JPH10289718A - Manufacture of graphite material for large-capacity lithium-ion secondary battery negative electrode - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stably provide a high discharge capacity under the application of a graphite material as a lithium secondary battery negative electrode material by thermally treating the graphite materia under a vacuum condition at a temperature equal to or above a specific level for causing a metallic content to drop to a level below a specific range. SOLUTION: A graphite material is thermally treated under a vacuum condition at a temperature equal to or above 2,200 deg.C, and a metallic content is thereby made to drop to a level equal to or bellow 20 ppm. Mesophase pitch carbon fiber as milled, as well as graphitized is preferable particularly as a graphite material. The mesophase pitch carbon fiber has easily graphitized quality and high battery characteristic as the graphite material. Moreover, the mesophase pitch carbon fiber has small fiber length. In other words, the ratio of fiber cross section is made to increase via a milling process, thereby allowing lithium ions to easily enter and leave. Also, an advantage such as an increase in battery charging capacity is available. As a result, a trace metallic content in the graphite, boron nitride resulting from a graphitizing process in the presence of a boron compound or the like can be reduced efficiently.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technique for improving a graphite material for a negative electrode of a high capacity lithium secondary battery. Specifically, the present invention is characterized by a method for producing a graphite material for a negative electrode of a lithium secondary battery in which a high discharge capacity can be stably obtained by reducing a metal component. Furthermore, the present invention relates to a method for efficiently removing boron nitride, boron carbide, and the like generated on the surface, which is a subject of a highly graphitized graphite material using a boron compound.

[0002]

2. Description of the Related Art In general, a secondary battery using an alkali metal, for example, lithium as a negative electrode active material has a high energy density and a high electromotive force. It has many advantages such as excellent storage, light weight and small size. Therefore, such a nonaqueous electrolyte lithium secondary battery is expected to be put to practical use as a high-performance battery for a power source of a portable electronic device, an electric vehicle, a power storage device, and the like.

The use of graphite as a negative electrode material of this lithium secondary battery has been studied. As negative electrode materials for lithium ion secondary batteries, graphite materials such as artificial graphite, which is obtained by graphitizing natural graphite and organic substances, have been particularly noted and studied. In the case of natural graphite, when the degree of graphitization is high, the chargeable / dischargeable capacity per unit weight is considerably large, but the current density that can be taken out without difficulty is low, and charging and discharging at a high current density will result in charging. There is a problem that the discharge efficiency is reduced, and such a material is required to take out a large current, and in order to shorten the charging time, it is desirable to charge at a high current density, for example, a high-load power supply, for example, It was not suitable for use as a negative electrode of a power supply for equipment having a drive motor and the like.

On the other hand, in a conventional negative electrode using artificial graphite,
If the degree of graphitization is high, one having a charge / discharge capacity exceeding 300 mAh / g has been obtained, but it is still far from the theoretical capacity value of graphite material of 370 mAh / g.
Further, as these artificial graphite materials, in particular,
As disclosed in JP-B-168725, it has been pointed out that a carbon fiber obtained by using a mesophase pitch as a starting material and subjected to graphitization is excellent from the measurement results of various battery characteristics. However, carbon fibers vary in crystallite size, shape, impurity content, etc., depending on the starting materials and production conditions, and the above-mentioned graphite fibers also have an internal structure of the graphite material for lithium ion secondary batteries. It is difficult to say that the structure is controlled to the optimal structure, and it can be said that there is still room for improvement at present.

The present applicant has disclosed in JP-A-7-85862 and Japanese Patent Application No. 8-99643 that it is important to reduce impurities as a negative electrode material of a battery.
Japanese Patent Application Laid-Open No. 8-100329 discloses a method for producing a carbon fiber mill containing low metal. However, these techniques mainly focus on the adjustment of raw materials and the reduction of impurities in the manufacturing process, and only the description of the heat treatment in a halogen gas atmosphere is given for the treatment of reducing the impurities of the graphite material itself.

[0006] In addition, Japanese Patent Application Laid-Open No. 7-168725 describes that it is preferable to use low-impurity carbon fibers (particularly PAN-based) as battery electrodes.
As for the reduction of impurities, it is only disclosed that the purity is increased at the stage of producing carbon fibers. Further, as a result of subsequent research, heat treatment in a halogen gas atmosphere has a sufficient effect of reducing impurities, but the halogen gas partially reacts with carbon in the graphite material, and the yield of the graphite material decreases. And high processing costs have arisen as problems.

Further, the applicant has disclosed in Japanese Patent Application Laid-Open No. 9-63584.
In the invention of JP-A No. 63585/1994, a lithium-based secondary battery obtained by graphitization treatment in the presence of a boron compound, having a large charge / discharge capacity, a high energy density, and excellent charge / discharge cycle characteristics. A graphite material for secondary batteries was proposed. Japanese Patent Application Laid-Open Nos. 8-31422 and 306359 also disclose similar techniques relating to graphitization in the presence of a boron compound.

However, as a result of the subsequent research, the graphitization treatment in the presence of a boron compound shows that in a nitrogen atmosphere, boron is incorporated into the surface of the graphite material in the form of boron nitride, and in a nitrogen-free atmosphere. Even in the graphitization treatment, a compound of carbon and boron is generated on the surface of the graphite material, and the presence of these compounds (boron nitride and a compound of carbon and boron: hereinafter referred to as boron nitride) hinders an improvement in battery capacity. It is emerging as.

[0009]

DISCLOSURE OF THE INVENTION The present invention is to reduce the content of impurities, particularly metals, in a graphite material.
Focusing on obtaining a high capacity battery, an object of the present invention is to efficiently reduce the metal content of a graphite material. further,
The present invention also relates to a method for efficiently removing boron nitride and the like generated on the surface, which is a problem of a graphite material highly graphitized using boron.

[0010]

As a result of various studies on the above problems, the present inventor has found that a trace amount of metal present in the graphite material and boron nitride on the surface layer of the graphite material produced by the graphitization treatment in the presence of a boron compound. And the like can be efficiently reduced by reheating at a high temperature under reduced pressure, and a high discharge capacity can be stably supplied when the treated graphite material is used as a negative electrode material of a lithium ion secondary battery. And completed the present invention. That is, the present invention:

A method for producing a graphite material for a negative electrode of a high-capacity lithium ion secondary battery, wherein the graphite material is heat-treated at a temperature of 2200 ° C. or more under reduced pressure to reduce the metal content to 20 ppm or less. It is also characterized in that the graphite material is a graphite fiber mill obtained by milling carbon fibers made of mesophase pitch as a raw material and then graphitizing. Further, the present invention is characterized in that the described graphite material is a graphite fiber mill obtained by milling a carbon fiber using mesophase pitch as a raw material, and then graphitizing the carbon fiber in the presence of a boron compound.

Hereinafter, the present invention will be described in detail. (1) Graphite material: The graphite material of the present invention is not particularly limited as long as it is a carbon material having a highly graphitized structure so as to satisfy the following properties. And those subjected to graphitization are particularly preferred. The mesophase pitch-based carbon fiber is easily graphitizable and has excellent battery characteristics as a graphite material.Furthermore, by shortening the fiber length, that is, by milling, the ratio of the fiber cross section is increased, and lithium There are advantages such that ions can easily enter and exit, and the packing density of the battery can be increased.

The structure of the graphite material used in the present invention includes:
Graphite interlayer distance (d ₀₀₂ ) by X-ray diffraction is 0.338 n
m or less, preferably 0.337 nm or less; the crystallite size (Lc) in the C-axis direction is 35 nm or more, preferably 45 nm or less.
nm or more; crystallite size (La) in the a-axis direction is 50 n
m or more, preferably 60 nm or more; and a peak ratio between the (101) diffraction peak and the (100) diffraction peak (P ₁₀₁ / P
₁₀₀ ) is 1.0 or more, preferably 1.15 or more. These characteristics are indices indicating the degree of graphitization of the graphite material, and satisfying all of them is required to improve the performance of the battery. Hereinafter, various X-ray parameters used for defining the structure of the graphite material will be briefly described.

(I) X-ray diffraction method: X-ray diffraction method refers to Cu
kα is an X-ray source, high-purity silicon is used as a standard substance, and a diffraction pattern is measured on carbon fibers and the like. Then, the graphite interlayer distance d ₍₀₀₂₎ , the crystallite size Lc ₍₀₀₂₎ in the c-axis direction, and the 110-axis diffraction pattern peak position and half width from the ₀₀₂ diffraction pattern peak position and half width Size of crystallite in direction La ₍₁₁₀₎
Is calculated based on the Gakushin method. (ii) Measurement of the peak ratio of 101/100: The peak ratio of 101/100 was measured by drawing a baseline on the obtained diffraction diagram, and 101 (2θ ≒ 44.5) from the baseline.
Measure the height of each peak of 100 (2θ ≒ 42.5),
It is determined by dividing the 101 diffraction peak height by the 100 diffraction peak height.

(2) Method for producing graphite material: An example of a method for producing a graphite fiber mill as a preferred graphite material used in the present invention will be described below. (i) Raw Material As the raw material for producing carbon fiber, any easily graphitizable hydrocarbon can be used. Examples thereof include condensed polycyclic hydrocarbon compounds such as naphthalene and phenanthrene, and condensed heterocyclic compounds such as petroleum and coal-based pitch. In particular, it is preferable to use a petroleum or coal pitch, preferably an optically anisotropic pitch, that is, a mesophase pitch. The mesophase pitch preferably has a mesophase content of 100%, but is not particularly limited as long as spinning is possible.

(Ii) Production of milled carbon fiber The above-mentioned raw material is spun and infusibilized by a conventional method, and then milled as it is or after light carbonization. (B) Spinning etc. The method for melt-spinning the raw material pitch is not particularly limited, and various methods such as melt spinning, melt blowing, centrifugal spinning, and vortex spinning can be used. From the viewpoint of the quality of the obtained fiber
Melt blowing is preferred.

The size of the spinning hole at the time of melt blowing is set to 0.1.
1 mmφ or more and 0.5 mmφ or less, preferably 0.15 m
It is not less than mφ and not more than 0.3 mmφ. If the size of the spinning hole exceeds 0.5 mmφ, the fiber diameter tends to be as large as 25 μm or more, and the fiber diameter tends to vary, which is not preferable in quality control. If the size of the spinning hole is less than 0.1 mmφ, clogging tends to occur at the time of spinning, and it is difficult to manufacture a spinning nozzle, which is not preferable. The spinning speed is 500 m / min or more, preferably 1500 m / min or more, more preferably 2000 m / min or more in terms of productivity. The spinning temperature varies somewhat depending on the raw material pitch, but may be any temperature above the softening point of the raw material pitch and below the temperature at which the pitch does not deteriorate.
00 ° C to 400 ° C, preferably 300 ° C to 38
0 ° C. or less.

The melt blow method also has an advantage that the graphite layer surface can be easily arranged parallel to the fiber axis by spinning at a low viscosity of several tens of poise or less and cooling at a high speed. Although the softening point of the raw material pitch is not particularly limited, a material having a low softening point and a high infusibilization reaction rate is advantageous in terms of production cost and stability, in view of the spinning temperature. Accordingly, the softening point of the raw material pitch is 230 ° C. or higher and 35
0 ° C. or less, preferably 250 ° C. or more and 310 ° C. or less.

(B) Infusibilization and the like The pitch fiber after spinning is infusibilized by a conventional method. Examples of the infusibilization method include a method of performing heat treatment in an oxidizing gas atmosphere such as nitrogen dioxide and oxygen, a method of performing treatment in an oxidizing aqueous solution such as nitric acid and chromic acid, and a method of treating light or γ.
It is possible to use a method of performing a polymerization treatment using a wire or the like. A simpler infusibilizing method is a method in which heat treatment is performed in the air.
Heat treatment at a rate of at least 350 ° C./min, preferably at least 5 ° C./min.

(C) Milling of carbon fiber, etc. Normally, the graphite material for a negative electrode of a secondary battery desirably has a small particle size in order to facilitate the inflow and outflow of lithium and to increase the packing density. Is required in the range of 10 to 50 μm. If the average particle size is more than 50 μm, the packing density cannot be increased, coating unevenness of the sheet tends to occur, and a graphite material having a large particle size that causes a short circuit is apt to be mixed. If the average particle size is less than 10 μm, the active graphite layer is exposed and easily reacts with the electrolyte, which is not preferable. For this reason, the graphite material of the present invention is also required to be milled (pulverized) so that the particle size falls within this range. The average particle size is calculated from a particle size distribution by a laser diffraction method.

The aspect ratio (the ratio of the length to the diameter of the fiber) of the carbon fiber is 1 or more and 30 or less, preferably 1 or less.
It is desirable that the number is not less than 20 and not more than 20. Aspect ratio is 3
If it exceeds 0, that is, if a milled fiber having a relatively long fiber length is used, the bulk density becomes low, the energy density per volume becomes small, and the positive and negative electrodes may be short-circuited. On the other hand, if the aspect ratio is less than 1, the number of fibers that cause longitudinal cracks in the fiber axis direction increases, which is not preferable. The above-mentioned aspect ratio is shown by the average value of the values of 100 pieces of the milled fibers obtained. The average particle size,
From the viewpoint of the aspect ratio, the diameter of the graphite material is 4 μm.
It is preferably at least 25 μm. The milling of the fiber can be performed after infusibilization, carbonization, or graphitization. However, it is preferable to perform milling after carbonization in a specific temperature range as described later.

(D) Carbonization etc. Usually, the infusibilized fiber is subjected to 25
The carbonization treatment is performed at 0 ° C or more and 2000 ° C or less, but in the present invention, it is 400 ° C or more and 1500 ° C or less, preferably 5 ° C or less.
Light carbonization at a temperature of 00 ° C or more and 1000 ° C or less is desired for suitably milling the fiber. Milling carbonized carbon fibers in this range can relatively prevent longitudinal cracking of the fiber, and even if a graphite layer surface newly exposed on the surface during milling occurs, it is then graphitized at a higher temperature. This is also advantageous in that the polycondensation / cyclization reaction sometimes tends to proceed, the surface activity is reduced, and the decomposition of the electrolytic solution is prevented.

On the other hand, in the milling after heat treatment (carbonization or graphitization) at a temperature exceeding 1500 ° C., cracks are apt to occur along the graphite layer surface developed in the fiber axis direction, and the manufactured milled is obtained. The ratio of the fracture surface area to the total surface area of the carbon fibers increases, and the decomposition of the electrolyte due to the localization of electrons on the fractured graphite layer surface is not preferable.
Further, unfusible fibers or milling after carbonization at a temperature of less than 400 ° C. is not preferable because the strength of the fibers is low, the fibers are pulverized more than necessary at the time of pulverization, and the yield deteriorates.

(E) Milling method of carbon fiber In order to mill lightly carbonized carbon fiber, it is effective to use a victory mill, jet mill, disk mill, cross flow mill or the like. In order to efficiently carry out milling suitable for the present invention, it is common to the above-mentioned various methods that, for example, by rotating a rotor attached with a plate at a high speed, the fibers are perpendicular to the fiber axis. The shredding method is appropriate. The fiber length of the milled fibers can be controlled by adjusting the number of rotations of the rotor, the angle of the plate, the size of the mesh of a filter attached around the rotor, and the like.

The milling may be performed by a method using a Henschel mixer, a ball mill, a grinding machine, or the like. However, according to these methods, pressure is applied in a direction perpendicular to the fiber, and longitudinal cracks are often generated in the direction of the fiber axis. It is not preferable. In addition, this method requires a long time for milling, and is not an appropriate milling method.

(Iii) Graphitization etc. Graphitization is usually carried out at 2000 ° C. or higher in a non-oxidizing atmosphere, for example, an inert atmosphere such as nitrogen or argon. In addition, the graphite material for the secondary battery negative electrode is capable of accepting lithium, that is, the battery capacity is substantially proportional to the degree of graphitization. At present, the temperature of graphitization is 2600 ° C. or higher, preferably 2600 ° C.
800 ° C. or higher is required. Further, in order to further develop graphitization, it is possible to use a boron compound at the time of graphitization. The graphitization method is described in Japanese Patent Application Laid-Open No.
No. 31422, 306359, JP-A-9-63584
And the method described in JP-A-63-585, etc., and an outline of graphitization of a carbon fiber mill is shown below.

(A) Types and shapes of boron compounds Boron compounds (B ₄ C), boron chloride, boric acid, boron oxide, sodium borate, borate, etc. may be used in addition to boron alone in graphitization. Potassium acid,
Organic boron compounds such as copper borate, nickel borate, boron trifluoride-methanol complex (BF ₃ .CH ₃ OH) and borane-dimethylamine complex [BH ₃ .HN (CH ₃ ) ₂ ]; May be used at least one. Usually, the addition of these boron compounds is preferably a method of directly adding a solid boron compound and uniformly mixing as necessary, but a method of dipping the boron compound in a solvent solution,
It is also possible to add a boron compound at the stage of raw material pitch. (B) Particle size of boron compound In order to obtain a homogeneous graphite material, it is preferable to use a mixture of a boron compound and a carbon fiber mill during graphitization.
For this reason, in the case of a solid boron compound, in order to uniformly mix with the carbon fiber mill, a finer one is desirable,
For example, the average particle size is 100 μm or less, preferably 1.7 to
It is desirable to use it as 70 μm.

(C) Addition amount of boron compound: The addition amount of the boron compound is 15% by weight or less, preferably 1 to 10% by weight as boron based on the material to be graphitized. If it is 1% by weight or less, there is no difference in the graphitization promoting effect, and the effect of the present invention is low. If it is 15% by weight or more, the effect on cost is reduced. Further, the residual amount of boron increases in the graphite fiber mill after graphitization, which causes a problem that the graphite materials adhere to each other, which is not preferable. Further, it is not preferable because boron nitride and the like generated on the surface of the graphite fiber mill increase. (D) Graphitization temperature: The principle of graphitization of boron compounds is unknown, but the melting point of boron compounds (the melting point of boron is 2
080 ° C., the melting point of boron carbide is 2450 ° C.), the effect of promoting graphitization is obtained from the temperature, and graphitization is further promoted by using a boron compound.
Even at a temperature lower than the normal graphitization temperature, a high-capacity battery performance can be obtained.
C. or higher, preferably 2400 ° C. or higher, more preferably 2600 ° C. or higher.

(3) heat treatment under reduced pressure; in order to reduce the content of impurities, particularly metals, in the graphite material, and in a graphite material obtained by graphitization treatment in the presence of a boron compound,
In particular, heat treatment is performed under reduced pressure in order to efficiently remove boron nitride and the like generated in the surface layer. (i) Metal content Graphite material contains oxygen (O), hydrogen (H), nitrogen (N), sulfur (S), etc. in addition to the carbon (C) element, depending on the starting material and the production method. Small amounts of organic elements (0.05 to 2
%), Iron (Fe), silica (Si), aluminum (Al),
Metals such as calcium (Ca), chromium (Cr), vanadium (V), and nickel (Ni) are contained.
According to the study of the inventor, the detailed reason is unknown,
Perhaps because the metal reacts with lithium ions to increase the irreversible capacity inside the battery, the content of the metal greatly influences the capacity of the battery, and the total content of the seven metals can be increased. It has been found that reducing only this leads to a remarkable improvement in battery capacity. The measurement of the metal content is performed by a plasma emission analysis method. Further, the content of the metal component in the graphite material of the present invention is defined as the total weight of seven types of metal elements of Fe, Si, Al, Ca, Ni, Cr and V obtained by the above-described measuring method.

(Ii) Boron nitride and the like As described above, the graphite material which has been graphitized in the presence of a boron compound has a surface in which boron is incorporated in the form of boron nitride in a nitrogen atmosphere and an atmosphere free of nitrogen. Also in the graphitization treatment, a compound of carbon and boron is formed on the surface of the graphite material, which is a factor that hinders an improvement in battery capacity. The detection of boron nitride or the like is performed by the ESCA method or elemental analysis (plasma emission analysis is used for boron, and distillation separation neutralization titration is used for nitrogen).

(Iii) Heat treatment under reduced pressure Metal components, boron nitride, etc. have a high melting point (especially, boron nitride exceeds 3000 ° C.), and it is difficult to efficiently remove it by ordinary heat treatment. For this purpose, heat treatment is generally performed in a halogen gas atmosphere. However, this process has the above-mentioned problems. (A) For this reason, as a result of various studies on other methods, the present inventor has found that heat treatment under reduced pressure has the same effect as heat treatment under a halogen gas atmosphere, and at a lower cost than halogen gas treatment. It has been found that a graphite material having a small decrease in the yield of the graphite material and a reduced metal content can be obtained. (B) Further, in the graphitized material which has been graphitized in the presence of a boron compound, the content of the metal component is similarly reduced, and the graphite material does not substantially contain boron nitride or the like in the surface layer portion of the graphite material. Material was obtained, and it was found that the performance of the battery was improved.

(C) Hereinafter, the heat treatment method under reduced pressure will be described in detail. The conditions of the heat treatment under reduced pressure are 1 torr or less, preferably 0.1 to 0.5 torr, and the heat treatment temperature is 2200 ° C. or more, preferably 2400 ° C. or more. In this case, the lower the pressure and the higher the heat treatment temperature, the more easily the metal component and boron nitride are removed by sublimation. Under a pressure exceeding 1 torr or at a heat treatment temperature lower than 2200 ° C., the metal component and boron nitride are removed. Sublimation does not occur and is not preferred. Further, a pressure of less than 0.1 torr is not preferable in terms of cost and safety of the apparatus. The upper limit of the heat treatment temperature is not particularly limited. However, from the viewpoint of cost and physical properties of the graphite material, the upper limit of the heat treatment temperature is generally equal to or lower than the heat treatment temperature for graphitization.
C. or less is sufficient. The heat treatment time is a function of the degree of pressure reduction and the heat treatment temperature, and cannot be uniquely determined. However, it is usually about 1 hour, preferably 30 minutes to 30 minutes.
It is desirable to hold for about one hour.

(Iv) Properties of graphite material after heat treatment under reduced pressure By heat treatment under reduced pressure, the total amount of the seven metals in the graphite material is 20 ppm, regardless of the raw material and the production method.
Or less, preferably 10 ppm or less. Further, boron nitride and the like in the surface layer of the graphite material are substantially completely removed. On the other hand, by performing the heat treatment under reduced pressure at a temperature lower than the temperature at the time of graphitization, the properties of the graphite structure do not change. Thus, even if the graphite structure is the same, the graphite material in which the metal content is reduced to the above specific range by the heat treatment under the reduced pressure, and particularly, in the graphite material graphitized in the presence of the boron compound, The graphite material in which boron nitride and the like in the surface layer are substantially completely removed, and the metal content is also reduced to the above-described specific range, is clearly improved in battery capacity as compared with the graphite material before this treatment, The effect of this treatment was confirmed.

(4) Negative electrode material for lithium ion secondary battery: The graphite material obtained according to the present invention is obtained by adding a binder such as polyethylene, polyvinylidene fluoride or polytetrafluoroethylene to a material suitable for forming a negative electrode, for example, After forming a pressure roll into a sheet or a plate, a high-performance negative electrode can be easily obtained by performing a reduction treatment using lithium metal as a counter electrode. The negative electrode thus produced has a large capacity per unit volume and is suitable for miniaturization of a battery.

Further, the graphite material according to the present invention is used for a negative electrode,
When a lithium ion secondary battery is prepared, the electrolytic solution may be any as long as it can dissolve the lithium salt, and an organic solvent having a large aprotic dielectric constant is particularly preferable.
As the organic solvent, for example, propylene carbonate, ethylene carbonate, tetrahydrofuran, 2-
Examples thereof include methyl tetrahydrofuran, dioxolan, 4-methyl-dioxolan, acetonitrile, dimethyl carbonate, methyl ethyl carbonate, and diethyl carbonate. These solvents can be used alone or in a suitable mixture.

As the electrolyte, a lithium salt that generates a stable anion, such as lithium perchlorate, lithium borofluoride, lithium antimonate hexachloride, and lithium hexafluorophosphate (LiPF ₆ ) are preferable. Examples of the positive electrode of the lithium ion secondary battery include metal oxides such as chromium oxide, titanium oxide, cobalt oxide, and vanadium pentoxide, and lithium manganese oxide (LiMn ₂ O).
₄ ) lithium metal oxides such as lithium cobalt oxide (LiCoO ₂ ) and lithium nickel oxide (LiNiO ₂ ); chalcogen compounds of transition metals such as titanium sulfide and molybdenum sulfide; and polyacetylene, polyparaphenylene, polypyrrole and the like. A conjugated polymer substance having conductivity or the like can be used.

A separator such as a nonwoven fabric made of synthetic fiber or glass fiber, a woven fabric, a polyolefin-based porous membrane, or a nonwoven fabric made of polytetrafluoroethylene is provided between the positive electrode and the negative electrode. In addition, a current collector can be used as in the case of a conventional battery. As the negative electrode current collector, an electrode, a conductor which is electrochemically inert to the electrolytic solution, for example, copper, nickel,
Metals such as titanium and stainless steel can be used in the form of plates, foils and bars. The secondary battery of the present invention uses the above-mentioned battery components such as the separator, current collector, gasket, sealing plate, and case and the specific negative electrode of the present invention, and has a form such as a cylindrical type, a square type, or a button type according to a conventional method. Can be assembled into a lithium ion secondary battery.

[0038]

In the present invention, a trace amount of metal present in the graphite material, boron nitride on the graphite material surface layer generated by the graphitization treatment in the presence of the boron compound, and the like are heat-treated under reduced pressure at a high temperature. It can be efficiently reduced, and a high discharge capacity can be stably supplied when the treated graphite material is used as a negative electrode material of a lithium ion secondary battery. That is, even if the graphite structure is the same, the graphite material in which the metal content is reduced by the heat treatment under reduced pressure in the present invention, and the graphite material which has been graphitized in the presence of the boron compound, such as boron nitride in the surface layer Is substantially completely removed and the metal content is also reduced. The graphite material before heat treatment under reduced pressure according to the present invention has an effect of clearly improving the battery capacity.

[0039]

The present invention will be described more specifically with reference to the following examples.
They do not limit the scope of the invention. (Example 1) Using a petroleum-based mesophase pitch having an optical anisotropy and a specific gravity of 1.25 as a raw material, a spinneret having a width of 3 mm and a spinning hole having 500 spinning holes of 0.2 mmφ in a line was used. , Heated air was blown out of the melt to pull the molten pitch to produce pitch fibers having an average diameter of 13 μm. At this time, the spinning temperature was 360 ° C and the discharge amount was 0.8 g.
/ H · min. The spun fiber is repaired by 20
It was collected on the belt while sucking from the back of the belt made of mesh stainless steel wire mesh.

The collected mat is placed in air at room temperature for 3 hours.
The infusibilization treatment was performed by raising the temperature to 00 ° C. at an average rate of 6 ° C./min. Subsequently, the infusibilized yarn was slightly carbonized at 650 ° C., and then pulverized with a cross flow mill to obtain an average particle size of 18%.
A μm carbon fiber mill was obtained. The carbon fiber mill is heated at a rate of 3 ° C./min to 3000 ° C. under a nitrogen atmosphere,
The temperature was maintained for one hour to obtain a milled graphite fiber.

When the degree of graphitization of the milled graphite fiber was measured by X-ray diffraction, the distance between graphite layers (d ₀₀₂ ) = 0.3362 n
m, crystallite size in the C-axis direction (Lc) = 48 nm, a
Axial crystallite size (La) = 70 nm, (10
1) Peak ratio P of diffraction peak and (100) diffraction peak
It was ₁₀₁ / P ₁₀₀ = 1.19. Further, when the content of the metal component was measured, seven types of metal components (Fe, Si, A
1, Ca, Ni, Cr, and V) were 90 ppm in total.

The graphite fiber mill was removed under reduced pressure (0.1 torr).
Heat treatment was performed at 2700 ° C. for 1 hour. When the degree of graphitization after the heat treatment was measured by X-ray diffraction, d ₀₀₂ = 0.3362 n
m, Lc = 48 nm, La = 70 nm, peak ratio P ₁₀₁
/ P ₁₀₀ = 1.19, indicating that there was no change compared to before the heat treatment under reduced pressure. On the other hand, when the content of the metal component was measured, the total content of the seven metal components was greatly reduced to 5 ppm.

Further, the charge / discharge capacity characteristics of the graphite fiber mill after the heat treatment under reduced pressure were measured. Measurement, the metal lithium used for the anode and the reference electrode, ethylene carbonate (EC) / dimethyl carbonate in a mixed carbonic ester solvent was adjusted to 1/1 volume ratio (DMC), lithium perchlorate as an electrolyte (LiCl0 ₄₎ Was carried out in an electrolytic solution having a concentration of 1 mol, and the charge / discharge capacity characteristics were measured. The measurement of the charge / discharge capacity characteristics was performed under a constant current charge / discharge of 100 mA / g, and the measured potential range was the reference electrode (0 to 0).
2V / Li / Li ⁺ ), and the measurement was repeated 10 times. Initial discharge capacity 312 mAh / g, charge / discharge efficiency 94%, 2
The third discharge capacity was 310 mAh / g, and the charge / discharge efficiency was 99.8.
%, All showed high values. In addition, the discharge capacity was 310 mAh / g and the charge / discharge efficiency was 100%, which were stable values from the third to the tenth.

Example 2 5% by weight of boron carbide having an average particle size of 32 μm was added to the milled carbon fiber obtained in Example 1, and the mixture was stirred and mixed so as to be uniform. The temperature is raised at a rate of 3 ° C./min.
After holding for a time, a graphite fiber mill was obtained. When the degree of graphitization of the graphite fiber is measured by X-ray diffraction, the distance between graphite layers (d ₀₀₂ ) =
0.3348 nm, crystallite size in the C-axis direction (Lc)
= 100 nm or more, crystallite size in the a-axis direction (La)
= 100 nm or more, (101) diffraction peak and (100)
The peak ratio P ₁₀₁ / P _{100 of the} diffraction peaks was 2.19. When the surface of the graphite fiber mill is analyzed by the ESCA method, a B-1S peak and an N-1S peak appear,
It was found that boron nitride and the like were formed on the surface layer.
Further, when the content of the metal component was measured, the total content of the seven metal components was 92 ppm.

The milled graphite fiber was heat-treated at 2700 ° C. for 1 hour under reduced pressure (0.1 torr) in the same manner as in Example 1.
When the degree of graphitization of the graphite fiber mill after the heat treatment under reduced pressure was measured by X-ray diffraction, d ₀₀₂ = 0.3348 nm and Lc = 10
0 nm or more, La = 100 nm or more, peak ratio P ₁₀₁ /
P ₁₀₀ = 2.19, indicating that there was no change compared to before the heat treatment under reduced pressure. On the other hand, when the surface was analyzed by the ESCA method, it was found that the B-1S peak and the N-1S peak disappeared, and that the boron nitride in the surface portion was removed. Further, when the content of the metal component was measured, the total content of the seven metal components was greatly reduced to 9 ppm.

With respect to the graphite fiber mill after the heat treatment under reduced pressure, the charge / discharge capacity characteristics were measured in the same manner as in Example 1. The initial discharge capacity was 362 mAh / g, and the charge / discharge efficiency was 9%.
3%, the second discharge capacity was 360 mAh / g, and the charge / discharge efficiency was 99.8%, all of which were high values. In addition, the discharge capacity was 360 mAh from the third time to the tenth time.
/ G, and a stable value of 100% charge / discharge efficiency.

Example 3 Optically anisotropic and specific gravity 1.2
Using a coal-based mesophase pitch containing a relatively large amount of metal as No. 8 as a raw material, spinning was performed in the same manner as in Example 1 to produce pitch fibers having an average particle diameter of 13 μm. The pitch fiber,
Infusibility treatment, carbonization treatment, milling treatment, and graphitization treatment were performed in the same manner as in Example 1 to obtain a graphite fiber mill having an average particle size of 16 μm. When the degree of graphitization of the graphite fiber mill was measured by X-ray diffraction, the distance between graphite layers (d ₀₀₂ ) = 0.3361 n
m, crystallite size in the C-axis direction (Lc) = 50 nm, a
Axial crystallite size (La) = 75 nm, (10
1) Peak ratio P of diffraction peak and (100) diffraction peak
It was ₁₀₁ / P ₁₀₀ = 1.21. When the contents of the metal components were measured, seven types of metal components (Fe, Si, A
1, Ca, Ni, Cr, and V) were 160 ppm in total.

The graphite fiber mill was removed under reduced pressure (0.1 torr).
Heat treatment was performed at 2700 ° C. for 1 hour. When the degree of graphitization after the heat treatment was measured by X-ray diffraction, d ₀₀₂ = 0.3361 n
m, Lc = 50 nm, La = 75 nm, peak ratio P ₁₀₁
/ P ₁₀₀ = 1.21, indicating that there was no change compared to before the heat treatment under reduced pressure. On the other hand, when the content of the metal component was measured, the total of the seven metal components was greatly reduced to 9 ppm.

When the graphite fiber mill after the heat treatment under reduced pressure was measured for charge / discharge capacity characteristics in the same manner as in Example 1, the initial discharge capacity was 312 mAh / g, the charge / discharge efficiency was 94%, and the second discharge was performed. The capacity was 310 mAh / g and the charge / discharge efficiency was 99.8%, all of which were high values. In addition, the discharge capacity is 310 m from the third time to the tenth time.
Ah / g and a charge / discharge efficiency of 100% showed stable values.

(Comparative Example 1) The graphite fiber mill obtained in Example 1 before the heat treatment under reduced pressure was subjected to the measurement of the charge / discharge capacity characteristics in the same manner as in Example 1. It shows a capacity of 302 mAh / g, a charge / discharge efficiency of 91%, a second discharge capacity of 300 mAh / g, and a charge / discharge efficiency of 99.8%, and shows a discharge capacity of 300 mAh / g and a charge / discharge efficiency of 100 from the third to the tenth time. %showed that. Even with a similar graphite structure, it is a lower value compared to the example,
It is determined that the reduction in the metal content has led to an increase in the capacity of the battery.

(Comparative Example 2) The graphite fiber mill obtained in Example 2 before the heat treatment under reduced pressure was subjected to the measurement of the charge / discharge capacity characteristics in the same manner as in Example 1. Capacity 305 mAh / g, charge / discharge efficiency 89%, second discharge capacity 302 mAh / g, charge / discharge efficiency 98%,
From the third time to the tenth time, the discharge capacity is 3
02 mAh / g and a charge / discharge efficiency of 100% were shown. Even with the same graphite structure, the value is lower than that of the example, and it is judged that the reduction of boron nitride and the like and the metal content led to the improvement of the capacity of the battery.

(Comparative Example 3) When the graphite fiber mill obtained in Example 3 before the heat treatment under reduced pressure was subjected to measurement of the charge / discharge capacity characteristics in the same manner as in Example 1, the first discharge was performed. A capacity of 294 mAh / g, a charge / discharge efficiency of 90%, a second discharge capacity of 290 mAh / g, and a charge / discharge efficiency of 99.8% are shown. From the third time to the tenth time, the discharge capacity is 290 mAh / g and the charge / discharge efficiency is 100. %showed that. Even with a similar graphite structure, it is a lower value compared to the example,
It is determined that the reduction in the metal content has led to an increase in the capacity of the battery. Tables 1 and 2 show the measurement results of the examples and comparative examples.

[0053]

[Table 1]

[0054]

[Table 2]

[0055]

As described above, according to the method of heat-treating a graphite material under reduced pressure and high temperature according to the present invention, a trace amount of metal present in the graphite material and a surface layer of the graphite material generated by the graphitization treatment in the presence of a boron compound. Has an effect that a high discharge capacity can be stably supplied when the treated graphite material is used as a negative electrode material of a lithium ion secondary battery.

Claims (3)

[Claims] 1. A graphite material for a negative electrode of a high-capacity lithium ion secondary battery, wherein the graphite material is heat-treated at a temperature of 2200 ° C. or more under reduced pressure to reduce the metal content to 20 ppm or less. Method. 2. The negative electrode for a lithium ion secondary battery according to claim 1, wherein the graphite material is a graphite fiber mill obtained by milling a carbon fiber made of mesophase pitch as a raw material and then graphitizing the milled carbon fiber. Manufacturing method of graphite material. 3. The lithium material according to claim 1, wherein the graphite material is a graphite fiber mill obtained by milling a carbon fiber made of mesophase pitch as a raw material and then graphitizing in the presence of a boron compound. A method for producing a graphite material for an anode of an ion secondary battery.