graphite is higher than that, the characteristics of the battery become closer to those of graphite alone. When the resin was a vinyl chloride resin or a chlorinated vinyl chloride resin, no powder was obtained even if any solvent was used.

[0029]

According to the present invention, a high performance carbon material for a negative electrode can be manufactured very easily, and by using this negative electrode material, a lithium ion secondary battery having a high charge / discharge capacity can be manufactured.

[Brief description of drawings]

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

FIG. 2 is a charge / discharge curve diagram of a battery according to the manufacturing method of the present invention.

FIG. 3 is a charge / discharge curve diagram of a battery that has undergone the steps of high shear kneading and pulverization.

FIG. 4 is a charge / discharge curve diagram at a high rate of a battery according to the manufacturing method of the present invention.

FIG. 5 is a charge / discharge curve diagram at a high rate of a battery that has undergone the steps of high shear kneading and pulverization.

Claims (2)

[Claims] 1. A graphite crystal fine powder and an organic binder are dissolved in an excess amount of solvent, and then the solvent is evaporated to obtain a composition comprising graphite and the organic binder, the composition being in an inert atmosphere or a non-volatile atmosphere. A step of carbonizing the contained organic binder to obtain a graphite / carbon composite carbon material by firing in an oxygen atmosphere, and using the graphite / carbon composite carbon material as a material for a negative electrode for a lithium ion secondary battery. A method for manufacturing a negative electrode for a lithium ion secondary battery, which comprises: 2. The method according to claim 1, wherein the weight ratio of the graphite crystal fine powder to the organic binder is in a range such that the composition becomes powdery after the solvent is evaporated.