Negative electrode for lithium secondary battery

The present invention relates to novel graphite particles, a method for producing graphite particles, a graphite paste using graphite particles, a negative electrode of a lithium secondary battery, and a lithium secondary battery. More specifically, a lithium secondary battery suitable for use in portable devices, electric vehicles, power storage, etc., which has excellent rapid charge / discharge characteristics, cycle characteristics, etc., and graphite particles for obtaining the lithium secondary battery, a method for producing graphite particles, etc. The present invention relates to a graphite paste using graphite particles and a negative electrode of 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 crushing these. These graphite particles are mixed with an organic binder and an organic solvent to form a graphite paste, and this 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 shown in Japanese Patent Publication No. 62-23433 (Patent Document 1), the problem of internal short circuit due to lithium dendrite is solved by using graphite for the negative electrode, and the cycle characteristics are improved.

However, natural graphite particles in which graphite crystals are developed and artificial graphite particles obtained by graphitizing coke have a weaker bonding force between the layers of the crystals in the c-axis direction than those in the plane direction of the crystals. The bonds between the layers are broken, resulting in so-called scale-like graphite particles with a large aspect ratio. Since these scale-shaped graphite particles have a large aspect ratio, when they are kneaded with a binder and applied to a current collector to prepare an electrode, the scale-shaped graphite particles are oriented in the plane direction of the current collector, and as a result. , The strain in the c-axis direction caused by repeated occlusion and release of lithium into graphite crystals causes destruction inside the electrode, which not only causes a problem of deterioration of cycle characteristics, but also tends to deteriorate rapid charge / discharge characteristics. .. Further, since the scale-like graphite particles having a large aspect ratio have a large specific surface area, not only the irreversible capacity of the first cycle of the lithium secondary battery obtained in some cases is large, but also the adhesion to the current collector is poor. There is a problem that requires many binders. If the adhesion to 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 and the like are lowered. Therefore, graphite particles that can improve the rapid charge / discharge characteristics and cycle characteristics or the irreversible capacity of the first cycle, the cycle characteristics or the irreversible capacity of the first cycle are small, and the rapid charge / discharge characteristics and the cycle characteristics of the lithium secondary battery can be improved. Is required.<patcit num="1"><text>Special Publication No. 62-23433</text></patcit>

<p> The present invention has excellent rapid charge / discharge characteristics and cycle characteristics or a small irreversible capacity in the first cycle, excellent cycle characteristics or a small irreversible capacity in the first cycle, and excellent rapid charge / discharge characteristics and cycle characteristics. It provides a negative electrode for a secondary battery.</p><p> The present invention is excellent in rapid charge / discharge characteristics and cycle characteristics, or has a small irreversible capacity in the first cycle, is excellent in cycle characteristics, or has a small irreversible capacity in the first cycle, and is excellent in rapid charge / discharge characteristics and cycle characteristics. It provides a secondary battery.</p>

<p> The present invention relates to graphite particles formed by assembling or combining a plurality of flat particles so that their orientation planes are non-parallel. The present invention also relates to graphite particles having an aspect ratio of 5 or less. The present invention also relates to graphite particles in which the graphite particles are an aggregate of graphite particles.</p><p> The present invention also relates to graphite particles having an aspect ratio of 5 or less. The present invention also relates to graphite particles having an aspect ratio of the graphite particles of 1.2 to 5. Further, the present invention has a specific surface area of 8 m.²For graphite particles of / g or less. Further, in the present invention, the specific surface area is 2 to 5 m.²Regarding graphite particles that are / g. The present invention also relates to graphite particles obtained by assembling or bonding a plurality of flat particles of the graphite particles so that the orientation planes are non-parallel. The present invention also relates to graphite particles having an aspect ratio of 5 or less.</p><p> Further, the present invention is characterized in that 1 to 50% by weight of a graphitizing catalyst is added to a graphitizable aggregate or graphite and a graphitizable binder, mixed, fired and then pulverized. Regarding the manufacturing method. The present invention also relates to a graphite paste obtained by adding an organic binder and a solvent to any of the above graphite particles or graphite particles produced by the above method and mixing them. The present invention also relates to a negative electrode for a lithium secondary battery, which is formed by applying the above graphite paste to a current collector and integrating it. Further, the present invention relates to a lithium secondary battery in which the above-mentioned negative electrode and positive electrode for a lithium secondary battery are arranged so as to face each other via a separator, and an electrolytic solution is injected around the negative electrode.</p>

The graphite particles of the present invention can be roughly classified into three types according to their characteristics. The first graphite particles of the present invention are a plurality of flat particles assembled or bonded so that their orientation planes are non-parallel. In the present invention, the flat particles are particles having a shape having a major axis and a minor axis, and are not completely spherical. For example, scaly, scaly, and some lumpy shapes are included in this. In graphite particles, the orientation planes of a plurality of flat particles are non-parallel, meaning that the flat planes of the shapes of the respective particles, in other words, the plane closest to the flattest, are the orientation planes of the plurality of flat particles. It refers to a state in which the respective orientation planes are assembled without being aligned in a certain direction.

In these graphite particles, the flat particles are aggregated or bonded, and the bond is a state in which the particles are chemically bonded to each other via carbonized carbonaceous substances such as tar and pitch. The term "aggregate" refers to a state in which particles are not chemically bonded to each other, but the shape of the aggregate is maintained due to its shape or the like. From the viewpoint of mechanical strength, those that are bonded are preferable. In one graphite particle, the number of aggregated or bonded flat particles is preferably 3 or more. The size of each flat particle is preferably 1 to 100 μm in diameter, and is 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 lithium is easily occluded and released into the negative electrode graphite, so that the rapid charge / discharge characteristics and cycle characteristics of the obtained lithium secondary battery are improved. Can be made to. 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 a cross section of the graphite particles. In (a), it can be observed that there are many fine scaly graphite particles, and the orientation planes of these particles are non-parallel and bonded 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 lithium can be easily occluded and released as described above. The aspect ratio is preferably 1.2 to 5. When the aspect ratio is less than 1.2, the contact area between particles is reduced, so that the conductivity tends to be lowered. For the same reason, the more preferred range is 1.3 or higher. 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 deteriorate. Therefore, a particularly preferable aspect ratio is 1.3 to 3. The aspect ratio is represented by A / B, where A is the length of the graphite particles in the major axis direction and B is the length in the minor axis direction. The aspect ratio in the present invention is obtained by magnifying graphite particles with a microscope, arbitrarily selecting 100 graphite particles, measuring A / B, and taking the average value thereof.

In the first graphite particles, those having an aspect ratio of 5 or less are preferable, those having an aspect ratio of 1.2 to 5 are more preferable, and those having an aspect ratio of 1.3 to 3 are even more preferable. The second graphite particles are preferably aggregates or conjugates of smaller graphite particles.

The third graphite particle of the present invention has a specific surface area of 8 m.²It is less than / g. Specific surface area is preferably 5 m²It is less than / g. When the graphite particles are used as 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. Specific surface area is 8m²If it exceeds / g, there is a problem that the irreversible capacity of the obtained lithium secondary battery in the first cycle becomes large, the energy density is small, and a large amount of binder is required when manufacturing the negative electrode. The specific surface area is 1.5 to 5 m because the rapid charge / discharge characteristics and cycle characteristics of the obtained lithium secondary battery are even better.²/ g is preferred, 2-5m²More preferably, it is / g. 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 such as the first graphite particles are aggregated or bonded so that the orientation planes are non-parallel, and the second graphite particles. Those having an aspect ratio of 5 or less are preferable, those having an aspect ratio of 1.2 to 5 are more preferable, and those having an aspect ratio of 1.3 to 3 are more preferable, such as graphite particles.

Further, the interlayer distance d (002) of the crystals in the X-ray wide-angle diffraction of each graphite particle used in the present invention is preferably 3.38 Å or less, more preferably 3.37 Å 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 interlayer distance d (002) of the crystal is small or the crystallite size Lc (002) in the c-axis direction is large, the discharge capacity tends to be large, which is preferable.

The method for producing each of the above-mentioned graphite particles of the present invention is not particularly limited, but after adding 1 to 50% by weight of a graphitizing catalyst to a graphitizable aggregate or graphite and a graphitizable binder, mixing, and firing. It can be obtained by crushing. As a result, pores are generated after the graphitization catalyst is removed, which gives the graphite particles of the present invention good properties. Further, each of the above graphite particles can be adjusted by appropriately selecting a mixing method of graphite or aggregate and a binder, adjustment of a mixing ratio such as a binder amount, crushing conditions after firing, and the like.

As the graphitizable aggregate, for example, coke powder, carbide of resin, etc. can be used, but there is no particular limitation as long as it is a graphitizable powder material. Of these, coke powder that is easily graphitized, such as needle coke, is preferable. As the graphite, for example, natural graphite powder, artificial graphite powder and the like can be used, but there is no particular limitation as long as it is in the form of powder. 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 metals such as iron, nickel, titanium, silicon and boron, carbides thereof and oxides can be used. Of these, carbides or oxides of silicon or boron are preferred. The amount of these graphitization catalysts added is preferably in the range of 1 to 50% by weight, more preferably in the range of 5 to 40% by weight, still more preferably in the range of 5 to 30% by weight, based on the obtained graphite particles. If it is less than 50% by weight, the aspect ratio and specific surface area of the graphite particles tend to be large and the development of graphite crystals tends to be poor, while if it exceeds 50% by weight, it is difficult to mix uniformly and workability tends to be poor. It is in.

As the binder, for example, in addition to tar and pitch, organic materials such as thermosetting resin and thermoplastic resin are preferable. The amount of the binder to be blended is preferably 5 to 80% by weight, more preferably 10 to 80% by weight, and 15 to 80% by weight, based on the flat graphitizable aggregate or graphite. Is even more preferable. 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 be large. The method for mixing graphitizable aggregate or graphite and binder is not particularly limited and is carried out 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, etc., 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 calcined and graphitized. The mixture may be formed into a predetermined shape before this treatment. Further, after molding, it may be pulverized before graphitization, the particle size may be adjusted, and then graphitization may be performed. The firing is preferably carried out under conditions in which the mixture is less likely to be oxidized, and examples thereof include a method of firing in a nitrogen atmosphere, an argon gas atmosphere, and a vacuum. The graphitization temperature is preferably 2000 ° C. or higher, more preferably 2500 ° C. or higher, and even more preferably 2800 ° C. to 3200 ° C. When the graphitization temperature is low, the graphitization crystal development is poor, the discharge capacity tends to be low, and the added graphitization catalyst tends to remain in the graphitized particles produced. If the graphitizing catalyst remains in the graphite particles to be produced, the discharge capacity is reduced. If the graphitization temperature is too high, the graphite may sublimate.

Next, it is preferable to pulverize the obtained graphitized product. The method for pulverizing the graphitized product is not particularly limited, but known methods such as a jet mill, a vibration mill, a pin mill, and a hammer mill can be used. The average particle size after pulverization is preferably 1 to 100 μm, more preferably 10 to 50 μm. If the average particle size becomes too large, the surface of the electrode to be manufactured tends to have irregularities. In the present invention, the average particle size can be measured by a laser diffraction particle size distribution meter.

According to the present invention, by going 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. And the specific surface area is 8m²Graphite particles of / g or less can be obtained.

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

As for the mixing ratio of the graphite particles and the organic binder, it is preferable to use 3 to 10 parts by weight of the organic binder with respect to 100 parts by weight of the graphite particles. The solvent is not particularly limited, and N-methyl2-pyrrolidone, dimethylformamide, isopropanol and the like are used. The amount of the solvent is not particularly limited and may be adjusted to a desired viscosity, but it is preferably used in an amount of 30 to 70% by weight based on the graphite paste.

The negative electrode for a lithium ion battery of the present invention is characterized by using each of the above-mentioned graphite particles. The 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 foil such as nickel or copper, or a metal current collector such as a mesh can be used. The integration can be performed by, for example, a molding method such as a roll or a press, or these may be combined and integrated. The negative electrode thus obtained is rapidly charged as compared with a lithium secondary battery using a conventional carbon material for the negative electrode by arranging the positive electrode facing each other via a separator and injecting an electrolytic solution. A lithium secondary battery having excellent discharge characteristics and cycle characteristics and a small irreversible capacity can be manufactured.

The material used for the positive electrode of the lithium secondary battery in the present invention is not particularly limited, and LiNiO₂, LiCoO₂, LiMn₂O₄Etc. can be used alone or in combination. As an electrolytic solution, LiClO₄, LiPF₆, LiAsF₆, LiBF₄, LiSO₃CF₃A so-called organic electrolytic solution in which a lithium salt such as ethylene carbonate, diethyl carbonate, dimethoxyethane, dimethyl carbonate, tetrahydrofuran, propylene carbonate or the like is dissolved in a non-aqueous solvent can be used.

As the separator, for example, a non-woven fabric containing polyolefin as a main component such as polyethylene or polypropylene, a cloth, a micropore film, or a combination thereof can be used. FIG. 2 shows a partial cross-sectional front view of an example of a cylindrical lithium secondary battery. In the cylindrical lithium secondary battery shown in FIG. 2, a positive electrode 1 processed into a thin plate and a negative electrode 2 processed in the same manner are wound by superimposing them 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 bonded to the positive electrode lid 6 via the positive electrode tab 4, and the negative electrode 2 is bonded to the bottom of the battery via the negative electrode tab 5. The positive electrode lid 6 is fixed to the battery can 7 with a gasket 8.

Hereinafter, examples of the present invention will be described with reference to the drawings. Example 1 (1) Adjustment 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 are mixed and stirred at 100 ° C. for 1 hour. did. Then, it was calcined at 2800 ° C. in a nitrogen atmosphere and then pulverized to obtain graphite particles having an average particle size 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 assembled 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 interlayer distance d (002) of the obtained graphite particles by X-ray wide-angle diffraction was 3.360 Å and the crystallite size Lc (002) was 1000 Å or more. Furthermore, the specific surface area by the BET method is 3.5 m.²It was / g.

(2) Preparation of lithium secondary battery A lithium secondary battery having the shape shown in Fig. 2 was manufactured as follows. LiCoO as positive electrode active material₂88% by weight, 7% by weight of scaly natural graphite having an average particle size of 1 μm as a conductive agent, and 5% by weight of polyvinylidene fluoride (PVDF) as a binder, to which N-methyl-2-pyrrolidone was added. (50% by weight of the paste, the same ratio was 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 the 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 the paste, the following example) was added thereto. However, the same ratio was added) and mixed to obtain a paste of the negative electrode mixture.

Next, the positive electrode mixture paste was applied to both sides of an aluminum foil having a thickness of 25 μm, and then vacuum dried at 120 ° C. for 1 hour. After vacuum drying, the electrodes were pressure-molded by a roller press to a thickness of 190 μm. The amount of positive electrode mixture applied per unit area is 49 mg / cm.²The 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 the positive electrode 1 having a length of 10 mm at both ends, and the aluminum foil is exposed, and the positive electrode tab 4 is crimped to one of them by ultrasonic bonding.

On the other hand, the paste of the negative electrode mixture was applied to both sides of a copper foil having a thickness of 10 μm, and then vacuum dried at 120 ° C. for 1 hour. After vacuum drying, the electrodes were pressure molded by a roller press to a thickness of 175 μm. The amount of negative electrode mixture applied per unit area is 20 mg / cm.²The negative electrode 2 was prepared by cutting it into a size having a width of 40 mm and a length of 290 mm. Similar to the positive electrode 1, the 10 mm long portions at both ends of the negative electrode 2 were not coated with the negative electrode mixture and the copper foil was exposed, and the negative electrode tab 5 was crimped to one of them by ultrasonic bonding.

For the separator 3, a polyethylene micropore membrane 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 these were wound to form an electrode group. This was inserted into an AA size battery can 7, the negative electrode tab 5 was welded to the bottom of the can, and a throttle portion for crimping the positive electrode lid 6 was provided. After that, 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 was used. After welding 4 to the positive electrode lid 6, the positive electrode lid 6 was crimped to obtain a lithium secondary battery. Using the obtained lithium secondary battery, charging / discharging was repeated at a charge / discharge current of 300 mA, a charge termination voltage of 4.15 V, and a discharge end voltage of 2.8 V. In addition, 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. 3 and 4.

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. Then, it was calcined at 2800 ° C. in a nitrogen atmosphere and then pulverized to obtain graphite particles having an average particle size 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 assembling or combining a large number of flat particles 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 4.8. The inter-story distance d (002) of the crystals obtained by X-ray wide-angle diffraction of the obtained graphite particles was 3.363 Å, and the crystallite size Lc (002) was 1000 Å or more. Furthermore, the specific surface area by the BET method is 4.3 m.²It was / g. The obtained graphite particles were subjected to the same steps as in Example 1 to prepare a lithium secondary battery, and the same battery characteristic test as in Example 1 was carried out. The results are shown in FIGS. 3 and 4.

Comparative Example 1 Coke powder having an average particle size of 20 μm was calcined 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 and a specific surface area of 11 m.²The crystal layer distance d (002) was 3.365 Å and the crystallite size Lc (002) was 800 Å scaly graphite. The obtained scaly graphite was subjected to the same steps as in Example 1 to prepare a lithium secondary battery, and the same battery characteristic test as in Example 1 was carried out. The results are shown in FIGS. 3 and 4.

The results of comparative tests on the storage and release 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 discharge capacity of the lithium secondary battery and the number of charge / discharge cycles when the lithium secondary battery is repeatedly charged and discharged. In FIG. 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. Is shown.

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

Furthermore, Fig. 4 shows the relationship between the charge / discharge current and the discharge capacity when rapid charge / discharge is performed. 12 is the discharge capacity of the lithium secondary battery obtained in Example 1, 13 is the discharge capacity of the lithium secondary battery obtained in Example 2, and 14 is 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 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. Then, it was calcined at 2800 ° C. in a nitrogen atmosphere and then pulverized to prepare graphite particles having an average particle size 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. The specific surface area of the obtained graphite particles by the BET method is 2.9 m.²It was / g, and the interlayer distance d (002) of the crystals by X-ray wide-angle diffraction of graphite particles was 3.360 Å and the crystallite size Lc (002) was 1000 Å or more. Further, according to a scanning electron micrograph (SEM photograph) of the obtained graphite particles, the graphite particles had a structure in which flat particles were assembled 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 terms of solid content and kneaded to obtain a graphite paste. This graphite paste is applied to a rolled copper foil with a thickness of 10 μm, further dried, and the surface pressure is 490 MPa (0.5 ton / cm).²), And used as a sample electrode. The thickness of the graphite particle layer is 75 μm and the density is 1.5 g / cm.³And said.

The prepared sample electrode was charged and discharged with a constant current by the 3-terminal method, and evaluated as a negative electrode for a lithium secondary battery. FIG. 5 is a schematic view of this lithium secondary battery, and the evaluation of the sample electrode is performed in the glass cell 15 as shown in FIG. 5 and LiPF as the electrolytic solution 16.₄Was dissolved in a mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) (EC and DMC in a volume ratio of 1: 1) at a concentration of 1 mol / liter, and the sample electrode (negative electrode) 17 , The separator 18 and the counter electrode (positive electrode) 19 were laminated and arranged, and the reference electrode 20 was further hung from above to prepare a lithium secondary battery. Metallic lithium was used for the counter electrode 19 and the reference electrode 20, and a polyethylene microporous film was used for the separator 18. Using the obtained lithium secondary battery, between the sample electrode 17 and the counter electrode 19, 0.3 mA / cm with respect to the area of the sample electrode.²5 mV (Vvs.Li / Li) with constant current⁺), 1V (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 rapid charge / discharge characteristic evaluation, 0.3mA / cm²Charge with constant current and discharge current 0.3, 2.0, 4.0 and 6.0mA / cm²Table 2 shows the discharge capacity when changed to.

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. Then, it was calcined at 2800 ° C. in a nitrogen atmosphere and then pulverized to prepare graphite particles having an average particle size of 20 μm. As a result of arbitrarily selecting 100 obtained graphite particles and measuring the average value of the aspect ratio, the result was 4.5. The specific surface area of the obtained graphite particles by the BET method is 4.9 m.²It was / g, and the interlayer distance d (002) of the crystals by X-ray wide-angle diffraction of graphite particles was 3.362 Å and the crystallite size Lc (002) was 1000 Å or more. Further, the obtained graphite particles had a structure in which flat particles were assembled or bonded so that a plurality of orientation planes were non-parallel.

Hereinafter, a lithium secondary battery was produced through the same steps as in Example 3, and the same tests as in Example 3 were 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. Also, as a rapid charge / discharge characteristic evaluation, 0.3mA / cm²Charge with constant current and discharge current 0.3, 2.0, 4.0 and 6.0mA / cm²Table 2 shows the discharge capacity when changed to.

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. Then, it was calcined at 2800 ° C. in a nitrogen atmosphere and then pulverized to prepare graphite particles having an average particle size of 20 μm. As a result of arbitrarily selecting 100 obtained graphite particles and measuring the average value of the aspect ratio, the result was 5. The specific surface area of the obtained graphite particles by the BET method is 6.3 m.²It was / g, and the interlayer distance d (002) of the crystals by X-ray wide-angle diffraction of graphite particles was 3.368 Å and the crystallite size Lc (002) was 700 Å. Further, 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.

Hereinafter, a lithium secondary battery was produced through the same steps as in Example 3, and the same tests as in Example 3 were 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. Also, as a rapid charge / discharge characteristic evaluation, 0.3mA / cm²Charge with constant current and discharge current 0.3, 2.0, 4.0 and 6.0mA / cm²Table 2 shows the discharge capacity when changed to.

Comparative Example 2 Coke powder having an average particle size of 22 μm was calcined 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 7 and a specific surface area of 8.5 m by the BET method.²It was a scale-like graphite with an interlayer distance d (002) of 3.368 Å and a crystallite size Lc (002) of 800 Å by / g, X-ray wide-angle diffraction.

Hereinafter, a lithium secondary battery was produced through the same steps as in Example 3, and the same tests as in Example 3 were 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. Also, as a rapid charge / discharge characteristic evaluation, 0.3mA / cm²Charge with constant current and discharge current 0.3, 2.0, 4.0 and 6.0mA / cm²Table 2 shows the discharge capacity when changed to.

<tables num="1"><img file="JP2008159595A_D0001.tif" /></tables>

<tables num="2"><img file="JP2008159595A_D0002.tif" /></tables>

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

[Effect of the invention]

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

<figref num="1">It is a scanning electron micrograph which shows the particle structure of the graphite particle according to this invention, (a) is a photograph of the outer surface of a particle, and (b) is a photograph of a cross section of a particle.</figref><figref num="2">It is a partial cross-sectional front view of a cylindrical lithium secondary battery.</figref><figref num="3">It is a graph which shows the relationship between the discharge capacity and the number of charge / discharge cycles.</figref><figref num="4">It is a graph which shows the relationship between a discharge capacity and a charge / discharge current.</figref><figref num="5">It is the schematic of the lithium secondary battery used for the measurement of charge / discharge characteristics and irreversible capacity in Examples 3, 4, 5 and Comparative Example 2.</figref>

Code description

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 lithium secondary battery obtained in Example 1 10 Discharge capacity of 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 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