JP4738039B2 - Method for producing graphite-based carbon material - Google Patents

Description

  The present invention relates to a method for producing a graphite-based carbon material, and more particularly to a method for producing a graphite-based carbon material capable of increasing the number of rhombohedral graphite layers.

In recent years, non-aqueous electrolyte secondary batteries have come into practical use as compact, lightweight, high-capacity chargeable / dischargeable batteries, and are used in portable electronic and communication devices such as small video cameras, mobile phones, and notebook computers. It came to be able to. This type of non-aqueous electrolyte secondary battery uses a carbon-based material that can occlude and release lithium ions as a negative electrode active material, and lithium such as LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ , LiFeO ₂ as a positive electrode active material. A transition metal oxide is used, and a nonaqueous electrolytic solution in which a solute composed of a lithium salt is dissolved in an organic solvent is used.

  By the way, the carbon-based material used for the negative electrode of such a nonaqueous electrolyte secondary battery has been attracting attention because of its excellent durability with little capacity deterioration during the charge / discharge cycle. This is because the carbon-based material can reversibly store and release lithium at a low potential, and it utilizes the fact that an intercalation compound between lithium and the carbon-based material is reversibly formed. . For example, a positive electrode containing a sufficient amount of lithium and a negative electrode using a carbon material as an active material are opposed to each other through a separator and inserted into a battery can, and a solute composed of a lithium salt is dissolved in the organic solvent. By injecting the non-aqueous electrolyte, the assembly of the battery is completed in a discharged state.

  In particular, when a graphite-based carbon material is used as the carbon material, a non-aqueous electrolyte secondary battery having a high energy density can be obtained. Therefore, such a graphite-based carbon material has been widely used. Research and development for the purpose of improving the characteristics is also actively conducted.

  Here, in the graphite-based carbon material, the peak intensity (P1) of the (100) plane by the X-ray diffraction method (XRD) of the hexagonal graphite layer and the (002) plane by the X-ray diffraction method of the hexagonal graphite layer. The intensity ratio (P1 / P2) with respect to the peak intensity (P2) is an index indicating the disorder of the orientation of the carbon material. By improving this disorder, the carbon material can be used even when a high current value is passed. Lithium ions are easy to enter.

On the other hand, there are various indices of graphite-based carbon materials. When the number of rhombohedral graphite layers increases in the diffraction of graphite-based carbon materials by the X-ray wide angle diffraction method, the turbulent layer structure increases and the solvated lithium Intercalation into the graphite layer is suppressed. Therefore, as shown in the following non-patent document 1, there is a report that the irreversible capacity in the initial charge / discharge of the carbon material decreases as the rhombohedral graphite layer increases. Further, Non-Patent Document 1 describes that the rhombohedral graphite layer increases when graphite is crushed.

Hitachi Powder Metallurgy Technical Report No. 1 [2002] Structural analysis of natural graphite as a negative electrode material for lithium ion batteries and optimization of negative electrode material properties

  However, increasing the rhombohedral graphite layer by crushing graphite requires a long time using a crusher such as a ball mill, which complicates the graphite production process and increases the production cost. Become. In addition, when a crusher is used, impurities may be mixed into the graphite due to rubbing between graphite and a part of a crusher such as a ball, which may adversely affect the characteristics of a battery using graphite.

  Therefore, the present invention has been made in consideration of the above problems, and a battery using a carbon material by easily increasing the ratio of rhombohedral graphite layers while suppressing an increase in manufacturing cost and mixing of impurities. An object of the present invention is to provide a method for producing a graphite-based carbon material capable of improving the initial charge / discharge efficiency.

In order to achieve the above object, the invention according to claim 1 of the present invention is a graphite-based carbon material having at least a rhombohedral graphite layer and a hexagonal graphite layer, wherein the hexagonal graphite layer comprises: The intensity ratio (P1 / P2) between the peak intensity (P1) of the (100) plane by the X-ray diffraction method and the peak intensity (P2) of the (002) plane by the X-ray diffraction method of the hexagonal graphite layer is 0.01 A method for producing a graphite-based carbon material, wherein only the centrifugal force is applied to the graphite-based carbon material as described above to increase the ratio of the rhombohedral graphite layer .

  With the above method, the ratio of the rhombohedral graphite layer can be easily increased by simply applying a centrifugal force to the graphite-based carbon material, so that the production cost of the graphite is complicated. Soaring can be suppressed. In addition, since only a centrifugal force is applied and there is no friction with a part of the apparatus, it is possible to suppress deterioration of battery characteristics due to impurities mixed into graphite.

Here, the reason why the strength ratio (P1 / P2) is limited to the graphite-based carbon material of 0.01 or more is as follows.
(1) Even if a centrifugal force is applied to a graphite-based carbon material having an intensity ratio (P1 / P2) of less than 0.01, the ratio of the rhombohedral graphite layer does not increase or decrease so much. The reason is that the intercalation of the solvated lithium into the graphite layer is not suppressed.
(2) When a centrifugal force is applied to the graphite-based carbon material, the strength ratio (P1 / P2) may decrease. From the beginning, the graphite-based carbon material having a strength ratio (P1 / P2) of 0.01 or more. Therefore, even if the strength ratio (P1 / P2) after applying centrifugal force is reduced, the strength ratio (P1 / P2) can be suppressed from becoming too small. Therefore, even when a high current value is applied, the effect that lithium ions easily enter the carbon material is maintained.

A second aspect of the invention is characterized in that, in the first aspect of the invention, the centrifugal force applied to the graphite-based carbon material is 10 ⁴ G (G is gravitational acceleration) or more.
Thus, when a centrifugal force of 10 ⁴ G or more is applied, a graphite-based carbon material in which the proportion of the rhombohedral graphite layer is increased can be produced in a short time. Is further demonstrated.

  Here, when the graphite-based carbon material of the present invention is used in a non-aqueous electrolyte secondary battery, the electrolyte of the battery is a cyclic carbonate, a chain carbonate, used as a normal non-aqueous solvent for batteries, Esters, cyclic ethers, chain ethers, nitriles, amides and the like can be used.

  Examples of the cyclic carbonate include ethylene carbonate, propylene carbonate, butylene carbonate and the like, and those in which a part or all of these hydroxyl groups are fluorinated can be used, for example, trifluoropropylene carbonate. And fluoroethyl carbonate.

Examples of the chain carbonate include dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methyl propyl carbonate, ethyl propyl carbonate,
Examples thereof include methyl isopropyl carbonate, and those in which a part or all of these hydrogens are fluorinated can also be used.
Examples of the esters include methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and γ-butyrolactone.

Examples of the cyclic ether include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide, 1,4-dioxane, 1,3,5. -Trioxane, furan, 2-methylfuran, 1,8-cineol, crown ether, etc. are mentioned.
Examples of the chain ether include 1,2-dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, methyl phenyl ether, ethyl phenyl ether, butyl phenyl ether, pentyl phenyl. Ether, methoxytoluene, benzyl ethyl ether, diphenyl ether, dibenzyl ether, o-dimethoxybenzene, 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, 1,1 -Dimethoxymethane, 1,1-diethoxyethane, triethylene glycol dimethyl ether, tetrae Glycol dimethyl and the like.

Examples of the nitriles include acetonitrile, and examples of the amides include dimethylformamide.
However, particularly from the viewpoint of voltage stability, it is desirable to use cyclic carbonates such as ethylene carbonate and propylene carbonate, and chain carbonates such as dimethyl carbonate, diethyl carbonate and dipropyl carbonate.

As the electrolyte, an electrolyte generally used for a non-aqueous electrolyte secondary battery can be used. For example, LiPF ₆ , LiAsF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiN (C _l F _{2l + 1} SO ₂ ) (C _m F _{2m + 1} SO ₂ ) (where l and m are integers of 1 or more), LiC (C _p F _{2p + 1} SO ₂ ) (C _q F _{2q + 1} SO ₂ ) (C _r F _{2r + 1} SO ₂ ) (where p, q, and r are integers of 1 or more), lithium difluoro (oxalato) borate (see the following chemical formula 1), and the like. These electrolytic solutions may be used alone or in combination of two or more. The electrolyte is preferably dissolved in the non-aqueous solvent at a concentration of 0.1 to 1.5 mol / liter, preferably 0.5 to 1.5 mol / liter.

Examples of the positive electrode active material include lithium-containing transition metal composite oxides such as LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ , LiMn ₂ , LiCo _0.5 Ni _0.5 O ₂ , LiNi _0.33 Co _0.33 Mn _0.34 O _2. .

  According to the present invention, there is an excellent effect that the ratio of the rhombohedral graphite layer can be easily increased while suppressing an increase in manufacturing cost and mixing of impurities.

  Hereinafter, the manufacturing method of the graphite-type carbon material which concerns on this invention is demonstrated below. In addition, the manufacturing method of the graphite-type carbon material in this invention is not limited to what was shown to the following best form, In the range which does not change the summary, it can change suitably and can implement.

(Best form)
When diffracted by the X-ray wide angle diffraction method (XRD) under the following conditions, the peak intensity (P1) of the (100) plane of the hexagonal graphite layer and the peak intensity of the (002) plane of the hexagonal graphite layer ( A graphite-based carbon material having a strength ratio (P1 / P2) to P2) of 0.0133 was prepared, and centrifugal force was applied to the graphite-based carbon material under the following conditions using a centrifuge.
The graphite-based carbon material thus produced is hereinafter referred to as the present invention carbon material a.

・ X-ray wide angle diffraction condition source: CuKα
Tube current: 40 mA
Tube voltage: 40 kV
Centrifugal centrifugal force: 1.7 × 10 ⁴ G (G is gravitational acceleration)
Time: 3 hours

(Comparison form)
When diffracted by the X-ray wide angle diffraction method (XRD) under the same conditions as the best mode, the peak intensity (P1) of the (100) plane of the hexagonal graphite layer and the (002) of the hexagonal graphite layer Prepare a graphite-based carbon material having an intensity ratio (P1 / P2) to the peak intensity (P2) of the surface of 0.0028, and centrifuge the graphite-based carbon material under the same conditions as in the above best mode. Centrifugal force was applied.
The graphite-based carbon material thus produced is hereinafter referred to as a comparative carbon material x.

(Experiment)
The carbon material a of the present invention and the carbon material before and after comparison carbon material x are diffracted by X-ray wide angle diffraction (XRD) (X-ray wide angle diffraction conditions are the same as those in the best mode), and hexagonal (100) plane peak intensity (P1) of hexagonal graphite layer, (002) plane peak intensity (P2) of hexagonal graphite layer, (101) plane peak intensity (P3) of rhombohedral graphite layer 1 to 4 and Table 1 show the results. 1 is an X-ray wide-angle diffraction graph of the carbon material a of the present invention after centrifugation, FIG. 2 is an X-ray wide-angle diffraction graph of the carbon material of the present invention before centrifugation, and FIG. 3 is an X-ray of the comparative carbon material x after centrifugation. FIG. 4 is an X-ray wide-angle diffraction graph before centrifugation of the comparative carbon material x. Table 1 shows the diffraction angle (2θ) of each peak intensity, and for reference, the peak intensity of the (101) plane of the hexagonal graphite layer is also described.

  Further, the intensity ratio [P1 / P2] of the peak intensity (P1) of the (100) plane of the hexagonal graphite layer and the peak intensity (P2) of the (002) plane of the hexagonal graphite layer, and rhombohedral Since the intensity ratio [P3 / P1] between the peak intensity (P3) of the (101) plane of the graphite layer and the peak intensity (P1) of the (100) plane of the hexagonal graphite layer was calculated, the results are also shown in Table 1. Also shown in FIG.

As is clear from Table 1, the strength ratio [P3 / P1] of the comparative carbon material x is 1.98 before centrifugation and 1.84 after centrifugation, which is decreased after centrifugation. In the carbon material a of the present invention, the strength ratio [P3 / P1] is 1.12 before centrifugation, 1.42 after centrifugation, and increases after centrifugation. This is because the comparative carbon material x has a strength ratio [P1 / P2] before centrifugation of 0.0028, which is very low, whereas the carbon material a of the present invention has a strength ratio [P1 / P2] before centrifugation. This is considered to be due to the fact that it is 0.0133, which is very high. Therefore, it can be seen that in order to increase the strength ratio [P3 / P1], it is necessary to use a graphite-based carbon material having a strength ratio [P1 / P2] before centrifugation of 0.01 or more.
In the carbon material a of the present invention, the strength ratio [P1 / P2] decreases after centrifugation, but it is still recognized that it is a sufficiently high value.

(Other matters)
In the above-mentioned best mode, when the centrifugal force is applied, the centrifugal force is set to 1.7 × 10 ⁴ G and the time is set to 3 hours. However, the present invention is not limited thereto, and the time is shortened by increasing the centrifugal force. Of course, the centrifugal force can be reduced to increase the time.

The case where the graphite-based carbon material produced by the production method of the present invention is applied to a non-aqueous electrolyte secondary battery (test cell) will be described below.
(Example)
Production of working electrode First, graphite carbon material a after centrifugation shown in the above-mentioned best mode as an active material and polyvinylidene fluoride (PVdF) as a binder are mixed at a mass ratio of 95: 5. Thus, a working electrode mixture was obtained. Next, an appropriate amount of N-methylpyrrolidone (NMP) solution was added to this working electrode mixture and mixed to prepare a slurry. Next, this slurry was applied onto a copper foil (thickness: 20 μm) as a current collector by a doctor blade method, then cut into a size of 2 cm × 2 cm, and a current collecting tab was attached to prepare a working electrode.

-Production of counter electrode A counter electrode was produced by cutting a lithium metal plate into a predetermined size and attaching a current collecting tab thereto.

-Preparation of non-aqueous electrolyte In an electrolyte prepared by mixing ethylene carbonate (EC) and diethyl carbonate (DEC) at a volume ratio of 1: 1, non-aqueous electrolyte was dissolved by dissolving LiPF ₆ as a lithium salt to 1 mol / liter. A water electrolyte was prepared.

-Preparation of test cell As shown in FIG. 5, under the inert atmosphere, the working electrode 1 and the counter electrode 2 are put in the test cell container 6 through the polyethylene separator (Hypore made by Asahi Kasei Co., Ltd.) 5. The test cell was produced by arranging and pouring the nonaqueous electrolyte 4 into the test cell container 6. Note that lithium metal was used for the reference electrode 3.
The cell thus produced is hereinafter referred to as the present invention cell A.

(Comparative example)
A positive electrode was produced in the same manner as in the above example except that the graphite-based carbon material a before centrifugation was used as the active material for the working electrode.
The cell thus produced is hereinafter referred to as a comparison cell Y.

(Experiment)
The present invention cell A and the comparative cell Y are charged and discharged under the following charge / discharge conditions for one cycle, the initial charge capacity and the initial discharge capacity per 1 g of the active material are examined, and the following (1) Since the initial charge / discharge efficiency was calculated according to the equation, the results are shown in Table 2. In addition, the charging / discharging curve of this invention cell A is shown in FIG. 6, and the charging / discharging curve of the comparison cell Y is shown in FIG.

Initial charge / discharge efficiency = (initial discharge capacity / initial charge capacity) × 100 (%) (1)

[Charging / discharging conditions]
-Charging conditions After charging to a potential of 0 V (Li / Li ⁺ ) at a charging current of 1/5 It, charging to a potential of 0 V (Li / Li ⁺ ) at a charging current of 1/10 It and further to a potential of 0 V at a charging current of 1/50 It ( Li / Li ⁺ ).
-Discharge conditions Conditions for discharging to a potential of 1.5 V (Li / Li ⁺ ) at a discharge current of 1/10 It.

  As is clear from Table 2, in the present invention cell A using the graphite-based carbon material after centrifugation, the initial charge / discharge efficiency was 91.2%, whereas the graphite-based carbon material before centrifugation was used. In the comparison cell Y, the initial charge / discharge efficiency is 90.6%, and it can be seen that the cell A of the present invention has an initial charge / discharge efficiency higher than that of the comparison cell Y.

  As described above, this is because the peak intensity (P3) of the (101) plane of the rhombohedral graphite layer and the peak intensity (P1) of the (100) plane of the hexagonal graphite layer are greater after centrifugation than before centrifugation. This is considered to be due to the fact that the intensity ratio [P3 / P1] is large.

  The present invention can be used not only for driving power sources of mobile information terminals such as mobile phones, notebook personal computers, and PDAs, but also for negative electrodes of large batteries such as in-vehicle power sources for electric vehicles and hybrid vehicles.

It is a X-ray wide angle diffraction graph after centrifugation in the carbon material a of the present invention. It is an X-ray wide angle diffraction graph before centrifugation in the carbon material a of the present invention. It is a X-ray wide angle diffraction graph after centrifugation in the comparative carbon material x. 3 is an X-ray wide angle diffraction graph of a comparative carbon material x before centrifugation. It is a perspective view of the test cell which concerns on the best form of this invention. It is a graph which shows the charging / discharging curve of this invention cell A. FIG. It is a graph which shows the charging / discharging curve of the comparison cell Y.

Explanation of symbols

1: Positive electrode 2: Negative electrode 4: Non-aqueous electrolyte

Claims (2)

A graphite-based carbon material having at least a rhombohedral graphite layer and a hexagonal graphite layer, wherein the (100) plane peak intensity (P1) and hexagonal graphite of the hexagonal graphite layer by X-ray diffraction method A rhombohedral graphite layer obtained by applying only centrifugal force to a graphite-based carbon material having an intensity ratio (P1 / P2) of 0.01 or more with respect to the peak intensity (P2) of the (002) plane by X-ray diffraction of the layer A method for producing a graphite-based carbon material, characterized by increasing the ratio of . The method for producing a graphite-based carbon material according to claim 1, wherein a centrifugal force applied to the graphite-based carbon material is 10 ⁴ G or more.