JP2000012034A - Graphite material for lithium secondary battery and its manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a graphite material for a lithium secondary battery with high charging/discharging capacity and high charging/discharging cycle characteristics by using a graphite material having a specific structure capable of identifying by Raman spectroscopic analysis. SOLUTION: Graphite material is obtained by graphitizing a carbon material under the existence of a boron compound at 2,500 deg.C or higher, and has a spacing of (d002) as determined by the X-ray diffraction of 0.338 nm or less, a crystallite size in the direction of a (c)-axis (Lc) of 35 nm or less, a crystallite size in the direction of an (a)-axis (La) of 50 nm or more, a peak ratio of (101) diffraction peak to (100) diffraction peak (P101/P100) is 1.0 or higher and moreover a peak intensity ratio (I1340/I1580) as determined by Raman spectroscopic analysis is 0.5 or higher, and also has shoulder-shape of 1,310-1,320 cm-1 in the peak in the vicinity of 1,340 cm-1. The graphite material is mesophase pitch family graphite milled fibers. When the carbon material is graphitized under the existence of the boron compound at 2,500 deg.C or higher, the carbon material is kept at a constant temperature between 2,200-2,400 deg.C for 20 minutes or longer.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement in a graphite material for a negative electrode of a lithium secondary battery obtained by performing a graphitization treatment in the presence of a boron compound (including boron alone). For details,
The present invention relates to a lithium secondary battery negative electrode having excellent battery performance, having a unique graphite structure in which the graphite structure is highly advanced as evident from X-ray diffraction and a shoulder-like peak specified by Raman analysis is present. Related to graphite materials. Furthermore,
The present invention relates to an improved method for producing a graphite material characterized in that special heat treatment conditions are maintained during the graphitization of a carbon material. The lithium secondary battery using the graphite material of the present invention has features that it has a large discharge / charge capacity, a high energy density, and excellent charge / discharge cycle characteristics.

[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. Such a non-aqueous electrolyte lithium secondary battery has been put to practical use as a power source for portable electronic devices, and is expected to be put into practical use as a high-performance battery for electric vehicles, power storage, and the like. The use of a carbon material as a negative electrode material of a lithium secondary battery has been studied and put to practical use. In particular, the use of a graphite material is becoming mainstream because of its advantages such as excellent cycle characteristics. However, the charge and discharge capacity of a graphite material that has been put to practical use is 310 m.
Ah / g, and the theoretical capacity of graphite material is 372 mAh /
Compared with g, there is still room for improvement, and research and development for higher capacity is ongoing.

The inventor of the present invention has developed a highly graphitized structure by using a graphite material (particularly a milled graphite fiber) obtained by graphitizing a carbon material in the presence of a boron compound. It has been found that it is possible to improve the graphite structure, which facilitates the inflow and outflow of lithium, improve the capacity of the battery, and reduce the deterioration of the cycle characteristics, as disclosed in JP-A-9-63584 and 9-63585. Already proposed. Further, the present inventor has disclosed a post-graphitization treatment in the presence of a boron compound in Japanese Patent Application No. 8-331.
Application No. 410, and a method of adding a boron compound to pitch is disclosed in Japanese Patent Application Nos. 9-68985 and 9-68985.
No.-72824 has already been filed.

[0004]

SUMMARY OF THE INVENTION An object of the present invention is to solve the problem that the conventional lithium secondary battery is still smaller than the theoretical capacity as described above.

[0005]

As described above, the present inventor has studied various graphite materials which have been graphitized in the presence of a boron compound as a carbon material for a negative electrode of a lithium secondary battery. As a result of examining the relationship between the discharge capacity and the structure of the graphite material, those with a unique graphite structure with a shoulder-shaped peak specified by Raman analysis have a large discharge capacity and high energy density. In addition, the present inventors have found that a graphite material for a lithium secondary battery having excellent charge-discharge cycle characteristics can be provided, and for that purpose, the present invention has been completed by maintaining special heat treatment conditions in the graphitization treatment.

That is, the present invention relates to a graphite material obtained by subjecting a carbon material to graphitization at a temperature of 2500 ° C. or more in the presence of a boron compound, and (a) a graphite interlayer distance (d ₀₀₂ ) obtained by X-ray diffraction of the graphite material. ) Is 0.338 nm
(B) The crystallite size (Lc) in the C-axis direction is 35
(c) When the crystallite size (La) in the a-axis direction is 50 nm or more, (d) the (101) diffraction peak and (10)
0) The peak ratio of diffraction peaks (P ₁₀₁ / P ₁₀₀ ) is 1.0
(E) peak intensity ratio (I ₁₃₄₀ / I ₁₅₈₀ ) by Raman spectrum analysis is 0.5 or more, and (f) 1340
The peak near cm ^-1 to provide a lithium secondary battery graphite material is present shoulder-shaped peak of 1310~1320cm ^-1. Another feature is that the graphite material is a graphite fiber mill made from mesophase pitch. In addition, when the carbon material is graphitized at a temperature of 2500 ° C. or more in the presence of a boron compound, the carbon material is held at a constant temperature in a temperature range of 2200 to 2400 ° C. for 20 minutes or more. A manufacturing method is provided.

Hereinafter, the present invention will be described in detail. (1) Regarding graphite material; (i) Structure of graphite material; The graphite material of the present invention is graphitized at a temperature of 2500 ° C. or more, preferably 2600 ° C. or more in the presence of boron, and a graphite structure develops. It is a material suitable for a negative electrode of a lithium secondary battery having the following characteristics. The upper limit of the heat treatment temperature is desirably as high as possible, and is not particularly limited. However, considering economic efficiency, about 3000 ° C. is sufficient. The form of the graphite material is not particularly limited, and includes a fibrous form, a milled fibrous form, a paper form, a film form, and a spherical form such as mesocarbon microbeads. In the present invention, as described below, a graphite material obtained by graphitizing milled carbon fibers (particularly, mesophase pitch type) is preferably used in terms of increasing the capacity of a battery. The structure of the graphite material after graphitization of the present invention is specified by X-ray diffraction and Raman analysis.

X-ray Diffraction The graphite material of the present invention has a graphite interlayer distance (d ₀₀₂ ) of 0.33.
8 nm or less, preferably 0.336 nm or less, the crystallite size (Lc) in the C-axis direction is 35 nm or more, preferably 45 nm or more, and the crystallite size (La) in the a-axis direction is 5
0 nm or more, preferably 60 nm or more, and (10
1) Peak ratio of diffraction peak to (100) diffraction peak (P
₁₀₁ / P ₁₀₀₎ is 1.0 or more, preferably 1.4 or more. The upper limit of the diffraction peak ratio is not limited uniformly because of the relationship with other factors, but it is usually preferably about 1.8. These are indices indicating the degree of graphitization of the graphite material, and satisfying all of them in the present invention is required to improve the performance of the battery.

The X-ray diffraction method uses Cukα as an X-ray source and high-purity silicon as a standard substance, and measures the diffraction pattern of carbon fibers and the like. , Respectively, the distance d between graphite layers
_(002), the c-axis direction of the crystallite size Lc _(002), and 1
The size La ₍₁₁₀₎ of the crystallite in the a-axis direction is calculated from the peak positions and the half widths of the ten diffraction patterns. The calculation method is based on the Gakushin method. In the measurement of the diffraction peak ratio of 101/100, a base line was drawn on the obtained diffraction diagram, and 101 (2θ ≒ 44.5) was obtained from the base line.
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.

Raman Analysis In the Raman spectrum of a carbon material, two peaks are observed around 1340 cm ⁻¹ and 1580 cm ⁻¹ . The former is derived from a carbon bond in an edge portion where the crystal structure of graphite is not developed, and the latter is derived from a carbon double bond in which the graphite structure is developed. Usually, for highly graphitized materials, typically 13
The peak around 40 cm ^-1 has disappeared or has a very low intensity. Therefore, the intensity ratio of the two peaks (I ₁₃₄₀ / I ₁₅₈₀ ) can represent the degree of graphitization of the surface of the carbon material. Generally, the intensity ratio of graphite material is 0.4 or less. However, Japanese Patent Application Laid-Open No. Hei 8-31422
As disclosed in Japanese Unexamined Patent Publication, a peak near 1340 cm ^-1 is remarkably present in the graphite material containing boron.

Therefore, in the present invention, a graphite material having an intensity ratio of 0.5 or more, preferably 0.6 or more is suitable for a lithium battery. In particular, the upper limit of the peak intensity ratio is not limited uniformly by the X-ray diffraction value, but is usually preferably about 0.9. The reason for the boron-containing graphite material is unknown, but from the results of X-ray diffraction and Raman analysis, a highly graphitized structure develops inside,
It is determined that the surface layer retains the state in which the carbon bond remains at the edge and the like. Further, the most important feature of the present invention is that
The peak around 40 cm ^-1 has 1310 to 1320 cm
^This is due to the presence of a shoulder-like peak near ^-1 .

[0012] This peak is considered to be derived from a structure in which part of carbon atoms in the hexagonal mesh plane of the graphite crystal is replaced by boron atoms. In such a state, boron atoms are present in the graphite crystal. It is considered that the electron energy level in the graphite crystal changes, thereby facilitating the intercalation of lithium between the graphite crystal layers, and increasing the charge / discharge capacity when used as a battery negative electrode material. Note that the Raman analysis of the present invention is performed using He at a wavelength of 632.8 nm.
Using Ne laser was performed at a resolution 0.2 cm ^-1, the measurement range 1000～1800Cm ^-1 at room temperature. Further, the peak intensity ratio was found to be around 1340 cm ⁻¹ and 15
The heights of the two peaks near 80 cm ⁻¹ were calculated, and the heights were calculated.

(2) Production of graphite material: An example of producing a graphite fiber mill, which is a graphite material suitable for the present invention, will be described below. (i) Carbon fiber raw material As the carbon fiber raw material used in the present invention, 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 pitch. In particular, the use of petroleum or coal pitch, preferably the use of an optically anisotropic pitch, ie, a mesophase pitch, is preferable from the viewpoint of increasing the capacity of a battery. The mesophase pitch is not particularly limited as long as it can be spun, but a mesophase content of 100% is preferable in terms of spinnability and quality stability in addition to increasing the capacity of the battery.

(Ii) Production of Milled Carbon Fiber The above 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 from 0.1 mmΦ to 0.5 mmΦ, preferably from 0.15 mmΦ to 0.3 mmΦ. When the size of the spinning hole exceeds 0.5 mmΦ, the fiber diameter becomes 25μ.
m or more, and the fiber diameter tends to vary, which is not preferable in quality control. Spinning hole size is 0.1mmΦ
If it is less than 1, clogging tends to occur during spinning, and production of a spinning nozzle is difficult, which is not preferable.

The spinning speed is 500 m / min from the viewpoint of productivity.
Above, preferably 1500 m / min or more, more preferably 2000 m / min or more. The spinning temperature varies somewhat depending on the raw material pitch, but may be any temperature as long as it is higher than the softening point of the raw material pitch and lower than the temperature at which the pitch does not deteriorate, and is usually 300 to 400 ° C, preferably 300 to 380 ° C. The melt blow method also has an advantage that the graphite layer surface is easily arranged in 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, those having a low softening point and a high infusibilization reaction rate are 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, etc. 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 1) Carbon fiber milling The carbon fiber used in the present invention is preferably a milled carbon fiber milled to a predetermined particle size before the graphitization treatment. Usually, graphite materials for negative electrodes of lithium secondary batteries are:
Those having a particle size of about 1 to 200 μm are used, and the average particle size of the milled carbon fiber of the present invention is preferably in the range of 5 to 50 μm. If the average particle size is smaller than the preferred range, the number of active surfaces is unnecessarily increased, the decomposition of the electrolytic solution becomes severe, the initial charge / discharge efficiency is small, and the cycle deterioration is severe. On the other hand, if it is large, the bulk density of the electrode becomes low and the energy density per volume becomes small, which is not preferable. It is also not preferable from the viewpoint of short circuit. 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 milled carbon fiber of the present invention is preferably 1 or more and 30 or less, more preferably 1 or more and 20 or less.

When the aspect ratio exceeds 30, that is, when a carbon fiber mill having a relatively long fiber length is used, the bulk density decreases, the energy density per volume decreases, and
This may cause a short circuit between the positive and negative electrodes, which is not preferable. 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 obtained carbon fiber milled samples. From the viewpoint of the average particle size and the aspect ratio, the fiber diameter before milling is 4 μm or more and 25 μm in consideration of the volume reduction during milling and graphitization.
m or less is preferable. The milling method is described below.

2) Production of milled carbon fiber Milling may be performed at the stage of the infusibilized fiber. However, the infusibilized fiber is treated at a temperature of 250 ° C or more and 1500 ° C or less, preferably 500 ° C or more. It is desirable to mildly carbonize in an inert gas at a temperature of 900 ° C. or less and then mill. Mild carbonization of the infusibilized fiber at a temperature of 250 ° C or more and 1500 ° C or less can be relatively prevented from longitudinal cracking of the fiber after the milling, and the graphite layer surface newly exposed to the surface during the milling can be more improved. At the time of graphitization treatment at a high temperature, the condensation polymerization / cyclization reaction tends to proceed easily, the activity of the surface is reduced, and the effect of inhibiting the decomposition of the electrolytic solution is advantageous. When the fiber is heat-treated (carbonized or graphitized) at a temperature of 1500 ° C. or more and milled, cracks easily occur along the graphite layer surface developed in the fiber axis direction, and the total surface area of the milled carbon fiber is increased. The proportion of the fracture surface area occupying becomes large, and the decomposition of the electrolytic solution due to the localization of electrons on the fractured graphite layer surface is not preferable. Also,
At a temperature of 250 ° C. or less, carbonization hardly occurs and there is no effect of treatment.

3) Milling technology In order to mill the fiber after infusibilization or after mild carbonization, it is effective to use a Victory mill, a jet mill, a high-speed rotation 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 fiber is controlled by adjusting the number of rotations of the rotor, the angle of the plate, and the like. The milling may be performed by a grinding machine such as a ball mill. However, according to these methods, a pressing force in a direction perpendicular to the fiber acts, and longitudinal cracks in the fiber axis direction increase, which is not preferable. In addition, this method requires a long time for milling, and is not an appropriate milling method.

(Iii) Graphitization treatment 1) Addition of boron compound, etc. In the method for producing a graphite material of the present invention, graphitization treatment is performed at a high temperature of 2500 ° C. or more, preferably 2600 ° C. or more in the presence of a boron compound. Thus, there is a great feature in that a graphite material having a graphite structure as described above is developed at a high degree, and a graphite material whose battery capacity is greatly improved is manufactured. For this reason,
Add boron compound to carbon material such as carbon fiber milled,
It needs to be graphitized. The method of adding the boron compound generally includes, but not limited to, a method of directly adding a solid boron compound and uniformly mixing as necessary, and a method of immersing the boron compound in a solvent solution. Further, as described in Japanese Patent Application Nos. 9-68985 and 9-72824, it is also possible to add a boron compound at the stage of the raw material pitch.

The boron compound is added in an amount of not more than 15% by weight, preferably 1 to 10% by weight, based on the material to be graphitized. If the amount is less than 1% by weight, the effect of the present invention is small, and if it exceeds 15% by weight, the effect on cost is reduced. In addition, the residual amount of boron in the carbon material after the graphitization increases, which causes a problem that the carbon materials adhere to each other, which is not preferable. As the boron compound, in addition to boron alone,
Examples include boron carbide (B ₄ C), boron chloride, boric acid, boron oxide, sodium borate, potassium borate, copper borate, and nickel borate. Examples of the solvent for forming the solvent solution include water, methanol, glycerin, acetone and the like, and may be appropriately selected according to the boron compound to be used. When used as a solid, it is preferable to use a boron compound having an average particle size of 500 μm or less, preferably 200 μm or less, in order to uniformly mix with a mill or the like.

2) Special heat treatment conditions In the present invention, the inside of a carbon material such as a milled carbon fiber is highly graphitized, and the peak intensity ratio (I
In ₁₃₄₀ / I ₁₅₈₀₎ of 0.5 or more, and 1340cm there is a shoulder-shaped peak of 1310～1320Cm ^-1 to the peak near ^-1, remainder carbon bond in the surface layer portion, replacing the boron atom is part carbon atoms It is important that the graphite material is used. For this purpose, in the presence of a boron compound,
25 ° C. at a rate of 30 ° C./min, preferably 2 to 8 ° C./min.
It is necessary to perform the graphitization treatment at a temperature of at least 00 ° C, preferably at least 2600 ° C. Although the principle of the action by the addition of the boron compound is unknown, the effect of further promoting graphitization is obtained from the temperature around the melting point of the boron compound (the melting point of boron is 2080 ° C. and the melting point of boron carbide is 2450 ° C.), and the peak intensity is obtained. Preferably, the ratio (I ₁₃₄₀ / I ₁₅₈₀ ) is 0.5 or more.

Further, although the reason is not clear, when the temperature of the carbon material is raised to 2500 ° C. or more and the carbon material is graphitized, 220 ° C.
By selecting conditions for maintaining the temperature at a constant temperature in a temperature range of 0 to 2400 ° C. for 20 minutes or more, preferably 30 to 150 minutes, more preferably 30 to 120 minutes, 131
A shoulder-like peak at ⁰ to 1320 cm ^-1 appeared, and it was found that an increase in battery capacity was observed. This 2200
The temperature range of 2400 ° C. is determined to be the temperature range in which the above-mentioned substitution of boron and carbon is most likely to occur, and it is considered significant to maintain the temperature at a specific temperature in this temperature range for a certain period of time. Therefore, if the holding time is as short as less than 20 minutes,
Although no shoulder-like peak appears, it is not preferable. On the other hand, the upper limit is not particularly limited. However, if it is too long, the cost of graphitization increases remarkably, so that about 150 minutes is sufficient. In addition, the graphitization of carbon material is performed in the absence of oxygen.
For example, it is preferably performed in an inert atmosphere such as nitrogen gas or argon gas. This is because oxygen reacts with carbon in carbon material,
This is because carbon dioxide gas and the like tend to be generated and the yield of carbon material tends to be reduced.

(3) Negative electrode material for lithium ion secondary battery: 1) Shape of negative electrode graphite material The graphite material obtained by the present invention is suitable for forming a negative electrode by adding a binder such as polyethylene or polytetrafluoroethylene. A high-performance negative electrode can be easily obtained by performing a reduction treatment using lithium metal as a counter electrode after forming a pressure roll into a sheet or plate, for example. The negative electrode made of the graphite material thus produced has a large capacity per unit volume and is suitable for miniaturization of a battery. 2) Electrolyte etc. When a graphite material according to the present invention is used for a negative electrode to produce a lithium ion secondary battery, the electrolyte may be any as long as it can dissolve a lithium salt. Organic solvents having a large dielectric constant are preferred. Examples of the organic solvent include propylene carbonate, ethylene carbonate, tetrahydrofuran, 2-methyltetrahydrofuran, dioxolan, 4-methyl-dioxolan, acetonitrile, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate and the like. 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, or lithium antimonate hexafluoride, is suitable.

3) Positive Electrode As the positive electrode of the lithium ion secondary battery, for example, metal oxides such as chromium oxide, titanium oxide, cobalt oxide, and vanadium pentoxide, lithium manganese oxide (LiMn ₂ O ₄ ), Lithium cobalt oxide (LiC
oO ₂ ), lithium metal oxides such as lithium nickel oxide (LiNiO ₂ ); transition metal chalcogen compounds such as titanium sulfide and molybdenum sulfide; and polyacetylene;
A conductive conjugated polymer substance such as polyparaphenylene or polypyrrole can be used. 4) Others 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, a conductor that is electrochemically inert to an electrode, an electrolyte, or the like, for example, a metal such as copper, nickel, titanium, or stainless steel can be used in the form of a plate, a foil, or a rod. 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.

[0027]

According to the present invention, the principle of action of a boron compound is not clear by graphitizing a carbon material such as a milled carbon fiber while the boron compound is present, but the graphitization is highly advanced. In addition, it is possible to provide a graphite material for a negative electrode of a lithium secondary battery having excellent performance with large charge / discharge capacity and charge / discharge efficiency due to a unique structure specified by Raman analysis.

[0028]

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 was heated in air from room temperature to 300 ° C. at an average heating rate of 6 ° C./min to perform infusibility treatment. Subsequently, the infusibilized yarn was lightly carbonized at 700 ° C. and then pulverized by a high-speed rotating mill to obtain a carbon fiber mill having an average particle size of 18 μm.

5% by weight of boron carbide having an average particle size of 10 μm was added to the milled carbon fiber, and the mixture was stirred and mixed so as to be uniform.
After heating at a speed of 2 minutes, the temperature is maintained at 2300 ° C. for 40 minutes,
Next, the temperature was raised to 3000 ° C. at a rate of 3 ° C./min, and further maintained at 3000 ° C. for 1 hour, followed by cooling to obtain a milled graphite fiber. Table 1 shows the measurement results of the X-ray diffraction of the graphite fiber mill.
FIG. 1 and Table 1 show the results of the Raman analysis. From FIG.
The presence of a shoulder-like peak at 1310 to 1320 cm ^-1 was confirmed.

Next, 4.85 g of the milled graphite fiber was added to 0.1 g of the milled graphite fiber.
After a pellet was prepared by kneading with 15 g of polytetrafluoroethylene to form a negative electrode, a charge / discharge test was performed in a three-electrode cell. The test uses metallic lithium for the anode and standard electrode,
Lithium perchlorate (LiClO) was used as an electrolyte in a mixed carbonate solvent in which ethylene carbonate (EC) / dimethyl carbonate (DMC) was adjusted to a volume ratio of 1/1.
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 charge / discharge capacity characteristics is as follows.
The measurement was performed under a constant current charge / discharge of 00 mA / g, and the measurement potential range was set to a standard potential (0 to 2 V / Li / Li ⁺ ), and the measurement was repeated 10 times. Table 1 shows the measurement results. The first discharge capacity was 350 mAh / g, the charge / discharge efficiency was 93%, and the tenth discharge capacity was 349 mAh / g, and the charge / discharge efficiency was 100%.

(Comparative Example 1) The milled carbon fiber obtained in Example 1 was heated to 3000 ° C at a rate of 3 ° C / min in an argon atmosphere without adding a boron compound.
After maintaining at 000 ° C. for 1 hour, the temperature was lowered to obtain a milled graphite fiber. Table 1 shows the measurement results of the X-ray diffraction of the graphite fiber mill.
FIG. 2 and Table 1 show the results of the Raman analysis. From FIG.
The presence of a shoulder-like peak at 1310 to 1320 cm ^-1 was not recognized. Also, using the graphite fiber mill,
A negative electrode was produced in the same manner as in Example 1, and the results obtained by measuring the charge / discharge capacity characteristics are also shown in Table 1.

(Comparative Example 2) To the milled carbon fiber obtained in Example 1, 5% by weight of boron carbide having an average particle diameter of 10 μm was added in the same manner as in Example 1, and the mixture was stirred and mixed so as to be uniform. Under an atmosphere, the temperature was raised to 3000 ° C. at a rate of 3 ° C./min, and further kept at 3000 ° C. for 1 hour, and then lowered to obtain a milled graphite fiber. Table 1 shows the measurement results of the X-ray diffraction of the graphite fiber mill, and FIG. 3 and Table 1 show the results of the Raman analysis.
Shown in From FIG. 3, the presence of a shoulder-like peak at 1310 to 1320 cm ⁻¹ was not recognized. A negative electrode was produced using the milled graphite fiber in the same manner as in Example 1, and the results of measuring the charge / discharge capacity characteristics are also shown in Table 1.

[0033]

[Table 1]

[0034]

According to the present invention, a carbon material is graphitized in the presence of a boron compound, thereby being highly graphitized, and having a unique structure that can be specified by Raman analysis, thereby increasing the charge / discharge capacity. In addition, it has become possible to provide a graphite material suitable for a negative electrode for a lithium secondary battery having excellent charge / discharge cycle characteristics.

[Brief description of the drawings]

FIG. 1 is a graph showing Raman analysis results of the graphite material of Example 1.

FIG. 2 is a graph showing a Raman analysis result of the graphite material of Comparative Example 1.

FIG. 3 is a graph showing a Raman analysis result of the graphite material of Comparative Example 2.

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Claims (3)

[Claims] 1. A method for producing a carbon material in the presence of a boron compound at 2500
A graphite material which has been graphitized at a temperature of at least 100 ° C., wherein the graphite material has an (a) graphite interlayer distance (d ₀₀₂ ) of 0.
338 nm or less, and (b) the crystallite size (L
c) is 35 nm or more, (c) the crystallite size (La) in the a-axis direction is 50 nm or more, and (d) the peak ratio (P ₁₀₁ / P ₁₀₀ ) of the (101) diffraction peak and the (100) diffraction peak. )
Is 1.0 or more, and by Raman spectrum analysis (e)
When the peak intensity ratio (I ₁₃₄₀ / I ₁₅₈₀ ) is 0.5 or more, (f)
The peak around 1340 cm ^-1 has 1310 to 1320 c
A graphite material for lithium secondary batteries, characterized by having a shoulder-shaped peak at m ^-1 . 2. The graphite material for a lithium secondary battery according to claim 1, wherein the graphite material is a graphite fiber mill made from mesophase pitch. 3. The method according to claim 1, wherein the carbon material is 2,500 in the presence of a boron compound.
When performing the graphitization treatment at a temperature of at least
The method for producing a graphite material for a lithium secondary battery according to claim 1, wherein the temperature is maintained at a constant temperature in a temperature range of 400 ° C. for 20 minutes or more.