JP2000133267A - Negative active material for lithium secondary battery and lithium secondary battery using this - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a negative electrode material capable of suppressing volume change attendant on charge/discharge cycles without increasing electrical resistance of a negative electrode by including at least one graphite material of flake graphite and globular graphite, and fibrous carbon forming secondary particles having specified particle size. SOLUTION: Secondary particles having a particle size of 10 μm or more but 30 μm or less are formed. Preferably, when the total amount of graphite material and fibrous carbon is 100 pts.wt. in a negative electrode material, ratio of the fibrous carbon is made 0.5 pts.wt. or more but 22.5 pts.wt. or less. By positioning secondary particles of fibrous carbon between flake or globular graphite particles, volume change attendant on adsorbing/releasing cycles of lithium ions is suppressed, disruption of the negative electrode structure caused by charge/discharge cycles is prevented. As the material for reinforcing the negative electrode structure, fibrous carbon is used, and electric conductivity of the negative electrode is ensured and at the same time, cycle characteristics of a lithium secondary battery are enhanced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a negative electrode active material for use in a lithium secondary battery utilizing occlusion and release of lithium ions, and more particularly to a negative electrode active material capable of forming a lithium secondary battery having good cycle characteristics. About the material.

[0002]

2. Description of the Related Art Lithium secondary batteries have a high energy density.
It has been put into practical use in the fields of information-related equipment, communication equipment and the like, and has been widely spread. One of the characteristics required for a secondary battery is that the cycle life is long.
As a matter of course, the lithium secondary battery is also required to have a battery capacity that does not significantly deteriorate due to repeated charging and discharging.

In a lithium secondary battery, each of a positive electrode and a negative electrode has an active material capable of inserting and extracting lithium ions, and the lithium ions flow between the active materials to perform charging and discharging. Therefore, the lithium ions are inserted into and released from the positive and negative electrode active materials during charge and discharge, whereby the positive and negative electrodes formed of the active material expand and contract. As a result, the thickness of the positive electrode and the negative electrode changes, which gradually destroys the battery structure,
The performance of the secondary battery is significantly deteriorated.

[0004] The positive electrode of a lithium secondary battery is often composed of a lithium transition metal composite oxide as a positive electrode active material, and since the composite oxide has low conductivity, the destruction of the electrode structure has a serious effect. Effect. Therefore, as means for preventing the structural change of the positive electrode, JP-A-9-02734 has
As disclosed in Japanese Patent Publication No. 4 (1993) -104, there has been proposed a technique of adding fibrous carbon to flaky graphite conventionally used as a positive electrode conductive material. This technology uses the action of added fibrous carbon to expand and expand the positive electrode even during heavy load charging and discharging.
This is intended to suppress the shrinkage and secure the conductivity of the positive electrode to extend the cycle life of the secondary battery.

[0005] However, the problem of expansion and contraction of the electrode due to charging and discharging of the electrode also accompanies the negative electrode. LiCoO, which is currently the mainstream, has been used to reduce the cost of the positive electrode active material in order to use lithium secondary batteries as power sources for electric vehicles.
_{2. The use of} a manganese-based lithium composite oxide as a positive electrode active material instead of LiNiO ₂ is also being studied. When this lithium manganese composite oxide is used as the positive electrode active material, the change in volume of the positive electrode is relatively small, and the influence of the expansion and contraction of the electrode on the cycle life is rather large in the negative electrode. When a carbon material is used as the negative electrode active material, the change in volume of the negative electrode can be up to about 10% in the thickness direction, and as a result, short-circuiting due to pressure between the electrodes can cause
A phenomenon such as peeling of the negative electrode from the current collector is caused.

As described above, suppression of the change in volume of the negative electrode is a very important technique for improving the performance of the lithium secondary battery. Conventionally, it is necessary to increase the amount of the binder in the negative electrode or to impart rigidity to the negative electrode. Has been done. But,
These measures result in an increase in the current-carrying resistance of the electrodes,
This is a cause of deteriorating the charge / discharge cycle characteristics at a high current density.

[0007]

SUMMARY OF THE INVENTION The present invention provides a negative electrode active material capable of suppressing a change in volume due to charging and discharging without increasing the current-carrying resistance of the negative electrode in a lithium secondary battery. An object is to improve the cycle characteristics of a lithium secondary battery by using a negative electrode active material as a negative electrode active material.

[0008]

Means for Solving the Problems The present inventor has made intensive efforts to mix and add fibrous carbon to a graphite material which has been conventionally used as a negative electrode active material so that the volume of the negative electrode accompanying charge and discharge is reduced. Focusing on the fact that the change is suppressed, the following invention has been reached. The present invention provides a lithium secondary battery comprising: at least one graphite material of flaky graphite or spheroidal graphite; and fibrous carbon forming secondary particles having a particle diameter of 10 μm or more and 30 μm or less. Negative electrode active material.

That is, according to the present invention, the secondary particles of fibrous carbon are located in the gaps between the flaky or spherical graphite particles, so that the negative electrode of the negative electrode using the negative electrode active material as the negative electrode active material can absorb the lithium ions. -It is possible to suppress a change in volume due to release, and to prevent collapse of the negative electrode structure due to repeated charging and discharging. By using fibrous carbon as a material for reinforcing the negative electrode structure, it is possible to improve the cycle characteristics of the lithium secondary battery while securing the electrical conductivity of the negative electrode. Further, by setting the secondary particles of the fibrous carbon to be mixed and added to have an appropriate size having an average particle diameter of 10 μm or more and 30 μm or less, it is possible to maintain more excellent cycle characteristics.

In the present invention, in the negative electrode active material, the proportion of the fibrous carbon to be added and mixed is 0.5 parts by weight or more when the total amount of the graphite material and the fibrous carbon is 100 parts by weight. Means for reducing the content to 0.5 parts by weight or less is also employed. Since the amount of fibrous carbon to be added and mixed greatly affects the cycle characteristics of the battery, by setting the amount of mixed and added in an appropriate range, a negative electrode active material capable of maintaining a larger battery capacity can be obtained. By configuring the lithium secondary battery using the negative electrode active material as the negative electrode active material, the lithium secondary battery has a low cycle capacity and a good cycle characteristic even after repeated charging and discharging. Next battery.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail based on embodiments. The description will be made on the graphite material and fibrous carbon constituting the negative electrode active material, then the configuration and fabrication of the negative electrode using the negative electrode active material as the negative electrode active material, and the configuration of the lithium secondary battery using the negative electrode Do.

<Graphite Material> In the negative electrode active material of the present invention, a graphite material is used as a main material. Graphite material is
In general, it is a material suitable as a negative electrode active material of a lithium secondary battery because a high discharge capacity can be achieved. Graphite materials can be classified into flaky graphite and spherical graphite according to their particle shapes, and any of the materials contained therein can be used as the negative electrode active material.

The flaky graphite is obtained, for example, by pulverizing and classifying natural graphite or artificial graphite. Natural graphite as a raw material of flaky graphite is mainly produced from Sri Lanka, Madagascar, China and the like. Since the electrode performance varies depending on the degree of the presence of impurities and the variation of the particle size, when natural graphite is used as the negative electrode active material, it is necessary to increase the purity and make the particle size appropriate. The discharge capacity is about 370 mAh /
g and the highest capacity among the graphitic materials, it is an excellent active material in this respect.

[0014] Artificial graphite is usually produced using petroleum coke or the like as a starting material and pitch as a binder. Extrusion or rolling is performed by mixing a binder into the ground coke, and after removing volatile components, the coke is heated at a temperature of 2600 ° C. to 3000 ° C. to be graphitized. Since artificial graphite is less graphitized than natural graphite, it has a discharge capacity of 350 to 360.
mAh / g is slightly inferior to natural graphite, but has the advantage of less impurities. Artificial graphite may be used as the spheroidal graphite. As a typical material that can be used as the negative electrode active material, graphitized mesocarbon microbeads (M
CMB). This graphitized MCMB is produced by heating optically anisotropic small spheres obtained in the process of heating pitches at around 400 ° C. at a high temperature of 2000 ° C. or higher to graphitize. Although the discharge capacity is 290 mAh / g, which is inferior to the above-mentioned flaky graphite, since it has a spherical shape, there is an advantage that the specific surface area can be minimized and the packing density can be improved. In addition, the internal structure of MCMB is such that crystallites are arranged in a lamellar shape, so that crystallite end faces are exposed on the particle surface, and lithium ions can be absorbed and released smoothly, making charging and discharging at large current densities possible. It becomes a suitable negative electrode active material. In addition, what made the natural graphite granulated may be used for spherical graphite.

As the negative electrode active material of the present invention, any one of the above-mentioned flaky graphite and spherical graphite can be selectively used in accordance with the use of the secondary battery or the like. Can also be used as a mixture. Regardless of the type of graphite material used, the particle size of the graphite material affects the performance of the secondary battery. Therefore, the graphite material has an average particle size in consideration of the particle size of fibrous carbon described later. Is 3 μm or more and 40
It is preferable to use a powder having a size of about μm or less. When the average particle diameter is less than 3 μm, the paste has a high viscosity and the filling rate of the electrode does not increase, resulting in an electrode having a low active material density. When the average particle diameter exceeds 40 μm, a thin film electrode having a thickness of about 50 μm on one side is provided. This is because, in the case of manufacturing a non-uniform, the coated surface becomes non-uniform.

<Fibrous Carbon> In the negative electrode active material of the present invention, a fibrous carbon material is used as a material to be added to the graphite material. There are various types of fibrous carbon, and any of them can be used. For example, carbon fibers obtained by heat-treating a pitch such as a polymer or coal tar such as polyacrylonitrile melt-spun into fibrous,
Mesophase pitch-based carbon fiber obtained by heat treatment of mesophase pitch having optical anisotropy, vapor-grown carbon fiber grown by vaporizing organic substances such as benzene on a high-temperature substrate, and growing carbon crystals with a catalyst It is. In addition, these may be graphitized by heat treatment. Further, these can be used alone, or two or more of them can be used as a mixture.

In the case of using any of the above fibrous carbons, it is necessary to appropriately pulverize the fibers. In order to take advantage of the fiber shape, it is important to produce a powder that retains the fiber shape. From the viewpoint of strength, it is advantageous that the fiber length is long and the secondary particles are large. On the other hand, since the occlusion / release of lithium ions increases in the cross section of the fiber, it is better to shorten the fiber length and increase the cross section of the fiber, so that the discharge capacity is larger. From this point, the smaller secondary particles are better. Considering these points comprehensively, and also considering the results of experiments described later, fibrous carbon is used in the form of powder in which the average particle diameter of the secondary particles is 10 μm or more and 30 μm or less. Is desirable.

When the fibers are thick or short, it is difficult to form secondary particles of 10 μm or more and 30 μm or less. Therefore, the fiber diameter of the fibrous carbon is 0.05 μm or more and 0.5 μm or less. And the fiber length is 5 μm or more and 50
It is desirable that it is not more than μm. <Structure and Fabrication of Negative Electrode> In the negative electrode active material of the present invention, the above graphite material and fibrous carbon are mixed, and this mixing ratio is also a factor that affects the capacity maintenance performance of the battery. The function of the fibrous carbon in the negative electrode is that it is located in the gap between the graphite material particles and suppresses expansion and contraction of the negative electrode. Therefore, considering this function, there is an appropriate mixing ratio for maintaining the battery capacity in a good range even after repeated charging and discharging. In view of the results of the experiments shown below, the proportion of fibrous carbon, when the total amount of graphite material and fibrous carbon is 100 parts by weight,
It is desirable that the amount be 0.5 part by weight or more and 22.5 parts by weight or less.

Using this negative electrode active material as a negative electrode active material, a negative electrode is manufactured. The negative electrode is formed by mixing a binder with a negative electrode active material and adding a suitable solvent to form a paste-like negative electrode mixture on the surface of a metal foil current collector and drying.
The binder plays a role of binding the particles of the graphite material and the fibrous carbon, which are the active material, and any material that is generally used to bind the negative electrode active material is used. Can also be used. For example, fluorine-containing resins such as polytetrafluoroethylene, polyvinylidene fluoride, and fluororubber, and thermoplastic resins such as polypropylene and polyethylene can be used. As a solvent for dispersing the particles and the binder, an organic solvent such as N-methyl-2-pyrrolidone can be used.

Instead of these materials, one or more cellulose ether-based substances selected from the group consisting of methylcellulose, carboxymethylcellulose and the like, and a styrene-butadiene rubber latex and a carboxy-modified styrene-butadiene rubber latex are used as a negative electrode binder. It is also possible to use a composite binder with a synthetic rubber-based latex-type adhesive such as, for example, and use water as a solvent. As the negative electrode current collector, a copper foil or the like can be used.

When the negative electrode active material is used, the mixing amount of the binder can be reduced to about half as compared with the conventional case. This is considered to be because the graphite material is held by fibrous carbon, so that only a small amount of the binder is attached to the protrusions of the secondary particles formed by the fibrous carbon, and the active material easily adheres to the electrode. Because. In order to ensure good battery performance, the negative electrode mixture needs to have the graphite material, fibrous carbon, and the binder kneaded and dispersed sufficiently and uniformly. Therefore, the kneading and dispersing step is desirably performed using a stirrer having rotating blades, a ball mill, a medium stirring mill, or the like.

The method of applying the negative electrode mixture to the surface of the current collector is not particularly limited, but a coater-type coating machine capable of continuously applying and drying the negative electrode mixture on the belt-shaped current collector is used. Convenient to use. Since the negative electrode mixture has a relatively high viscosity, it is preferable to employ a coating method such as a comma coat, a squeeze coat, and a lip coat in the coating section of the coating machine. Above all, the reverse comma roll system using three rolls of a coating roll, a backup roll, and a comma roll is excellent in that a uniform coating thickness can be obtained and the viscosity can be easily changed. The coating thickness of the negative electrode mixture can be arbitrarily set in a range of 70 μm to 200 μm depending on the use of the battery.

After forming the negative electrode by coating and drying the negative electrode mixture, pressing after drying to increase the density of the negative electrode is effective to increase the energy density of the battery. However, the degree to which the battery capacity is maintained by repeated charging and discharging is closely related to the negative electrode density.
It is desirable that the concentration be not less than cm ^{3 and not} more than 1.5 g / cm ³ . This is because when the negative electrode density is less than 1.0 g / cm ³ , the sheet resistance of the electrode increases, and as a result, the rate characteristics decrease. When the negative electrode density exceeds 1.5 g / cm ³ ,
This is because the activity of the active material inside the electrode decreases, and the battery capacity decreases.

<Structure of Lithium Secondary Battery> A lithium secondary battery mainly includes a positive electrode and a negative electrode, a separator, and a non-aqueous electrolyte. Since the above-described element may be used for the negative electrode, other components except the negative electrode will be described here. In addition, about a component except a negative electrode, a well-known thing can be used in general, The following is an example and is not limited to this.

The positive electrode used in the lithium battery is obtained by mixing a conductive material and a binder with a positive electrode active material capable of inserting and extracting lithium ions, and adding an appropriate solvent to form a paste-like positive electrode mixture as in the case of the negative electrode. Then, it is formed by coating and drying on the surface of a current collector made of metal foil. LiCoO ₂ , Li
One or more kinds of lithium composite oxide powders such as NiO ₂ and LiMn ₂ O ₄ can be used, provided that they can occlude and release lithium ions. Absent.

The conductive material used for the positive electrode is for ensuring the electrical conductivity of the positive electrode, and may be one or more of carbon material powders such as carbon black, acetylene black, and graphite.
A mixture of more than one species can be used. Like the negative electrode, a fluorine-containing resin such as polytetrafluoroethylene, polyvinylidene fluoride, or fluororubber, or a thermoplastic resin such as polypropylene or polyethylene can be used as the binder. An organic solvent such as N-methyl-2-pyrrolidone can be used as a solvent for dispersing the active material, the conductive material, and the binder. An aluminum foil or the like can be used for the positive electrode current collector. The method of manufacturing the positive electrode
It can be similar to the negative electrode.

The separator sandwiched between the positive electrode and the negative electrode separates the positive electrode from the negative electrode and holds the electrolyte, and a microporous membrane such as polyethylene or polypropylene can be used. The non-aqueous electrolyte is a solution in which an electrolyte is dissolved in an organic solvent.Examples of the organic solvent include aprotic organic solvents such as ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, γ-butyrolactone, acetonitrile, and dimethoxyethane. , Tetrahydrofuran, dioxolan, methylene chloride, or a mixture of two or more thereof. Also, as the electrolyte to be dissolved,
LiI, LiClO ₄ , LiAsF ₆ , LiBF ₄ , Li
Lithium salts such as PF ₆ can be used.

The lithium secondary battery constituted as described above can be formed in various shapes such as a cylindrical type, a stacked type and a coin type. Regardless of the shape used, a separator is sandwiched between the positive electrode and the negative electrode to form an electrode body, and a current collecting lead extends from the positive electrode current collector and the negative electrode current collector to the positive electrode terminal and the negative electrode terminal that lead to the outside. And the like, and sealed in a battery case together with the non-aqueous electrolyte.

[0029]

EXAMPLES Various lithium secondary batteries were actually manufactured, and various experiments were conducted on these lithium secondary batteries. The lithium secondary batteries using the negative electrode active material of the present invention were repeatedly charged. It was confirmed that the secondary battery had good cycle characteristics that could maintain a large discharge capacity even by discharging. The experiments performed and the results are described below. The experiment was performed on the relationship between the mixed addition ratio of fibrous carbon and the capacity retention, the relationship between the negative electrode density and the capacity retention, and the relationship between the average secondary particle diameter of the fibrous carbon and the capacity retention.

<Experiment 1: Relationship between Mixed Addition Ratio of Fibrous Carbon and Capacity Retention Ratio> First, the lithium secondary battery used in this experiment will be described. The negative electrode active material of this battery includes
Graphitized mesocarbon microbeads which are spheroidal graphite as a graphite material (manufactured by Osaka Gas Chemical: MCMB25-2)
8: average particle size of 25 μm) and vapor-grown carbon fiber (VGCF, manufactured by Showa Denko KK) was used as the fibrous carbon.

The preparation of the negative electrode mixture was first performed by preliminary kneading for adjusting the particle size of the secondary particles of fibrous carbon.
The preliminary kneading is performed using polyvinylidene fluoride (PV) as a binder.
DF) with N-methyl-2-pyrrolidone (NM
P) was prepared at a concentration of 12 wt%, and fibrous carbon was mixed with this solution so that the weight ratio of fibrous carbon and PVDF was 1: 1. (Manufactured by Kogyo Co., Ltd.) for 10 minutes.

After the preliminary kneading, spheroidal graphite was added so that the spheroidal graphite and fibrous carbon had the following ratio, and kneading was continued for further 30 minutes to prepare a paste-like negative electrode mixture. The prepared negative electrode mixture was 99: 1, 95: 1, 80:20, 7:99 by weight ratio of spherical graphite to fibrous carbon.
5:25, 50:50. Further, for comparison, a negative electrode mixture in which fibrous carbon was not mixed and added and the negative electrode active material was only spherical graphite was also prepared. As a result of these series of kneading, the average particle size of the secondary particles of fibrous carbon was 18 μm.
Had become.

Each negative electrode mixture thus obtained was
After adjusting the viscosity by MP, a coating thickness of 170 μm per side was applied to both sides of the copper foil current collector having a thickness of 10 μm using a coater.
Coated at μm and dried. Thereafter, compression molding was performed by a roll press to produce a sheet-shaped negative electrode having a layered negative electrode having a thickness of 110 μm per side. The density of each of the obtained negative electrodes is 1.2 g / cm ^{3 to} 1.4 g / c.
m ³ .

The positive electrode facing the negative electrode has L as an active material.
A powder of iMn ₂ O ₄ was prepared using spherical graphite (TB5500 manufactured by Tokai Carbon) as a conductive material and PVDF as a binder. 5 parts by weight of spheroidal graphite and 5 parts by weight of PVDF are mixed with 90 parts by weight of LiMn ₂ O ₄ , NMP is added as a solvent and kneaded, and the obtained paste-like positive electrode mixture is mixed with aluminum having a thickness of 20 μm. A positive electrode was prepared by applying and drying both sides of the foil current collector and performing roll pressing.

The produced negative electrode and positive electrode are interposed via a separator.
Then, a rolled electrode body was formed. This electrode body
A 18650 type battery with a diameter of 18 mmφ and a length of 65 mm
Insert into a can, impregnate with non-aqueous electrolyte and seal to complete battery
I let it. A 20μm thick microporous polyp is used for the separator.
The propylene film is used for the non-aqueous electrolyte
And diethyl carbonate in a volume ratio of 1: 1
LiPF in mixed solvent ₆At a concentration of 1M as an electrolyte
The dissolved solution was used.

A charge / discharge cycle test was performed on each of the batteries thus produced, that is, each battery having a different mixing ratio of spherical graphite and fibrous carbon in the negative electrode. The charge / discharge cycle test was performed at a temperature of 20 ° C. for 1.0
At a current density of mA / cm ² , constant current charging was performed up to a charging end voltage of 4.2 V. After reaching 4.2 V, charging was performed at a constant voltage for 2 hours. After the charging was completed, a current density of 1.0 mA / cm ² was obtained. The one that performs constant current discharge up to the discharge end voltage of 3.0 V
Cycle. And repeat up to 500 cycles,
The discharge capacity in each cycle was measured.

Prior to the test, the discharge capacity in each cycle with respect to the initial discharge capacity measured by charging and discharging at a constant current of 0.25 mA / cm ² was taken as a percentage to obtain a capacity retention rate. The cycle characteristics of the battery are evaluated by comparing the capacity retention ratio in each cycle. Table 1 below
Fig. 1 shows the capacity retention ratio after 100 cycles with respect to the mixing ratio of spheroidal graphite and fibrous carbon, and Fig. 1 shows the relationship of the capacity retention ratio of each cycle up to 500 cycles with respect to the mixing ratio of spheroidal graphite and fibrous carbon. Show.

[0038]

[Table 1] As can be seen from FIG. 1, in the battery in which fibrous carbon was not added as the negative electrode active material, the capacity retention rate sharply decreased from around 150 cycles. On the other hand, in a battery using a negative electrode active material mixed with fibrous carbon,
No rapid decrease in capacity retention rate was observed up to 500 cycles. Therefore, it was confirmed that mixing and adding fibrous carbon to a graphite material as the negative electrode active material is advantageous for improving the cycle durability of the lithium secondary battery.

Further, as can be seen from FIG. 1 and Table 1, when the ratio of the fibrous carbon exceeds 25 parts by weight when the total amount of the graphite material and the fibrous carbon is 100 parts by weight, the capacity retention ratio increases as the cycle progresses. The degree of decrease became large.
Therefore, it is desirable that the mixing ratio of the fibrous carbon in the negative electrode active material be 0.5 to 22.5 parts by weight when the total amount of the graphite material and the fibrous carbon is 100 parts by weight. It could be confirmed.

<Experiment 2: Relationship between negative electrode density and capacity retention ratio> Using the same manufacturing method as in Experiment 1, spherical graphite was used.
A negative electrode mixture was prepared in which the mixing ratio of fibrous carbon and PVDF was 95 parts by weight: 5 parts by weight: 5 parts by weight. The negative electrode mixture was applied to both surfaces of the negative electrode current collector with a thickness of 110 μm, 140 μm, 180 μm, 220 μm,
The coating was applied to a thickness of 50 μm, dried, and compression-molded by a roll press to prepare a sheet-shaped negative electrode having a negative electrode thickness of 110 μm per side. The respective negative electrode densities were 0.75 g / cm ³ , 1.00 g / cm ³ ,
1.25 g / cm ³ , 1.50 g / cm ³ , 1.75 g / cm ³
cm ³ .

Using this negative electrode, a secondary battery was manufactured in the same manner as in Experiment 1 except that the positive electrode, the separator, the nonaqueous electrolyte, the battery can, and the like were used. A charge / discharge cycle test was performed on each battery under the same conditions as in Experiment 1 to determine the discharge capacity at the second cycle per unit weight of the negative electrode. The discharge capacity at the 100th cycle relative to the discharge capacity at the second cycle was obtained. The capacity retention rate was compared with the percentage of the capacity retention rate, and the relationship between the negative electrode density and the capacity retention rate was examined. The results are shown in Table 2 and FIG.

[0042]

[Table 2] As can be seen from the results, the capacity retention rate was dependent on the negative electrode density. If the negative electrode density is less than 1.0 g / cm ³ or more than 1.5 g / cm ³ , a sufficient capacity retention cannot be obtained, so that fibrous carbon was mixed with the graphite material. When a secondary battery having better cycle characteristics is to be obtained using a negative electrode active material, it is confirmed that the negative electrode density is desirably 1.25 g / cm ³ or more and 1.5 g / cm ³ or less. did it.

<Experiment 3: Relationship between Average Secondary Particle Diameter of Fibrous Carbon and Capacity Retention Ratio> In the preliminary kneading of the fibrous carbon and the binder, the secondary time of the fibrous carbon was changed by changing the kneading time. It has been found that the size of the particles changes. Experiment 1
In the same manner as in the above, the preliminary kneading is performed by adding 12% by weight of PVDF to NMP.
Was prepared, and fibrous carbon was mixed with this solution so that the weight ratio of fibrous carbon and PVDF was 1: 1. The kneading time is 5 minutes, 10 minutes respectively.
Minutes, 30 minutes, 60 minutes, and 120 minutes.

After the preliminary kneading, spherical graphite was added so that the ratio of spherical graphite and fibrous carbon was 95 parts by weight: 5 parts by weight, and kneading was further continued for 30 minutes to produce a paste-like negative electrode mixture. did. Further, a negative electrode mixture in which spherical graphite and fibrous carbon were directly mixed without performing preliminary kneading was also produced. Then, a sheet-shaped negative electrode on which a negative electrode was formed was produced in the same manner as in Experiment 1 using these negative electrode mixtures. Each of the formed negative electrodes was observed by a scanning electron microscope (SEM) to determine an average particle diameter of secondary particles of fibrous carbon.

Next, a secondary battery having the same configuration as in Experiment 1 was manufactured, and a charge / discharge cycle test was performed on these batteries under the same conditions as in Experiment 1 to obtain a battery with 100% of the initial capacity.
The relationship between the average secondary particle diameter of the fibrous carbon and the capacity retention rate was examined with the percentage of the discharge capacity at the cycle as the capacity retention rate. The results are shown in Table 3 below.

[0046]

[Table 3] As can be seen from Table 3, the average particle size of the secondary particles of the fibrous carbon that has not been pre-kneaded is 51 μm,
When the preliminary kneading was performed, it was confirmed that the average particle diameter was reduced to 12 μm in the initial stage of kneading, and that secondary particles grew as the kneading time elapses thereafter. Then, it was confirmed that a high capacity retention ratio was obtained when the average particle size of the secondary particles of the fibrous carbon was in the range of 10 μm to 30 μm. Therefore, in a secondary battery using a negative electrode material obtained by mixing fibrous carbon with a graphite material, the average secondary particle diameter of the fibrous carbon is set to 10 μm or more and 30 μm or less in order to improve cycle characteristics. It was confirmed that it was desirable.

When an X-ray diffraction analysis was performed on these negative electrodes, a peak of the (110) plane characteristic of fibrous carbon was observed in each case. It was confirmed that it was located in the state. Further, according to SEM observation, it was confirmed that the negative electrode in which the fibrous carbon was mixed had many vacancies in the negative electrode, and it was considered that lithium ions could enter and exit well. Can be expected to improve.

[0048]

The present invention is characterized in that a mixture of a graphite material and fibrous carbon is used as a negative electrode active material for a lithium secondary battery. The negative electrode mixed and added with fibrous carbon having an appropriate secondary particle size suppresses the volume change due to charge and discharge without increasing the current-carrying resistance of the negative electrode.The lithium secondary battery using this negative electrode has a high current density. , A large discharge capacity can be maintained even by repeated charge and discharge, and good cycle characteristics can be obtained.

[Brief description of the drawings]

FIG. 1 is a diagram showing a relationship between a mixed addition ratio of fibrous carbon and a capacity retention ratio.

FIG. 2 is a diagram showing a relationship between a negative electrode density and a capacity retention ratio.

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 5H029 AJ05 AK03 AL07 AM02 AM07 BJ02 BJ03 BJ04 DJ15 DJ16 HJ01 HJ05

Claims (3)

[Claims] 1. A lithium secondary battery comprising: at least one graphite material of flaky graphite or spherical graphite; and fibrous carbon forming secondary particles having an average particle diameter of 10 μm or more and 30 μm or less. Negative electrode active material for secondary batteries. 2. When the total amount of the graphite material and the fibrous carbon is 100 parts by weight, the ratio of the fibrous carbon is 0.5.
The negative electrode active material for a lithium secondary battery according to claim 1, which is not less than 2 parts by weight and not more than 22.5 parts by weight. 3. A lithium secondary battery using the negative electrode active material for a lithium secondary battery according to claim 1 as a negative electrode active material.