JPH10162829A - Negative electrode material for lithium ion secondary battery and manufacture thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery to stably supply high discharging capacity by forming a boron nitride uncontained graphite material by performing reheating treatment at a high temperature on a graphite material containing a boron nitride on which graphitization processing is performed, and using it as a negative electrode. SOLUTION: A boron compound is made to exist, and a graphitization processing is performed in a nitrogen atmosphere, and a graphite material on which an inter-graphite layer distance by X-ray diffraction is not more than 0.338nm and the size of a crystallite is not less than 35nm in the (c) axis direction and is not less than 50nm in the (a) axis direction and the ratio of a diffraction peak is not less than 1.0, is obtained. Heat treatment of a temperature not less than 2000 deg.C is also performed on this boron nitride containing graphite under prescribed reduced pressure or in a halogen gas containing atmosphere. Therefore, a baron nitride regarded as hindering the improvement of battery performance is removed, and when it is graphitized to a high degree and is used as a negative electrode, high discharge capacity is stably obtained, and performance of a lithium ion secondary battery can be improved.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to an improved technique for a graphite material for a negative electrode material of a lithium secondary battery, which is obtained by graphitization in a nitrogen atmosphere in the presence of a boron compound (including boron alone in the present invention). About. Furthermore, the present invention is a graphite material for a lithium secondary battery negative electrode material, which is highly graphitized at a relatively low temperature than ordinary graphitization, and a high discharge capacity is stably obtained,
And its manufacturing method.

[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 a carbon material or a graphite material as a negative electrode material of this lithium secondary battery has been studied. For example, as the carbon material, coal, coke, PAN-based carbon fiber, isotropic pitch-based carbon fiber, and the like have been studied.

However, since these carbon materials have a small graphite crystallite size and disordered crystal arrangement, the charge / discharge capacity is insufficient. If the current density at the time of charge / discharge is set high, decomposition of the electrolytic solution will occur. And many problems such as a reduction in cycle life. Currently, graphite materials such as natural graphite and artificial graphite obtained by graphitizing the above carbon materials and the like have been attracting attention and studied as negative electrode materials for lithium ion secondary batteries. 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. Such a material is used for a negative electrode of a high-load power supply which needs to take out a large current and which is preferably charged at a high current density in order to shorten a charging time, for example, a power supply for a device having a drive motor or the like. Was unsuitable.

In a conventional negative electrode using artificial graphite,
If the degree of graphitization is high, the capacity between the graphite layers as a whole is sufficient and a large charge / discharge capacity can be obtained, but it is still unsuitable for charge / discharge at a high current density. In addition, in the current lithium ion secondary battery using a negative electrode containing a graphite material, the current density during charging is generally 20 to 35 mA / g, and a charging time of about 10 hours is required in view of the charging capacity. However, if charging at a high current density becomes possible, for example, about 3 hours at 100 mA / g and 3 hours at 600 mA / g.
It should be possible to reduce the charging time to about 0 minutes.

[0006] Among these graphite materials,
As disclosed in JP-A-6-168725, it has been pointed out that a carbon fiber obtained by using a mesophase pitch as a starting material is graphitized is excellent in 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, etc., and even in the above-mentioned graphite fibers, the internal structure of the fibers is a graphite material for lithium ion secondary batteries. It is difficult to say that the structure is controlled to an optimal structure, and at present, a structure that satisfies all of the cycle life and charge / discharge capacity has not been developed.

As examples of using boron, JP-A-6-333601 and JP-A-7-73898 disclose that a carbon material in which a part of carbon atoms constituting a graphite layer is substituted by boron atoms is lithium. A lithium secondary battery having a large charge / discharge capacity of lithium used as a carbon material for a secondary battery is described. However, any of the disclosed methods uses boron chloride (BCl ₃ ) and benzene (C ₆ H ₆ ). C using
This is a method of synthesizing by the VD method. In addition, in order to replace the carbon atoms constituting the crystal lattice of the graphite layer with other atoms, a special complicated device is required and the degree of substitution is controlled. It has the disadvantage of requiring a fairly high degree of skill.

Japanese Patent Application Laid-Open No. 3-245458 discloses that a furfuryl alcohol-maleic anhydride copolymer or polyamide fiber is fired at a low temperature of about 1200 ° C. and that boron of 0.1 to 2.0% by weight is obtained. It has been proposed to use the contained carbon material or carbon fiber as a negative electrode material of a lithium secondary battery. In this case, the increase in the charge / discharge capacity was not sufficient even with the increase in the residual boron, and no improvement was shown in the battery voltage in particular. JP-A-5-251080
The JP, was added H ₃ BO ₃ and the like natural graphite 1000 ° C.
The carbon material calcined in the above can easily take in lithium ions and improve the battery performance as a negative electrode material.
It is disclosed that boron is added up to WT%, but the mechanism is not elucidated at all.

First, the applicant has filed Japanese Patent Application No. 7-23207.
In the invention of No. 1,232072 (hereinafter referred to as "the prior invention"), a charge / discharge capacity is large, a high energy density is obtained, and a charge / discharge cycle is obtained by graphitization treatment in the presence of a boron compound. A graphite material with excellent characteristics for lithium secondary batteries was proposed. Further, Japanese Patent Application Laid-Open No. 8-31
No. 422 also discloses a similar technique relating to graphitization in the presence of a boron compound.

However, as a result of subsequent research, when the graphitization treatment in the presence of a boron compound is performed in a nitrogen atmosphere, boron is incorporated into the graphite material surface in the form of boron nitride, and the presence of this boron nitride In addition, there is a problem that the improvement of the battery capacity is hindered.

[0011]

SUMMARY OF THE INVENTION Accordingly, the present invention relates to a method for improving the battery capacity of a graphite material obtained by graphitization in a nitrogen atmosphere in the presence of a boron compound. The goal is to efficiently remove.

[0012]

Means for Solving the Problems The present inventors further re-heat-treated a graphite material containing boron nitride graphitized in a nitrogen atmosphere in the presence of a boron compound at a high temperature under specific conditions. A graphite material substantially free of boron nitride was obtained, and it was found that a high discharge capacity could be stably supplied when the graphite material was used as a negative electrode material of a lithium ion secondary battery, and the present invention was completed. That is, the present invention:

[0013] A graphite material which has been graphitized in a nitrogen atmosphere in the presence of a boron compound and further heat-treated under reduced pressure or a halogen gas atmosphere, wherein the graphite material is X
Graphite interlayer distance (d ₀₀₂ ) by X-ray diffraction is 0.338 nm
Hereinafter, the crystallite size (Lc) in the C-axis direction is 35 nm or more, the crystallite size (La) in the a-axis direction is 50 nm or more, and the peak ratio between the (101) diffraction peak and the (100) diffraction peak ( (P ₁₀₁ / P ₁₀₀ ) is 1.0 or more, and a graphite material for lithium ion secondary batteries, substantially free of boron nitride is provided. It is also characterized in that carbon fibers made from mesophase pitch are milled and then graphitized. Also,

[0014] Using mesophase pitch as a raw material, spinning and infusibilization are carried out by a conventional method.
Milling after mild carbonization at a temperature of 0 ° C or less,
Thereafter, a boron compound is added, and under a nitrogen atmosphere,
The present invention provides a method for producing a carbon material for a lithium ion secondary battery as described above, wherein the carbon material is graphitized at a temperature of 200 ° C. or more, and further heat-treated at a temperature of 2,000 ° C. or more under reduced pressure.
Also, using mesophase pitch as a raw material, spinning and
Infusibilize, further milled as it is or after mild carbonization at a temperature of 1,500 ° C. or less, then add a boron compound and graphitize at a temperature of 2,200 ° C. or more under a nitrogen atmosphere, Further, the present invention provides the method for producing a graphite material for a lithium ion secondary battery described above, wherein the reheat treatment is performed at a temperature of 2,000 ° C. or more in a halogen gas atmosphere.

Hereinafter, the present invention will be described specifically. (1) Graphite material; (i) Carbon material to be graphitized; The carbon material to be graphitized in a nitrogen atmosphere in the presence of a boron compound in the present invention is one that develops a graphite structure by heat treatment. It is not particularly limited as long as it has various shapes such as fibrous, milled fiber, paper, and film-like carbon materials and spherical carbon materials such as mesocarbon microbeads. In the present invention, milled carbon fibers (particularly, mesophase pitch-based carbon fibers) are preferably used in the present invention, as in the prior application invention.

(Ii) the structure of the graphite material;
As disclosed in the prior application, the structure of a graphite material which has been graphitized in a nitrogen atmosphere in the presence of a boron compound has a graphite interlayer distance (d ₀₀₂ ) by X-ray diffraction of 0.338 nm or less, preferably 0.338 nm or less. 0.336 nm or less, crystallite size (Lc) in the C-axis direction is 35 nm or more, preferably 45 nm
As described above, the crystallite size (La) in the a-axis direction is 50 nm or more, preferably 60 nm or more, and the peak ratio between the (101) diffraction peak and the (100) diffraction peak (P ₁₀₁ / P ₁₀₀ ).
Is 1.0 or more. These are indices indicating the degree of graphitization of the graphite material, and it is required that all of them be satisfied in order 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.

1) The X-ray diffraction method means that Cukα is an X-ray source,
It uses high-purity silicon as a standard substance and measures diffraction patterns of carbon fibers and the like. And that 00
From the peak position and half width of the two diffraction patterns, the graphite interlayer distance d ₍₀₀₂₎ and the crystallite size Lc in the c-axis direction are respectively obtained.
The crystallite size La ₍₁₁₀₎ in the a-axis direction is calculated from the ₍₀₀₂₎ and the peak position and the half width of the 110 diffraction pattern.
The calculation method is based on the Gakushin method. 2) For the measurement of the peak ratio of 101/100, a baseline was drawn on the obtained diffraction diagram, and 10%
The height of each peak of 1 (2θ ≒ 44.5) and 100 (2θ ≒ 42.5) was measured, and the diffraction peak height of 101 was set to 10
It is determined by dividing by zero diffraction peak height.

The reheat treatment is performed at a temperature lower than the graphitization temperature in principle from the viewpoint of cost, and the graphite structure itself is not particularly changed by the reheat treatment.

(Iii) Milled carbon fiber: As the carbon material used in the present invention, as disclosed in the prior application, it is preferable to use milled carbon fiber, particularly, mesophase pitch-based carbon fiber. . Mesophase pitch-based carbon fibers are easily graphitized and have excellent battery characteristics as a graphite material.Furthermore, by shortening the fiber length, that is, by milling, the proportion of the fiber cross section is increased, and lithium ion It is preferable because it is easy to access and the filling density of the battery can be increased.

However, it should be noted that if the fibers are unnecessarily pulverized, the active graphite layer is exposed and reacts with the electrolytic solution, resulting in disadvantages such as a decrease in capacity. Therefore, generally, the average particle size of the milled carbon fiber used as the electrode material for the lithium secondary battery is 10 to 5 mm.
0 μm, preferably in the range of 12 to 30 μm. If the average particle size is less than 10 μm, the number of active surfaces is unnecessarily increased, the decomposition of the electrolyte solution becomes severe, the initial charge / discharge efficiency is small, and the cycle deterioration is also severe. On the other hand, 50 μm
If it is larger, the bulk density of the electrode is reduced, and the energy density per volume is undesirably reduced. It is also not preferable from the viewpoint of short circuit.

The average particle size is calculated from the particle size distribution by a laser diffraction method. The aspect ratio (the ratio of the length to the diameter of the milled carbon fiber) of the milled carbon fiber of the present invention is preferably 1 or more and 30 or less, and more preferably 1 or more and 20 or less. If the aspect ratio exceeds 30, 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 short circuit between the positive electrode and the negative electrode may occur, 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 value of 100 pieces of milled fibers obtained. From the viewpoint of the average particle diameter and the aspect ratio, the fiber diameter before milling is 4 μm in consideration of the volume reduction during milling and graphitization.
m to 25 μm, preferably 4 μm to 18 μm.

(2) Production of milled carbon fiber: An example of a preferred method for producing milled carbon fiber used in the present invention will be described below. (i) Raw Material As the raw material for carbon fiber 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, it is preferable to use petroleum or coal pitch, preferably to use 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 overflow spinning can be used. From the viewpoint of the properties and 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 it is difficult to manufacture a spinning nozzle, which is not preferable.

The spinning speed is 500 m / min in terms 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-blowing method, which is a preferred spinning method, also has an advantage that the graphite layer surface can be easily arranged in parallel with 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. As a result, the softening point of the raw material pitch is 230 ° C. or more and 350 ° C. or less, preferably 2 ° C. or less.
It is 50 ° C. or more and 310 ° C. or less.

(B) Infusibilization etc. The pitch fiber after spinning is infusibilized by a conventional method. As the infusibilization method, for example, a method of performing heat treatment in an oxidizing gas atmosphere such as nitrogen dioxide or oxygen, a method of performing treatment in an oxidizing aqueous solution such as nitric acid or chromic acid, and further, by light or γ-rays or the like It is possible to use a method of performing a polymerization treatment and the like. A simpler infusibilizing method is a method of performing a heat treatment in the air.
The heat treatment is performed at a rate of at least 5 minutes / minute, preferably at least 5 ° C./minute, to about 350 ° C.

(C) Milling method of fiber and the like The infusibilized fiber is then milled. At this time,
The infusibilized fiber is heated to 1,500 ° C or less, preferably 250 ° C to 1,500 ° C, more preferably 500 ° C or less.
It is also possible to mildly carbonize in an inert gas at a temperature of not less than 900C and not more than 900C, followed by milling. Therefore, if it is mildly carbonized and milled at such a temperature, longitudinal cracking of the fiber after milling can be relatively prevented, and the graphite layer surface newly exposed to the surface during milling can be graphitized at a higher temperature. Occasionally, the condensation polymerization / cyclization reaction tends to proceed easily, the activity of the surface is reduced, and there is an effect of preventing the decomposition of the electrolytic solution, which is advantageous.

In the milling after the heat treatment (carbonization or graphitization) at a temperature exceeding 1,500 ° C., cracks are apt to occur along the graphite layer surface developed in the fiber axis direction, and the manufactured milled steel is formed. The ratio of the fracture surface area to the total surface area of the broken carbon fiber becomes large, and the decomposition of the electrolytic solution due to the localization of electrons on the fractured graphite layer surface is not preferable. If the temperature is lower than 250 ° C., carbonization hardly occurs and there is no effect of the treatment.

In order to mill the fiber after infusibilization or after mild carbonization, it is effective to use a Victory mill, a jet mill, a 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, a pressure is applied in a direction perpendicular to the fiber, and longitudinal cracks are generated in the fiber axis direction. It is not preferable because it increases. In addition, these methods require a long time for milling, and are not appropriate milling methods. In particular, in order to manufacture a secondary battery negative electrode having a high bulk density, it is preferable to adjust the manufacturing conditions so that the aspect ratio of the milled fiber after graphitization is 1 or more and 30 or less, preferably 1 or more and 20 or less. .

(3) Graphitization The graphitization of the present invention is carried out at a high temperature of 2200 ° C. or more in a nitrogen atmosphere in the presence of a boron compound to obtain a highly graphitic structure (graphite interlayer by X-ray diffraction). Distance (d002)
Is 0.338 nm or less). After infusibilization by the above method or at 1,500 ° C
A boron compound is added to the milled fiber after mild carbonization at the following temperature, and the fiber is graphitized.

The addition of the boron compound is usually carried out by a method in which a solid boron compound is directly added and uniformly mixed as necessary, or a method in which the boron compound is immersed in a solvent solution, but is not particularly limited. . Further, it is sufficiently possible to add a boron compound at the stage of the raw material pitch. The amount of the boron compound to be added is 15% by weight or less as boron based on the material to be graphitized, preferably 1 to 1%.
0% by weight. 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. Further, the residual amount of boron in the graphite material after the graphitization increases, which causes a problem that the graphite materials adhere to each other, which is not preferable.

Examples of the boron compound include, in addition to boron alone, boron carbide (B ₄ C), boron chloride, boric acid, boron oxide, sodium borate, potassium borate, copper borate, nickel borate and the like. . 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. Also, when used as a solid,
500μm average particle size for uniform mixing with milled etc.
Or less, preferably 200 μm or less, more preferably 1.
It is preferable to use a boron compound having a size of 5 μm or more and 150 μm or less.

In the present invention, it is important to highly graphitize the milled carbon fiber and the like.
2,200 ° C or higher, preferably 2,400 ° C or higher, more preferably 2,2 ° C or higher under a nitrogen atmosphere in the presence of a boron compound.
It is necessary to perform the graphitization treatment at a temperature of 400 ° C or more and 3000 ° C or less. Although the principle of action of the boron compound is unknown, the melting point of the boron compound (the melting point of boron is 2,080
° C, the melting point of boron carbide is around 2,450 ° C), an effect of further promoting graphitization and an effect of increasing the charge / discharge capacity when used as a battery negative electrode material are obtained.

In general, graphitization of a carbonaceous material proceeds as the heat treatment temperature is increased. However, when a mesophase pitch-based carbon fiber is used as a raw material, the graphite for a battery having a high graphite structure defined in the present invention is used. In order to obtain a material, it is possible to graphitize in the presence of a boron compound at a temperature of about 2200 ° C., whereas if no boron compound is used, a temperature as high as about 700 ° C. and a high temperature of 2,900 ° C. or more Cost.
Furthermore, even in the graphite material having a structure similar to that shown by the X-ray diffraction measurement result, the graphite material graphitized in the presence of the boron compound has a higher discharge capacity than the graphite material in the case where the boron compound is not used. Excellent results were shown in terms of battery performance such as charge and discharge efficiency.

Therefore, in order to obtain the same battery performance as that of the graphite material obtained in the present invention, a higher temperature of about 3,100 ° C. is required unless a boron compound is used. The high temperature of 3,100 ° C. is not only cost-effective, but also in the state of the art, stable continuous operation is difficult, and the sublimation temperature of carbon (3,3
(70 ° C.).

The graphitizing treatment is preferably performed in the absence of oxygen. This is because oxygen tends to react with carbon in the carbon material to generate carbon dioxide gas and the like, thereby decreasing the yield of the graphite material. For this reason, in the boron compound added during the graphitization treatment, a compound containing oxygen, for example, boron oxide (B ₂ O ₃ ; melting point of 450 ° C. or more,
(Boiling point 1,500 ° C. or higher), etc., the effect of promoting graphitization can be obtained, but the yield of the graphite material tends to decrease.

(4) Re-heat treatment of graphite material under specific conditions; the present inventor has proposed that the graphite material obtained by graphitizing in the presence of a boron compound has a large charge / discharge capacity in the present invention. It has been shown that a carbon material for a lithium-based secondary battery having a high energy density and excellent characteristics of charge-discharge cycle characteristics can be provided. However, it has been found that it is difficult to stably obtain a high discharge capacity when the graphite material subjected to the graphitization treatment in a nitrogen atmosphere is used as a negative electrode material for a lithium ion secondary battery. As a result of intensive studies on the cause, it has been found that boron nitride is formed mainly in the surface layer portion of the graphite material, and this boron nitride is a factor inhibiting lithium ion intercalation.

Although boron nitride has a high melting point of 3,000 ° C. or more, it is difficult to remove it efficiently by ordinary heat treatment. However, the present inventor has made various attempts to remove boron nitride efficiently. As a result of the examination, (a) under reduced pressure or (b)
By performing re-heat treatment under specific conditions under a halogen gas atmosphere, a graphite material substantially free of boron nitride is obtained at a re-heat treatment temperature lower than the graphitization temperature, and the performance of the battery is improved. I found that. The detection of boron nitride can be performed by ESCA method or elemental analysis (plasma emission analysis method for boron, and distillation separation neutralization titration method for nitrogen for nitrogen). less than,
The reheating method under the above specific conditions will be described in detail.

(A) Reheat treatment under reduced pressure: Since the graphite material of the present invention contains boron nitride in its surface layer as it is, it is necessary to perform heat treatment under reduced pressure. As the heat treatment conditions under reduced pressure, 2,000 ° C. or more, preferably 2,300 ° C. or more, more preferably 2,300 ° C. under a reduced pressure of 10 torr or less, preferably 1.0 to 0.5 torr.
Not less than 2,500 ° C. Since the heat treatment time is a function of the degree of pressure reduction and the heat treatment temperature and the like, it cannot be uniquely determined, but is usually about 1 hour, preferably 30 minutes to
It is desirable to hold for about one hour. In this case, the higher the pressure reduction and the higher the heat treatment temperature, the more easily boron nitride is removed by sublimation.
If the heat treatment temperature is lower than 0 ° C., sublimation of boron nitride occurs, which is not preferable. Further, a reduced pressure of less than 0.5 torr is not preferable in terms of cost and safety of the apparatus, and the upper limit of the temperature of the reheat treatment is not particularly limited. However, from the viewpoint of cost and physical properties of the graphite material, graphitization is difficult. It is preferable that the temperature is not higher than the heat treatment temperature.

(B) Re-heat treatment in a halogen gas atmosphere;
The conditions of the reheat treatment in the halogen gas atmosphere include a mixed gas containing an inert gas such as nitrogen or argon and a halogen gas having a halogen gas concentration of 5 vol% to 70 vol%, preferably about 10 to 30 vol%. ,
2,000 ° C. or higher, preferably 2,300 ° C. or higher, more preferably 2,300 ° C. or higher, and 2,500 ° C. or lower. Since the heat treatment time is a function of the concentration of the halogen gas and the heat treatment temperature, it is unique. However, it is usually desirable to hold for about 3 hours, preferably for about 2 to 3 hours. The amount of the halogen gas used may be adjusted so as to be 1.5 times, preferably about 2 to 20 times, the amount of boron in the graphite material to be reheated, and then charged into the heat treatment furnace.

The higher the concentration of the halogen gas and the higher the heat treatment temperature, the more easily the boron nitride is removed by the reaction with the halogen gas.
If the heat treatment temperature is lower than the above, a reaction between boron nitride and a halogen gas occurs, which is not preferable. When the concentration of halogen gas is 7
If it exceeds 0 vol%, the amount of halogen gas used for removal tends to increase significantly, which is not preferable in terms of cost. Further, the temperature of the reheat treatment is the same as in the case of the reheat treatment under reduced pressure (a), from the viewpoint of cost and physical properties of the graphite material,
It is preferable that the temperature is not higher than the heat treatment temperature for graphitization. Also,
Examples of the type of halogen gas include chlorine, fluorine, and iodine. Use of chlorine or fluorine is desirable from the viewpoint of operability, cost, and availability.

(3) Anode 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, forming a pressure roll into a shape suitable for forming a negative electrode, for example, a sheet or plate, and then forming a counter electrode. By performing a reduction treatment using lithium metal, a high-performance negative electrode can be easily obtained. 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 which 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.

[0047]

The graphite material for a lithium ion secondary battery according to the present invention comprises:
(A) By subjecting the milled carbon fibers and the like to graphitization in a nitrogen atmosphere while the boron compound is present,
Although the principle of action of the boron compound is not clear, graphitization progresses to a high degree, charging and discharging efficiency is large, voltage efficiency is stabilized, and (b) after the above graphitization, re-heat treatment under specific conditions By doing so, boron nitride, which is considered to hinder the performance improvement of the battery as described above, since the graphite material is not substantially contained, a high discharge capacity can be obtained stably, and for lithium ion secondary batteries Performance can be further improved.

[0048]

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) <Reheat treatment under reduced pressure (a)> A petroleum-based mesophase pitch having an optical anisotropy and a specific gravity of 1.25 was used as a raw material, and a diameter of 0.3 mm was formed in a slit having a width of 3 mm.
Using a spinneret with 500 2mmφ spinning holes in a row,
Hot air was blown out of the slit 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 rate was 0.8 g / H · min. The spun fibers were collected on the belt while the repaired portion was suctioned from the back surface of the belt made of a stainless steel wire mesh of 20 mesh.

The collected mat was 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. 5% by weight of boron carbide having an average particle diameter of 10 μm was added to the milled 000 precursor, and the mixture was stirred and mixed so as to be uniform.
The temperature was raised to 3 ° C. at a rate of 3 ° C./min, and kept at that temperature for 1 hour.

When the degree of graphitization after graphitization was measured by X-ray diffraction, the distance between graphite layers (d ₀₀₂ ) = 0.3355 nm, the crystallite size in the C-axis direction (Lc) = 74 nm, and the crystallinity in the a-axis direction Child size (La) = 100 nm or more, (101)
Peak ratio of diffraction peak to (100) diffraction peak (P ₁₀₁
/ P ₁₀₀ ) = 1.72. When the surface of this milled graphite fiber was analyzed by the ESCA method, a B-1S peak and an N-1S peak appeared, and it was found that boron nitride was formed on the surface layer portion.

The milled graphite fiber was removed under reduced pressure (1.0 to
rr) and heat-treated at 2300 ° C. for 1 hour. When the degree of graphitization after the heat treatment was measured by X-ray diffraction, d ₀₀₂ = 0.3355 n
m, Lc = 74 nm, La = 100 nm or more, P ₁₀₁ /
P ₁₀₀ = 1.72, indicating that there was no change compared to before the heat treatment under reduced pressure. The milled graphite fiber is ES
When the surface was analyzed by the CA method, the B-1S peak and the N-
The 1S peak disappeared, indicating that boron nitride in the surface layer had been removed.

Further, the charge and discharge capacity characteristics of the milled graphite fibers were measured. That is, metal lithium is used for the anode and the reference electrode, and ethylene carbonate (EC) /
Dimethyl carbonate (DMC) in a mixed carbonic ester solvent was adjusted to 1/1 by volume, lithium perchlorate (LiCl0 ₄₎ was performed in an electrolytic solution obtained by dissolving 1 molar concentration as the electrolyte, the charge-discharge capacity The properties were measured. The measurement of the charge / discharge capacity characteristic was performed under a constant current charge / discharge of 100 mA / g, and the potential range to be measured was from the reference electrode (0 to 2 V / Li / L).
i ⁺ ), the measurement was repeated 10 times. Initial discharge capacity 3
40 mAh / g, a charge / discharge efficiency of 91%, a second discharge capacity of 338 mAh / g, and a charge / discharge efficiency of 99.8% showed high values. In addition, the discharge capacity was 338 mAh / g and the charge / discharge efficiency was 100 from the second time to the tenth time.
% And a stable value.

(Example 2) <Reheat treatment under halogen gas (b)> 100 g of the milled graphite fiber obtained in Example 1 before the heat treatment under reduced pressure was subjected to chlorine gas in nitrogen gas. 1
Heat treatment was performed at 2,300 ° C. for 2 hours in an atmosphere in which a mixed gas containing 0 vol% was introduced at a rate of 20 L. When the degree of graphitization after the heat treatment was measured by X-ray diffraction, d ₀₀₂ = 0.33
55 nm, Lc = 74 nm, La = 100 nm or more, P
₁₀₁ / P ₁₀₀ = 1.72, indicating that there was no change compared to before the heat treatment under chlorine gas. When the surface of the milled graphite fiber was analyzed by the ESCA method, it was found that B-1
The S peak and the N-1S peak disappeared, and it was found that the boron nitride in the surface layer had been removed. Further, when the milled graphite fiber was evaluated by a charge / discharge test using a three-electrode cell in the same manner as in Example 1, the initial discharge capacity was 339 mAh / g. Other battery characteristics were also measured and are shown in Table 1.

(Comparative Example 1) The milled graphite fiber obtained in Example 1 before the heat treatment under reduced pressure was evaluated by a charge / discharge test using a three-electrode cell in the same manner as in Example 1.
The initial discharge capacity was 05 mAh / g. Other battery characteristics were also measured and are shown in Table 1. The obtained milled graphite fiber has a lower value than that of the example even if it has a similar graphite structure, and it is determined that the presence of boron nitride has an adverse effect. The results are summarized in Table 1 below.

[0055]

[Table 1]

[0056]

According to the present invention, (a) a carbon material is graphitized in a nitrogen atmosphere in the presence of a boron compound, and is highly graphitized, so that the charge / discharge capacity is large and the charge / discharge cycle characteristics are high. In addition, (b) by re-heat treating the graphitized graphite material under specific conditions,
To provide a graphite material suitable for a negative electrode for a lithium ion secondary battery, which does not substantially contain boron nitride which is considered to hinder the performance improvement of the battery as described above and has a high discharge capacity stably obtained. Enabled.

Claims (4)

[Claims] 1. A graphite material which has been graphitized in a nitrogen atmosphere in the presence of a boron compound and further reheated under reduced pressure or a halogen gas atmosphere, wherein the graphite material is a graphite interlayer by X-ray diffraction. When the distance (d ₀₀₂ ) is 0.338 nm or less, the crystallite size (L
c) is 35 nm or more, and the crystallite size (L
a) is 50 nm or more, and (101) diffraction peak and (1)
00) The peak ratio of diffraction peaks (P ₁₀₁ / P ₁₀₀ ) is 1.
A negative electrode material for a lithium ion secondary battery, characterized by using a graphite material that is 0 or more and does not substantially contain boron nitride. 2. The negative electrode material for a lithium ion secondary battery according to claim 1, wherein the carbon fibers made from the mesophase pitch are milled and then graphitized. 3. Using mesophase pitch as a raw material, spinning and infusibilization by a conventional method, and further as it is or 1,500
Mildizing after mild carbonization at a temperature of not more than 2 ° C., then adding a boron compound and adding 2,2
The method for producing a graphite material for a lithium secondary battery according to claim 1, wherein after the graphitization treatment at a temperature of 00 ° C or higher, the heat treatment is further performed at a temperature of 2,000 ° C or higher under reduced pressure. 4. Using mesophase pitch as a raw material, spinning and infusibilizing by a conventional method, and further as it is or 1,500
Mildizing after mild carbonization at a temperature of not more than 2 ° C., then adding a boron compound and adding 2,2
2. The method for producing a graphite material for a lithium secondary battery according to claim 1, wherein after the graphitization treatment is performed at a temperature of at least 00 ° C., the heat treatment is further performed at a temperature of at least 2,000 ° C. in a halogen gas atmosphere. .