JP2001266878A - Negative electrode active material for lithium ion secondary battery and lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a negative electrode active material for lithium ion secondary battery and a lithium ion secondary battery that surpass the performance of lithium ion secondary battery using the current graphite system carbon material. SOLUTION: The negative electrode for lithium ion secondary battery and the lithium ion secondary battery using the same are characterized in that the negative electrode active material contains a silicic particle that contains a compound made from silicon, carbon and boron dispersed minutely at least in the particle.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a negative electrode active material for a lithium ion secondary battery and a lithium ion secondary battery.

[0002]

2. Description of the Related Art Lithium ion secondary batteries have a high energy density and are therefore used as power supplies for mobile communication and portable information terminals. The market has been rapidly expanding with the spread of terminals. Along with this, in order to further pursue the small size and light weight, which are the characteristics of the terminal device, there is a demand for a battery occupying a large volume in the device to further improve the performance of the small size and light weight.

At present, the negative electrode active material used in the secondary battery is mainly a graphite-based carbonaceous material, and is a key material that affects battery performance. However, the amount of lithium that can be reversibly inserted and desorbed into the material is limited to 1 atom of lithium to 6 atoms of carbon, and the electric capacity is 372 mAh / g, which is the theoretical limit for charging and discharging of the carbon material. Capacity. Since current secondary batteries are used at a level close to this limit capacity, a dramatic improvement in performance in the future cannot be expected.

Under such circumstances, search for materials other than carbon, for example, alloys and inorganic compounds having a capacity much larger than 372 mAh / g is gradually being conducted. Among them, it has been found that a crystalline or amorphous oxide material containing tin and silicon exhibits a discharge capacity close to 1000 mAh / g (see, for example, JP-A-7-220721).
JP, JP-A-7-249409, etc.). Also,
Recently, when silicon simple substance is used for the negative electrode active material, 3000 m
It shows an initial discharge capacity of around Ah / g (38th Battery Symposium, 3A16 (1997)), and shows an initial discharge capacity of around 1500 mAh / g when silicon oxide is used for the negative electrode active material (38th case). Battery Symposium, 3A17 (199
7)), it has been reported that the graphite-based carbonaceous material has a discharge capacity far exceeding the limit capacity. However, each material has a large initial charge capacity with respect to its initial discharge capacity, that is, a very large capacity loss at the time of charge / discharge (both materials are about 1000 mAh / g), and low cycle characteristics (after the start of charge / discharge). However, the capacity was reduced by half in several cycles).

On the other hand, as a material containing boron in silicon, a silicon boride alloy structure in which n is 0.1 to 3 in the general formula SiB _n (JP-A-53-1366)
No. 30), SiB _{4 having} n in the range of 3.2 to 6.6.
Boron compound powder containing silicon as the main component (JP-A-8-13)
No. 8744). Although the above-mentioned alloy structure shows a large discharge capacity far exceeding the original graphite-based carbonaceous material of silicon, since it is an electrode formed by impregnating a current collecting matrix in an alloy bath, lithium ion which has been put into practical use in recent years It was difficult to mass-produce compared to an electrode formed by applying a slurry of an active material powder together with a binder on a current collector foil as seen in a battery and forming the slurry. In addition, since it is difficult to reduce the film thickness of this alloy structure, at a large current density, lithium is not sufficiently diffused in the electrode, causing a large polarization resistance, and the absorbed lithium is not sufficiently absorbed. Since it cannot be pulled out, there is a large problem in reversibility such as an increase in capacity loss at the time of charging and discharging. On the other hand, the latter SiB
It was difficult to obtain a higher discharge capacity with a boron compound powder of silicon mainly composed of ₄ or the like than with a graphite-based carbonaceous material.

[0006]

SUMMARY OF THE INVENTION It is an object of the present invention to provide the above-mentioned problems which occur when silicon powder is used as a negative electrode active material for a lithium ion secondary battery, that is, it is unsuitable for mass production. It solves the problems of large reversibility due to large capacity loss during charge and discharge, low cycle characteristics, and the ability to obtain a lower discharge capacity than graphite-based carbonaceous materials. It is an object of the present invention to provide a negative electrode active material for a lithium ion secondary battery having performance exceeding that of a conventional lithium ion secondary battery, and a lithium ion secondary battery using the same.

[0007]

Means for Solving the Problems The present inventors have conducted intensive studies on silicon powder containing boron and carbon based on the electrochemical characteristics of the silicon powder itself. Boron carbide (B
_It has been found that by mixing 4C) at an appropriate concentration and heat-treating under certain conditions, a large number of fine compounds of silicon, carbon and boron are generated and dispersed inside the silicon particles. Further, the present inventors have found that when the siliconaceous powder having such a structure is used, the capacity loss is significantly improved while maintaining a large discharge capacity far exceeding the graphite-based carbonaceous material originally possessed by silicon. As a result, it has been found that excellent electrode characteristics with good cycle characteristics are exhibited. The present invention has been completed based on such findings.

That is, the present invention provides: (1) a lithium ion secondary battery, wherein the negative electrode active material comprises siliconaceous particles in which a compound comprising silicon, carbon, and boron is finely dispersed in at least particles. (2) The lithium ion secondary material according to (1), wherein the compound comprising silicon, carbon, and boron is mainly represented by (C, Si, B) ₃ B _12. Negative electrode active material for a battery, (3) the negative electrode active material,
The negative electrode active material for a lithium ion secondary battery according to (1) or (2), further comprising carbonaceous particles. Further, the present invention provides (4) a lithium ion secondary battery containing a positive electrode active material, a negative electrode active material, and a non-aqueous electrolyte, wherein the negative electrode active material is any one of (1) to (3). A lithium ion secondary battery, which is a negative electrode active material for an ion secondary battery.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The specific contents of the present invention will be described below.

The negative electrode active material for a lithium ion secondary battery of the present invention contains silicon particles. Here, the siliconaceous particles in the present invention are those containing silicon and capable of recognizing a silicon peak by X-ray diffraction. Due to the fine dispersion of the compound composed of silicon, carbon and boron inside the siliconaceous particles, when used as a negative electrode active material for a lithium ion secondary battery, it has a high discharge capacity exceeding the theoretical value of graphite. However, it shows high initial efficiency of 90% or more and high cycle characteristics.

Here, the fact that the compound composed of silicon, carbon and boron is finely dispersed inside the silicon particles is as follows.
The volume of each of the compound particles composed of silicon, carbon, and boron dispersed in each of the silicon particles is 1/10 or less of the volume of the silicon particles. This volume ratio is
Preferably 1/100, more preferably 1/1000
It is desirable that: When the volume of each compound of silicon, carbon, and boron dispersed inside the siliceous particles is larger than 1/10 of the volume of the siliceous particles,
In particular, the initial efficiency may be reduced, and the cycle characteristics may be reduced.

When a silicon-containing particle in which a compound composed of silicon, carbon and boron is finely dispersed is used as a negative electrode active material for a lithium ion secondary battery, a high discharge exceeding the theoretical value of graphite is obtained. While having the capacity
The reason for the high initial efficiency of 90% or more and high cycle characteristics is not clear, but is estimated as follows. That is, the mechanism by which the silicon powder in the negative electrode active material for a lithium ion secondary battery of the present invention inserts and releases lithium ions is described in the report on tin metals and oxides (J. Electr.
ochem. Soc., 144, 6, 2045 (1997)), it is presumed that this is basically an alloying / dealloying reaction between silicon powder and lithium. It is expected that the particles will expand and contract. Here, the compound composed of silicon, carbon, and boron finely dispersed inside the siliceous particles enhances the diffusion of lithium and resists mechanical destruction of the particles due to expansion and contraction of the negative electrode active material due to charge and discharge. It is presumed to express sex.

The ratio of the total volume of the compound comprising silicon, carbon and boron dispersed in the silicon particles to the volume of the silicon particles is preferably 5% or more and 90% or less, more preferably 10% or less. It is preferably at least 80%. When this ratio is less than 5%, the best effect of the initial efficiency and cycle characteristics due to the fine dispersion of the compound comprising silicon, carbon and boron in the siliceous particles is sufficiently recognized, but the silicon and carbon The effect is limited due to the small amount of the boron compound. On the other hand, if this ratio exceeds 90%, the discharge capacity is reduced, and the merit as compared with the graphite-based active material is reduced.

In the negative electrode active material for a lithium ion secondary battery of the present invention, it is preferable that the compound comprising silicon, carbon and boron in the silicon particles is mainly represented by (C, Si, B) ₃ B _12. .

When the compound comprising silicon, carbon and boron is mainly represented by (C, Si, B) ₃ B ₁₂ , the reason why the negative electrode active material for a lithium ion secondary battery of the present invention exhibits good characteristics. Although it is not clear, it is considered that one of the causes is that the compound can be finely dispersed in the siliconaceous particles. It is also conceivable that (C, Si, B) ₃ B ₁₂ , for example, its crystal structure, electrochemical properties, interface or surface structure, and other structures or properties contribute to it.

With respect to the particle size of the negative electrode active material of the present invention, the powder preferably has a 50% cumulative diameter (d ₅₀ ) of 1 to 100 μm, more preferably 10 to ₅₀ μm. If d ₅₀ of less than 1μm, the powder of a small particle size often and that handling properties is observed a tendency that deteriorate because it contains the energy density per unit volume required many binders and conductive agent is reduced It is not preferable because there is a possibility. On the other hand, if d ₅₀ exceeds 100 μm, it may be difficult for lithium to diffuse into the active material powder, or the current lithium ion battery has an electrode thickness of about 200 μm or less, making electrode fabrication difficult. It is not preferable because it may become possible.

The specific surface area of the negative electrode active material of the present invention calculated by desorption of nitrogen is preferably 100 m ² / g or less, more preferably 10 m ² / g. If the specific surface area exceeds 100 m ² / g, the surface area is large, which may cause a reaction with the electrolyte at the time of initial charging to increase the capacity loss or occlusion in the material during the charge / discharge cycle. The reactivity between the lithium and the solvent in the electrolyte may increase, and the safety may decrease.

As long as the characteristics of the negative electrode active material for a lithium ion secondary battery of the present invention are not hindered, silicon, carbon and boron are contained in the silicon powder in the negative electrode active material for a lithium ion secondary battery of the present invention. In addition to the above compounds, other compounds may be present regardless of the form of the impurities, whether in the form of impurities or the like or intentionally for another purpose. Examples of such compounds include, but are not limited to, silicon carbide, silicon boride, iron silicide, iron boride, silicon oxide, aluminum oxide, and the like.

The negative electrode active material for a lithium ion secondary battery of the present invention is a silicon material in which a compound composed of silicon, carbon and boron is finely dispersed as long as the characteristics of the negative electrode active material for a lithium ion secondary battery are not impaired. Compounds and powders other than particles may be contained, irrespective of their form, whether in the form of impurities or the like or intentionally for another purpose. Examples of such compounds and powders include, but are not limited to, carbonaceous powders such as natural graphite powders, tin, aluminum, iron, tin oxide and the like, or composite particles and mixtures thereof.

Here, the negative electrode active material for a lithium ion secondary battery of the present invention contains carbonaceous particles simultaneously with the siliconaceous particles in which a compound of silicon, carbon and boron is finely dispersed. , It has higher cycle characteristics. Here, the siliconaceous powder and the carbonaceous particles may be mixed, or one may be present on the other surface or inside. Even when the carbonaceous powder is contained, the compound composed of silicon, carbon and boron in the siliconaceous particles is mainly composed of (C, Si, B) ₃ B
_It is preferably represented by ₁₂ .

When the carbonaceous particles or powder are used as a negative electrode active material or a negative electrode material for a lithium ion secondary battery, they are basically of any kind as long as there is no problem in the battery manufacturing process or the like. It may be. In addition to artificial coal derived from coal tar, artificial graphite derived from petroleum tar, MCMB,
Examples thereof include carbon fiber, natural graphite, quiche graphite and the like, and those containing boron therein. However, when a material having low characteristics as a negative electrode material or having poor properties as a powder is used, it tends to be difficult to exhibit excellent performance when coexisting with siliconaceous particles. Accordingly, the carbonaceous particles have a discharge capacity of 300 mAh /
g or more, preferably 330 mAh / g or more, and an initial efficiency of 85% or more, preferably 90% or more, and a low specific surface area of 5 m ² / g or less, preferably 3 m ² / g or less.

It is not clear why the negative electrode active material for a lithium ion secondary battery of the present invention has higher cycle characteristics because it contains carbonaceous particles at the same time as the siliconaceous particles. The present inventors, by absorbing the expansion and contraction of the silicon particles during charge and discharge by the carbonaceous powder,
It is presumed that maintaining the structure of the electrode body applied on the current collector may have the cause.

Consisting of silicon, carbon and boron (C, S
i, B) a compound represented by ₃ B ₁₂ is conveniently its presence is detected by the X-ray diffraction measurement. The compound is POWDER DIF
FRACTION FILE (published by JOINT COMMITTEE ON POW
DER DIFFRACTION STANDARDS, 1974) or Von A. Lipp an
d M. Roder, Z. anorg. Allgem. Chem., vol. 334, 225
According to (1966), the three strongest peaks in the X-ray diffraction of the powder have a plane spacing of 0.384 nm (relative intensity 65) and 0.26 nm.
It appears at the positions of 11 nm (relative intensity 60) and 0.2402 nm (relative intensity 100).
2), (104) and (201).

The distribution of the compound of silicon, carbon and boron in the silicon particles can be easily known by embedding and polishing the silicon particles and then performing element mapping by EPMA or the like. The ratio of the volume of the siliceous particles to the volume of each of the silicon, carbon and boron compounds present therein can be estimated by the ratio of the area of appearance on the polished surface. Further, the ratio of the total volume of the compound consisting of silicon, carbon, and boron dispersed in the siliconaceous particles to the volume of the siliconaceous particles can also be estimated from the ratio of the appearance area on the polished surface. .

Further, the distribution of the compound represented by (C, Si, B) ₃ B ₁₂ consisting of silicon, carbon and boron in the siliceous particles was determined by appropriately slicing the siliceous particles. And observed with a transmission electron microscope, (C, Si, B)
₃ dark-field image can be known also by taking for each diffraction spot from compounds represented by the by B _12. In this case, the ratio between the volume of the siliceous particles and the volume of each of the compounds represented by (C, Si, B) ₃ B ₁₂ consisting of silicon, carbon and boron present therein is the flake observed. It can be estimated by the ratio of the appearance area in. Also,
The ratio of the total volume of the compound represented by (C, Si, B) ₃ B ₁₂ consisting of silicon, carbon and boron dispersed in the siliconaceous particles to the volume of the siliconaceous particles was also observed. It can be estimated from the ratio of the appearance area in the thin section.

The silicon powder in the negative electrode active material for a lithium ion secondary battery of the present invention is prepared by the following exemplified method, but is not limited thereto. For example, d ₅₀
Is d _{50 with} respect to 80 parts by mass of silicon powder having a particle size of 1 to 100 μm.
Is 1/10 or less of silicon powder (B)
₄ C) 20 parts by mass of the powder were mixed, and the mixture was
After the temperature is raised from 200 ° C. to 1250 ° C., the temperature is maintained for 50 to 100 hours, then rapidly cooled to 600 ° C. at a rate of about 15 ° C./min, and then cooled to about room temperature at a rate of about 5 ° C./min.
The particle size of the powder obtained by the heat treatment is adjusted by crushing and classification as needed.

Another method for producing the silicon powder in the negative electrode active material for a lithium ion secondary battery of the present invention is, for example, as follows. d ₅₀ of a mixture of powders 25 parts by weight of several tens of μm of silicon powder 75 parts by weight and the same degree of particle size of boron carbide (B ₄ C), carefully mechanical alloying by ball, those obtained 1000 ° C to 1200 ° C
Heat treatment in argon gas for 50 to 100 hours, and then quenching to 600 ° C. at a rate of about 15 ° C./min.
Cool to near room temperature at a rate of about 5 ° C./min. The obtained product is pulverized to adjust the particle size.

Still another method for producing the silicon powder in the negative electrode active material for a lithium ion secondary battery of the present invention is, for example, as follows. 75 parts by mass of silicon powder, 20 parts by mass of boron powder and 5 parts by mass of carbon powder were mixed and mixed with 17 parts by mass.
Melts at about 00 ° C or higher to form a uniform melt. Next, the melt is blown out from a nozzle into an argon gas atmosphere in the form of a mist, and rapidly cooled to obtain a powder. The powder thus obtained is optionally heat-treated at 1000 ° C. to 1200 ° C. in argon gas for 10 to 100 hours, and then quenched to 600 ° C. at a rate of about 15 ° C./min. Cool to near room temperature at a moderate rate.

The silicon powder used as a raw material here may intentionally contain various elements other than silicon for impurities or other purposes. For example, carbon, oxygen, hydrogen, nitrogen, sulfur, phosphorus, halogen, alkali metal, alkaline earth metal, transition metal, Al, Ga, In, Ge, Sn, P
At least one kind of b, Sb, Bi, etc. may be included.
The total content of these elements is preferably from 0 to 10% by mass. Further, the amount is more preferably 0 to 5% by mass. In addition, the boron carbide (B ₄ C) powder that plays an important role in the production of the negative electrode active material for a lithium ion secondary battery of the present invention contains boron and carbon other than boron and carbon intentionally for impurities or other purposes. May be included. For example, oxygen, hydrogen, nitrogen, sulfur, phosphorus, halogen, alkali metal, alkaline earth metal, transition metal,
It may include at least one of Al, Ga, In, Ge, Sn, Pb, Sb, Bi, and the like. Boron carbide (B
₄ C), the structure due molar ratio of carbon and boron exactly 1: There may not be 4, there is no any problem as a raw material for producing a siliceous powder of the present invention.

In the above-exemplified production method, in addition to silicon powder, boron carbide powder, boron powder and carbon powder, a small amount of another compound is mixed or another compound is intentionally added for some other purpose. This is also acceptable as long as the effects of the present invention obtained by the exemplified manufacturing method can be exhibited.

As for the mixing method, it is sufficient that the samples are sufficiently uniformly mixed. For example, a V-blender, a kneader, a ball mill and the like can be suitably used, but the invention is not particularly limited thereto. The particle size and specific surface area of the calcined siliceous powder can be adjusted by methods generally used in industry. For example,
Ball mills, pin mills, disc mills, impeller mills, jet mills, roller mills, stamp mills, cutting mills, etc. are preferably used for pulverization, and air classifiers, sieves, etc. are suitably used for classification, but are not particularly limited thereto. is not.

The obtained siliconaceous powder may be mixed or complexed with other compounds or powders by the above-mentioned method or other known methods to form the negative electrode active material for a lithium ion secondary battery of the present invention. I can do it.

The lithium ion secondary battery of the present invention is obtained by combining the above-mentioned negative electrode active material containing siliconaceous particles with a positive electrode active material and a non-aqueous electrolyte. Regarding the molding of the negative electrode active material of the present invention, if the performance of the negative electrode active material for a lithium ion secondary battery of the present invention is sufficiently drawn out, and the shapeability is high, it is chemically and electrochemically stable. Although not limited thereto, for example, a conductive agent such as carbon black may be added to the negative electrode active material for a lithium ion secondary battery of the present invention as needed, and a fluorine-based material such as polytetrafluoroethylene may be further added. There is a method of mixing and kneading after adding a resin powder or a dispersion solution. Further, a conductive agent such as carbon black is added to the negative electrode active material for a lithium ion secondary battery of the present invention as necessary, and a resin mixture such as polyethylene and polyvinyl alcohol is added. There is also a method of molding by hot pressing. Further, a conductive agent such as carbon black is added as necessary to the negative electrode active material for a lithium ion secondary battery of the present invention, and a water-soluble binder such as fluorinated resin powder such as polyvinylidene fluoride or carboxymethyl cellulose is used as a binder. Then, a slurry can be prepared by mixing using a solvent such as N-methylpyrrolidone, dimethylformamide or water, alcohol, etc., applied onto a current collector, and dried to form a slurry.

The positive electrode active material and the organic solvent-based electrolyte used in the lithium ion secondary battery of the present invention are not particularly limited as long as they can be generally used for a lithium ion secondary battery.

As the positive electrode active material used in the lithium ion secondary battery of the present invention, for example, a lithium-containing transition metal oxide LiM (1) _x O ₂ (where x is a numerical value in the range of 0 ≦ x ≦ 1) In the formula, M (1) represents a transition metal, Co, N
i, Mn, Cr, Ti, V, Fe, Zn, Al, In,
Sn) or LiM (1)
_y M (2) _2-y O ₄ (where y is a numerical value in the range of 0 ≦ y ≦ 1, where M (1) and M (2) represent transition metals and Co,
Ni, Mn, Cr, Ti, V, Fe, Zn, B, Al,
In, consisting of at least one kind of Sn), a transition metal chalcogenide (TiS _2, NbSe _3, etc.), vanadium oxide _{(V 2 O 5, V 6} O 13, V 2 O 4, V 3 O 8, etc. ) And its Li compound, a general formula M _x Mo ₆ Ch _8-y (where x is 0
≦ x ≦ 4, y is a numerical value in the range of 0 ≦ y ≦ 1;
Represents a metal such as a transition metal, and Ch represents a chalcogen element), a activated carbon, an activated carbon fiber, or the like.

The organic solvent in the organic solvent-based electrolyte is not particularly limited. For example, propylene carbonate, ethylene carbonate, butylene carbonate, chloroethylene carbonate, dimethyl carbonate, diethyl carbonate, 1,1- and 1,1- 2
-Dimethoxyethane, 1,2-diethoxyethane, γ-
Butyrolactone, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolan, 4-methyl-
1,3-dioxolan, anisole, diethyl ether, sulfolane, methylsulfolane, acetonitrile,
Chloronitrile, propionitrile, trimethyl borate, tetramethyl silicate, nitromethane, dimethylformamide, N-methylpyrrolidone, ethyl acetate, trimethyl orthoformate, nitrobenzene, benzoyl chloride, benzoyl chloride, benzoyl bromide, tetrahydrothiophene, dimethyl sulfoxide, 3 A single solvent such as -methyl-2-oxazolidone, ethylene glycol, sulfite, dimethyl sulfite and the like, or a mixed solvent of two or more thereof can be used.

As the electrolyte, any of conventionally known electrolytes can be used. For example, LiClO ₄ , Li
BF ₄ , LiPF ₆ , LiAsF ₆ , LiB (C ₆ H ₅ ),
LiCl, LiBr, LiCF ₃ SO ₃ , LiCH ₃ S
O ₃ , Li (CF ₃ SO ₂ ) ₂ N, Li (CF ₃ SO ₂ )
₃ C, Li (CF ₃ CH ₂ OSO ₂ ) ₂ N, Li (CF ₃ CF
₂ CH ₂ OSO ₂ ) ₂ N, Li (HCF ₂ CF ₂ CH ₂ OS
O ₂ ) ₂ N, Li ((CF ₃ ) ₂ CHOSO ₂ ) ₂ N, LiB
One or a mixture of two or more such as [C ₆ H ₃ (CF ₃ ) ₂ ] ₄ can be mentioned.

The methods for measuring various physical properties used for defining the negative electrode active material for a lithium ion secondary battery of the present invention are described below.

(1) 50% Cumulative Diameter (d ₅₀ ) The particle size distribution was analyzed by calculating the diffraction pattern when the dispersed particles were irradiated with parallel rays, and the particle size distribution was calculated when the cumulative mass became 50%. The particle size was determined as a 50% cumulative diameter (d ₅₀ ). Usually, about 0.2 g of each sample was placed in 20 cc of water as a dispersion medium, and a drop of a commercially available surfactant was added to the sample, and the particle size was measured with a particle size distribution analyzer LMS-24 manufactured by Seishin Enterprise.

(2) Specific surface area The specific surface area was determined by analyzing using the BET method based on the adsorption curve for each nitrogen partial pressure when nitrogen was adsorbed on the sample. Usually, it measured by BELSORP-36 made from Japan Bell Co., Ltd. using 1-2 g of each sample.

[0041]

The present invention will be specifically described below with reference to examples.
These examples serve to better illustrate the invention but do not limit it in any way.

Example 1 Boron carbide powder (purity 98%, d ₅₀ = 1.5 μm) was added to silicon powder (purity 99%, d ₅₀ = 25 μm) in an amount of 40% in terms of mass and thoroughly mixed using a kneader. Thereafter, the mixture was heated to 1230 ° C. in an argon stream, kept at this temperature for 70 hours, and then heated at a rate of about 15 ° C./min to 60 ° C.
After rapidly cooling to 0 ° C., it was cooled to around room temperature at a rate of about 5 ° C./min. The heat-treated product thus obtained was crushed by an impeller mill, and the particle size was adjusted using an air classifier to obtain a powder having a 50% cumulative diameter (d ₅₀ ) of 20 μm.

The specific surface area of the powder was 0.8 m ² / g. As a result of X-ray diffraction measurement of this material, it was found that a compound of silicon, carbon and boron (C, Si, B) ₃ B ₁₂ was formed in the material. Furthermore, after embedding and polishing the obtained siliconaceous powder, about 10 pieces were analyzed by EPMA. As a result, the compound composed of silicon, carbon and boron is very finely precipitated in the silicon particles, and the volume of the silicon, carbon and boron compound particles precipitated in the silicon particles is the volume of the silicon particles. Was about 1/100 or less.

With respect to 70% by mass of the silicon powder, 20% by mass of carbon black as a conductive agent and 10% by mass of polytetrafluoroethylene powder as a binder were added and kneaded to prepare an electrode sheet having a thickness of about 0.1 mm. And 1
A negative electrode was prepared by cutting out into a cm square (about 21 mg in mass) (about 15 mg in terms of a silicon material) and pressing it on a Cu mesh as a current collector.

In order to evaluate the monopolar electrode characteristics of the molded electrode, a three-electrode cell using lithium metal for the counter electrode and the reference electrode was used. In the electrolyte, a mixed solvent of ethylene carbonate and diethyl carbonate (1: 1 mixture by volume ratio)
Was used in which LiPF ₆ was dissolved at a rate of 1 mol / l. Regarding the charge / discharge test, both charging and discharging were performed at a constant current (0.5 mA / cm ² ) under regulation of potential. The potential range was 0 V to 2.0 V (based on lithium metal).
As a result of the evaluation under these conditions, the silicon-based powder had an initial charge capacity of 2050 mAh / g and an initial discharge capacity of 15
20 mAh / g, indicating a high discharge capacity. In addition,
The initial capacity loss is 530 mAh / g.
Of the initial capacity loss of mAh / g, about 330 mAh / g is derived from carbon black added as a conductive agent and polytetrafluoroethylene powder added as a binder.
It was estimated to be 0 mAh / g. Further, almost no capacity loss was observed after the second time. Also, in the second and subsequent charging and discharging, the discharge capacity was hardly changed, and excellent cycle characteristics were exhibited.

Example 2 Boron carbide powder (purity 98%, d ₅₀ = 1.5 μm) was added to silicon powder (purity 99%, d ₅₀ = 25 μm) in an amount of 20% in terms of mass, and thoroughly mixed using a kneader. Thereafter, the mixture was sufficiently mechanically alloyed by a ball mill, and then heated to 1050 ° C. in an argon stream, held at this temperature for 70 hours, and then rapidly cooled to 600 ° C. at a rate of about 15 ° C./min. It was cooled to around room temperature at a rate of about 5 ° C./min. The heat-treated product thus obtained was crushed by an impeller mill, and the particle size was adjusted using an air classifier, so that the 50% cumulative diameter (d ₅₀ ) was 23 μm.
m were obtained.

The specific surface area of the powder was 0.7 m ² / g. As a result of X-ray diffraction measurement of this material, it was found that a compound of silicon, carbon and boron (C, Si, B) ₃ B ₁₂ was formed in the material. Furthermore, after embedding and polishing the obtained siliconaceous powder, about 10 pieces were analyzed by EPMA. As a result, the compound composed of silicon, carbon and boron is very finely precipitated in the silicon particles, and the volume of the silicon, carbon and boron compound particles precipitated in the silicon particles is the volume of the silicon particles. Of about 1/200 or less.

The silicon powder was evaluated for electrodes in the same manner as in Example 1. As a result, the silicon powder had an initial charge capacity of 2630 mAh / g and an initial discharge capacity of 20.
30 mAh / g, indicating a high discharge capacity. In addition,
The initial capacity loss is 600 mAh / g.
Of the initial capacity loss of mAh / g, about 330 mAh / g is derived from carbon black added as a conductive agent and polytetrafluoroethylene powder added as a binder.
It was estimated to be 0 mAh / g. Further, almost no capacity loss was observed after the second time. Also, in the second and subsequent charging and discharging, the discharge capacity was hardly changed, and excellent cycle characteristics were exhibited.

Example 3 A silicon powder (purity 99%, d ₅₀ = 25 μm) was combined with a carbon powder (purity 99%, d ₅₀ = 14 μm) by 10% by mass conversion.
Boron powder (purity 99%, d ₅₀ = 25 μm) was added in an amount of 40% in terms of mass, mixed well using a kneader, and then melted at about 1700 ° C. or more to form a uniform melt. Mist was blown out from the nozzle into an argon gas atmosphere and rapidly cooled to obtain a silicon powder. The obtained powder was heat-treated at 1050 ° C. in an argon gas for 70 hours, then rapidly cooled to 600 ° C. at a rate of about 15 ° C./min, and then cooled to about room temperature at a rate of about 5 ° C./min. The heat-treated product thus obtained was crushed by an impeller mill, and the particle size was adjusted using an air classifier to obtain a powder having a 50% cumulative diameter (d ₅₀ ) of 21 μm.

The specific surface area of the powder was 0.7 m ² / g. As a result of X-ray diffraction measurement of this material, it was found that a compound of silicon, carbon and boron (C, Si, B) ₃ B ₁₂ was formed in the material. Furthermore, after embedding and polishing the obtained siliconaceous powder, about 10 pieces were analyzed by EPMA. As a result, the compound composed of silicon, carbon and boron is very finely precipitated in the silicon particles, and the volume of the silicon, carbon and boron compound particles precipitated in the silicon particles is the volume of the silicon particles. Of about 1/200 or less.

The silicon powder was subjected to electrode evaluation in the same manner as in Example 1. As a result, the silicon powder had an initial charge capacity of 1960 mAh / g and an initial discharge capacity of 14
40 mAh / g, indicating a high discharge capacity. In addition,
The initial capacity loss is 520 mAh / g.
Of the initial capacity loss of mAh / g, about 330 mAh / g is derived from carbon black added as a conductive agent and polytetrafluoroethylene powder added as a binder.
It was estimated to be 0 mAh / g. Further, almost no capacity loss was observed after the second time. Also, in the second and subsequent charging and discharging, the discharge capacity was hardly changed, and excellent cycle characteristics were exhibited.

Example 4 50 parts by mass of the silicon powder obtained in Example 1 and an average particle size of 15
50 parts by mass of natural graphite having a particle diameter of 50 μm were mixed to obtain a negative electrode powder for a lithium ion secondary battery.

The evaluation of the obtained powdery electrode was performed by adding 70% by mass of carbon black as a conductive agent and 10% by mass of polytetrafluoroethylene powder as a binder to 70% by mass of the powder, and kneading the mixture. 90
10% by mass of a polytetrafluoroethylene powder as a binder was added to the mass% and kneaded (without adding a conductive agent in particular), and otherwise the same method as in Example 1 was used. As a result, this silicon powder has an initial charge capacity of 1040 mAh / g and an initial discharge capacity of 950 mAh / g.
g, indicating high discharge capacity and high initial efficiency. Further, almost no capacity loss was observed after the second time.
Also, in the second and subsequent charging and discharging, the discharge capacity was hardly changed, and excellent cycle characteristics were exhibited.

Comparative Example 1 Silicon powder (purity 99%, d ₅₀ = 25 μm) was used as a negative electrode active material for a lithium secondary battery. This material was subjected to electrode evaluation under the same conditions as in Example 1. As a result, although the initial discharge capacity of this silicon powder was large at 2100 mAh / g, the initial charge capacity was 3730 mAh / g, and the initial capacity loss due to carbon black added as a conductive agent and polytetrafluoroethylene powder added as a binder was increased. The initial capacity loss of the siliconaceous powder from which the
It was as large as 00 mAh / g. By repeating the charge and discharge further, the capacity loss is 800 mAh /
g, then gradually decreased and finally reached 0 at the 7th time
mAh / g. For this reason, the total capacity loss was very large. In addition, the discharge capacity rapidly decreased with the progress of the cycle and dropped to 200 mAh / g at the tenth time, indicating that it was not practically usable as a negative electrode active material for a lithium ion secondary battery.

[0055]

The negative active material for a lithium ion secondary battery according to the present invention provides a material having a discharge capacity exceeding the theoretical value of graphite, a high initial efficiency and a high cycle characteristic. This also contributes to the weight reduction and size reduction of the lithium ion secondary battery. In addition, the lithium ion secondary battery of the present invention has a high energy density,
An object of the present invention is to provide a battery having high cycle characteristics.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Tsutomu Sugiura 20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel Corporation Technology Development Division (72) Inventor Taro Kawano 2-6-3 Otemachi, Chiyoda-ku, Tokyo F-term in Nippon Steel Corporation (reference) 5H029 AJ03 AJ05 AK02 AK03 AK04 AL01 AL06 AM03 AM04 AM05 AM07 5H050 AA07 AA08 BA17 CA02 CA08 CA09 CA11 CB01 CB07 DA10 EA08 FA17

Claims (4)

[Claims] 1. A negative electrode active material for a lithium ion secondary battery, wherein the negative electrode active material comprises silicon particles in which a compound comprising silicon, carbon and boron is finely dispersed in at least particles. 2. The negative electrode for a lithium ion secondary battery according to claim 1, wherein said compound comprising silicon, carbon and boron is mainly represented by (C, Si, B) ₃ B _12. Active material. 3. The negative electrode active material for a lithium ion secondary battery according to claim 1, wherein the negative electrode active material further contains carbonaceous particles. 4. A lithium ion secondary battery containing a positive electrode active material, a negative electrode active material and a non-aqueous electrolyte, wherein the negative electrode active material is used for a lithium ion secondary battery according to claim 1. A lithium ion secondary battery, which is a negative electrode active material.