JP2000012020A - Carbon material for lithium secondary battery negative electrode - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a powdery carbon material, which is high in discharge capacity, and excellent in initial charging/discharging efficiency and cycle characteristics, by specifying the 10% cumulative diameter (d10) of a graphitized carbon powder containing boron and nitrogen. SOLUTION: In a graphitized powder containing boron and nitrogen, a 10% cumulative diameter (d10) of the carbon powder is set 5 to 25 μm. Preferably, the carbon powder contains boron of 0.1 to 10% in terms of atomic ratio and nitrogen of 10% or less in terms of atomic ratio, and a spacing (d002) of a carbon mesh surface layer by an X-ray wide-angle diffraction method satisfies d002<=0.337 nm, and the size (Lc) of crystallite in a C-axis direction satisfies Lc>=40 nm. More preferably, surfaces of the powder satisfy C(N)/(C(B)+C(C)+C (N))<0.3, where the concentration of boron atoms, the concentration of carbon atoms, and the concentration of nitrogen atoms on the surfaces measured by a photoelectric spectrophotometer are C(B), C(C), and C(N), respectively, and boron nitride is used as a compound coating the surfaces.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a material for a negative electrode of a lithium secondary battery. More specifically, the present invention relates to a graphitized carbon material for a negative electrode of a lithium secondary battery, which can be industrially manufactured in large quantities, has a large discharge capacity, and has a small capacity loss during charge and discharge.

[0002]

2. Description of the Related Art Since lithium secondary batteries have a high energy density, they are used as power sources for mobile communication and portable information terminals, and the market is rapidly expanding with the spread of terminals. However, the performance improvement of the battery occupying a large part of the volume, which is a characteristic of the terminal device, for further reduction in size and weight is being promoted.

The negative electrode material currently used for such secondary batteries is a carbon material, which is a key material that affects battery performance. Among the carbon materials, it is known that natural graphite, carbon fiber, and mesoface microspheres exhibit high performance as a negative electrode material for a lithium secondary battery by itself or simply by heat treatment at a high temperature (for example, Carb
on, 13, 337 (1975), JP-A-64-2258, J.
Electrochem. Soc., 142, 2564 (1995), 34th Battery Symposium 3A07). However, natural graphite has poor load characteristics due to excessive advancement of the graphite crystal in the preferred orientation, and carbon fiber and mesoface spheroids have a problem in production cost as easily estimated from the production method.

As a candidate for a graphite-based negative electrode material which has high performance and can be mass-produced at low cost, boron-doped graphite materials are disclosed in JP-A-8-31422 and JP-A-5-2908.
No. 43, and the like. By firing at a practical heat treatment temperature level in the coexistence with boron, a carbon material having highly developed graphite crystals can be obtained as compared with a case where carbon powder as a raw material is fired alone. The highly graphitized carbon material thus obtained has high electrode performance (discharge capacity, initial efficiency, cycle characteristics, etc.) as a negative electrode for a lithium secondary battery.

[0005] The present inventors have succeeded in producing such a graphitized carbon material by adding boron using an experimental furnace and finding out the effect thereof. Was. That is, a boron compound is added to a large amount of carbon powder as a raw material, and an Acheson furnace (an indirect current heating method in which current is applied to the filling powder) and LW
Production of a high-performance carbon material for a lithium secondary battery was performed using a G furnace (direct current heating method for a crucible). However, even though the carbon powder produced in large quantities exhibited a sufficient graphitizing catalytic effect as in the case of the experimental furnace, it was found that the irreversible capacity was particularly large when it was incorporated into the battery. .

[0006] As a result of investigating the cause, it was confirmed that insulating boron nitride was formed on the surface of a large amount of carbon powder. Boron nitride itself is considered to be electrochemically inert, but when measured, no lithium intercalation reaction occurs, but an irreversible capacity corresponding to the decomposition of the solvent is observed on the surface during reduction. Was done. Therefore, it was presumed that the irreversible capacity of the carbon powder produced in large quantities was increased due to the side reaction of boron nitride present on the surface.

[0007] Since the industrial firing furnace is operated in an open system,
The furnace atmosphere is filled with carbon monoxide and nitrogen in a temperature range where graphitization of the carbon powder occurs. In the temperature range, nitrogen in the atmosphere reacts with boron added as a graphitization catalyst to produce boron nitride (Physical Dictionary, Third Edition, 833).
page). Therefore, it is considered that the carbon powder mass-produced inevitably produces boron nitride on its surface. Further, since the heat capacity of an industrial firing furnace is very large, a long time is required for heating and cooling. Therefore, in the mass production of carbon powder in an industrial firing furnace, the time in which nitrogen in the atmosphere and boron as a catalyst are in a temperature range in which boron nitride is generated is considerably long, so that the boron nitride generation reaction proceeds sufficiently. Would.

[0008] From the above-mentioned knowledge, it is necessary to control the gas atmosphere in the sintering furnace and control the heating and cooling rates in order to sinter a large amount of carbon powder using an industrial sintering furnace while preventing the formation of boron nitride. However, its implementation requires enormous costs and is practically difficult. Therefore, simply firing a large amount in an industrial firing furnace is disclosed in JP-A-8-31422,
It has not been possible to obtain a material exhibiting the same electrode performance as the boron graphitized carbon material disclosed in JP-A-5-290843 and the like.

[0009]

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems that occur in carbon powder produced by heat-treating a boron compound and carbon powder in an industrial sintering furnace, that is, inevitably, the powder surface is inevitable. Mass production of graphitized carbon powder for negative electrodes of lithium secondary batteries that solves the problem of irreversible capacity increasing due to side reactions occurring on the surface of boron nitride formed while maintaining the highly developed graphite crystal structure inside the material It is to provide a method.

[0010]

Means for Solving the Problems The inventors of the present invention have eagerly studied a boron-added graphitized carbon powder having boron nitride produced in an industrial furnace formed on its surface, based on the electrochemical characteristics of boron nitride itself. As a result of the investigation, it has been found that the irreversible capacity is largely improved by removing the fine powder portion of the graphitized material and defining the particle size distribution in a certain range in order to reduce the area as boron nitride. The present invention has been completed based on such findings.

That is, the graphitized carbon powder containing boron and nitrogen according to the present invention is characterized in that the carbonaceous powder has a 10% cumulative diameter (d ₁₀ ) of 5 to 25 μm. Preferably, the carbon powder contains boron in an atomic ratio of 0.1 to 10% and nitrogen in an atomic ratio of 10% or less, and the plane spacing (d ₀₀₂₎ of the carbon mesh layer in the X-ray wide-angle diffraction method. ) And the crystallite size (Lc) in the C-axis direction is d ₀₀₂ ≦ 0.337 nm and Lc ≧ 40 nm. More preferably, when the surface of the powder has a boron atom concentration, a carbon atom concentration, and a nitrogen atom concentration measured by photoelectron spectroscopy as C (B), C (C), and C (N), respectively, C ( N) / (C (B) + C (C) + C (N)) <0.3, and the compound covering the surface is boron nitride.

[0012]

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

According to the present invention, carbon powder produced by heat-treating a boron compound and carbon powder in an industrial firing furnace is highly developed by the same graphitizing catalytic effect of boron as carbon powder produced in a research furnace. It is possible to exhibit high discharge capacity and high initial charge / discharge efficiency derived from the obtained graphite crystal structure.

That is, according to the present invention, a carbon powder produced by heat-treating a boron compound and carbon powder in an industrial sintering furnace retains high graphite crystallinity which cannot be obtained by ordinary heat treatment by adding boron, and Reduce the effect of boron nitride by defining the particle size distribution of the material finally manufactured through pulverization and classification operations after graphitization, with the aim of becoming a material with a small area of boron nitride generated on the surface It focuses on that.

Although it is not clear about the irreversible reaction occurring on the surface of boron nitride, it has a crystal structure almost identical to that of graphite, and its charge-discharge curve in a non-aqueous electrolyte has a shoulder portion which appears especially during initial reduction. Considering that the potential is close to the potential corresponding to the decomposition of the electrolyte when graphite is used as the electrode, and there is no capacity to be taken out during discharge, co-insertion of solvated ions between layers at the graphite electrode triggers the solvent Was presumed to have undergone reductive decomposition.

The reason why the influence of boron nitride on the electrode performance is reduced by defining the specific surface area of the final product in the present invention is as follows. That is, the graphitized carbon powder obtained by the graphitization treatment is in a strongly sintered state through the boron carbide generated partially between the powder particles, so that it is necessary to adjust the particle size as a product by grinding or the like. . It is considered that some or most of the boron nitride on the surface of the graphitized carbon powder is mechanically peeled off by pulverization or the like at the time of adjusting the particle size, depending on the conditions. The fine powder containing a large amount of finely pulverized boron nitride generated in the process is removed by a classification operation so that the 10% cumulative diameter (d ₁₀ ) of the graphitized carbonaceous powder becomes 5 to 25 μm. The amount and area of boron nitride on the surface of the material can be considerably reduced. For example, if the 10% cumulative diameter is less than 5 μm, for example, if only the pulverizing operation is performed, or if the subsequent classification operation is not sufficient, pulverized pieces of boron nitride generated by the pulverization remain contained and the boron nitride surface However, the initial efficiency is greatly deteriorated, which is a serious problem in practical use. When the 10% cumulative diameter exceeds 25 μm, the final electrode thickness of 1
Not only is it difficult to reduce the thickness to about 00 μm, but also the contact area between the particles and the current collector is so small that the particles can be easily separated from the current collector.

The particle size distribution of the graphitized carbon powder can be adjusted by a method generally used in industry. For example, ball mills, pin mills, disk mills, impeller mills, jet mills, roller mills, stamp mills, cutting mills, etc. are preferably used for pulverization, and air classifiers, sieves, etc. are preferably used for classification, but are not particularly limited thereto. Not something.

As a result of examining the contents of boron and nitrogen contained in the graphitized carbon powder after firing, in order to obtain a high discharge capacity and a high initial efficiency, the content of boron in the material should be 0 in atomic ratio. It has been found that the content of nitrogen is preferably not less than 0.1% and not more than 10%, and the content of nitrogen is preferably not more than 10% in atomic ratio. When the boron content exceeds 10%, boron exceeding the solid solubility limit of boron in graphite remains in the graphitized product as boron carbide, but is inactive electrochemically, and accordingly, the amount of discharge increases. The capacity will decrease. In addition, boron content 0.1
% Of the graphitized powder, the catalytic effect of the added boron is not sufficiently exhibited, and is substantially the same as that of a normal heat-treated product, so that various physical properties required for a battery are not improved. On the other hand, in the case of a material having a nitrogen content of more than 10%, the electric resistivity of the material itself increases, and the overvoltage at the time of charging and discharging increases, so that the doping / dedoping amount of lithium cannot be increased and the discharge capacity is greatly reduced. Resulting in.

Regarding the degree of graphitization, which is an index of the degree of development of the graphite structure, as parameters by the X-ray diffraction method for defining the carbonaceous material, the plane spacing (d ₀₀₂ ) of the carbon mesh layer and the C-axis direction of the crystallite The size (Lc) is d ₀₀₂ ≦ 0.337
It was found that it was necessary to satisfy nm and Lc ≧ 40 nm.

When d ₀₀₂ exceeds 0.337 nm and L
When c is less than 40 nm, the degree of development of the graphite structure is low, so that the doping amount of lithium becomes small and a high discharge capacity cannot be obtained.

The surface chemical species of the boron-added graphitized carbon material produced in the industrial furnace is boron nitride, and the boron atom concentration, carbon atom concentration, and nitrogen atom concentration on the surface measured by photoelectron spectroscopy are represented by C, respectively. (B), C (C), C
It was found that it was necessary to satisfy C (N) / (C (B) + C (C) + C (N)) <0.3 when (N) was set. C (N)
When / (C (B) + C (C) + C (N)) is 0.3 or more, the adverse effect of boron nitride on the electrode performance reaches a nonnegligible level, and in particular, side reactions on the boron nitride surface Progress and it is not possible to obtain a practically high initial efficiency. The reason why the surface species was identified as boron nitride is that the peak position derived from the 1s orbital of the nitrogen atom measured by photoelectron spectroscopy and the peak position derived from the 1s orbital of the boron atom were the respective peaks measured for boron nitride alone. That is, they correspond to the positions, and the ratio of the nitrogen atom concentration to the boron atom concentration determined from the peak area is approximately 1: 1.

The carbon powder as a raw material used in the present invention is a material that easily forms a graphite structure (regular arrangement regularity of graphite layers) as a highly graphitized carbon powder for a negative electrode of a lithium secondary battery. Carbon fiber as raw material,
Examples include pitch coke and mesoface small spheres, but are not particularly limited to these. Also,
When pitch is used as a raw material of the carbon powder, there is no particular restriction on the pitch used, but it is easy for graphite crystallinity to develop by firing, so-called graphitization (easy graphitization). Essentially important, for example, petroleum pitch, asphalt pitch, coal tar pitch, naphthalene pitch, crude cracking pitch, petroleum sludge pitch, pitch obtained by pyrolysis of a high-molecular polymer, etc. Further, these pitches may be subjected to a hydrogenation treatment or the like.

The average particle size of the carbon powder as a raw material is 50 μm.
It is desirable that: This is because when forming an electrode using carbon powder as a negative electrode material for a lithium secondary battery, it is necessary to form a thin film having a thickness of about 100 μm on the current collector. Adjustment of the particle size distribution of the raw material carbon powder can be performed by a method generally used in industry, similarly to the preparation of the particle size distribution of the graphitized carbon powder.

With regard to the molding of the graphitized carbon powder provided by the present invention, it is possible to mold the powdered battery active material used for a lithium battery by a method generally used,
The performance of the carbonaceous powder is sufficiently extracted, and the shapeability of the powder is high, and it is not limited to this as long as it is chemically and electrochemically stable. There is a method in which a powder or dispersion solution of a fluororesin such as polytetrafluoroethylene is added, followed by mixing and kneading. In addition, polyethylene,
There is also a method of adding a resin powder such as polyvinyl alcohol, inserting the dry mixture into a mold, and molding by hot pressing. Further, a carbonaceous powder is mixed with a fluorine-based resin powder such as polyvinylidene fluoride or a water-soluble binder such as carboxymethyl cellulose as a binder, and mixed with a solvent such as N-methylpyrrolidone, dimethylformamide or water or alcohol. Can be formed by applying a slurry on a current collector and drying the slurry.

The carbon material of the present invention can be used in an appropriate combination with a positive electrode active material and an organic solvent-based electrolyte. These organic solvent-based electrolytes and the positive electrode active material are usually used in lithium secondary batteries. This is not particularly limited as long as it is possible.

As the positive electrode active material, for example, a lithium-containing transition metal oxide LiM (1) _x O ₂ (where X is 0 ≦ X
≦ 1 where M (1) represents a transition metal, Co, Ni, Mn, Cr, Ti, V, Fe, Zn, A
l, In, Sn) or L
iM (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 and Ni , Mn, Cr, Ti, V, Fe, Z
n, B, Al, In, Sn), transition metal chalcogenides (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 compounds, the general formula M _x
Mo ₆ Ch _8-y (where X is 0 ≦ X ≦ 4, Y is 0 ≦ Y ≦ 1
Wherein M represents a metal such as a transition metal, and Ch represents a chalcogen element), a chevrel phase compound represented by the following formula, activated carbon, 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 , L
iCH ₃ SO ₃ , Li (CF ₃ SO ₂ ) ₂ N, Li (C
F 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 OSO ₂ ) ₂ N, Li ((C
F ₃ ) ₂ CHOSO ₂ ) ₂ N, LiB [C ₆ H ₃ (CF
_{3) 2]} can be exemplified one or two or more thereof, such as _4.

Hereinafter, various physical property expression methods used for defining the carbon material for a negative electrode of a lithium secondary battery according to the present invention, and
The measurement method will be described.

(1) d ₀₀₂ , Lc Monochromatic X-rays are collimated into a parallel beam and irradiated to a carbon powder to which high-purity silicon is added as an internal standard.
The peak corresponding to the 02 plane is measured. By correcting the peak position and half-value width with the internal standard silicon peak as a standard, the distance d ₀₀₂ between the layer planes and the C
The size Lc in the axial direction is calculated. A more specific evaluation method is described in, for example, "Carbon Fiber", pp. 733 to p.

(2) Element concentration in the carbon powder surface region by photoelectron spectroscopy The carbon powder fixed on the sample table is irradiated with monochromatic X-rays under high vacuum, and the inner electrons of each element appearing at that time are emitted. By measuring the number and multiplying by the sensitivity coefficient of each element, the concentration of each element in the carbon powder surface region is measured. In the measurement using the carbon powder of the present invention, only boron, carbon, and nitrogen are used as measurement elements, and their atomic concentrations are standardized to be 1 when these three are added.

(3) Particle Size Distribution The particle size distribution was determined by calculating the diffraction pattern when the dispersed particles were irradiated with parallel rays (Franhofer diffraction). Usually, about 0.2 g of each sample is put in 20 cc of water as a dispersion medium, and a particle size distribution analyzer L manufactured by Seishin Enterprise Co., Ltd.
Measured by MS-24.

[0033]

EXAMPLE 1 Boron carbide was added to carbonaceous powder having an average particle size of 20 μm from coal tar pitch as a raw material in an amount of 3% by weight of boron and mixed well, and the mixture was stored in a graphite container. The temperature was raised to about 2900 ° C. in an Acheson furnace, maintained at this temperature for about 1 hour, and then gradually cooled to near room temperature. Remove the heat-treated carbon powder from the graphite container,
After pulverized by an impeller mill, fine particles are removed using an air classifier, so that 10% of the carbonaceous powder is removed.
A highly graphitized carbon powder for a negative electrode of a lithium secondary battery having a cumulative diameter (d ₁₀ ) of 10 μm was obtained. The amount of boron and the amount of nitrogen contained in the calcined powder are 2.5% and 1% in atomic ratio, respectively.
The crystal structure of the graphitized powder is d ₀₀₂ = 0.3358 nm, L
c = 80 nm. The boron atom concentration C (B), the carbon atom concentration C (C), and the nitrogen atom concentration C (N) on the surface of the carbon powder were measured by photoelectron spectroscopy as described above. ) = 0.21, C (C) = 0.5
9, C (N) = 0.20, indicating that boron nitride was present on the carbon powder surface. Also, C (N) / (C
(B) + C (C) + C (N)) was 0.20, which was almost １ ／ of the value of the carbonaceous powder after the graphitization treatment.

To the carbonaceous powder thus prepared, 5% by weight of a polytetrafluoroethylene powder as a binder was added and kneaded to prepare an electrode sheet having a thickness of about 0.1 mm, which was cut into about 15 mg (for carbon material). Approximately 14
mg), and pressed against a Ni mesh as a current collector to form a negative electrode.

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)
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 potential regulation. The potential range was 0 V to 1.0 V (based on lithium metal).
Initial charge capacity is 380 mAh / g, initial discharge capacity is 34
At 5 mAh / g, the initial charge / discharge efficiency was extremely high at just over 90%, and remained stable at almost 100% after the second cycle.
Further, even in the second and subsequent charging / discharging, the discharge capacity was hardly changed, and excellent cycle characteristics were exhibited.

Example 2 A spherical carbon powder having an average particle diameter of 18 μm prepared by using a mesoface pitch obtained from coal tar pitch as a raw material was mixed with 90
After carbonization at 0 ° C., light grinding was performed with an impeller mill, and boric acid was added to the carbon powder in an amount of 2% by weight of boron.
After the addition and thorough mixing, the mixture was stored in a graphite container, heated to about 2900 ° C. in an Acheson furnace, kept at this temperature for about 1 hour, and then gradually cooled to near room temperature. The heat-treated carbon powder is taken out of the graphite container, crushed by a jet mill, and fine particles are removed using an air classifier.
% Graphitized carbon powder for a negative electrode of a lithium secondary battery having a cumulative diameter (d ₁₀ ) of 6 μm was obtained. The amount of boron and the amount of nitrogen contained in the calcined powder were 1.6% and 0.8% in atomic ratio, respectively, and the crystal structure of the graphitized powder was d ₀₀₂ = 0.3362n.
m, Lc = 50 nm. The boron atom concentration C (B), the carbon atom concentration C (C), and the nitrogen atom concentration C (N) on the surface of the carbon powder were measured by photoelectron spectroscopy as described above. ) = 0.28, C (C) =
0.44 and C (N) = 0.28, indicating that boron nitride was present on the surface of the carbon powder. Also C (N)
/ (C (B) + C (C) + C (N)) was 0.26.

The carbonaceous powder thus prepared was evaluated for electrode characteristics in the same manner as in Example 1. As a result, the initial charge capacity was 385 mAh / g, the initial discharge capacity was 330 mAh / g, the initial charge / discharge efficiency was as high as over 85%,
After the second cycle, it was stable at almost 100%. Further, even in the second and subsequent charging / discharging, the discharge capacity was hardly changed, and excellent cycle characteristics were exhibited.

Example 3 Boron oxide was added to carbonaceous powder having a mean particle size of 30 μm from petroleum pitch as a raw material in an amount of 3% by weight of boron and mixed well. The mixture was stored in a graphite container, The temperature was raised to about 2900 ° C. in a furnace, maintained at this temperature for about 1 hour, and then gradually cooled to near room temperature. The heat-treated carbon powder is taken out of the graphite container, crushed by an impeller mill, and fine particles are removed by using an air classifier to obtain a 10% cumulative diameter (d
₁₀ ) to obtain a highly graphitized carbon powder for a negative electrode of a lithium secondary battery having a thickness of 20 μm. The amount of boron and the amount of nitrogen contained in the calcined powder are 2.5% and 1.5% in atomic ratio, respectively, and the crystal structure of the graphitized powder is d ₀₀₂ = 0.3355 nm, Lc = 10
It was 0 nm. Boron atom concentration C (B), carbon atom concentration C (C), nitrogen atom concentration C on the surface of this carbon powder
(N) was measured by photoelectron spectroscopy in the manner described above. As a result, C (B) = 0.11, C (C) = 0.79,
C (N) = 0.10, indicating that much boron nitride was present on the carbon powder surface. Also, C (N) /
(C (B) + C (C) + C (N)) is 0.10.
The value was approximately 1/5 of the value of the carbonaceous powder after the graphitization treatment.

The carbonaceous powder thus prepared was evaluated for electrode characteristics in the same manner as in Example 1. As a result, the initial charge capacity was 388 mAh / g, the initial discharge capacity was 348 mAh / g, and the initial charge / discharge efficiency was as high as about 90%, and was stable at about 100% after the second cycle.
Further, even in the second and subsequent charging / discharging, the discharge capacity was hardly changed, and excellent cycle characteristics were exhibited.

Comparative Example 1 The same operation was performed using the same carbonaceous powder as in Example 1 to graphitize the graphitized carbonaceous powder.

The carbon powder after the heat treatment is taken out of the graphite container, and is only crushed by an impeller mill, whereby the carbonaceous powder has a 10% cumulative diameter (d ₁₀ ) of 2 μm and has a high graphitization for a negative electrode of a lithium secondary battery. A carbon powder was obtained. The amount of boron and the amount of nitrogen contained in the calcined powder are 2.8% and 2%, respectively, in atomic ratio, and the crystal structure of the graphitized powder is d _002.
= 0.3359 nm and Lc = 70 nm. The boron atom concentration C (B), the carbon atom concentration C (C), and the nitrogen atom concentration C (N) on the surface of the carbon powder were measured by photoelectron spectroscopy as described above. ) = 0.
44, C (C) = 0.11 and C (N) = 0.45, indicating that a great amount of boron nitride was present on the surface of the carbon powder. Also, C (N) / (C (B) + C
((C) + C (N)) was 0.45.

The electrode properties of the carbonaceous powder thus prepared were evaluated in the same manner as in Example 1. As a result, while the initial charge capacity is as large as 452 mAh / g, the boron nitride on the particle surface is insulative and the overvoltage of the electrode is large, so that the lithium inserted in the material is not sufficiently released, so that the initial discharge capacity is 308 mAh / g. And the initial charge / discharge efficiency was as very low as about 68%. In addition, the discharge capacity increased with the repetition of the cycle thereafter and recovered to 322 mAh / g at the fifth cycle. However, since the irreversible capacity was large and a large amount of passive film was formed on the electrode surface,
Despite having a high degree of graphitization, it did not show a large discharge capacity. In addition, the charge / discharge efficiency also changes at 95 to 98% after the second cycle, and finally reaches 10% at the sixth cycle.
Reached 0%. In addition, the discharge capacity showed a tendency to decrease again after the fifth charge and discharge, and did not show high electrode performance, such as being reduced to about 70% of the fifth cycle at the 100th cycle.

[0043]

As is clear from the above description, the carbon anode material for a lithium secondary battery of the present invention is produced when a boron compound is added to carbon powder as a raw material and heat-treated at a high temperature in a large amount. Solving the problem of irreversible capacity increase due to the formation of boron nitride on the carbon powder surface, high discharge capacity,
A powdered carbon material having excellent initial charge / discharge efficiency and cycle characteristics can be provided.

Continued on the front page (72) Inventor Ken Hamada 3-35-1 Ida, Nakahara-ku, Kawasaki City, Kanagawa Prefecture New Nippon Steel Corporation Technology Development Division (72) Inventor Tsutomu Sugiura 3-35 Ida, Nakahara-ku, Kawasaki City, Kanagawa Prefecture -1 New Nippon Steel Corporation Technology Development Division F term (reference) 4G046 CA07 CB02 CB09 5H003 AA02 AA04 BB01 BB02 BC01 BC05 BC06 BD00 BD02 BD03 5H014 AA02 CC07 EE05 EE08 HH00 HH01 5H029 AJ03 AJ05 AK02 AM01 AL01 AM01 AM03 AM04 AM05 AM07 DJ12 DJ16 DJ17 HJ00 HJ01 HJ05

Claims (4)

[Claims] 1. A negative electrode material for a lithium secondary battery, comprising a graphitized carbon powder containing boron and nitrogen, wherein the carbonaceous powder has a 10% cumulative diameter (d ₁₀ ) of 5 to 25 μm. . 2. A graphitized carbon powder containing boron and nitrogen, comprising boron in an atomic ratio of 0.1 to 10% and nitrogen in an atomic ratio of 10% or less. Item 7. The negative electrode material for a lithium secondary battery according to Item 1. 3. A graphitized carbon powder containing boron and nitrogen, wherein a plane distance (d ₀₀₂ ) of a carbon netting layer and a size of a crystallite in a C-axis direction (Lc) in a wide angle X-ray diffraction method.
2. The negative electrode material for a lithium secondary battery according to claim 1, wherein d satisfies d ₀₀₂ ≦ 0.337 nm and Lc ≧ 40 nm. 4. The surface of a graphitized carbon powder containing boron and nitrogen, wherein the surface has a boron atom concentration, a carbon atom concentration, and a nitrogen atom concentration measured by photoelectron spectroscopy, respectively, of C (B),
When C (C) and C (N) are used, the compound satisfying C (N) / (C (B) + C (C) + C (N)) <0.3 and the compound covering the surface is boron nitride The carbon material for a negative electrode of a lithium secondary battery according to claim 1, wherein: