JP2001006669A - Graphite particles for lithium secondary battery negative electrode, manufacture of the particles, negative electrode for lithium secondary battery, and lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a carbonic material for a lithium secondary battery negative electrode allowing a reduction of the irreversible capacity in the first cycle and enhancement of the safety, establish a method of stably manufacturing such carbon materials, and offer a negative electrode and secondary battery equipped with the mentioned characteristics. SOLUTION: The graphite particles for a lithium secondary battery negative electrode are manufactured through such procedures that a catalyst for turning into graphite is added to a binder capable of being turned into graphite and graphite or an aggregate capable of being turned into graphite followed by agitation for mixing together, baking to turn into graphite, crushing, and heating at a temp. over 500 deg.C in a low oxygen concentration atmosphere or in the non-oxidating atmosphere. The obtained graphite particles have such a form that a plurality of flat particles are gathered or coupled together so that their orientation planes are out of parallelelism, wherein the aspect ratio is below 5 and the specific surface area is below 3 m2/g. A resultant lithium secondary battery includes a negative electrode 2 containing such graphite particles and a positive electrode 1 containing lithium compound.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a graphite particle for a negative electrode of a lithium secondary battery having a non-aqueous electrolyte, a method for producing the same,
The present invention relates to a high-capacity lithium secondary battery and a negative electrode using the same.

[0002]

2. Description of the Related Art Conventional graphite particles include, for example, natural graphite particles,
Examples include artificial graphite particles obtained by graphitizing coke, organic polymer materials, artificial graphite particles obtained by graphitizing pitch and the like, and graphite particles obtained by pulverizing these. These 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 ion secondary battery. . For example, as shown in JP-B-62-23433,
By using graphite for the negative electrode, the problem of internal short circuit due to lithium dendrite is eliminated, and the cycle characteristics are improved.

[0003] However, natural graphite particles in which graphite crystals are developed and artificial graphite particles obtained by graphitizing coke are:
Since the bonding force between the layers of the crystal in the c-axis direction is weaker than the bonding in the plane direction of the crystal, the bonding between the graphite layers is broken by pulverization,
So-called graphite particles having a large aspect ratio are obtained. Since the scale-like graphite particles have a large aspect ratio, the scale-like graphite particles are oriented in the surface direction of the current collector when the electrode is produced by kneading with a binder and applying the mixture to the current collector. Distortion in the c-axis direction caused by the repeated insertion and extraction of lithium into and from the particles causes the destruction of the inside of the electrode, which causes not only the problem of reduced cycle characteristics but also the tendency of rapid charge / discharge characteristics to deteriorate.

Further, scale-like graphite particles having a large aspect ratio have a large specific surface area, so that they have poor adhesion to a current collector and require a large amount of binder. If the adhesion to the current collector is poor, there is a problem that the current collecting effect is reduced and the discharge capacity, rapid charge / discharge characteristics, cycle characteristics, and the like are reduced.
Further, scale-like graphite particles having a large specific surface area have a problem that the irreversible capacity in the first cycle of a lithium secondary battery using the particles is large. Furthermore, scale-like graphite particles having a large specific surface area have low thermal stability in a state where lithium is stored, and have a problem in safety when used as a negative electrode material for a lithium secondary battery. Therefore, graphite particles capable of improving rapid charge / discharge characteristics, cycle characteristics, irreversible capacity in the first cycle, and safety are required.

[0005] For the purpose of answering the above requirements, graphite particles are obtained by assembling or binding flat particles so that a plurality of oriented planes are non-parallel, and have an aspect ratio of 5 or less and a specific surface area of 10 m ² / g or less, measured by the mercury intrusion method
10 ⁵ total pore volume of the nm range is 0.4~2.0cc / g, d
Graphite particles having a 002 of 0.338 nm or less, a Lc size of 50 nm or more, and a La size of 100 nm or less have been proposed (JP-A-10-158005, JP-A-10-236808, JP-A-10-236808). 10-236809
No.). Such graphite particles have rapid charge and discharge characteristics,
It can be suitably used for a lithium ion secondary battery having excellent cycle characteristics, irreversible capacity in the first cycle, and safety.

[0006]

However, further reduction of the irreversible capacity and further improvement of safety in the first cycle are required, and a method for stably producing graphite particles which fulfills this requirement is required. Has been required. The present invention provides a carbon material for a negative electrode of a lithium secondary battery, in which the irreversible capacity in the first cycle is reduced and safety is improved, and a carbon for a negative electrode of a lithium secondary battery, which can stably produce the material. A method for producing a material is provided. The present invention also provides a negative electrode for a lithium secondary battery and the lithium secondary battery having the above characteristics.

[0007]

SUMMARY OF THE INVENTION The present invention provides a graphitizable aggregate or a graphite and a graphitizable binder, which are mixed with a graphitizing catalyst, calcined, graphitized, pulverized, and pulverized. 500 ° C in oxygen concentration atmosphere or non-oxidizing atmosphere
The present invention relates to a method for producing graphite particles for a negative electrode of a lithium secondary battery, which is characterized by performing a heat treatment. Further, the present invention has a shape obtained by assembling or binding flat particles obtained by the above-mentioned production method so that a plurality of orientation planes are non-parallel, and has an aspect ratio of 5 or less and a specific surface area of 3 m ^2. The present invention relates to graphite particles for a negative electrode of a lithium secondary battery, which are not more than / g.

[0008] The present invention also relates to a negative electrode for a lithium secondary battery containing the above graphite particles or the graphite particles produced by the production method described above. Furthermore, the present invention relates to a lithium secondary battery having the above-mentioned negative electrode and a positive electrode containing a lithium compound.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The graphite particles of the present invention are prepared by adding a graphitizing catalyst to a graphitizable aggregate or graphite and a graphitizable binder, preferably 1 to 50% by weight (graphitizable aggregate or graphite). And the total amount of binders that can be graphitized), mixed, calcined, graphitized, ground, and heat-treated at 500 ° C or more in a low oxygen concentration atmosphere or a non-oxidizing atmosphere. You. During this step, particularly by performing the above-described heat treatment step, it is possible to improve the safety when used as an electrode, increase the capacity in rapid charge and discharge, and reduce the irreversible capacity. Further, graphite particles having the above characteristics can be stably produced.

The graphite particles obtained according to the present invention have a shape obtained by assembling or bonding flat particles such that a plurality of orientation planes are non-parallel, and have an aspect ratio of 5 or less and a specific surface area of 3 m ² / It is preferably not more than g. In the present invention, flat particles are particles having a major axis and a minor axis, and are not perfectly spherical. For example, scaly,
Shapes such as scaly and massive are included in this. By using graphite particles as a form in which flat particles are aggregated or bonded so that a plurality of oriented surfaces are non-parallel, when used as a negative electrode, the flat particles are unlikely to be oriented on the current collector, and the negative electrode graphite Facilitates the insertion and extraction of lithium ions, thereby improving the rapid charge and discharge characteristics and cycle characteristics of the obtained lithium secondary battery.

Furthermore, by setting the aspect ratio of the graphite particles to 5 or less, the flat particles are less likely to be oriented on the current collector, and the rapid charge / discharge characteristics and cycle characteristics of the lithium secondary battery are further improved. be able to. The graphite particles preferably have an aspect ratio of 3 or less. If the aspect ratio exceeds 5, the flat particles are likely to be oriented on the current collector, and the rapid charge / discharge characteristics and cycle characteristics tend to deteriorate.

In the present invention, the specific surface area of the graphite particles is 3
It is preferred to be not more than m ² / g. If the specific surface area exceeds 3, the stability of the lithium-graphite intercalation compound formed by occluding lithium ions tends to deteriorate, and the intercalation compound is rapidly decomposed at the time of short-circuiting of the battery and at the time of heating, thereby generating heat. The heat generated increases the risk of the organic electrolyte igniting and exploding.

In the present invention, the graphite particles can be specifically produced, for example, as follows. Various cokes such as fluid coke and needle coke can be used as the graphitizable aggregate. Moreover, what has already been graphitized, such as natural graphite or artificial graphite, may be used as an aggregate. As a binder that can be graphitized,
Various pitches and tars such as coal-based, petroleum-based, and man-made can be used. As the graphitization catalyst, iron, nickel, titanium, boron, etc., and their carbides, oxides, nitrides and the like can be used.

It is preferable to add 1 to 50% by weight of a graphitization catalyst to the graphitizable aggregate or graphite and the graphitizable binder. If it is less than 1% by weight, the development of graphite particles becomes poor, and the charge / discharge capacity tends to decrease. on the other hand,
If it exceeds 50% by weight, it becomes difficult to mix uniformly, and the workability tends to decrease.

The mixture of the above materials is then fired and graphitized. The firing and graphitization are preferably performed in an atmosphere in which the mixture is hardly oxidized, and examples include a method of firing in a nitrogen atmosphere, an argon gas, or a vacuum. The temperature for firing and graphitization is preferably 2000 ° C or higher, more preferably 2500 ° C or higher, even more preferably 2800 to 3200 ° C. If the graphitization temperature is less than 2000 ° C., the development of graphite crystals becomes worse, and the graphitization catalyst tends to remain in the produced graphite particles, and in any case, the charge / discharge capacity tends to decrease.

Next, the obtained graphitized product is pulverized. The method for pulverizing the graphitized material is not particularly limited, but a known method such as a jet mill, a vibration mill, a pin mill, a hammer mill, or the like can be used. Average particle size after pulverization is 100
μm or less, more preferably 10 to 50 μm. If the average particle diameter exceeds 100 μm, the surface of the prepared electrode is likely to be uneven.

Next, the graphite particles obtained by the pulverization are heat-treated at a temperature of 500 ° C. or more in a low oxygen concentration atmosphere or a non-oxidizing atmosphere. The graphite particles obtained in the pulverizing step generally have a specific surface area exceeding 3 m ² / g, and when used as an electrode, there may be a problem in safety. By subjecting such graphite particles to the heat treatment, the specific surface area of 3 m ² / g or less of the present invention can be obtained.

The low-oxygen-concentration atmosphere is prepared by, for example, burying a graphite case filled with sample powder in silica sand and heating the case.
This can be achieved by suppressing the intrusion of oxygen from the outside air. Examples of the non-oxidizing atmosphere include a nitrogen atmosphere, an argon gas, and a vacuum. If the oxygen concentration in the atmosphere in contact with the sample during heating is high, the sample may be consumed by oxidation and the specific surface area may increase. If the heating temperature is lower than 500 ° C., the non-surface area of the graphite particles cannot be sufficiently reduced. There is no particular limitation on the heating time, but it is preferable to maintain the treatment temperature at 500 ° C. or higher for 1 minute to 10 hours because a predetermined effect can be obtained.

The graphite particles for a negative electrode of a lithium secondary battery of the present invention obtained by the above-mentioned method are generally kneaded with an organic binder and a solvent and formed into a sheet, pellet, or the like, and then formed into a negative electrode. It can be. As the organic binder, for example, polyethylene, polypropylene, ethylene propylene polymer, butadiene rubber, styrene butadiene rubber, butyl rubber, and a polymer compound having a high ionic conductivity can be used.

As the polymer having a high ionic conductivity, polyvinylidene fluoride, polyethylene oxide, polyepihydrin, polyphosphazene, polyacrylonitrile and the like can be used. The content of the organic binder is 3 to 3 with respect to the total amount of the graphite particles and the organic binder.
It is preferably 20% by weight. The graphite particles are kneaded with an organic binder and a solvent to adjust the viscosity, and then applied to, for example, a current collector and integrated with the current collector to form a negative electrode for a lithium secondary battery.

As the current collector, for example, a foil or mesh of nickel, copper, or the like can be used. The integration can be performed by a molding method such as a roll and a press. The negative electrode obtained in this way is arranged in such a manner that a positive electrode containing a lithium compound is opposed to each other with a separator interposed therebetween, and an electrolyte solution is injected, thereby comparing with a conventional lithium secondary battery using a carbon material. Excellent in rapid charge / discharge characteristics and cycle characteristics,
A lithium secondary battery with small irreversible capacity and excellent safety can be manufactured.

The lithium compound contained in the positive electrode of the lithium secondary battery according to the present invention is not particularly limited.
NiO ₂ , LiCoO ₂ , LiMn ₂ O _{4 and the} like can be used alone or in combination. Further, as the _{electrolyte, LiClO 4, LiPF 6, LiAsF} 6, LiB
A so-called organic electrolyte obtained by dissolving a lithium salt such as F ₄ or LiSO ₃ CF _{3 in} a non-aqueous solvent such as ethylene carbonate, diethyl carbonate, dimethoxyethane, dimethyl carbonate, tetrahydrofuran or propylene carbonate can be used.

As the separator, for example, a nonwoven fabric, cloth, microporous film, or a combination thereof, containing a polyolefin such as polyethylene or polypropylene as a main component can be used.

FIG. 1 is a partial cross-sectional front view of an example of a cylindrical lithium secondary battery. The cylindrical lithium 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. This is inserted into a battery can 7 made of metal or the like, and sealed. The positive electrode 1 is connected to a positive electrode cover 6 via a positive electrode tab 4, and the negative electrode 2 is connected to a battery bottom via a negative electrode tab 5. The positive cover 6 is a gasket 8
To the battery can (positive electrode can) 7.

[0025]

Embodiments of the present invention will be described below. Example 1 50 parts by weight of coke powder having an average particle diameter of 5 μm, 20 parts by weight of tar pitch, 7 parts by weight of silicon carbide having an average particle diameter of 48 μm, and 10 parts by weight of coal tar were mixed and mixed at 200 ° C. for 1 hour. The obtained mixture is pulverized, pressed into pellets, then fired at 3000 ° C. in a nitrogen atmosphere, and then pulverized using a hammer mill to have an average particle diameter of 20 μm.
Was produced. The specific surface area of the graphite particles measured by the BET method was 3.6 m ² / g. After heating the graphite particles in a nitrogen atmosphere at 500 ° C. for 1 hour, the specific surface area was measured again. As a result, the specific surface area was 2.6 m ² / g. As a result of measuring the pore size distribution of the obtained graphite particles by a mercury intrusion method, the graphite particles had pores in a range of 10 ¹ to 10 ⁵ nm, and the total pore volume was 0.9 cc / g.
Further, 100 obtained graphite particles were arbitrarily selected, and the aspect ratio was measured. As a result, it was 2.0, and the interlayer distance d of the graphite particles by the X-ray wide-angle diffraction of the graphite particles was obtained.
(002) is 0.336 nm and the crystallite size Lc (0
02) was 100 nm or more. Further, according to a scanning electron microscope (SEM) photograph of the obtained graphite particles, the graphite particles had a structure in which flat particles were aggregated or bonded so that a plurality of oriented planes became non-parallel.

Next, 90 parts by weight of the obtained graphite particles are
10 parts by weight of polyvinylidene fluoride (PVDF) dissolved in N-methyl-2-pyrrolidone was added as a solid content and kneaded to prepare a graphite paste. This graphite paste has a thickness of 10
μm rolled copper foil, and then dried, surface pressure 490Mp
It was compression-molded at a (0.5 ton / cm ² ) pressure to obtain a sample electrode. The thickness of the graphite particle layer is 90 μm and the density is 1.6 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 a lithium secondary battery used in the experiment. LiPF ₆ is used as an electrolyte solution 10 in a glass cell 9 in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) (EC: DEC = 1: 1 (volume ratio)) so as to have a concentration of 1 mol / liter. The dissolved solution is put therein, and the sample electrode 11, the separator 12, and the counter electrode 13 are placed.
And the reference electrode 14 was suspended from above. Lithium metal was used for the counter electrode 13 and the reference electrode 14, and a polyethylene microporous membrane was used for the separator 4. At a constant current of 0.3 mA / cm ² , 5 mV (V vs L
i / Li ⁺ ) and 1 V (V vs Li / L)
A test for discharging to i ⁺ ) was performed, and the charge / discharge capacity was measured. Table 1 shows the charge capacity, discharge capacity and irreversible capacity per unit weight of the graphite particles in the first cycle.

Example 2 In the same manner as in Example 1, graphite particles having an average particle size of 20 μm and a specific surface area of 3.6 m ² / g were produced. After heating the graphite particles in a nitrogen atmosphere at 700 ° C. for 1 hour, the specific surface area was measured again. As a result, the specific surface area was 2.5 m ² / g, and a decrease in the specific surface area was recognized. Hereinafter, a lithium secondary battery was manufactured in the same manner as in Example 1, and the same test as in Example 1 was performed.
Table 1 shows the charge capacity, discharge capacity and irreversible capacity per unit weight of the graphite particles in the first cycle.

Example 3 In the same manner as in Example 1, graphite particles having an average particle size of 20 μm and a specific surface area of 3.6 m ² / g were produced. After heating the graphite particles in a nitrogen atmosphere at 1000 ° C. for 1 hour, the specific surface area was measured again. As a result, the specific surface area was 2.4 m ² / g, and a decrease in the specific surface area was recognized. Hereinafter, a lithium secondary battery was manufactured in the same manner as in Example 1, and the same test as in Example 1 was performed. Table 1 shows the charge capacity, discharge capacity and irreversible capacity per unit weight of the graphite particles in the first cycle.

Example 4 In the same manner as in Example 1, graphite particles having an average particle diameter of 20 μm and a specific surface area of 3.6 m ² / g were produced. This graphite particles are filled in a graphite case, and the case is covered with silica sand.
Heated at 900 ° C. for 1 hour. When the specific surface area of the obtained powder was measured, it was 2.5 m ² / g, and a decrease in the specific surface area was recognized. Hereinafter, a lithium secondary battery was manufactured in the same manner as in Example 1, and the same test as in Example 1 was performed. Table 1 shows the charge capacity per unit weight of the graphite particles in the first cycle,
Shows discharge capacity and irreversible capacity.

Comparative Example 1 In the same manner as in Example 1, graphite particles having an average particle diameter of 20 μm and a specific surface area of 3.6 m ² / g were produced. After heating the graphite particles in a nitrogen atmosphere at 400 ° C. for 1 hour, the specific surface area was measured again. As a result, the specific surface area was 3.5 m ² / g, and no reduction in the specific surface area was observed. Hereinafter, a lithium secondary battery was manufactured in the same manner as in Example 1, and the same test as in Example 1 was performed. Table 1 shows the charge capacity, discharge capacity and irreversible capacity per unit weight of the graphite particles in the first cycle.

Comparative Example 2 Graphite particles having an average particle size of 20 μm and a specific surface area of 3.6 m ² / g were prepared in the same manner as in Example 1. After placing the graphite particles in a porcelain dish and heating at 600 ° C. for 1 hour in a static air atmosphere, the specific surface area was measured to be 4.5 m ² / g, indicating an increase in the specific surface area.

[0033]

[Table 1]

[0034]

According to the method for producing graphite particles for a negative electrode of a lithium secondary battery of the present invention, the specific surface area is small, the capacity in rapid charge and discharge is high, the irreversible capacity in the first cycle is small, and Graphite particles having high properties can be stably produced. As a result, the yield is improved, and the battery cost can be reduced. The obtained graphite particles for a lithium secondary battery negative electrode, the negative electrode for a lithium secondary battery, and the lithium secondary battery have high capacity in rapid charge and discharge, small irreversible capacity in the first cycle, and high safety. Becomes

[Brief description of the drawings]

FIG. 1 is a schematic view showing one example of a lithium secondary battery of the present invention.

FIG. 2 is a schematic view of a lithium secondary battery used for measurement of charge and discharge characteristics in Examples and Comparative Examples.

[Explanation of symbols]

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

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 4G046 EA05 EB02 EB04 EC02 EC05 EC06 5H003 AA02 AA10 BA01 BA03 BA04 BB01 BC01 BD00 BD02 BD05 5H014 AA01 AA06 BB01 BB06 EE08 HH00 HH01 HH06 HH08 5H029 A0703 A0703 BJ02 BJ14 CJ00 CJ02 CJ08 CJ28 DJ16 HJ00 HJ07 HJ14

Claims (4)

[Claims] A graphitizable aggregate or graphite and a graphitizable binder are mixed with a graphitizing catalyst, calcined and graphitized, and then pulverized, and then pulverized in a low oxygen concentration atmosphere or non-oxidizing atmosphere. A method for producing graphite particles for a negative electrode of a lithium secondary battery, comprising performing a heat treatment at 500 ° C. or more in an atmosphere. 2. The method according to claim 1, which is obtained by:
Graphite particles for negative electrodes of lithium secondary batteries having a shape formed by assembling or combining flat particles such that a plurality of orientation planes are non-parallel and having an aspect ratio of 5 or less and a specific surface area of 3 m ² / g or less. . 3. A negative electrode for a lithium secondary battery comprising the graphite particles produced by the production method according to claim 1. 4. A lithium secondary battery comprising the negative electrode according to claim 3 and a positive electrode containing a lithium compound.