JP4135162B2 - Negative electrode for lithium secondary battery - Google Patents

Description

  The present invention relates to novel graphite particles, a method for producing graphite particles, a graphite paste using graphite particles, a lithium secondary battery negative electrode, and a lithium secondary battery. More specifically, a lithium secondary battery excellent in rapid charge / discharge characteristics, cycle characteristics, etc. suitable for use in portable equipment, electric vehicles, power storage, etc., and graphite particles for obtaining the same, a method for producing graphite particles, The present invention relates to a graphite paste using graphite particles and a negative electrode for a lithium secondary battery.

  Conventional graphite particles include, for example, natural graphite particles, 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 graphite 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 secondary battery. . For example, as disclosed in Japanese Examined Patent Publication No. 62-23433 (Patent Document 1), the use of graphite for the negative electrode eliminates the problem of internal short circuit due to lithium dendrite and improves the cycle characteristics.

However, natural graphite particles in which graphite crystals are developed and artificial graphite particles graphitized from coke have a weaker bonding force between crystals in the c-axis direction than in the crystal plane direction. Bonds between layers are broken and so-called scaly graphite particles having a large aspect ratio are obtained. Since the scaly graphite particles have a large aspect ratio, when the electrodes are produced by kneading with a binder and applying to the current collector, the scaly graphite particles are oriented in the surface direction of the current collector, and as a result In addition, the c-axis direction strain generated by repeated insertion and extraction of lithium into and from the graphite crystal causes breakdown inside the electrode, resulting in a problem that cycle characteristics deteriorate, and rapid charge / discharge characteristics tend to deteriorate. . Furthermore, the scale-like graphite particles having a large aspect ratio have a large specific surface area, so that in some cases, not only the irreversible capacity of the first cycle of the obtained lithium secondary battery is large, but also the adhesion with the current collector is poor, There is a problem that requires many binders. If the adhesiveness with the current collector is poor, there is a problem that the current collecting effect is lowered and the discharge capacity, rapid charge / discharge characteristics, cycle characteristics, etc. are lowered. Therefore, graphite particles that can improve the rapid charge / discharge characteristics and cycle characteristics of the lithium secondary battery with low rapid charge / discharge characteristics and cycle characteristics or small irreversible capacity of the first cycle and low cycle characteristics or irreversible capacity of the first cycle. Is required.
Japanese Examined Patent Publication No. 62-23433

  The present invention is a lithium having excellent rapid charge / discharge characteristics and cycle characteristics, excellent in rapid charge / discharge characteristics and cycle characteristics, or small in irreversible capacity in the first cycle, excellent in cycle characteristics or small in irreversible capacity in the first cycle. A negative electrode for a secondary battery is provided.

  The present invention is a lithium having excellent rapid charge / discharge characteristics and cycle characteristics, excellent in rapid charge / discharge characteristics and cycle characteristics, or small in irreversible capacity in the first cycle, excellent in cycle characteristics or small in irreversible capacity in the first cycle. A secondary battery is provided.

  The present invention relates to graphite particles obtained by assembling or bonding a plurality of flat particles so that their orientation planes are non-parallel. The present invention also relates to a graphite particle having an aspect ratio of 5 or less. The present invention also relates to a graphite particle in which the graphite particle is an aggregate of graphite particles.

The present invention also relates to graphite particles having an aspect ratio of 5 or less. The present invention also relates to a graphite particle having an aspect ratio of 1.2 to 5 of the graphite particle.
The present invention also relates to graphite particles having a specific surface area of 8 m ² / g or less. The present invention also relates to graphite particles having a specific surface area of ² to 5 m ² / g. The present invention also relates to a graphite particle obtained by collecting or combining a plurality of flat particles of the graphite particles such that the orientation planes are non-parallel. The present invention also relates to a graphite particle having an aspect ratio of 5 or less.

In addition, the present invention provides graphite particles characterized by adding 1 to 50% by weight of a graphitization catalyst to a graphitizable aggregate or graphite and a graphitizable binder, mixing, firing and pulverizing. It relates to the manufacturing method.
The present invention also relates to a graphite paste obtained by adding and mixing an organic binder and a solvent to any of the above graphite particles or the graphite particles produced by the above method.
The present invention also relates to a negative electrode for a lithium secondary battery obtained by applying and integrating the above graphite paste to a current collector.
Furthermore, the present invention relates to a lithium secondary battery in which the negative electrode for a lithium secondary battery and a positive electrode are arranged to face each other with a separator interposed therebetween and an electrolyte is injected into the periphery thereof.

The graphite particles of the present invention are roughly divided into three according to their characteristics.
The first graphite particles of the present invention are obtained by collecting or combining a plurality of flat particles so that their orientation planes are non-parallel.
In the present invention, flat particles are particles having a major axis and a minor axis, and are not completely spherical. For example, those having a shape such as a scale shape, a scale shape, or a part of a lump shape are included.
In graphite particles, the orientation planes of a plurality of flat particles are non-parallel. The flat surfaces in the shape of each particle, in other words, the plane that is closest to the plane is the orientation plane, and the plurality of flat particles are A state in which the orientation planes are gathered together in a certain direction.

In this graphite particle, the flat particles are aggregated or bonded, but the bond is a state in which the particles are chemically bonded through carbonaceous carbonized binder such as tar and pitch. The term “aggregate” refers to a state in which the particles are not chemically bonded, but the shape of the aggregate is maintained due to the shape and the like. From the viewpoint of mechanical strength, those bonded are preferable.
In one graphite particle, the number of flat particles aggregated or bonded is preferably 3 or more. The size of the individual flat particles is preferably 1 to 100 μm in particle size, and preferably 2/3 or less of the average particle size of the aggregated or bonded graphite particles.

When the graphite particles are used for the negative electrode, the graphite crystals are less likely to be oriented on the current collector, and it becomes easier to occlude and release lithium into the negative electrode graphite, improving the rapid charge / discharge characteristics and cycle characteristics of the resulting lithium secondary battery. Can be made.
FIG. 1 shows a scanning electron micrograph of the particle structure of an example of the graphite particles of the present invention. In FIG. 1, (a) is a scanning electron micrograph of the outer surface of the graphite particles according to the present invention, and (b) is a scanning electron micrograph of the cross section of the graphite particles. In (a), it can be observed that there are many fine scaly graphite particles that are bonded with the orientation planes of these particles non-parallel to form graphite particles.

The second graphite particles of the present invention have an aspect ratio of 5 or less. The graphite particles tend to be difficult to orient on the current collector, and it is easy to occlude and release lithium as described above.
The aspect ratio is preferably 1.2-5. If the aspect ratio is less than 1.2, the contact area between particles tends to decrease, and the conductivity tends to decrease. For the same reason, the more preferable range is 1.3 or more.
On the other hand, the upper limit of the aspect ratio of the graphite particles is more preferably 3 or less. When the aspect ratio is larger than this, the rapid charge / discharge characteristics tend to be deteriorated. Therefore, a particularly preferable aspect ratio is 1.3 to 3.
The aspect ratio is represented by A / B, where A is the length in the major axis direction of the graphite particles and B is the length in the minor axis direction. The aspect ratio in the present invention is obtained by enlarging graphite particles with a microscope, arbitrarily selecting 100 graphite particles, measuring A / B, and taking the average value.

The first graphite particles preferably have an aspect ratio of 5 or less, more preferably have an aspect ratio of 1.2 to 5, and still more preferably 1.3 to 3.
The second graphite particles are preferably smaller graphite particle aggregates or bonded bodies.

The third graphite particles of the present invention have a specific surface area of 8 m ² / g or less. The specific surface area is preferably 5 m ² / g or less. When the graphite particles are used for the negative electrode, the rapid charge / discharge characteristics and cycle characteristics of the obtained lithium secondary battery can be improved, and the irreversible capacity in the first cycle can be reduced. When the specific surface area exceeds 8 m ² / g, the irreversible capacity in the first cycle of the obtained lithium secondary battery becomes large, the energy density is small, and a large amount of binder is required when producing a negative electrode. There is a problem. The specific surface area is preferably 1.5 to ⁵ m ² / g, more preferably ^{2 to 5} m ² / g, from the viewpoint that the rapid charge / discharge characteristics, cycle characteristics, and the like of the obtained lithium secondary battery are further improved. More preferred. The specific surface area can be measured by a known method such as the BET method (nitrogen gas adsorption method).
The third graphite particles are preferably graphite particles in which a plurality of flat particles like the first graphite particles are aggregated or bonded so that the orientation planes are non-parallel, and the second graphite particles Like graphite particles, those having an aspect ratio of 5 or less are preferred, those having an aspect ratio of 1.2 to 5 are more preferred, and those having an aspect ratio of 1.3 to 3 are more preferred.

  Further, the crystal interlayer distance d (002) in X-ray wide angle diffraction of each graphite particle used in the present invention is preferably 3.38 mm or less, and more preferably 3.37 mm or less. The crystallite size Lc (002) in the c-axis direction is preferably 500 Å or more, and more preferably 1000 Å or more. When the crystal interlayer distance d (002) is reduced or the crystallite size Lc (002) in the c-axis direction is increased, the discharge capacity tends to increase, which is preferable.

Although there is no restriction | limiting in particular in the manufacturing method of each said graphite particle of this invention, After adding 1-50 weight% of graphitization catalysts to the aggregate which can be graphitized, or graphite and a graphitizable binder, and baking it, It can be obtained by grinding. This creates pores after the graphitization catalyst is removed, giving the graphite particles of the present invention good properties.
Moreover, each said graphite particle can also be adjusted by selecting suitably the mixing method of graphite or an aggregate, a binder, adjustment of mixing ratios, such as the amount of binders, and the grinding | pulverization conditions after baking.

Examples of the aggregate that can be graphitized include coke powder and resin carbide, but there is no particular limitation as long as it is a powder material that can be graphitized. Among these, coke powder that is easily graphitized such as needle coke is preferable.
Moreover, as graphite, natural graphite powder, artificial graphite powder, etc. can be used, for example, but there is no restriction | limiting in particular if it is a powder form. The particle size of the graphitizable aggregate or graphite is preferably smaller than the particle size of the graphite particles produced in the present invention.

Further, as the graphitization catalyst, for example, a graphitization catalyst such as a metal such as iron, nickel, titanium, silicon, boron, or a carbide or oxide thereof can be used. Of these, silicon or boron carbides or oxides are preferred.
The addition amount of these graphitization catalysts is preferably 1 to 50% by weight, more preferably 5 to 40% by weight, and further preferably 5 to 30% by weight with respect to the obtained graphite particles. If it is less than% by weight, the aspect ratio and specific surface area of the graphite particles tend to increase and the development of graphite crystals tends to deteriorate. On the other hand, if it exceeds 50% by weight, it is difficult to mix uniformly and workability tends to deteriorate. It is in.

As the binder, for example, an organic material such as a thermosetting resin and a thermoplastic resin is preferable in addition to tar and pitch. The blending amount of the binder is preferably 5 to 80% by weight, more preferably 10 to 80% by weight, and more preferably 15 to 80% by weight based on the flat graphitizable aggregate or graphite. More preferably. If the amount of the binder is too large or too small, the aspect ratio and specific surface area of the graphite particles to be produced tend to increase.
The method for mixing the graphitizable aggregate or graphite and the binder is not particularly limited and is performed using a kneader or the like, but it is preferable to mix at a temperature equal to or higher than the softening point of the binder. Specifically, when the binder is pitch, tar or the like, 50 to 300 ° C is preferable, and when the binder is a thermosetting resin, 20 to 100 ° C is preferable.

Next, the above mixture is fired and graphitized. In addition, you may shape | mold the said mixture in a predetermined shape before this process. Furthermore, after forming and pulverizing before graphitization to adjust the particle size, graphitization may be performed. Firing is preferably performed under conditions where the mixture is not easily oxidized, and examples thereof include a method of baking in a nitrogen atmosphere, an argon gas atmosphere, and in a vacuum. The graphitization temperature is preferably 2000 ° C. or higher, more preferably 2500 ° C. or higher, and further preferably 2800 ° C. to 3200 ° C.
When the graphitization temperature is low, the development of graphite crystals tends to be poor, the discharge capacity tends to be low, and the added graphitization catalyst tends to remain in the graphite particles produced. When the graphitization catalyst remains in the graphite particles to be produced, the discharge capacity decreases. If the graphitization temperature is too high, the graphite may sublime.

  Next, it is preferable to grind the obtained graphitized material. The method for pulverizing the graphitized material is not particularly limited, and a known method such as a jet mill, a vibration mill, a pin mill, a hammer mill or the like can be used. As for the particle size after pulverization, the average particle size is preferably 1 to 100 μm, and more preferably 10 to 50 μm. When the average particle size becomes too large, the surface of the electrode to be produced tends to be uneven. In the present invention, the average particle diameter can be measured with a laser diffraction particle size distribution meter.

In the present invention, by passing through the steps shown above, a plurality of flat particles can be assembled or combined so that the orientation planes are non-parallel, and graphite particles having an aspect ratio of 5 or less can be obtained. Further, graphite particles having a specific surface area of 8 m ² / g or less can be obtained.

The graphite paste of the present invention is produced by mixing the material containing the graphite particles, the organic binder and the solvent.
As the organic binder split, for example, polyethylene, polypropylene, ethylene propylene terpolymer, butadiene rubber, styrene butadiene rubber, butyl rubber, a polymer compound having high ionic conductivity, and the like can be used.
In the present invention, polyvinylidene fluoride, polyethylene oxide, polyepichlorohydrin, polyphosphazene, polyacrylonitrile and the like can be used as the polymer compound having a high ionic conductivity.
Among these, a polymer compound having a high ionic conductivity is preferable, and polyvinylidene fluoride is particularly preferable.

The mixing ratio of the graphite particles and the organic binder is preferably 3 to 10 parts by weight of the organic binder with respect to 100 parts by weight of the graphite particles.
There is no restriction | limiting in particular as a solvent, N-methyl 2-pyrrolidone, a dimethylformamide, isopropanol etc. are used.
There is no restriction | limiting in particular in the quantity of a solvent, Although it should just adjust to a desired viscosity, It is preferable to use 30 to 70 weight% with respect to a graphite paste.

The negative electrode for a lithium ion battery of the present invention is characterized by using each of the above graphite particles. This negative electrode for a lithium ion battery can be obtained by molding the graphite paste into a sheet shape, a pellet shape or the like.
As the current collector, for example, a metal current collector such as 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 or a press, or these may be combined and integrated.
The negative electrode obtained in this manner is disposed more quickly than the lithium secondary battery using a conventional carbon material for the negative electrode by placing the positive electrode opposite to each other with a separator and injecting an electrolyte. A lithium secondary battery having excellent discharge characteristics and cycle characteristics and a small irreversible capacity can be produced.

There is no particular limitation on the material used for a cathode of a lithium secondary battery of the present invention may be used alone or as a mixture of _{LiNiO 2, LiCoO 2, LiMn 2} O 4 or the like.
As an electrolytic solution, a lithium salt such as LiClO ₄ , LiPF ₆ , LiAsF ₆ , LiBF ₄ , LiSO ₃ CF ₃ is dissolved in a non-aqueous solvent such as ethylene carbonate, diethyl carbonate, dimethoxyethane, dimethyl carbonate, tetrahydrofuran, and propylene carbonate. So-called organic electrolytes can be used.

As the separator, for example, a nonwoven fabric, a cloth, a microporous film, or a combination of these having a polyolefin such as polyethylene or polypropylene as a main component can be used.
FIG. 2 shows a partial cross-sectional front view of an example of a cylindrical lithium secondary battery. The cylindrical lithium secondary battery shown in FIG. 2 is formed by winding a thin plate-like positive electrode 1 and a similarly processed negative electrode 2 with a separator 3 such as a polyethylene microporous membrane overlapped. This is inserted into a battery can 7 made of metal or the like and sealed. The positive electrode 1 is bonded to the positive electrode lid 6 via the positive electrode tab 4, and the negative electrode 2 is bonded to the battery bottom via the negative electrode tab 5. The positive electrode lid 6 is fixed to the battery can 7 with a gasket 8.

Embodiments of the present invention will be described below with reference to the drawings.
Example 1
(1) Preparation of graphite particles 70 parts by weight of coke powder having an average particle size of 10 μm, 20 parts by weight of tar pitch, 10 parts by weight of iron oxide and 20 parts by weight of coal tar were mixed and stirred at 100 ° C. for 1 hour. Subsequently, it was fired in a nitrogen atmosphere at 2800 ° C. and then pulverized to obtain graphite particles having an average particle diameter of 20 μm. According to the scanning electron micrograph (SEM photograph) of the obtained graphite particles, the graphite particles had a structure in which a large number of flat particles were aggregated or bonded so that the orientation planes were non-parallel. As a result of arbitrarily selecting 100 obtained graphite particles and measuring the average value of the aspect ratio, it was 1.8. The obtained graphite particles had a crystal interlayer distance d (002) by X-ray wide-angle diffraction of 3.360 mm and a crystallite size Lc (002) of 1000 mm or more. Further, the specific surface area by the BET method was 3.5 m ² / g.

(2) Production of Lithium Secondary Battery A lithium secondary battery having the shape shown in FIG. 2 was produced as follows. 88% by weight of LiCoO ₂ as a positive electrode active material, 7% by weight of flaky natural graphite having an average particle diameter of 1 μm as a conductive agent, and 5% by weight of polyvinylidene fluoride (PVDF) as a binder are added to this. -Methyl-2-pyrrolidone (50% by weight of the paste, the same proportion is added in the following examples) was added and mixed to prepare a paste of the positive electrode mixture. Similarly, 90% by weight of the graphite powder obtained in (1) as a negative electrode active material and 10% by weight of PVDF as a binder were added thereto, and N-methyl-2-pyrrolidone (50% by weight of paste, the following example) However, the same proportion was added) and mixed to obtain a paste of a negative electrode mixture.

Next, the paste of the positive electrode mixture was applied to both surfaces of an aluminum foil having a thickness of 25 μm, and then vacuum-dried at 120 ° C. for 1 hour. After vacuum drying, the electrode was pressure-formed by a roller press to a thickness of 190 μm. The coating amount of the positive electrode mixture per unit area was 49 mg / cm ² , and the positive electrode 1 was produced by cutting it into a size having a width of 40 mm and a length of 285 mm. However, the positive electrode mixture is not applied to the portions of both ends of the positive electrode 1 that are 10 mm in length, and the aluminum foil is exposed, and the positive electrode tab 4 is pressure-bonded to this one by ultrasonic bonding.

On the other hand, the paste of the negative electrode mixture was applied to both surfaces of a copper foil having a thickness of 10 μm, and then vacuum-dried at 120 ° C. for 1 hour. After vacuum drying, the electrode was pressure-formed by a roller press to a thickness of 175 μm. The coating amount of the negative electrode mixture per unit area was 20 mg / cm ² , and the negative electrode 2 was produced by cutting it into a size of 40 mm in width and 290 mm in length. Similarly to the positive electrode 1, the negative electrode mixture was not applied to the 10 mm long portions at both ends of the negative electrode 2 and the copper foil was exposed, and the negative electrode tab 5 was pressure-bonded to this one by ultrasonic bonding.

As the separator 3, a polyethylene microporous film having a thickness of 25 μm and a width of 44 mm was used. Next, as shown in FIG. 2, the positive electrode 1, the separator 3, the negative electrode 2, and the separator 3 were superposed in this order, and this was wound to form an electrode group. This was inserted into an AA size battery can 7, and the negative electrode tab 5 was welded to the bottom of the can to provide a constricted portion for caulking the positive electrode lid 6. Thereafter, an electrolytic solution (not shown) in which 1 mol / liter of lithium hexafluorophosphate was dissolved in a mixed solvent of ethylene carbonate and dimethyl carbonate having a volume ratio of 1: 1 was injected into the battery can 7, and then the positive electrode tab. 4 was welded to the positive electrode lid 6, and then the positive electrode lid 6 was caulked to obtain a lithium secondary battery.
Using the obtained lithium secondary battery, charge / discharge was repeated at a charge / discharge current of 300 mA, a charge end voltage of 4.15 V, and a discharge end voltage of 2.8 V. Moreover, the charge / discharge current was changed in the range of 300 mA to 900 mA, and rapid charge / discharge was also performed. The results are shown in FIGS.

Example 2
70 parts by weight of coke powder having an average particle size of 10 μm, 10 parts by weight of tar pitch, 2 parts by weight of iron oxide, and 20 parts by weight of coal tar were mixed and stirred at 100 ° C. for 1 hour. Subsequently, it was fired in a nitrogen atmosphere at 2800 ° C. and then pulverized to obtain graphite particles having an average particle diameter of 20 μm. As a result of observing the graphite particles obtained with an electron microscope, it was confirmed that the graphite particles were formed by aggregating or bonding so that a large number of flat particles and the orientation planes are non-parallel. As a result of arbitrarily selecting 100 obtained graphite particles and measuring the average value of the aspect ratio, it was 4.8. The obtained graphite particles had a crystal interlayer distance d (002) of 3.363 mm and a crystallite size Lc (002) of 1000 mm or more by X-ray wide angle diffraction. Furthermore, the specific surface area by the BET method was 4.3 m ² / g.
A lithium secondary battery was produced from the obtained graphite particles through the same steps as in Example 1, and a battery characteristic test similar to that in Example 1 was performed. The results are shown in FIGS.

Comparative Example 1
Coke powder having an average particle size of 20 μm was fired at 2800 ° C. in a nitrogen atmosphere to obtain graphite particles having an average particle size of 20 μm. The obtained graphite particles have an average aspect ratio of 6, a specific surface area of 11 m ² / g, a crystal layer distance d (002) of 3.365 mm and a crystallite size Lc (002) of 800 mm. It was graphite.
The obtained scaly graphite was subjected to the same steps as in Example 1 to produce a lithium secondary battery, and the same battery characteristic test as in Example 1 was performed. The results are shown in FIGS.

  The comparative test results regarding the insertion and extraction of lithium in the lithium secondary batteries obtained in Examples 1 and 2 of the present invention and Comparative Example 1 are shown below. FIG. 3 is a graph showing the relationship between the battery discharge capacity and the number of charge / discharge cycles when the lithium secondary battery is repeatedly charged and discharged. 3, 9 is the discharge capacity of the lithium secondary battery obtained in Example 1, 10 is the discharge capacity of the lithium secondary battery obtained in Example 2, and 11 is the discharge capacity of the lithium secondary battery obtained in Comparative Example 1. Indicates.

  In FIG. 3, the maximum discharge capacity of the lithium secondary battery obtained in Example 1 was 750 mAh, and the capacity decrease rate with respect to the maximum capacity at the 500th cycle was 8%. The maximum discharge capacity of the lithium secondary battery obtained in Example 2 was 720 mAh, and the capacity decrease rate with respect to the maximum capacity of the discharge capacity at the 500th cycle was 12%. In addition, the maximum discharge capacity of the lithium secondary battery obtained in the comparative example was 650 mAh, and the capacity decrease rate with respect to the maximum capacity of the discharge capacity at the 500th cycle was 31%.

Further, FIG. 4 shows the relationship between the charge / discharge current and the discharge capacity when rapid charge / discharge is performed. 12 shows the discharge capacity of the lithium secondary battery obtained in Example 1, 13 shows the discharge capacity of the lithium secondary battery obtained in Example 2, and 14 shows the discharge capacity of the lithium secondary battery obtained in Comparative Example 1. At a charge / discharge current of 900 mA, the discharge capacity of the lithium secondary battery obtained in Example 1 was 630 mAh, and the discharge capacity of the lithium secondary battery obtained in Example 2 was 520 mAh, whereas the discharge capacity was obtained in Comparative Example 1. The discharge capacity of the lithium secondary battery was 350 mAh. The capacity reduction rate with respect to the discharge capacity at a charge / discharge current of 300 mAh was 16% for the lithium secondary battery obtained in Example 1, 28% for the lithium secondary battery obtained in Example 2, and the lithium obtained in Comparative Example 1 The secondary battery was 46%.
From the test results of Examples 1 and 2 and Comparative Example 1, it was confirmed that the lithium secondary battery according to the example of the present invention has a high capacity, excellent charge / discharge cycle characteristics, and rapid charge / discharge characteristics. .

Example 3
50 parts by weight of coke powder having an average particle size of 10 μm, 20 parts by weight of tar pitch, 10 parts by weight of silicon carbide, and 20 parts by weight of coal tar were mixed and stirred at 100 ° C. for 1 hour. Subsequently, it was fired at 2800 ° C. in a nitrogen atmosphere and then pulverized to produce graphite particles having an average particle diameter of 20 μm. As a result of arbitrarily selecting 100 obtained graphite particles and measuring the average value of the aspect ratio, it was 1.5. Further, the specific surface area of the obtained graphite particles by BET method is 2.9 m ² / g, and the interlayer distance d (002) of the crystals by X-ray wide angle diffraction of the graphite particles is 3.360 mm and the crystallite size Lc. (002) was 1000 kg or more. Further, according to the scanning electron micrograph (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 orientation planes were non-parallel.

Next, 10% by weight of polyvinylidene fluoride (PVDF) dissolved in N-methyl-2-pyrrolidone was added to 90% by weight of the obtained graphite particles in a solid content and kneaded to obtain a graphite paste. This graphite paste was applied to a rolled copper foil having a thickness of 10 μm, further dried, and compression molded at a surface pressure of 490 MPa (0.5 ton / cm ² ) to obtain a sample electrode. The graphite particle layer had a thickness of 75 μm and a density of 1.5 g / cm ³ .

The prepared sample electrode was subjected to constant current charge / discharge by the three-terminal method, and evaluated as a negative electrode for a lithium secondary battery. FIG. 5 is a schematic view of the lithium secondary battery. Evaluation of the sample electrode was performed by using a glass cell 15 and LiPF ₄ as an electrolytic solution 16 in ethylene carbonate (EC) and dimethyl carbonate (DMC) (see FIG. 5). EC and DMC are mixed at a volume ratio of 1: 1) in a mixed solvent so as to have a concentration of 1 mol / liter, and a sample electrode (negative electrode) 17, a separator 18 and a counter electrode (positive electrode) 19 are laminated. Then, the reference electrode 20 was further suspended from the upper part to produce a lithium secondary battery. The counter electrode 19 and the reference electrode 20 were made of metallic lithium, and the separator 18 was made of a polyethylene microporous film. Using the obtained lithium secondary battery, the sample electrode 17 and the counter electrode 19 were charged to 5 mV (Vvs. Li / Li ⁺ ) with a constant current of 0.3 mA / cm ^{2 with} respect to the area of the sample electrode. The test of discharging to 1 V (Vvs. Li ^/ Ll ⁺ ) was repeated. Table 1 shows the charge capacity per unit weight of the graphite particles in the first cycle, the discharge capacity per unit weight of the graphite particles, the irreversible capacity, and the discharge capacity per unit weight of the graphite particles in the 50th cycle. In addition, as a quick charge / discharge characteristic evaluation, charging was performed at a constant current of 0.3 mA / cm ² , and discharging was performed when the discharge current was changed to 0.3, 2.0, 4.0, and 6.0 mA / cm ^2. The capacity is shown in Table 2.

Example 4
50 parts by weight of coke powder having an average particle size of 10 μm, 10 parts by weight of tar pitch, 5 parts by weight of silicon carbide, and 10 parts by weight of coal tar were mixed and stirred at 100 ° C. for 1 hour. Subsequently, it was fired at 2800 ° C. in a nitrogen atmosphere and then pulverized to produce graphite particles having an average particle diameter of 20 μm. As a result of arbitrarily selecting 100 obtained graphite particles and measuring the average value of the aspect ratio, it was 4.5. Further, the specific surface area of the obtained graphite particles by BET method is 4.9 m ² / g, and the interlayer distance d (002) of the crystals by X-ray wide angle diffraction of the graphite particles is 3.362３６ and the crystallite size Lc. (002) was 1000 kg or more. Further, the obtained graphite particles had a structure in which flat particles were assembled or bonded so that the plurality of orientation planes were non-parallel.

Thereafter, a lithium secondary battery was produced through the same steps as in Example 3, and the same test as in Example 3 was performed. Table 1 shows the charge capacity per unit weight of the graphite particles in the first cycle, the discharge capacity per unit weight of the graphite particles, the irreversible capacity, and the discharge capacity per unit weight of the graphite particles in the 50th cycle. In addition, as a rapid charge / discharge characteristic evaluation, the discharge capacity when charging was performed at a constant current of 0.3 mA / cm ² and the discharge current was changed to 0.3, 2.0, 4.0, and 6.0 mA / cm ^2. Is shown in Table 2.

Example 5
50 parts by weight of coke powder having an average particle size of 10 μm, 5 parts by weight of tar pitch and 5 parts by weight of coal tar were mixed and stirred at 100 ° C. for 1 hour. Subsequently, it was fired at 2800 ° C. in a nitrogen atmosphere and then pulverized to produce graphite particles having an average particle diameter of 20 μm. As a result of arbitrarily selecting 100 obtained graphite particles and measuring the average value of the aspect ratio, it was 5. Further, the specific surface area of the obtained graphite particles by BET method is 6.3 m ² / g, the crystal interlayer distance d (002) by X-ray wide angle diffraction of the graphite particles is 3.368 mm, and the crystallite size Lc. (002) was 700 kg. Furthermore, the obtained graphite particles had a structure in which a plurality of flat particles were aggregated or bonded so that the orientation planes were non-parallel.

Thereafter, a lithium secondary battery was produced through the same steps as in Example 3, and the same test as in Example 3 was performed. Table 1 shows the charge capacity per unit weight of the graphite particles in the first cycle, the discharge capacity per unit weight of the graphite particles, the irreversible capacity, and the discharge capacity per unit weight of the graphite particles in the 50th cycle. In addition, as a rapid charge / discharge characteristic evaluation, the discharge capacity when charging was performed at a constant current of 0.3 mA / cm ² and the discharge current was changed to 0.3, 2.0, 4.0, and 6.0 mA / cm ^2. Is shown in Table 2.

Comparative Example 2
Coke powder having an average particle size of 22 μm was fired at 2800 ° C. in a nitrogen atmosphere to obtain graphite particles having an average particle size of 20 μm. The obtained graphite particles had an average aspect ratio of 7, a specific surface area by BET method of 8.5 m ² / g, a crystal interlayer distance d (002) by X-ray wide angle diffraction of 3.368 mm, and a crystallite size of The scale Lc (002) was 800 Å scale graphite.

Thereafter, a lithium secondary battery was produced through the same steps as in Example 3, and the same test as in Example 3 was performed. Table 1 shows the charge capacity per unit weight of the graphite particles in the first cycle, the discharge capacity per unit weight of the graphite particles, the irreversible capacity, and the discharge capacity per unit weight of the graphite particles in the 50th cycle. In addition, as a rapid charge / discharge characteristic evaluation, the discharge capacity when charging was performed at a constant current of 0.3 mA / cm ² and the discharge current was changed to 0.3, 2.0, 4.0, and 6.0 mA / cm ^2. Is shown in Table 2.

  As shown in Tables 1 and 2, the lithium secondary batteries obtained in the examples of the present invention have a large discharge capacity, a small irreversible capacity in the first cycle, and excellent cycle characteristics and rapid discharge characteristics. it is obvious.

[The invention's effect]
The negative electrode for a lithium secondary battery according to the present invention is excellent in rapid charge / discharge characteristics and cycle characteristics, or has low irreversible capacity in the first cycle, excellent in cycle characteristics or small in irreversible capacity in the first cycle, and is rapidly charged / discharged. It is a negative electrode for a lithium secondary battery suitable for a lithium secondary battery having excellent characteristics and cycle characteristics.

  The lithium secondary battery according to the present invention is excellent in rapid charge / discharge characteristics and cycle characteristics or small in the first cycle irreversible capacity, excellent in cycle characteristics or small in the first cycle irreversible capacity, It is a lithium secondary battery with excellent cycle characteristics.

It is a scanning electron micrograph which shows the particle structure of the graphite particle | grains which become this invention, (a) is a photograph of the outer surface of particle | grains, (b) is a photograph of the cross section of particle | grains. It is a partial cross section front view of a cylindrical lithium secondary battery. It is a graph which shows the relationship between discharge capacity and the number of charge / discharge cycles. It is a graph which shows the relationship between discharge capacity and charging / discharging electric current. In Example 3, 4, 5 and the comparative example 2, it is the schematic of the lithium secondary battery used for the measurement of a charging / discharging characteristic and an irreversible capacity | capacitance.

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 lid 7 Battery can 8 Gasket 9 Discharge capacity of the lithium secondary battery obtained in Example 1 10 Discharge capacity of the lithium secondary battery obtained in Example 2 11 Comparison Discharge capacity of the lithium secondary battery obtained in Example 1 12 Discharge capacity of the lithium secondary battery obtained in Example 1 13 Discharge capacity of the lithium secondary battery obtained in Example 2 14 Lithium secondary battery obtained in Comparative Example 1 Battery discharge capacity 15 Glass cell 16 Electrolyte 17 Sample electrode (negative electrode)
18 Separator 19 Counter electrode (positive electrode)
20 reference electrode

Claims (5)

A graphite paste having a specific surface area of 8 m ² / g or less and having an organic binder and solvent added to and mixed with graphite particles having pores observed in a scanning micrograph on the cross section of the particles is collected. A negative electrode for a lithium secondary battery, which is applied to an electric body and integrated.   An organic binder and a solvent are added to graphite particles having a crystallite size Lc (002) in the c-axis direction of 500 mm or more and having pores observed in a scanning micrograph in the cross section of the particles, A negative electrode for a lithium secondary battery, in which a graphite paste obtained by mixing is applied to a current collector and integrated.   3. The negative electrode for a lithium secondary battery according to claim 1, wherein the graphite particles are aggregated or bonded such that a plurality of flat particles and the orientation planes are non-parallel. 4.   The negative electrode for a lithium secondary battery according to any one of claims 1 to 3, wherein the graphite particles have an aspect ratio of 5 or less.   A lithium secondary battery in which the negative electrode for a lithium secondary battery according to any one of claims 1 to 4 and a positive electrode are arranged to face each other with a separator interposed therebetween, and an electrolyte is injected into the periphery thereof.