JPH10223223A - Anode material for lithium secondary battery and manufacture thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a material which is suitable for an anode material for a lithium secondary battery, has high discharge capacity, high charging and discharging electric efficiency from cycle starting stages, a good cydic property, and load property, and moreover to provide high sieving yield ratio of graphitized powder and lower the production cost as a whole at the time of producing the material. SOLUTION: In graphtized powder produced by thermal treatment of a carbon powder using pitch as a raw material, the powder has plane distance (d002 ) of laminated carbon net planes by X-ray wide angle diffraction method, d002 <=0.337nm and the size (Lc) of the crystallite Lc >=40nm and contains 0.1-10%, by atomic ratio, of boron and 0.1-10% of nitrogen. The powder is produced by adding 0.1-25wt.% (based on boron) of a boron compound and 0.1-25wt.% (based on nitrogen) of a nitrogen compound with respect to a carbon powder produced by using pitch as a raw material, mixing the resultant mixture, and heating the obtained mixture at 2500 deg.C or higher in an inert or reducing atmosphere for 0.1 hours or longer.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention
The present invention relates to a negative electrode material for a lithium secondary electrode utilizing an elimination reaction and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, lithium secondary batteries having a high energy density have been mounted as power supplies for portable electronic communication devices, and the market for lithium secondary batteries has rapidly expanded with the expansion of the market for these electronic communication devices. I have. The negative electrode material currently used for the secondary battery is a carbon material, which is a key material that affects battery performance. However, although there are carbon materials having various structures, structures, and forms, there is a great difference in electrode performance such as operating voltage during charging and discharging. Currently, graphite crystalline materials and amorphous materials are separated according to the usage environment / conditions of the electronic devices on the mounting side. However, from the viewpoint of easy control of battery voltage and energy density per volume, etc. The demand for graphite crystalline materials is expected to increase.

Investigations focused on natural graphite having an ideal graphite crystal structure have been conducted for a long time (for example, J. E.
lectrochem. Soc., 117, 222 (1970), Carbon, 13, 337 (197
5), JP-A-64-2258, etc.), however, in these crystal structures, since the carbon network layer has a preferential orientation in a specific direction, the diffusion direction of lithium in the material is limited, and Its diffusion distance is very long. Therefore, it has been confirmed that a high discharge capacity is obtained only under a very small charge / discharge current (for example, Electrochimica Acta, 38 (9), 11).
79 (1993)). However, since the current density cannot be increased from a practical point of view, it is considered that the range of use is greatly restricted. It has also been pointed out that there is a problem in the cycle characteristics of mesoface microspheres (mesocarbon microbeads) prepared by collecting the optically anisotropic phase of the mesoface pitch at the stage where the mesophase pitch is formed into a sphere (for example, The 34th Battery Symposium 3A07).

[0004] Similarly to pitch coke, pitch-based carbon fibers are graphitized in an ultra-high temperature range to have an interlayer distance of a carbon netting layer close to that of natural graphite, and the diffusion direction of lithium is from the fiber outer periphery toward the inside. Multi-directional, and
Since the diffusion distance is as short as about 5 μm, which is half of the fiber diameter from the fiber outer periphery to the fiber axis, the powder obtained by pulverizing this carbon fiber has a large diffusion coefficient compared to other graphitizable materials and is resistant to heavy loads. It is expected. In fact, it has recently been reported that good heavy load characteristics have been confirmed using pitch-based carbon fibers (J.
Electrochem. Soc., 142, 8, 2564 (1995)).
However, even if the heat treatment temperature is increased because the fiber form is maintained, the development of the crystal structure is not hindered even when the heat treatment temperature is increased, so that the discharge capacity does not increase. And the like, there is a problem that a higher production cost is required as compared with other materials.

[0005] Pitch coke belongs to the class of graphitizable materials. Graphite treatment in an ultra-high temperature range approaches the interlayer distance of a carbon netting layer close to natural graphite. Due to the structure, graphitization does not develop as much as natural graphite, and in these crystal structures, the carbon network layer does not have a preferred orientation in a specific direction. Therefore, there is no restriction on the range of use under a high current density found in natural graphite, and it is a very promising material as a negative electrode material for lithium secondary batteries, and many studies have been made so far (for example, 63-12
1257, JP-A-1-204361, JP-A-4-206276, etc.). However, the discharge capacity of the ultra-high temperature heat-treated (2000-3000 ° C. fired) product of ordinary coke is lower (<300 mAh / g) compared to the theoretical capacity (372 mAh / g), and further improvement in performance has been required. .

[0006] Recently, several studies have been made on materials having a structure similar to graphite. For example, a gas phase reaction (CV) at 900 ° C. by the vapor of boron chloride and benzene
D) Report on carbon powder synthesized by D) in which part of carbon atoms are replaced with boron (Physical Review B, Vol. 46, No.
3,1697 (1992), JP-A-7-73898), carbon powder synthesized by adding a boron compound to pitch coke and graphitizing at 2400 ° C., wherein a part of carbon atoms is replaced by boron and nitrogen atoms. Report on Japanese Patent
-290843), a report on carbon powder synthesized by adding H ₃ BO ₃ or the like to the pitch and carbonizing at 1000 ° C. (JP-A-5-251080, JP-A-5-266880); Report on carbon powder synthesized by gas phase reaction (CVD), in which carbon atoms are partially substituted with nitrogen (J. Electrochem. Soc., 14
1,900 (1994), Electrochemistry, 64 (11), 1180 (1996)).

[0007] In Japanese Patent Application Laid-Open No. H10-157, it has been proposed that the substitution of a part of the carbon six-membered ring skeleton with boron enhances the electron acceptability in the graphite plane and increases the discharge capacity by incorporating more lithium as an electron donor. It was aimed. According to the report, the crystal structure of the material obtained by the CVD method is close to the structure of the amorphous material, the interlayer distance of the graphite sheet is large (0.351 nm ≧ d ₀₀₂ ≧ 0.337 nm), and c
The crystallite size, which is an index of the size of the crystals stacked in the axial direction and the a-axis direction, is also small (30 nm>Lc> 10n).
m). Therefore, this material exhibits a charge / discharge behavior in which the battery voltage changes greatly similar to the conventional amorphous material,
Because it is a manufacturing method that is not suitable for mass production such as VD method,
It is considered that it is not necessarily suitable for a field where demand growth is expected in the future and a property close to that of a graphite crystalline material is required.

[0008] In the same manner, although the method is different, graphite is converted into a compound (BC ₃ , BC ₃ N) in which a part of carbon atoms forming a graphite skeleton is replaced by a boron atom or a nitrogen atom. The purpose is to increase the in-plane electron acceptability and improve the lithium doping amount (discharge capacity). Do you expect to replace some of the carbon atoms that form the graphite skeleton with boron atoms (boron substitution), or to insert boron compounds between the layers of the carbon netting layer (boron insertion) in the same way as described above? Although it is not clear, it seems that boron insertion aims at increasing the doping amount (discharge capacity) of lithium by expanding the graphite layer by the same mechanism as that of boron substitution. However, since the electrical properties of the carbon powder obtained above are generally semiconductive, the overvoltage during charging and discharging increases, and in a practical charging and discharging voltage region, lithium inserted into the carbon material is efficiently extracted. And high initial efficiency cannot be obtained. Furthermore, since this semiconductor property causes a large IR drop at the time of charge and discharge, there is a problem that a high discharge capacity cannot be obtained.

The crystal structure of the carbon powder obtained in step (1) is similar to that of the amorphous material, and the interlayer distance of the graphite sheet is large (d ₀₀₂ ≧ 0.337 n).
m) The crystallite size, which is an index of the size of the stack of crystals in the c-axis direction and the a-axis direction, is extremely small. Therefore, the charge and discharge curve of the material is very similar to the conventional amorphous material, the battery voltage is greatly changed, and the manufacturing method is not suitable for mass production such as the CVD method. In addition, it is considered that it is not necessarily suitable for a field in which characteristics close to those of a graphite crystalline material are required.

The present inventors have conducted extensive studies on the electrode characteristics of a wide range of carbonaceous powders from the viewpoint of the crystal structure.
It was found that the higher the degree of graphitization, the greater the discharge capacity, and reported (Electrochemical and Industrial Physical Chemistry, 61 (2), 1
383 (1993)). However, in a graphitized product obtained by heat-treating a series of graphitizable carbon materials to a normal temperature of about 3000 ° C., a crystal structure cannot be developed beyond a certain graphitization degree, and there is a limit in increasing the discharge capacity. . Therefore, in order to further improve the performance, it is essentially the most important to develop a material having a higher degree of graphitization by a method other than ordinary ultra-high temperature heat treatment.

As a method for obtaining a carbon powder having a higher degree of graphitization than a carbon powder prepared by ordinary ultra-high temperature heat treatment,
There are physical methods such as heat treatment under tension in the production of high-grade carbon fiber and treatment under stress of pyrolytic carbon, and chemical methods using a graphitization catalyst. Of these, graphitization promotion by the graphitization catalyst refers to improving the degree of graphitization of carbonaceous material having low crystallinity by a catalytic action of a metal or an inorganic compound.

The mechanism of action of the graphitization catalyst has been reported so far (for example, see Ber. Deut. Keram. Ges., 45, 224 (196).
8)) two mechanisms are considered. One is a so-called "dissolution-reprecipitation" mechanism, in which a catalyst dissolves carbonaceous material having a lower degree of graphitization, and moves through carbon while precipitating graphite. The other is called the "carbide formation-decomposition" mechanism, in which carbonaceous material with a low degree of graphitization reacts with the catalyst to form carbides, which are further decomposed at higher temperatures to form graphite. is there.

That is, the essence of the function of adding the graphitization catalyst to the carbon material is to promote the development of the graphite structure and the structure during firing in the presence of the catalyst. Upon calcination, the graphitization catalyst dissolves in the graphite crystal, which has the effect of removing the crystal distortion, and as a result, the graphite structure is said to be developed (carbon, 102 (1980) 118).

Research on improving the degree of graphitization of carbonaceous powders using a graphitization catalyst having an effect of promoting graphitization has been carried out for a long time (for example, see US Patent, 568323 (1896)). ), Carbon, 41, 18 (196
5), Carbon, 3,387 (1966), Carbon, 7,185 (1969), Journal of the Ceramic Society of Japan, 86 (12), 56 (1978), etc.). However, all of these methods focus on manufacturing methods aimed at reducing costs to improve the degree of graphitization at as low a temperature as possible and achieve the same effect as high-temperature processing, or densely molded products using a graphitization catalyst. It has been limited to ones that focus on improving mechanical strength by utilizing chemical conversion. On the other hand, the use of a carbonaceous powder whose degree of graphitization has been improved by a graphitization catalyst as a functional material such as lithium insertion-desorption reaction in a negative electrode of a lithium secondary battery has been studied. 8-
There is 31422 gazette. However, the publication does not utilize a nitrogen compound in the progress of the graphitization reaction, and the electrode performance of the material presented therein is a load characteristic under conditions similar to those actually used as a battery. Was not enough and further improvement was needed. In the case of firing under the same conditions, the production yield of the final product was not high because the grains became coarse after firing.

[0015]

SUMMARY OF THE INVENTION In view of the above problems, the present invention provides a lithium secondary battery having a large discharge capacity, high charge / discharge efficiency from the initial stage of the cycle, and excellent load characteristics with excellent cycle characteristics. The object of the present invention is to provide a negative electrode material for use, and to provide an industrially superior production method capable of reducing the overall production cost with a high classification yield of graphitized powder in producing the material. It is.

[0016]

The present inventors have conducted intensive studies on the graphitizing catalytic effect of a series of boron compounds. As a result, by further adding a nitrogen compound, the electrode characteristics of the calcined graphitized powder, particularly the It has been found that the properties are improved. Furthermore, when the boron compound and the nitrogen compound are used, the particle size of the powder after firing does not become coarse compared to the case of using the boron compound alone. Can be omitted. As a result, the manufacturing efficiency of the final product is high, which leads to simplification of the process.
The present invention has been completed based on such findings.

That is, the graphitized carbon powder according to the present invention is a graphitized powder prepared by heat-treating a carbon powder using pitch as a raw material, and has a plane spacing (d ₀₀₂ ) of a carbon mesh plane layer in the X-ray wide-angle diffraction method. And the size (Lc) of the crystallite in the c-axis direction is d ₀₀₂ ≦ 0.337 nm, Lc ≧ 40 nm, and boron is 0.1 to 10% in atomic ratio, and nitrogen is 0.1 to 0.1 in atomic ratio. It is characterized by containing 10%.

The method for producing a carbon negative electrode material for a lithium secondary battery according to the present invention is characterized in that the weight ratio of the boron compound to the carbon powder from the pitch is 0.1 to 25 in terms of boron.
% And a nitrogen compound in a weight ratio of 0.1 to 25 in terms of nitrogen.
%, Mixed and mixed under an inert or reducing atmosphere.
The heat treatment is performed at a temperature of 00 ° C. or more for 0.1 hour or more.

[0019]

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

According to the present invention, a carbonaceous powder belonging to the class of graphitizable carbon materials is heat-treated together with a boron compound and a nitrogen compound, whereby the compound to be added promotes the graphitization of the carbon material, In addition to high discharge capacity and high initial charge / discharge efficiency due to the development of a crystal structure so that the powder cannot be obtained by ordinary heat treatment, it is possible to exhibit excellent load characteristics due to the presence of nitrogen in the material. Is what you do. In addition, since the powder after firing does not become coarse due to the physical properties of the compound to be added, the final product can be manufactured with a high classification yield.

That is, the present invention provides a graphitization degree higher than that obtained by ordinary graphitization of a carbon powder made from pitch belonging to the class of graphitizable materials which is promising as a carbon anode material for lithium secondary batteries. For the purpose of obtaining a material containing boron and nitrogen in a certain range, the present invention focuses on applying a boron compound and a nitrogen compound having a graphitization catalytic action. Therefore, in the present invention, it is essentially important that the graphitization reaction of the carbon powder is promoted by the catalytic action of the graphitization reaction held by the compound calcined with the carbon powder, and a part of the added catalyst is a graphite crystal. It is not important to penetrate between the layers or to replace carbon atoms forming an in-plane six-membered ring network lattice.

The present inventors have conducted intensive studies on various graphitized carbon materials. As a result, the structure, crystal structure, and orientation of crystallites in the material surface layer are very important factors.
A graphitizing material utilizing the graphitizing catalytic effect of boron carbide, boron oxide, and boric acid was found to have much higher electrode characteristics than conventional negative electrode materials, and was filed earlier (Japanese Patent Application Laid-Open No. 8-31422). . In the present invention, among the electrode characteristics, particularly improved load characteristics, and, as a result of intensive study to improve the production efficiency by suppressing the coarsening of the powder in the firing process, the graphitization catalyst effect even boron compounds other than the above. By adding a nitrogen compound to the boron compound, the carbon powder catalyzed by the graphitization exhibits a high discharge capacity and a high initial efficiency without impairing the graphitization catalytic effect of the boron compound. In addition to the above, it has been found that the electrode has more excellent load characteristics as compared with a system in which boron, boron carbide, boron oxide, and boric acid are added alone.

The good load characteristics of the material are correlated with the amount of nitrogen present in the material. Although the mechanism is not clear at present, it has been found that the presence of nitrogen is important for improving the load characteristics. . Also, when the boron compound and the nitrogen compound coexist, coarsening of the powder after graphitization hardly occurs, and as a result, a final product having a target particle size range can be manufactured with high efficiency so that the classification step can be omitted. I found it to be. In particular, when boron nitride is used as a boron compound or a nitrogen compound, it is remarkably observed. This is because boron compounds other than boron nitride as in the prior application patent have a melting point below the graphitization temperature (≧ 2500 ° C.), whereas boron nitride has no melting point and sublimates around 3000 ° C. It seems to be due to the physical nature of doing so. That is, in the former case, the carbon particles of the raw materials are fused together through the liquid layer in which the catalyst is melted in the sintering step, and as a result, the baked product is coarsened, whereas in the latter case, such a phenomenon occurs. It is presumed that since the catalyst graphitization reaction proceeds without causing the carbon powder as the raw material, fusion and coarsening of the carbon powder are unlikely to occur.

The boron compound in the present invention is not particularly limited as long as it contains boron, but is preferably a metal compound of boron, boric acid, boron oxide, boron carbide, boron nitride or borate. At least one of them. The nitrogen compound is not particularly limited as long as it contains nitrogen therein, but is preferably made of at least one of nitride, nitrate, ammonium salt and cyanide. More preferably, it is boron nitride.

When the above compound is added, the compound can be added in a state of being dissolved or dispersed in advance with an organic solvent such as water or alcohol.

Regarding the degree of graphitization, which is an index of the degree of development of the graphite structure, as parameters according to the X-ray diffraction method for defining the carbonaceous material, d ₀₀₂ ≦ 0.337 nm, Lc ≧ 40
It has been found necessary to satisfy nm. A carbonaceous material having this degree of graphitization cannot be obtained except for the heat treatment with the graphitization catalyst according to the present invention. When the carbonaceous powder is fired at a normal heat treatment temperature, d ₀₀₂ > 0.3
Since 37 nm and Lc <40 nm, the degree of development of the graphite structure is low, the doping amount of lithium is small, and a high discharge capacity cannot be obtained.

As a result of studying the contents of boron and nitrogen contained in the graphitized carbon powder after firing, in order to obtain excellent load characteristics while maintaining a high discharge capacity and a high initial efficiency, the boron content in the material was determined. Content is 0.1% by atomic ratio
It was found that the content of nitrogen is preferably not less than 0.1% and not more than 10% in atomic ratio. When the boron content exceeds 10%, boron exceeding the solid solution limit of boron in graphite remains in the graphitized product as B ₄ C,
Since it does not participate in the doping / dedoping reaction of lithium at all, the discharge capacity is reduced. In addition, in the case of less than 0.1% of the graphitized powder, the catalytic effect of the added boron is not sufficiently exhibited, and in that case, the characteristics of each electrode are almost the same as those of a normal heat-treated product and are not improved at all. 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 during charging and discharging increases, so that the amount of doping / dedoping of lithium cannot be increased and the discharge capacity is greatly reduced. Resulting in. Further, the interaction between the doped lithium and nitrogen is strengthened, and the ratio of lithium trapped in the material is increased, so that the initial efficiency is greatly reduced. Further, a material having a nitrogen content of less than 0.1% does not have sufficient load characteristics under conditions similar to those when actually used as a battery.

The carbon powder using pitch as a raw material used in the present invention is a material which is easy to form a graphite structure (regular arrangement regularity of graphite layers) as a carbon material for a negative electrode of a lithium secondary battery. Examples of the raw material include carbon fiber, pitch coke, and mesoface small spheres, but are not particularly limited thereto. There is no particular limitation on the pitch, which is the raw material, but it is essentially important that graphite crystallinity is easily developed by firing, that is, easy to graphitize (easy graphitization). For example, petroleum pitch, asphalt pitch, coal tar pitch, naphthalene pitch, crude oil cracking pitch, petroleum sludge pitch, pitch obtained by thermal decomposition of a polymer, and the like can be mentioned. May be applied.

The shape of the carbonaceous powder used in the present invention has an average particle size of 50 μm or less, whereby the charge / discharge cycle characteristics when molded using a binder can be improved. If the average particle size exceeds 50 μm, it becomes difficult to insert lithium into the inside of the carbon powder, and the utilization rate of the carbonaceous powder is reduced to decrease the discharge capacity. 1 to be mounted on
When an electrode having a thickness of about 00 μm is formed, it is difficult to mold the electrode into a uniform thickness, and the performance of the carbonaceous powder cannot be sufficiently brought out.

The method of pulverizing the carbonaceous material as described above is not particularly limited as long as the method and apparatus satisfy the above-mentioned average particle size range. For example, a friction pulverization type ball mill, A roller mill, an impact compression crushing type vibration disk mill, a vibration ball mill, a pin mill, a jet mill, a stamp mill, a shearing crushing type cutting mill, an impeller mill and the like can be used.

The amount of the boron compound to be calcined together with the carbon powder made from pitch such as pitch coke, that is, the boron compound effective for graphitizing the carbon powder made from pitch such as pitch coke before calcination is as follows: The weight ratio is desirably 0.1% to 25% in terms of boron. Since the temperature becomes extremely high at the time of calcination, the temperature reaches the melting point or the boiling point of the boron compound itself, which is a graphitization catalyst, and a part of the added boron compound partially disappears during the heat treatment. Therefore, the content of boron present in the carbon powder after firing is smaller than the content in terms of boron of the boron compound added before firing. However, when the boron compound added before firing exceeds 25% by weight in terms of boron, the boron content in the carbon powder after firing exceeds the atomic ratio of 10%, so that a large amount of boron carbide is mixed. As a result, the discharge capacity is reduced. On the other hand, when the amount of the boron compound to be added is less than 0.1% by weight in terms of boron, the compound does not sufficiently function as a graphitization catalyst in the firing step. On the other hand, the amount of the nitrogen compound to be calcined together with the carbon powder from the pitch such as pitch coke, that is, the nitrogen compound effective for graphitizing the carbon powder from the pitch such as pitch coke before calcining, is calculated in terms of nitrogen. Desirably, the weight ratio is 0.1% to 25%. Regarding nitrogen compounds, if the added compound exceeds 25% by weight in terms of nitrogen, the nitrogen content in the carbon powder after firing exceeds 10% by atomic ratio.
This will cause a decrease in the initial efficiency. When the nitrogen compound to be added is less than 0.1% by weight in terms of nitrogen, the nitrogen content in the fired carbon powder is less than 0.1% in atomic ratio, and therefore, it is actually used as a battery. Insufficient load characteristics under similar conditions.

The method of mixing the carbonaceous material with the boron compound and the nitrogen compound as described above is not particularly limited as long as they are uniformly mixed, but they may be directly stirred. Stirring may be performed using a liquid such as water or an organic solvent such as alcohol as a medium.

The heat treatment temperature is desirably as high as possible for the purpose of developing the graphite crystallinity of the heat-treated material, and is preferably a temperature of 2500 ° C. or more under an inert atmosphere (nitrogen or argon gas) or a reducing atmosphere. And heat treatment for 0.1 hour or more is desirable. Heat treatment temperature is 2
If the temperature is lower than 500 ° C., the graphite structure of the graphitized carbon powder does not sufficiently develop, so that the discharge capacity is small and the current efficiency in the first charge / discharge cycle is particularly low. Further, if the heat treatment time is less than 0.1 hour, it is not enough to function the catalytic action of the graphitization catalyst, the graphite structure of the carbonaceous powder after the heat treatment does not develop, and both the discharge capacity and the initial efficiency are high. I can't get it. Here, the heat treatment temperature is desirably as high as possible in order to promote graphitization, but the limit of the capacity of the graphitization furnace equipment,
The temperature is preferably set to 3300 ° C. or less in view of problems such as accelerated wear of equipment due to a rise in temperature, a decrease in the weight of the fired product due to an increase in the vapor pressure of carbon in proportion to the rise in temperature, and a decrease in accuracy of temperature measurement. Regarding the heat treatment time, it is desirable to perform calcination as long as possible in order to function the catalytic action of the graphitization catalyst.
Even if the heat treatment is performed for less than an hour, the catalytic graphitization reaction proceeds and the crystal structure of the carbonaceous powder is sufficiently developed, so that the heat treatment for more than 1000 hours is not appropriate for economic reasons. Further, by performing heat treatment for more than 1000 hours, the carbon powders as raw materials are sintered, and the particle size of the carbon powder after firing tends to increase. It is not preferable because it cannot be obtained.

The classification of the powder after calcination is not limited in any way if it is a method or an apparatus which satisfies the above-mentioned average particle size range. For example, a sieve classifier or an airflow classifier may be used. Can be suitably used.

With respect 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 (2) _2-y O ₄ (where y is a numerical value in the range of 0 ≦ y ≦ 1, where M (2) represents a transition metal, Co, Ni, M
n, Cr, Ti, V, Fe, Zn, B, Al, In, S
n), transition metal chalcogenides (TiS ₂ , NbSe ₃ , etc.), vanadium oxides (V ₂ O ₅ , V ₆ O ₁₃ , V ₂ O ₄ , V ₃ O ₈ , e
tc. ) And its Li compound, general formula M _x Mo ₆ Ch
_8-y (where x is a numerical value in the range of 0 ≦ x ≦ 4, y is a numerical value in the range of 0 ≦ y ≦ 1, and M is a metal including transition metals, Ch
Represents a chalcogen element), activated carbon, activated carbon fiber or the like.

The organic solvent in the organic solvent-based electrolyte is not particularly restricted but includes, for example, propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, 1,1- and 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 ((CF
₃ ) ₂ CHOSO ₂ ) ₂ N, LiB (C ₆ H ₃ (C
F _{3) 2)} can be exemplified one or two or more thereof, such as _4.

[0040]

【Example】

Example A pitch coke lump using coal tar pitch as a raw material was pulverized using a pin mill to obtain a powder having an average particle size of 20 μm. The various compounds shown in Table 1 were added to the pitch coke powder classified with a 75 μm sieve, mixed well, and then each was graphitized for 1 hour in an argon atmosphere. Here, the heat treatment temperature was 3000 ° C. except that the heat treatment temperature was 2500 and 2700 ° C. for samples 7 and 8, respectively. Table 1 shows the graphite crystal structure of the obtained graphitized powder, the amounts of boron and nitrogen contained in the calcined powder, and the yield when the calcined powder was classified using a 75 μm vibration sieve. The crystal structure of the obtained graphitized powder is an ideal graphite crystal structure (d
₀₀₂ = 0.3354 nm), the pitch coke developed its graphite structure by the addition of boron nitride. Also,
The amount of nitrogen contained in the fired powder depended on the amount of boron nitride added before firing. In addition, as a result of measuring the powder after firing with a laser diffraction scattering type particle size distribution analyzer, the particle size distribution of the powder hardly changed before and after firing, and it was very high when actually classified by vibrating the graphitized powder. A powder having a desired particle size range could be obtained with high efficiency.

[0041]

[Table 1]

5% by weight of a polytetrafluoroethylene powder as a binder was added to the calcined powder thus prepared and kneaded to prepare an electrode sheet having a thickness of about 0.1 mm, which was cut into about 10.5 mg (carbon material). Converted to 1
0 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 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 1.0 V (based on lithium metal).
The heavy load characteristic test was performed by a constant current-constant voltage method in which the charging (Li occlusion) process was performed at a constant current (0.5 mA / cm ² ) and the voltage was maintained at 0 V. Discharge (Li release) with a total charge time of 12 hours
The process was performed by a constant current method of 0.5, 1, ^{2, 3} , 4 mA / cm ² . The potential range was 0 V to 1.0 V (based on lithium metal).

Tables 2 and 3 show the results of the electrode characteristics. In the initial lithium doping of the carbon material, 0.8
Since the potential flat portion (plateau) in the vicinity of V is very small, the initial charge / discharge efficiency is very high, and is approximately 1 after the third cycle.
It was stable at 00%. The discharge capacity per weight was high, and the capacity decrease accompanying the charge / discharge cycle was small, and the electrode performance was excellent. The heavy load characteristics correlated with the amount of nitrogen contained in the baked powder, and it was found that as nitrogen remained, the change in discharge capacity due to the difference in current density was small and the heavy load characteristics were excellent.

[0045]

[Table 2]

[0046]

[Table 3]

COMPARATIVE EXAMPLE 1 The same pitch coke powder as used in the examples was added at a rate of 10 / min.
Temperature at a rate of 2500C, 2500, 2700, 2900, 3
Graphitization treatment was performed at 000 ° C. for 1 hour. Table 4 shows the graphite crystal structure of the obtained graphitized powder, the amounts of boron and nitrogen contained in the calcined powder, and the classification yield after classification with a vibration sieve of 75 μm. Since no catalyst is added, there is almost no change in weight before and after firing, and the particle size of the powder does not increase even after firing. However, firing at such a high temperature is the interlayer distance (d ₀₀₂ ) which is an index of the graphite crystal structure. Even 0.335
It does not shrink to 4 nm.

[0048]

[Table 4]

When the pitch coke powder thus prepared was molded, the same method as in the example was used. The charge / discharge test was also performed according to the examples, and the results are shown in Table 5. In the initial lithium doping of carbon powder, a long potential flat portion (plateau) of 0.6 V to 0.9 V was observed, and the voltage difference until the open circuit state after lithium doping was large. Since the diffusion of lithium did not easily proceed, the charge / discharge efficiency was very low. The charge / discharge reaction reached almost 100% only after the fifth cycle, and thereafter changed to 100%. However, the discharge capacity was low, and the capacity decrease accompanying the charge / discharge cycle was large. The load characteristics are also 1 mA / cm
^The capacity starts dropping from ² and 0.5 mA at 4 mA / cm ²
/ Cm ² , to 80% of the discharge capacity.

[0050]

[Table 5]

Comparative Example 2 The pitch coke powder used in the examples was mixed with 6 wt% of various boron compounds in terms of boron under the conditions shown in Table 6, mixed and heated at a rate of 10 ° C./min. Graphite crystal structure of graphitized powder obtained by performing graphitization treatment for one hour,
Table 6 shows the amount of boron and the amount of nitrogen contained in the baked powder, and the classification yield after classification with a vibration sieve of 75 μm.
In each case, the interlayer distance (d ₀₀₂ ), which is an index of the graphite crystal structure, was closer to 0.3354 nm as compared with Comparative Example 1 in which no catalyst was added. On the other hand, especially in the boron oxide and boric acid addition systems, the weight loss before and after calcination was large, and the particle size of the powder after calcination was large in all cases, and the classification yield was low.

[0052]

[Table 6]

An electrode sheet was formed on the thus prepared pitch coke powder in the same manner as in the example, and a negative electrode was formed. All of the methods for evaluating the electrode characteristics of the above-mentioned molded electrode with a single electrode were performed in accordance with the examples. Table 7 shows the results of the electrode characteristics. The charge / discharge curve was almost the same as that in the case of adding boron nitride, and the value of the charge / discharge efficiency was high. In addition, the discharge capacity was high, the capacity decrease accompanying the charge / discharge cycle was small, and the cycle characteristics were good. On the other hand, as for the load characteristics, the capacity starts decreasing from 1 mA / cm ² and is 4 mA / c.
At m ² , the discharge capacity was reduced to nearly 90% of the discharge capacity at 0.5 mA / cm ² .

[0054]

[Table 7]

[0055]

As is apparent from the above description, the method for producing a carbon negative electrode material for a lithium secondary battery according to the present invention uses a carbonaceous powder belonging to the class of graphitizable carbon materials as a graphitization catalyst. Both boron compounds and nitrogen compounds are heat-treated, and it is possible to obtain graphite crystals with a high degree of graphitization that cannot be obtained by ordinary ultra-high temperature heat treatment, and to obtain a material containing nitrogen in the material. It is. Thereby, a powdery carbon material having high discharge capacity, initial charge / discharge efficiency, and excellent load characteristics can be provided.

 ────────────────────────────────────────────────── ─── Continuing from the front page (72) Inventor Takeshi Sasaki 1618 Ida, Nakahara-ku, Kawasaki-shi, Kanagawa Prefecture Nippon Steel Corporation Technology Development Division (72) Inventor Koki Inada 1618 Ida, Nakahara-ku, Kawasaki-shi, Kanagawa Address: Nippon Steel Corporation Technology Development Division (72) Inventor Tsutomu Sugiura 1618 Ida, Nakahara-ku, Kawasaki City, Kanagawa Prefecture Nippon Steel Corporation Technology Development Division

Claims (5)

[Claims] 1. A graphitized powder prepared by heat-treating a carbon powder using pitch as a raw material, wherein the distance between the carbon mesh plane layers (d ₀₀₂ ) and the c-axis direction of crystallites are measured by X-ray wide-angle diffraction. The size (Lc) is d ₀₀₂ ≦ 0.337 nm, Lc ≧ 4
0 nm, and an atomic ratio of boron of 0.1 to 10
% And nitrogen in an atomic ratio of 0.1 to 10%. 2. A method according to claim 1, wherein said carbon powder is made from pitch.
A boron compound is added and mixed in a weight ratio of 0.1 to 25% in terms of boron, and a nitrogen compound is added and mixed in a weight ratio of 0.1 to 25% in terms of nitrogen.
A method for producing a negative electrode material for a lithium secondary battery, comprising heat-treating at a temperature of at least 100C for at least 0.1 hour. 3. The method according to claim 1, wherein the boron compound is metallic boron, boric acid,
The method according to claim 2, wherein the method is at least one of boron oxide, boron carbide, boron nitride, and borate. 4. The method according to claim 2, wherein the nitrogen compound is at least one of a nitride, a nitrate, an ammonium salt, and a cyanide. 5. The method according to claim 4, wherein the nitrogen compound is boron nitride.