JPH11219704A - Lithium secondary battery, its negative electrode and its manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a negative electrode suppressing the excessive deformation of grains and the orientation of the graphitic grains caused by the fluctuation of the manufacturing condition by containing a mixture of two or more kinds of graphitic grains different in bulk specific gravity. SOLUTION: This negative electrode contains two or more kinds of graphitic grains having the bulk specific gravity in the ranges of 0.4-0.65 and 0.65-0.85. At least one kind of graphitic grains preferably have a structure collected or coupled with multiple flat grains so that orientation faces are made nonparallel. A graphitizing catalyst is added to a material containing a graphitizable aggregate or graphite and a graphitizable binder, and the mixture is baked, graphitized, then pulverized to obtain graphitic grains. Two or more kinds of graphitic grains thus obtained are mixed to form a negative electrode material. A good cycle characteristic, a high charging/discharging capacity and a quick charging/discharging characteristic can be applied to the negative electrode.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithium secondary battery, a negative electrode thereof, and a method of manufacturing the same, and particularly relates to a charge / discharge capacity,
The present invention relates to a lithium secondary battery having excellent charge / discharge characteristics and cycle characteristics, a negative electrode thereof, and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, there has been an increasing demand for small, lightweight, and high energy density secondary batteries for portable equipment, electric vehicles, and power storage. In response to such demands, non-aqueous electrolyte secondary batteries, especially lithium secondary batteries, have attracted attention as batteries having high voltage and high energy density.

As a negative electrode material for a lithium secondary battery, lithium metal, low graphitized carbon particles, and highly graphitized carbon particles are used. Metal lithium can achieve high charge / discharge capacity, but due to its high reactivity, it reacts with the solvent in the electrolytic solution with the progress of the charge / discharge cycle, the capacity decreases, and dendritic metal lithium is easily generated, There is a problem that a short circuit is likely to occur through the separator provided between the positive electrode and the negative electrode. Low graphitized carbonaceous materials have the characteristics of low reactivity with the electrolytic solution and the difficulty of forming dendritic lithium.However, the charge / discharge capacity is generally low, and the true density is low. It has the drawback of low capacity and has not achieved a secondary battery with high energy density. On the other hand, highly graphitized carbon particles
Recently, it has a higher charge / discharge capacity than low graphitized carbon particles, has a higher reactivity with electrolyte than lithium metal, and has a characteristic that lithium dendritic metal is less likely to be generated. Investigations have been actively made as materials.

As highly graphitized carbon particles, highly purified natural graphite particles and artificial graphite particles produced by carbonizing and graphitizing coke, pitch or a synthetic organic polymer material are used. In these highly graphitized carbon particles, the graphite crystal is highly developed, and thus the shape is a scale-like shape having a large aspect ratio. For this reason, when an electrode is produced by kneading with a binder and applying the mixture to a current collector, the flaky graphite particles are oriented at high density in the surface direction of the current collector, and as a result, the electrolyte solution into the negative electrode layer The charge / discharge capacity decreases, the high-speed charge / discharge characteristics decrease, and the thickness characteristics of the graphite particles caused by repeated occlusion and release of lithium into graphite particles cause the particles to peel easily, resulting in poor cycle characteristics. And other problems occur. On the other hand, in order to avoid the above-mentioned problem, when the density of the graphitic particles in the electrode is reduced, there arises a problem that the charge / discharge capacity per volume is reduced.

As a method for solving such a problem, attempts have been made to improve the characteristics of highly graphitized particles. Patent No. 26
No. 37305, the use of graphitized particles having a spherical or microstructure orientation radial or Brooks-Taylor type obtained by graphitizing mesophase microspheres extracted from a mesophase pitch, and the orientation of the microstructure being lamellar or The use of Brooks-Tailor type carbon fiber has been proposed, but the former has a charge / discharge capacity of 280 to 300 mA.
/ G, which is relatively low, and is expensive due to the need for steps of extraction and separation from the mesophase pitch. The latter is difficult to increase the density of the electrodes. There is a problem.

Japanese Patent Application Laid-Open No. 7-335216 proposes particles in which graphite crystallites are randomly oriented and produced by pulverizing a high-density graphite molded body produced using an aggregate and a binder as starting materials. However, the production method of a molded article using the cold isostatic pressing method has poor productivity. As a method for pulverizing the graphitized molded body to obtain graphite particles, besides WO9
5/28011 and JP-A-9-231974. The graphite powder obtained by pulverizing these graphitized compacts has a high bulk density and a high strength, and the graphite crystals are randomly oriented within the particles. This is an effective means in that it is suppressed and a space between particles through which the electrolyte can penetrate is secured. However, the fact that the particles have a high bulk density, that is, a dense property, suppresses the permeation of the electrolyte solution into the particles, and causes a limitation in improving the rapid charge / discharge characteristics.

[0007] Further, there has been proposed a technique in which highly graphitized carbon particles and other materials are mixed and used. JP-A-4-237
No. 971 proposes to prevent the particles from being separated due to repetition of charge / discharge by combining spherical graphitic carbon particles and carbon fibers, but this is because spherical particles having a relatively low charge / discharge capacity are proposed. Is used. JP-A-6-36760 proposes that a mixture of highly graphitized carbon particles and low graphitized carbon particles is used to prevent a rapid voltage drop at the end of discharge and prepare for determination of the end point of the battery capacity. However, there is a problem that the highly graphitized particles are oriented in the direction of the current collector surface, and when the amount of the low graphitized carbon particles is large, the discharge voltage decreases.

Japanese Patent Application Laid-Open No. 6-111818 proposes a combination of spherical graphitized carbon particles and graphitized carbon short fibers, thereby increasing the electrode strength, suppressing the destruction of the electrode layer accompanying the charge / discharge cycle, and reducing the short fibers. , The rapid charge / discharge characteristics can be improved by improving the conductivity in the electrode layer, but only spherical graphitized carbon particles having a relatively low charge / discharge capacity are used. Further, when the amount of the graphitized carbon short fibers is large, there is a problem that the electrode density is reduced and the charge / discharge capacity per volume is reduced. JP-A-6-302315
In Japanese Patent Application Publication No. 2002-214, it has been proposed to combine a spherical graphite particle with a chemically and electrochemically inactive metal-coated whisker to increase the strength of the electrode and prevent exfoliation of the particle. No mention is made, and the added whiskers do not contribute to charging and discharging, so that when the added amount is large, the charging and discharging capacity is reduced.

Japanese Patent Application Laid-Open No. 8-180864 discloses a spherical graphite particle and an average particle diameter of 1.3 to 4.
By adding non-spherical graphite particles having an average particle diameter of 0 and ground carbon fiber, the electron conductivity in the electrode is improved and the charge / discharge cycle characteristics are improved. Among them, it is mentioned that non-spherical particles (artificial graphite, natural graphite) exist in various directions between the spherical graphite particles, and the above-mentioned scale-like graphite particles are directed toward the current collector surface. Although the existence of spherical graphite particles has been shown to have an effect on suppressing orientation, it is necessary to precisely control the particle diameter of spherical particles and non-spherical particles, which is a problem in terms of productivity. There is. In JP-A-8-83608 and JP-A-8-83609, graphite crystals are collected at a high density by adding graphitized carbon fiber powder to artificial graphite or natural graphite particles in the form of blocks, flakes and granules. It is described that the particles are hardly oriented in the direction of the current collector surface, and that the separation of particles from the current collector with the progress of the charge / discharge cycle is suppressed. However, it is mentioned that this effect can be obtained when the amount of graphitized carbon fiber powder added is up to 20% by weight, and when the amount exceeds this, the electrode performance is reduced.

[0010] The above-mentioned mixed systems of highly graphitized carbonaceous particles and other materials each have their own problems, and especially when combined with graphitized carbon fibers, the particle shape is greatly different because of the large difference in particle shape. And there is a common problem that it is difficult to manufacture a lithium secondary battery exhibiting stable performance. Further, a system containing spherical graphite particles obtained by graphitizing mesophase small spheres has a problem that the charge and discharge capacity of the spherical graphite particles is relatively low and the cost is high as described above. .

[0011]

SUMMARY OF THE INVENTION According to the first to fourth aspects of the present invention, excessive deformation of particles due to fluctuations in electrode manufacturing conditions and suppression of the orientation of graphitic particles are suppressed, and charge / discharge is performed with a particularly high charge / discharge current. With a large charge / discharge capacity and a small decrease in charge / discharge capacity due to charge / discharge cycles, that is, with good cycle characteristics, high charge / discharge capacity and rapid charge / discharge. An object of the present invention is to provide a negative electrode for a lithium secondary battery having discharge characteristics. Claim 5
The described invention suppresses excessive deformation of particles due to fluctuations in electrode manufacturing conditions and suppresses the orientation of graphitic particles, and particularly when charging / discharging is performed at a high charging / discharging current, the amount of occlusion / release of lithium is large. A method for producing a negative electrode for a lithium secondary battery having a large capacity and a small decrease in charge / discharge capacity due to charge / discharge cycles, that is, having good cycle characteristics, and having high charge / discharge capacity and rapid charge / discharge characteristics. To provide. The invention according to claim 6 suppresses excessive deformation of particles due to fluctuations in electrode manufacturing conditions and suppresses the orientation of graphitic particles. A lithium secondary battery having a large charge / discharge capacity and a small decrease in the charge / discharge capacity due to charge / discharge cycles, that is, a lithium secondary battery having good cycle characteristics, high charge / discharge capacity, and rapid charge / discharge characteristics. Things.

[0012]

SUMMARY OF THE INVENTION The present invention relates to a negative electrode for a lithium secondary battery containing a mixture of two or more types of graphite particles having different bulk specific gravities. In the present invention, the mixture of two or more types of graphite particles having different bulk specific gravities may be a mixture of graphite particles having a bulk specific gravity in the range of 0.4 to 0.65, and a bulk specific gravity of 0.65 to 0.85. (Except for the case of 0.65) which includes the graphite particles in the range described above. Further, in the present invention, at least one of the graphite particles has a plurality of flat particles,
The present invention relates to a negative electrode for a lithium secondary battery having a structure in which the alignment surfaces are assembled or combined so as to be non-parallel.

Further, the present invention provides a lithium battery having a structure in which two or more types of graphitic particles having different bulk specific gravities are respectively aggregated or bonded so that a plurality of flat particles are non-parallel in the orientation plane. The present invention relates to a negative electrode for a secondary battery. Further, the present invention is a process of adding a graphitization catalyst to a material containing a graphitizable aggregate or graphite and a graphitizable binder, and mixing.
Graphitic particles are produced by a method including the steps of calcining and graphitizing and pulverizing steps, and separately producing graphite particles having a different bulk specific gravity from the graphite particles by a method including the same steps as described above. The present invention also relates to a method for producing a negative electrode for a lithium secondary battery, comprising mixing two or more kinds of produced graphite particles and using the mixture as a negative electrode material. Further, the present invention relates to a lithium secondary battery having the negative electrode and the positive electrode according to any of the above.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In general, a lithium secondary battery using a carbon material has a negative electrode, a positive electrode, and a non-aqueous electrolyte made of a carbonaceous substance that occludes and releases lithium ions. For the negative electrode, the carbonaceous material is
It is characterized by containing a mixture of two or more types of graphite particles having different bulk specific gravities. Here, in the case of using graphite particles having a small bulk specific gravity with one kind of graphite particles, the particles are excessively deformed depending on the electrode manufacturing conditions, and the graphite crystals are easily oriented in the plane direction of the current collector, and the cycle characteristics are reduced. In addition, rapid charge / discharge characteristics are deteriorated, and charge / discharge capacity is reduced. On the other hand, when the bulk specific gravity is large, the permeation of the electrolytic solution into the particles is insufficient, and the rapid charge / discharge characteristics deteriorate. By using two or more kinds of graphite particles having different bulk specific gravities as in the present invention, excellent cycle characteristics, excellent rapid charge / discharge characteristics, and high charge / discharge capacity can be achieved in a well-balanced manner. The bulk specific gravity in the present invention is a value measured by a method described in JIS K-1469.

The two or more types of graphite particles include a graphite particle having a bulk specific gravity of 0.4 to 0.65, and a graphite particle having a bulk specific gravity of 0.65 to 0.85. It is preferable to include Here, the bulk specific gravity of 0.65 is set as a boundary because only graphite particles having a bulk specific gravity of 0.65 or less easily deform graphite particles, deteriorating cycle characteristics and rapid charge / discharge characteristics. Discharge capacity tends to decrease, and only graphite particles having a bulk specific gravity of 0.65 or more have a small amount of deformation of graphite, insufficient permeation of electrolyte into the particles as a negative electrode, and rapid charge / discharge characteristics. This is because it tends to decrease.

The reason why the range of the bulk specific gravity of the graphite particles having a low bulk specific gravity is preferably set to 0.4 to 0.65 is that if the lower limit is less than the lower limit, the viscosity of the slurry used for producing the electrode becomes high. This is because there is a tendency that defects such as the need to increase the amount of binder added tend to occur, and when the amount exceeds the upper limit, the rapid charge / discharge characteristics tend to deteriorate as described above. In addition, in terms of good rapid charge / discharge characteristics, 0.1%
It is more preferably in the range of 45 to 0.64,
The range of 45 to 0.60 is more preferable. On the other hand, the range of the bulk specific gravity of the graphite particles having a large bulk specific gravity is 0.65 to 0.65.
The reason why 0.85 is set to the preferable range is that if the lower limit is less than the lower limit, excessive deformation of the graphitic particles due to the electrode production conditions is likely to occur, and the cycle characteristics and rapid charge / discharge characteristics tend to decrease. If it exceeds, the rapid charge / discharge characteristics tend to decrease. Further, from the viewpoint of good rapid charge / discharge characteristics, it is more preferably in the range of 0.7 to 0.85, and further preferably in the range of 0.7 to 0.8.

The mixing ratio of the two types of graphite particles having a bulk specific gravity is not particularly limited, and is selected according to the intended design of the lithium secondary battery. The mixing ratio is 0 in terms of the weight ratio of the graphite particles having a large bulk specific gravity / the graphite particles having a small bulk specific gravity, in order to prevent excessive deformation of the particles at the time of forming the electrode and to show good rapid charge / discharge characteristics. .25 to 49, more preferably 1 to 9. When three or more types of graphitic particles are included, the bulk specific gravity is 0.4 to 0.1 in that good discharge characteristics are maintained.
It is preferable that one or more graphite particles in the range of 65 are contained in an amount of 0.2 to 0.98, more preferably 0.5 to 0.95, more preferably 0.5 to 0.95, based on the weight of all the graphite particles. Is preferably one or more graphitic particles having a bulk specific gravity of 0.65 to 0.85.

In the present invention, the lattice spacing d002 of the (002) plane, the crystallite size Lc in the c-axis direction, and the true density of each of the two or more graphite particles are 0.338, respectively.
nm or less, 50 nm or more, and 2.21 g / cm ³ or more are preferable from the viewpoint of increasing the charge / discharge capacity of the entire negative electrode. Further, the charge / discharge capacity of each of the graphitic particles measured alone is preferably substantially the same, each value is 300 mA / g or more, and the difference between different graphitic particles is 10% or less. Preferably, there is. Thereby, the effect of combining two or more graphitic particles can be obtained without changing (decreasing) the charge / discharge capacity. here,
The discharge capacity measured alone means the first cycle discharge capacity measured by a known method using a negative electrode prepared by using a known method using each of the graphite particles and using lithium as a counter electrode.

In the present invention, the measurement of the discharge capacity is as follows.
Specifically, it can be performed by the following method. 10% by weight of solid content of polyvinylidene fluoride (PVDF) dissolved in N-methyl-2-pyrrolidone
In addition, the mixture is kneaded to produce a graphite paste, the graphite paste is applied to a rolled copper foil having a thickness of 10 μm, and further dried to obtain a negative electrode. The prepared sample electrode is 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 the lithium secondary battery used for this measurement. As shown in FIG.
As a solution, LiPF ₄ was dissolved in a mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) (EC and DMC in a volume ratio of 1: 1) so as to have a concentration of 1 mol / liter, and a sample electrode ( Negative electrode) 11,
The separator 12 and the counter electrode (positive electrode) 13 are stacked and arranged, and the reference electrode 14 is suspended from above to produce a lithium secondary battery. Lithium metal is used for the counter electrode 13 and the reference electrode 14, and a polyethylene microporous membrane is used for the separator 12. 5 mV at a constant current of 0.5 mA / cm ²
(V vs Li / Li ⁺ ), and 1V (V v
s Li / Li ⁺ ) The discharge capacity is measured by a test that discharges.

When the discharge capacity of each graphitic particle measured by this method is less than 300 mA / g, the improvement in charge / discharge capacity, rapid charge / discharge characteristics and cycle characteristics when used in combination may be small or low. is there.

It is appropriate that the graphite particles having different bulk specific gravities are approximately equal in shape. Specifically, the aspect ratio is preferably 5 or less, more preferably 1 to 3. . Thus, when a negative electrode is configured by mixing two or more graphite particles having different pore volumes,
Uniform distribution of these graphitic particles can be easily realized, and a lithium secondary battery with good characteristics with little variation can be obtained. The aspect ratio is defined as A / A, where A is the length in the major axis direction of the graphite particles, and B is the length in the minor axis direction.
It is represented by B. 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.

The specific surface area of the two or more graphitic particles constituting the negative electrode is suitably approximately equal, and specifically, the specific surface area is preferably in the range of 0.5 to 5.0 m ² / g. Preferably, even if a negative electrode is produced by combining two or more graphitic particles having different pore volumes, a irreversible capacity is not increased, and a mixture of the graphitic particles, a binder, and a solvent used in producing the negative electrode Can be minimized.

The structure of the two or more graphite particles constituting the negative electrode may be at least one of the two or more graphite particles.
It is preferable that the particles have a structure in which a plurality of kinds, more preferably two or more kinds, of flat particles are aggregated or bonded so that the orientation planes are non-parallel. Here, the flat particles refer to particles having a major axis and a minor axis and are not perfectly spherical. For example, a shape such as a scaly shape, a scaly shape, or a partial lump shape is included in this. In the plurality of flat particles, the orientation plane is non-parallel, and the flat surface having the shape of each particle, in other words, the plane closest to the flat surface is the orientation plane, and the plurality of particles have each orientation plane constant. Refers to a state in which they are gathered in the same direction. Each of the flat particles is preferably made of a graphitizable aggregate or graphite.

In the graphitic particles, the flat particles are aggregated or bonded. The term “bond” refers to a state in which the particles are bonded to each other via a binder or the like. Is a state in which the shape of the aggregate is maintained due to its shape and the like, although not bonded. From the standpoint of mechanical strength, it is preferable to combine them. When the graphite particles having such a structure are used for the negative electrode, the graphite crystals are less likely to be oriented on the current collector, and lithium is easily absorbed and released into the negative electrode graphite. The characteristics can be improved.

There is no particular limitation on the method for producing the graphite particles used in the present invention. However, since the graphite particles having the above-mentioned properties, shapes and structures can be obtained relatively easily, at least one of them can be obtained.
The steps of adding and mixing a graphitization catalyst to a graphitizable aggregate or a material containing graphite and a graphitizable binder, firing, graphitizing, and pulverizing the seeds, more preferably all of them. It is preferable that it is manufactured by a method including: In this method, several methods can be mentioned more specifically. The first method is to use graphitizable aggregate or graphite, tar or pitch as a graphitizable binder, add a graphitization catalyst to the mixture, mix, and then calcine and graphitize and then pulverize. Is the way.

As the graphitizable aggregate, various cokes such as fluid coke and needle coke are preferable. In addition, those already graphitized such as natural graphite and artificial graphite can be used as the aggregate. As the binder that can be graphitized, various pitches and tars such as coal-based, petroleum-based, and man-made are used. The amount of the binder is not particularly limited, but for the aggregate or graphite that can be graphitized,
It is preferably added in an amount of 5 to 80% by weight, more preferably 10 to 80% by weight, and even more preferably 15 to 80% by weight. If the amount of the binder is too large or too small, the graphite particles to be produced tend to have an increased aspect ratio and specific surface area.

The method of 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, the temperature is preferably 50 to 300 ° C. As a graphitization catalyst,
Oxides, carbides, nitrides, and the like of iron, nickel, titanium, boron, silicon, and the like can be used. The graphitization catalyst is preferably added to the graphitizable aggregate or the graphite and the graphitizable binder in an amount of 1 to 50% by weight. If the addition amount is less than 1% by weight, the development of the crystals of the graphitic particles becomes worse, and the charge / discharge capacity tends to decrease. On the other hand, if the content exceeds 50% by weight, it becomes difficult to mix uniformly, and the workability tends to deteriorate and the characteristic variation of the obtained graphite particles tends to increase.

A graphitizing catalyst is added to a graphitizable aggregate or graphite and a binder, mixed, fired and graphitized. Before firing, the mixture may be shaped into a suitable shape, if necessary. The firing is preferably performed in an atmosphere in which the mixture is hardly oxidized, and examples thereof include a method of firing in a nitrogen atmosphere, an argon gas, and a vacuum. The graphitization temperature is preferably 2000 ° C. or higher, more preferably 2500 ° C. or higher, and further preferably 2800 to 3200 ° C. If the graphitization temperature is low, the development of graphite crystals becomes worse, and the graphitization catalyst tends to remain in the prepared graphitic particles, and in any case, the charge / discharge capacity tends to decrease. On the other hand, if the graphitization temperature is too high, the graphite may sublime.

Next, the obtained graphitized product is pulverized. There is no particular limitation on the method of pulverizing the graphitized material, but known methods such as a jet mill, a vibration mill, a pin mill, a hammer mill and the like and a combination of a plurality of these methods can be used. The average particle size after pulverization is preferably from 1 to 100 μm, more preferably from 10 to 50 μm. If the average particle size is too large, the formed electrode surface is likely to be uneven.

The obtained graphitic particles can be used as they are, but are further subjected to 400 ° C. in a non-oxidizing atmosphere.
Heat treatment may be performed at the above temperature. By this treatment, the specific surface area can be reduced, and the safety and irreversible capacity of the lithium secondary battery can be improved. Examples of the non-oxidizing atmosphere include a nitrogen atmosphere, an argon atmosphere, and a vacuum.

As a second method, a graphitizing catalyst is added to a graphitizable aggregate or a graphite and a graphitizable binder by 1 to 50 times.
% By mixing, pulverizing, infusibilizing, calcining and graphitizing. The difference between this method and the first method is that the mixture of the materials is pulverized and then subjected to the infusibilizing treatment. In the pulverization, the average particle diameter of the graphite particles finally obtained is 100
It is preferable to select the particle size of the mixture so as to be not more than μm, preferably not more than 50 μm. The pulverizing method is not particularly limited, but a pulverizing apparatus such as a hammer mill, a pin mill, a vibration mill, a Jett mill and a plurality of these can be used in combination. If necessary, the particles obtained by pulverization can be classified. The classification method is not particularly limited, but an optimal model is selected from a mechanical classifier, a wind classifier, and the like in a timely manner.

The infusibilizing treatment method is not particularly limited as long as the mixture powder can be prevented from fusing together in the firing step. An oxidizing agent generally used for infusibilizing various pitches ( Air, oxygen, NO ₂ , chlorine, bromine, etc.) and a wet method using a nitric acid aqueous solution, a chlorine aqueous solution, a sulfuric acid aqueous solution, a hydrogen peroxide aqueous solution, etc. It can be achieved by a method combining these. Also, by coating the surface of the mixture powder with a thermosetting resin, it is possible to prevent the particles from fusing in the intended firing step. Although there is no particular limitation on the thermosetting resin to be coated, any resin can be used as long as it cures at or below the melting temperature of the binder to be used, and a phenol resin, a furfuryl alcohol resin, a polyimide resin, a cellulose resin,
Polyvinylidene chloride resin is preferred. After the infusibilization treatment, if necessary, pulverization and classification may be performed again.
The infusible mixture powder can be fired and graphitized according to the first method.

As a third method, a graphitizable aggregate or graphite and a thermosetting resin as a graphitizable binder are used, a graphitization catalyst is added thereto, mixed, and pulverized.
Then, it is a method of producing by firing and graphitizing. This method is different from the first method in that a thermosetting resin is used as a binder and the mixture is pulverized. As thermosetting resins, phenolic resins, furfuryl alcohol resins,
Polyimide resin, cellulose resin, polyvinylidene chloride resin, chlorinated polyvinyl chloride resin and the like can be used. The method of mixing the graphitizable aggregate or graphite with the binder is not particularly limited, and is performed using a kneader or the like. The temperature is preferably 20 to 100 ° C. in the case of a thermosetting resin.

The compounding ratio of the graphitizable aggregate or graphite and the thermosetting resin as a binder is not particularly limited, but the thermosetting resin is used to such an extent that particles do not fuse during the firing of the pulverized material. It is necessary to set the compounding amount. On the other hand, when the proportion of the thermosetting resin is excessively increased, the degree of graphitization of the obtained graphitic particles decreases, and the charge / discharge capacity decreases, which is not preferable. From these points, the compounding amount of the thermosetting resin is 5 to 80% by weight based on the graphitizable aggregate or graphite.
It is preferred to add. The grinding conditions for the mixture are the second
Method can be followed. The firing and graphitizing conditions can be in accordance with the first method.

In the present invention, at least two types of graphitic particles are particles produced by the first, second or third method, respectively, and it is preferable to mix these particles. Two or more graphitic particles having different bulk specific gravities obtained as described above are uniformly mixed with an organic binder for binding the graphitic particles, and then molded under pressure or using an organic solvent or the like. A negative electrode for a lithium secondary battery can be obtained by a known method, such as forming into a paste, applying it on a current collector, drying and pressing.

As the organic binder, for example, polyethylene, polypropylene, ethylene propylene polymer, butadiene rubber, styrene-butadiene rubber, and a high ion conductive polymer compound can be used. As the ion-conductive polymer compound, polyvinylidene fluoride, polyethylene oxide, polyepihydrin, polyphosphazene, polyacrylonitrile, and the like can be used. The content of the organic binder is preferably 3 to 20% by weight based on the mixture of the graphite particles and the organic binder. As the current collector, for example, a foil or mesh of nickel, copper, or the like can be used.

The negative electrode for a lithium secondary battery obtained as described above constitutes a lithium secondary battery in combination with a positive electrode composed of a chargeable / dischargeable lithium-containing active material.
As the positive electrode active material as used herein, Li _x M _y O
_z (where M = V, at least one selected from Mn, Fe, Co, Ni, x = 0.05 to 1.2, y =
1 or 2, z = 1.5 to 5), and a transition metal oxide containing lithium. These include alkali metals other than lithium, alkaline earth metals, and M
Other transition metals, or elements of groups 13 to 15 of the periodic table (Al, Ga, In, Si, Ge, Sn, Pb, Sb,
Bi, P, B) may be included. On the positive electrode, MnO ₂ , MoO ₃ , V ₂ O ₅ , TiO ₂ , T
Inorganic compounds such as iS ₂ , FeS and activated carbon, and high molecular compounds such as polyaniline can also be selected. In this case, a predetermined amount of lithium is previously stored in the negative electrode,
Alternatively, a predetermined amount of lithium can be pressed and used.

The lithium secondary battery further contains a non-aqueous electrolyte. As the non-aqueous electrolyte, a solution in which a lithium salt is dissolved in an organic solvent having a high dielectric constant is preferable. There is no particular limitation on the lithium salt, and LiClO ₄ , LiP
F ₆ , LiBF ₄ , LiCF ₃ SO _{3 and the} like can be used. Further, the organic solvent may be any as long as it dissolves the lithium salt to provide electrochemical stability and has electrochemical stability to the constituent negative and positive electrode materials. For example, ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, 1,2-dimethoxyethane, tetrahydrofuran, acetonitrile, sulfolane, γ-butyrolactone, a mixture thereof and the like are used. In addition, an organic electrolyte contained in a solid polymer electrolyte such as polyvinylidene fluoride can be used as the electrolyte.

In the lithium secondary battery of the present invention, when a liquid electrolyte is used, in addition to the positive electrode, the negative electrode, and the non-aqueous electrolyte, contact between the two electrodes is prevented, and the electrolyte is retained. A separator having a function of passing through
It is preferable to use in combination with a current collector having a function of holding an electrode material and collecting current. As a separator,
For example, a porous film of polyethylene, polypropylene, polytetrafluoroethylene, or the like, a nonwoven fabric, a woven fabric, and the like can be given. The thickness of the separator is preferably about 20 to 200 μm.

As the current collector, a conductor which is electrochemically stable with respect to the positive and negative electrode active materials can be used. For example, nickel, titanium, stainless steel, copper, and aluminum are mentioned. In addition, the lithium secondary battery of the present invention provided with a negative electrode containing two or more graphite particles having different bulk specific gravities can be used in various forms such as a cylindrical type, a box type, a coin type, a button type, a paper type, and a card type. Shape.

In the lithium secondary battery thus obtained, if the particles contained in the negative electrode are only one kind of graphite particles, for example, graphite particles having a bulk specific gravity in the range of 0.4 to 0.65, the It has excellent rapid charge / discharge characteristics and cycle characteristics when there is no excessive deformation, but flat particles are oriented parallel to the current collector surface when excessive deformation of the particles occurs due to the negative electrode manufacturing conditions etc. In addition, the voids in the particles and between the particles are also reduced, so that doping and undoping of lithium ions are less likely to occur, and rapid charge / discharge characteristics and cycle characteristics are deteriorated. Therefore, when graphite particles having a different bulk specific gravity, for example, a bulk specific gravity in the range of 0.65 to 0.85 are added to the graphite particles, the graphite particles are relatively dense, and thus the graphite particles are relatively dense. Excessive deformation of the particles is suppressed, and as a result, rapid charge / discharge characteristics and cycle characteristics are improved. Further, the graphite particles themselves have a high charge / discharge capacity, so that the rapid charge / discharge characteristics are relatively good, and further, the characteristics such as the shape and the true density are similar to those of the graphite particles. Therefore, uniform mixing can be easily realized, so that a lithium secondary battery having a high charge / discharge capacity can be stably manufactured.

[0042]

EXAMPLES Examples and comparative examples of the present invention will be described below. Example 1 (Production of Lithium Secondary Battery) FIG. 1 shows a partial cross-sectional front view of an example of a cylindrical lithium ion secondary battery. In FIG. 1, 1 is a positive electrode, 2 is a negative electrode, 3 is a separator, 4 is a positive electrode tab, 5 is a negative electrode tab, 6 is a positive electrode cover, 7 is a battery can, and 8 is a gasket. The lithium secondary battery shown in FIG. 1 was manufactured as follows.

(Preparation of positive electrode) LiC as positive electrode active material
An average particle size of 1 μm as a conductive agent was added to 288 parts by weight of oO.
m-scale natural graphite (7 parts by weight) and polyvinylidene fluoride (5 parts by weight) as a binder were added thereto.
-A slurry of the positive electrode mixture was prepared by adding and mixing pyrrolidone. Next, this positive electrode mixture was applied to both surfaces of an aluminum foil (thickness: 25 μm) as a positive electrode current collector by a doctor blade method, dried, and then pressure-formed by roller press. This was cut out into a size having a width of 40 mm and a length of 285 mm to produce a positive electrode 10. However, positive electrode 1
The positive electrode mixture was not applied to the 10 mm long portions of both ends of 0 and the aluminum foil was exposed, and the positive electrode tab 13 was pressure-bonded to one of the portions by ultrasonic bonding.

(Preparation of Graphite Particle A) The average particle diameter is 5 μm.
m coke powder, 50 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 was pulverized and formed into a pellet under pressure, then heated to 900 ° C. in a nitrogen atmosphere, and then heated to 3000 ° C. in the same nitrogen atmosphere to perform graphitization. The obtained graphitized material is pulverized using a hammer mill,
Graphitic particles having an average particle size of 20 μm were prepared. The bulk specific gravity of the graphite particles was 0.56. The specific surface area of the graphite particles measured by the BET method was 3.6 m ² / g. As a result of arbitrarily selecting 100 obtained graphite particles and measuring the aspect ratio, it was 2.0. The interlayer distance d (002) of the graphite particles by X-ray wide-angle diffraction was 0.336 nm, and The crystallite size Lc (002) is 10
It was 0 nm or more. Further, according to a scanning electron microscope (SEM) photograph of the obtained graphitic particles, the graphitic particles have a structure in which a plurality of flat particles are aggregated or bonded so that the orientation planes are non-parallel. I was The graphite particles produced as described above are hereinafter referred to as sample A.

(Preparation of Graphite Particle B) The average particle size is 5 μm.
50 parts by weight of coke powder, 30 parts by weight of tar pitch,
3 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 was pulverized and formed into a pellet under pressure, then heated to 900 ° C. in a nitrogen atmosphere, and then heated to 3000 ° C. in the same nitrogen atmosphere to perform graphitization. The obtained graphitized product was pulverized using a hammer mill to prepare graphitic particles having an average particle size of 20 μm. The bulk specific gravity of the graphite particles was 0.78. BE of this graphitic particle
The specific surface area according to the T method was 3.3 m ² / g. In addition, 100 obtained graphite particles were arbitrarily selected and the aspect ratio was measured. The result was 1.8, and the interlayer distance d (002) of the graphite particles by X-ray wide-angle diffraction was 0.336.
nm and the crystallite size Lc (002) were 100 nm or more. Further, according to a scanning electron microscope (SEM) photograph of the obtained graphitic particles, the graphitic particles have a structure in which a plurality of flat particles are aggregated or bonded so that the orientation planes are non-parallel. I was The graphite particles produced as described above are hereinafter referred to as a B sample.

(Preparation of Graphite Particle C) The average particle size is 5 μm.
50 parts by weight of coke powder, 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 resulting mixture was ground. The mixture is then placed in air for 2 hours.
Heat treatment was performed at 50 ° C. for 30 minutes to make the tar pitch infusible. The infusibilized mixture was heated to 900 ° C. in a nitrogen atmosphere and then heated to 3000 ° C. in a nitrogen atmosphere to graphitize. The average particle size of the obtained graphitic particles is 2
It was 3 μm. The bulk specific gravity of the graphite particles is 0.62.
Met. The specific surface area of the graphite particles measured by the BET method was 2.5 m ² / g. Further, the obtained graphitic particles were
As a result of measuring zero aspect ratio and measuring aspect ratio,
1.7, and the interlayer distance d (002) of the graphite particles by X-ray wide-angle diffraction of the graphite particles was 0.336 nm, and the crystallite size Lc (002) was 100 nm or more. Further, according to a scanning electron microscope (SEM) photograph of the obtained graphitic particles, the graphitic particles have a structure in which a plurality of flat particles are aggregated or bonded so that the orientation planes are non-parallel. I was The graphitic particles produced as described above are hereinafter referred to as a C sample.

(Preparation of Graphite Particle D) The average particle diameter is 5 μm.
50 parts by weight of coke powder, 20 parts by weight of tar pitch,
Novolac type phenolic resin (trade name cash register top PGA)
-2504, manufactured by Gunei Chemical Co., Ltd.), 10 parts by weight, 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 was pulverized, heated to 900 ° C. in a nitrogen atmosphere, and then heated to 3000 ° C. in a nitrogen atmosphere to perform graphitization. Graphite particles having an average particle size of 21 μm were obtained. The bulk specific gravity of the graphite particles is 0.1.
64. The specific surface area of the graphite particles measured by the BET method was 2.6 m ² / g. In addition, 100 obtained graphite particles were arbitrarily selected, and the aspect ratio was measured. As a result, the ratio was 1.7. Is 0.336
nm and the crystallite size Lc (002) were 100 nm or more. Further, according to a scanning electron microscope (SEM) photograph of the obtained graphitic particles, the graphitic particles have a structure in which a plurality of flat particles are aggregated or bonded so that the orientation planes are non-parallel. I was The graphitic particles produced as described above are hereinafter referred to as a D sample.

(Measurement of Discharge Capacity of Graphite Particles) Polyvinylidene fluoride (PVDF) dissolved in N-methyl-2-pyrrolidone was added to 90% by weight of graphite particles at a solid content of 10% by weight and kneaded. A graphite paste was prepared. This graphite paste was applied to a rolled copper foil having a thickness of 10 μm and dried to obtain a negative electrode. 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. As shown in FIG.
A sample electrode was prepared by dissolving LiPF ₄ in a mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) (EC and DMC in a volume ratio of 1: 1) so as to have a concentration of 1 mol / liter. (Negative electrode) 1
1, a separator 12 and a counter electrode (positive electrode) 13 were stacked and arranged, and a reference electrode 14 was suspended from above to produce a lithium secondary battery. Counter electrode 13 and reference electrode 14
, Metal lithium was used, and the separator 12 was a polyethylene microporous membrane. At a constant current of 0.5 mA / cm ² , 5
mV (V vs. Li / Li ⁺ ) and charge 1V (V
vs Li / Li ⁺ ).
Table 1 shows the obtained results.

[0049]

[Table 1]

(Preparation of Negative Electrode) 90 parts by weight of Sample A and 10 parts by weight of Sample B were uniformly mixed, and then the mixed graphite and PVDF as a binder were mixed at a weight ratio of 90:10. This was dispersed in a solvent (N-methyl-2-pyrrolidone) to form a slurry, which was then applied to both surfaces of a copper foil (thickness: 10 μm) as a negative electrode current collector by a doctor blade method, dried, and then dried. The electrode was pressure-formed by a roller press to obtain a negative electrode. This is 40 mm wide and 290 long.
A negative electrode was prepared by cutting out to a size of mm. In the same manner as the positive electrode, the negative electrode was pressure-bonded by ultrasonic bonding to one of the portions where the negative electrode mixture having a length of 10 mm was not applied, on one side.

(Preparation of electrolyte solution) LiPF was added to an equal volume mixed solvent of ethylene carbonate and dimethyl carbonate.
₆ was dissolved at 1 mol / liter to prepare an electrolytic solution. (Preparation of Battery) The positive electrode, a separator made of a porous film made of polyethylene (thickness 25 μm, width 44 mm) and the negative electrode were laminated in this order, and then spirally wound so that the negative electrode was positioned outside. To form an electrode group. Each of the electrode groups was housed in a stainless steel battery can, and the negative electrode tab was welded to the bottom of the can to provide a throttle portion for caulking the positive electrode lid. Thereafter, the electrolyte was injected into the battery can, and then the positive electrode tab was welded to the positive electrode cover, and the positive electrode cover was caulked to assemble a cylindrical lithium secondary battery.

Example 2 The mixing ratio of Sample A and Sample B in the preparation of the negative electrode was 8
A cylindrical lithium secondary battery was assembled in the same manner as in Example 1 except that 0 parts by weight and 20 parts by weight were used.

Example 3 The mixing ratio of Sample A and Sample B in the preparation of the negative electrode was 7
A cylindrical lithium secondary battery was assembled in the same manner as in Example 1 except that 0 parts by weight and 30 parts by weight were used.

Example 4 The mixing ratio of the A sample and the B sample in the production of the negative electrode was 6
A cylindrical lithium secondary battery was assembled in the same manner as in Example 1 except that 0 parts by weight and 40 parts by weight were used.

Example 5 The mixing ratio of the A sample and the B sample in the preparation of the negative electrode was 5
A cylindrical lithium secondary battery was assembled in the same manner as in Example 1, except that 0 parts by weight and 50 parts by weight were used.

Comparative Example 1 The mixing ratio of Sample A and Sample B in the preparation of the negative electrode was 1
A cylindrical lithium secondary battery was assembled in the same manner as in Example 1 except that the amount was 00 parts by weight and 0 parts by weight.

Comparative Example 2 In the preparation of the negative electrode, the mixing ratio of Sample A and Sample B was set to 0
A cylindrical lithium secondary battery was assembled in the same manner as in Example 1 except that the weight was changed to 100 parts by weight.

Example 6 A cylindrical lithium secondary battery was assembled in the same manner as in Example 1 except that the mixing ratio of the sample C and the sample B was changed to 90 parts by weight and 10 parts by weight, respectively.

Example 7 A cylindrical lithium secondary battery was assembled in the same manner as in Example 1 except that the mixing ratio of the sample C and the sample B was changed to 80 parts by weight and 20 parts by weight, respectively.

Example 8 A cylindrical lithium secondary battery was assembled in the same manner as in Example 1 except that the mixing ratio of the sample C and the sample B was changed to 70 parts by weight and 30 parts by weight, respectively.

Example 9 A cylindrical lithium secondary battery was assembled in the same manner as in Example 1 except that the mixing ratio of the sample C and the sample B was changed to 60 parts by weight and 40 parts by weight, respectively.

Example 10 A cylindrical lithium secondary battery was assembled in the same manner as in Example 1 except that the mixing ratio of the sample C and the sample B was changed to 50 parts by weight and 50 parts by weight, respectively.

Comparative Example 3 A cylindrical lithium secondary battery was assembled in the same manner as in Example 1 except that the mixing ratio of the sample C and the sample B was changed to 100 parts by weight and 0 part by weight, respectively.

Example 11 A cylindrical lithium secondary battery was assembled in the same manner as in Example 1 except that the mixing ratio of the D sample and the B sample was 90 parts by weight and 10 parts by weight, respectively, in the preparation of the negative electrode.

Example 12 A cylindrical lithium secondary battery was assembled in the same manner as in Example 1 except that the mixing ratio of the D sample and the B sample was changed to 80 parts by weight and 20 parts by weight, respectively, in the preparation of the negative electrode.

Example 13 A cylindrical lithium secondary battery was assembled in the same manner as in Example 1 except that the mixing ratio of the D sample and the B sample was 70 parts by weight and 30 parts by weight, respectively, in the preparation of the negative electrode.

Example 14 A cylindrical lithium secondary battery was assembled in the same manner as in Example 1 except that the mixing ratio of Sample D and Sample B was changed to 60 parts by weight and 40 parts by weight, respectively, in the preparation of the negative electrode.

Example 15 A cylindrical lithium secondary battery was assembled in the same manner as in Example 1 except that the mixing ratio of Sample D and Sample B was changed to 50 parts by weight and 50 parts by weight, respectively, in the preparation of the negative electrode.

Comparative Example 4 A cylindrical lithium secondary battery was assembled in the same manner as in Example 1, except that the mixing ratio of the C sample and the B sample was changed to 100 parts by weight and 0 part by weight, respectively.

The obtained Examples 1 to 15 and Comparative Examples 1 to 4
The end-of-charge voltage of the lithium secondary battery was 4.15.
V, the discharge end voltage is 2.8 V, and the charge / discharge current is 200
The discharge capacity at the time of rapid charge and discharge was measured by changing the range from mA to 800 mA. The result was compared with the charge / discharge current 20 of Comparative Example 1.
Tables 2 and 3 show the discharge capacity at 0 mA as 100%. The charge / discharge cycle characteristics of each battery were measured at a charge / discharge current of 200 mA. The results are shown in Tables 4 and 5 assuming that the discharge capacity when the number of cycles is 1 in Comparative Example 1 is 100%.

[0071]

[Table 2]

[0072]

[Table 3]

[0073]

[Table 4]

[0074]

[Table 5]

As is clear from Tables 2 and 3, the rapid charging / discharging characteristics of the examples are better than those of the comparative examples, and the reduction in discharge capacity is extremely small even at a large charging / discharging current. Further, as is clear from Tables 4 and 5, the cycle characteristics of the examples are better than those of the comparative examples, and it can be seen that a large discharge capacity can be maintained even after a high number of cycles.

[0076]

The negative electrode for a lithium secondary battery according to any one of claims 1 to 4, has an excessive deformation of particles due to a change in electrode manufacturing conditions,
Graphite-like particles whose orientation is suppressed, the charge / discharge capacity is large, the charge / discharge capacity is large, and the decrease in charge / discharge capacity due to the charge / discharge cycle is small especially when charging / discharging is performed with a high charge / discharge current. That is, it has good cycle characteristics and high charge / discharge capacity and rapid charge / discharge characteristics. According to the method for manufacturing a negative electrode for a lithium secondary battery according to claim 5, excessive deformation of particles due to fluctuations in electrode manufacturing conditions and suppression of the orientation of the graphite particles were suppressed, and charge and discharge were performed with a particularly high charge and discharge current. In this case, the charge / discharge capacity is large due to the large amount of occlusion / release of lithium, and the decrease in charge / discharge capacity due to charge / discharge cycles is small, that is, it has good cycle characteristics, and has high charge / discharge capacity and rapid charge / discharge. A negative electrode for a lithium secondary battery having characteristics is obtained. Claim 6
The lithium secondary battery described above suppresses excessive deformation of particles due to fluctuations in electrode manufacturing conditions, suppresses the orientation of graphitic particles, and particularly stores and stores lithium when charging and discharging is performed at a high charging and discharging current.
It has a large release amount, a large charge / discharge capacity, and a small decrease in the charge / discharge capacity due to a charge / discharge cycle, i.e., has good cycle characteristics, and has high charge / discharge capacity and rapid charge / discharge characteristics. .

[Brief description of the drawings]

FIG. 1 is a partial cross-sectional front view of a cylindrical lithium secondary battery.

FIG. 2 is a schematic view of a lithium secondary battery used for measuring the discharge capacity of graphite particles alone.

[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 (negative electrode) 12 Separator 13 Counter electrode (positive electrode) 14 Reference electrode

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Atsushi Fujita 3-3-1 Ayukawacho, Hitachi City, Ibaraki Prefecture Inside the Yamazaki Plant of Hitachi Chemical Co., Ltd. (72) Kazuo Yamada 3-chome Ayukawacho, Hitachi City, Ibaraki Prefecture No. 1 Inside the Yamazaki Plant of Hitachi Chemical Co., Ltd.

Claims (6)

[Claims] 1. A negative electrode for a lithium secondary battery comprising a mixture of two or more types of graphitic particles having different bulk specific gravities. 2. A mixture of two or more types of graphite particles having different bulk specific gravities, comprising a graphite particle having a bulk specific gravity in the range of 0.4 to 0.65 and a graphite bulk having a bulk specific gravity of 0.65 to 0.85. 2. The negative electrode for a lithium secondary battery according to claim 1, wherein the negative electrode contains graphitic particles in a range (excluding the case of 0.65). 3. 3. The lithium secondary material according to claim 1, wherein at least one of the graphite particles has a structure in which a plurality of flat particles are aggregated or bonded so that their orientation planes are non-parallel. Negative electrode for battery. 4. A structure in which two or more types of graphitic particles having different bulk specific gravities are formed by combining or bonding a plurality of flat particles, respectively, such that their orientation planes are non-parallel. Or the negative electrode for a lithium secondary battery according to 3. 5. A method comprising the steps of adding a graphitizing catalyst to a graphitizable aggregate or a material containing graphite and a graphitizable binder, mixing, calcining / graphitizing, and pulverizing. In the production of graphite particles, separately, graphite particles having a different bulk specific gravity from the graphite particles by a method including the same steps as described above are produced, and two or more types of produced graphite particles are mixed, A method for producing a negative electrode for a lithium secondary battery, comprising using this as a negative electrode material. 6. A lithium secondary battery comprising the negative electrode according to claim 1 or the negative electrode obtained by the production method according to claim 5, and a positive electrode.