JP2003077471A - Graphite material for lithium ion secondary battery negative electrode and its manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable to obtain stably a higher discharge capacity and charge and discharge efficiency compared with the graphite material that is graphitized under the presence of a boron compound. SOLUTION: (1), (A) This is a manufacturing method of a graphite material for a lithium ion secondary battery negative electrode in which a graphite material, that has a specific graphite structure obtained by graphitization of the carbon material under the presence of a boron compound and contains B4 C, is (B) heated at the temperature of 1,800 deg.C or more. (2), Further, (C) it is heated at the temperature of 500-900 deg.C in the presence of oxygen gas atmosphere, or (C)' heated at the temperature of 2,000 deg.C or more under reduced pressure. (3) As an alternative method, (i) the graphitization process is carried out in argon atmosphere, and (ii) the carbon material that is graphitized is a milled carbon fiber using the mesophase pitch as a base material.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD 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 a graphitization treatment in the presence of a boron compound (including a simple substance of boron). Furthermore, the present invention is a lithium secondary battery in which high discharge capacity and charge / discharge efficiency are stably obtained by performing a specific post-treatment as compared with a conventional graphite material graphitized in the presence of a boron compound. It is characterized by the method for producing the graphite material for the negative electrode material.

[0002]

2. Description of the Related Art Generally, a secondary battery using an alkali metal such as lithium as a negative electrode active material has a high energy density and a high electromotive force, and since it uses a non-aqueous electrolyte, it has a wide operating temperature range and a long operating time. It has many advantages such as excellent storage and light weight and small size. Therefore, such a non-aqueous electrolyte lithium secondary battery is expected to be put to practical use as a high-performance battery for power supplies for portable electronic devices, electric vehicles, and power storage. The use of a carbon material or a graphite material as the 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 the size of graphite crystallites of these carbon materials is small and the arrangement of crystals is disordered, the charge / discharge capacity is insufficient, and when the current density during charge / discharge is set high, decomposition of the electrolytic solution occurs and the cycle It had many problems such as a shortened life. At present, graphite materials such as natural graphite and artificial graphite obtained by subjecting the above carbon materials to graphitization are attracting attention and studied as negative electrode materials for lithium ion secondary batteries. With natural graphite, if the degree of graphitization is high,
Although the chargeable / dischargeable capacity per unit weight is considerably large, there is a problem that the current density that can be taken out reasonably is small, and that charging / discharging at a high current density lowers the charge / discharge efficiency.

Further, in the conventional negative electrode using artificial graphite,
By increasing the degree of graphitization, the capacity between the graphite layers as a whole can be improved, and the charge / discharge capacity can also be improved.
As such a graphite material, among others, JP-A-6-16
As disclosed in Japanese Patent No. 8725, it is pointed out that carbon fiber obtained by graphitizing carbon fiber using mesophase pitch as a starting material was favorable from measurement results of various battery characteristics. However, even with these artificial graphites, no material having satisfactory charge / discharge capacity and charge / discharge efficiency has been obtained.

Attempts have also been made to use boron. For example, JP-A-3-245458 discloses a furfuryl alcohol-maleic anhydride copolymer or a polyamide-based fiber obtained by low-temperature firing at about 1200 ° C. and containing 0.1 to 2.0% by weight of boron. It has been proposed to use a carbon material or carbon fiber as a negative electrode material for a lithium secondary battery. In this case, even if the residual boron is increased, the charging / discharging capacity is not sufficiently increased, and in particular, no improvement is shown in the battery voltage. JP-A-5-251080
The JP, was added H ₃ BO ₃ and the like natural graphite 1000 ° C.
Since the carbon material fired at 3 becomes easy to take in lithium ions and improves the battery performance as a negative electrode material, a maximum of 10
Although it is disclosed to add boron up to wt%,
The mechanism has not been elucidated at all.

Further, in JP-A-9-63584 and JP-A-9-63585, the charge-discharge capacity is high and the energy density is high, which is obtained by graphitizing in the presence of a boron compound. Graphite materials for lithium-based secondary batteries, which have excellent charge / discharge cycle characteristics, have been proposed. Further, JP-A-8-31422 also discloses a similar technique relating to graphitization treatment in the presence of a boron compound. However, the graphite material that has been graphitized in the presence of a boron compound has some improvement in charge / discharge capacity, but the theoretical capacity of graphite (372 mAh /
It cannot be said that the value is sufficiently high with respect to g), and the expected improvement in terms of charge / discharge efficiency and the like is not necessarily observed, and further improvement in performance is desired. In addition, when the graphitization treatment in the presence of boron is performed in a nitrogen atmosphere, boron is taken into the surface of the graphite material in the form of boron nitride, and the presence of this boron nitride hinders improvement in battery capacity. It was happening.

[0007]

The present invention solves the above-mentioned conventional problems and improves the graphitization degree for improving the performance of a graphite material obtained by graphitizing in the presence of a boron compound. An object of the present invention is to provide a method for producing a graphite material for a negative electrode of a lithium ion secondary battery, which exhibits excellent characteristics in terms of charge / discharge efficiency by performing a specific post-treatment while improving the charge / discharge capacity.

[0008]

The present inventors have further improved the graphitization degree of a graphite material obtained by subjecting a carbon material to a graphitization treatment in the presence of a boron compound to increase the charge / discharge capacity as compared with the conventional case. As a result of intensive studies to achieve excellent charge-discharge efficiency, it was found that it is effective to perform a specific post-treatment again after the graphitization treatment, and the present invention has been completed. . That is, the present invention: (A) A carbon material is obtained by graphitizing in the presence of a boron compound, and the graphite interlayer distance (d ₀₀₂ ) by X-ray diffraction is 0.338 nm or less, and the intensity of the (101) diffraction peak is: And a graphite material having a (100) diffraction peak intensity ratio (P ₁₀₁ / P ₁₀₀ ) of 2.0 or more and containing B ₄ C,
(B) A method for producing a graphite material for a negative electrode of a lithium ion secondary battery, which comprises heat treatment at a temperature of 1,800 ° C. or higher. Further, (C) Furthermore, in an atmosphere in which oxygen gas is present, 500 ° C or higher 9
It is also characterized in that it is heat-treated at a temperature of 00 ° C or lower. Further, (C) 'is further characterized in that it is heat-treated at a temperature of 2,000 ° C or higher under reduced pressure. Also,

It is also characterized in that the graphitization treatment of the carbon material in the presence of the boron compound is performed in an argon atmosphere. It is also characterized in that (A) the carbon material that is graphitized in the presence of the boron compound is a carbon fiber obtained by milling mesophase pitch as a raw material. Moreover, the graphite material for lithium ion secondary battery negative electrodes obtained by the method in any one of is provided. Also provided is a lithium ion secondary battery negative electrode comprising the graphite material described above. Also provided is a lithium ion secondary battery comprising the negative electrode described above.

The present invention will be described in detail below. (1) Material to be graphitized In the present invention, the carbon material to be graphitized in the presence of the boron compound is not particularly limited as long as the graphite structure is developed by heat treatment, Various shapes such as fibrous, milled fibrous, paper-like, and film-like carbon materials and spherical carbon materials such as mesocarbon microbeads can be used. In particular, in the present invention, mesophase pitch is used as a raw material, spun and infusibilized by a conventional method, and as it is or 1,50
Carbon fibers that have been mildly carbonized at a temperature of 0 ° C. or less and then milled (in the present invention, sometimes referred to as carbon fiber milled) are preferably used. The mesophase pitch-based carbon fiber is easily graphitizable and has excellent battery characteristics as a graphite material. Further, the fiber length is short, that is, by milling, the proportion of the fiber cross section is increased to increase the lithium ion content. There are advantages that it is possible to easily move in and out, and the packing density of the electrodes can be increased.

(2) Production of Carbon Fiber Milled An example of a suitable production method of the carbon fiber milled material preferably used in the present invention will be described below. (i) 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, it is preferable to use petroleum or coal pitch, preferably optically anisotropic pitch, that is, mesophase pitch. The mesophase pitch is preferably a mesophase content of 100%, but is not particularly limited as long as spinning is possible. The softening point of the raw material pitch is also not particularly limited, but one having a low softening point and a high infusibilization reaction rate is advantageous in terms of production cost and stability in view of the relationship with the spinning temperature. . As a result, the softening point of the raw material pitch is 2
30 ° C or higher and 350 ° C or lower, preferably 250 ° C or higher and 31
It is 0 ° C or lower.

(Ii) Manufacture of carbon fiber milled The above raw materials are spun and infusibilized by a conventional method, and as they are or after mild carbonization treatment, they are milled. (B) Spinning, etc.The method for melt spinning the carbon fiber raw material, particularly the pitch raw material, is not particularly limited, and various methods such as melt spinning, melt blowing, centrifugal spinning, and overflow spinning may be used. However, the melt-blowing method is preferable from the viewpoint of the productivity during spinning and the quality of the obtained fiber. The size of the spinning holes during meltblowing is 0.1 mmφ or more and 0.5 mmφ or less, and preferably 0.15 mmφ or more and 0.3 mmφ or less. Spindle hole size is 0.5 mm
If φ is exceeded, the fiber diameter tends to increase to 25 μm or more, and the fiber diameter tends to fluctuate, which is not preferable in quality control. If the size of the spinning hole is less than 0.1 mmφ, clogging during spinning tends to occur, and it is difficult to manufacture a spinning nozzle, which is not preferable. From the viewpoint of productivity, the spinning speed is 500 m / min or more, preferably 1500 m / min or more, more preferably 20 m / min.
It is more than 00m. The spinning temperature is somewhat changed depending on the raw material pitch, but may be a temperature below the softening point of the raw material pitch and at which the pitch does not change, and is usually 300 ° C or higher and 400 ° C or lower, preferably 300 ° C or higher and 380 ° C or lower. . The melt-blowing method, which is a preferable spinning method, 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 poise or less and cooling at high speed.

(B) Infusibilization, etc. The pitch fiber after spinning is infusibilized by a conventional method. As the infusibilizing method, for example, a method of heat treatment in an atmosphere of an oxidizing gas such as nitrogen dioxide or oxygen, a method of treating in an oxidizing aqueous solution such as nitric acid or chromic acid, and further by light or γ rays It is possible to use a method of polymerizing. A simpler infusibilizing method is a method of heat treatment in air, which is slightly different depending on the raw material, but has an average heating rate of 3 ° C /
Heat treatment is performed for at least 5 minutes, preferably at 5 ° C./minute or more, while raising the temperature to about 350 ° C.

(C) Milling Method of Fiber The infusibilized fiber is then milled. -At this time, the infusible treated fiber is 1,500 ° C or lower,
It is also possible to mildly carbonize in an inert gas at a temperature of preferably 250 ° C. or higher and 1,500 ° C. or lower, more preferably 500 ° C. or higher and 900 ° C. or lower, and then mill. Therefore, if mildly carbonized and milled at such temperatures,
Longitudinal cracking of the fiber after milling can be relatively prevented, and the graphite layer surface newly exposed on the surface during milling tends to facilitate the condensation polymerization / cyclization reaction during the graphitization treatment at a higher temperature, This is advantageous because the activity of the surface is lowered and the decomposition of the electrolytic solution is prevented. In the milling after heat treatment (carbonization or graphitization) at a temperature over 1,500 ° C,
Cleavage is likely to occur along the graphite layer surface developed in the fiber axis direction, the ratio of the fracture surface area to the total surface area of the manufactured milled carbon fiber is large, and the presence of electrons on the fracture graphite layer surface is high. It is not preferable because the electrolyte solution is decomposed due to oxidization. Further, at a temperature of less than 250 ° C., carbonization hardly occurs and there is no effect of treating. In order to mill the fibers after infusibilization or after slight 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 the milling suitable for the present invention, it is common to the above various methods.
For example, it is suitable to cut the fibers in a direction perpendicular to the fiber axis by rotating the rotor having the plate attached thereto at a high speed. -The fiber length of the milled fiber is the number of rotations of the rotor,
It is possible to control by adjusting the angle of the plate and the size of the mesh of the filter attached around the rotor. -For the milling, a Henschel mixer, a ball mill,
Although there are methods using a grinder or the like, these methods are not preferable because a pressure force in the direction perpendicular to the fiber acts and vertical cracks occur in the axial direction of the fiber. Further, these methods require a long time for milling, and it is difficult to say that they are suitable milling methods.

(D) Particle diameter of carbon fiber milled, etc. Also, if the fibers are mischievously pulverized, on the contrary, the active graphite layer is exposed and reacts with the electrolytic solution, which causes disadvantages such as capacity reduction. Be careful. -For this reason, the average particle diameter of carbon fiber milled as an electrode material for lithium secondary batteries is generally 10 to 50 μm.
m, preferably 12 to 30 μm. When the average particle size is less than 10 μm, the number of active surfaces becomes unnecessarily large, the decomposition of the electrolytic solution becomes severe, the initial charge / discharge efficiency becomes small, and the cycle deterioration becomes severe. On the other hand, when it is larger than 50 μm, the bulk density of the electrode is low and the energy density per volume is low, which is not preferable. It is also not preferable from the viewpoint of short circuit. -The above average particle size is calculated from the particle size distribution by the laser diffraction method.

The aspect ratio (ratio of length to diameter of carbon fiber mill) of the carbon fiber mill of the present invention is 1
It is desirable to be 30 or more and preferably 1 or more and 20 or less. If the aspect ratio exceeds 30, that is, if a carbon fiber milled with a relatively long fiber length is used, the bulk density becomes low, the energy density per volume becomes low, and
It is not preferable because it may cause a short circuit of the negative electrode. Further, if the aspect ratio is less than 1, many fibers are longitudinally cracked in the fiber axis direction, which is not preferable. -The above-mentioned aspect ratio shows the average value of the value of 100 pieces of the carbon fiber milled obtained. From the viewpoint of the average particle diameter and the aspect ratio, the fiber diameter before milling is preferably 4 μm or more and 25 μm or less, and more preferably 4 μm or more and 18 μm or less, in consideration of volume reduction during milling and graphitization. .

(3) Graphitization Treatment The graphitization of the present invention is conducted at 2200 ° C. in the presence of a boron compound.
By treatment graphitized at a temperature higher than, there is a great feature in that to produce a high graphite structure [peak intensity ratio by X-ray diffraction (P ₁₀₁ / P _{1 00)} of 2.0 or more]. After infusibilization by the above method or after mild carbonization treatment at a temperature of 1,500 ° C. or less, to a carbon fiber milled,
A boron compound is added and graphitized. (A) Addition of boron compound-The addition of a boron compound is usually performed by a method of directly adding a solid boron compound and uniformly mixing when necessary, and a method of immersing the boron compound in a solvent solution, and the like. It is not limited. It is also possible to add the boron compound at the stage of the raw material pitch. The amount of the boron compound added is 15% by weight or less, preferably 2 to 15 as boron with respect to the material to be graphitized.
%, And more preferably 3% to 10% by weight. If it is less than 2% by weight, the desired sufficiently high graphitization degree may not be obtained, which is not preferable. Also, 15% by weight
If it exceeds, the effect on the cost decreases. In addition, the amount of residual boron in the graphite material after graphitization increases, and the graphite materials stick to each other, which is undesirable.

Examples of boron compounds include boron carbide (B ₄ C), boron chloride, boric acid, boron oxide, sodium borate, potassium borate, copper borate, nickel borate, and the like, in addition to elemental boron. To be -The solvent for forming the solvent solution includes, for example, water, methanol, glycerin, acetone, etc., and may be appropriately selected according to the boron compound used. When it is used as a solid, it has an average particle size of 500 μm or less, preferably 200 μm or less, more preferably 1.5 μm or more 15 in order to uniformly mix with carbon fiber milled etc.
It is preferably used as a boron compound having a particle size of 0 μm or less.

(B) Graphitizing conditions: In the present invention, it is important to highly graphitize the milled carbon fiber and the like. For this purpose, in the presence of a boron compound, at 2,200 ° C. or higher, Preferably 2,400
It is necessary to perform graphitization at a temperature of ℃ or more. Although the principle of action of the boron compound is unknown, the effect of further promoting graphitization from a temperature in the vicinity of the melting point of the boron compound (the melting point of boron is 2,080 ° C, the melting point of boron carbide is 2,450 ° C) ( That is, a graphitization catalyst effect) and an effect of increasing charge / discharge capacity when used as a battery negative electrode material are obtained. -For the graphitization treatment, a commonly used graphitization treatment method is used and is not particularly limited. For example, a so-called Acheson type furnace, which performs graphitization in an air atmosphere, can be used. -In the present invention, it is desirable to perform the graphitization treatment in an inert atmosphere preferably containing no nitrogen, for example, an argon gas atmosphere. By graphitizing in an argon gas atmosphere, etc., the nitrogen compound of boron, which has a negative effect on the negative electrode characteristics, is not substantially generated on the surface of the graphite material after the treatment, and the materials adhere to each other due to the nitrogen compound. By preventing the above, it is possible to add a high amount of a boron compound and perform graphitization treatment, which is preferable.

(4) Post-Treatment of Graphite Material By the above-mentioned method, the graphite material (A) is graphitized in the presence of a boron compound, whereby the graphite interlayer distance (d ₀₀₂ ) by X-ray diffraction is 0. 338 nm, the ratio of the intensity of the (101) diffraction peak and the intensity of the (100) diffraction peak (P ₁₀₁
/ P ₁₀₀₎ is 2.0 or more, graphite material having a high degree of graphitization are obtained. However, it has been found that the graphite material thus obtained does not exhibit the expected charge / discharge capacity despite the high degree of graphitization. As a result of investigating the cause, it was considered that the graphite material contains boron carbide (B ₄ C) after the graphitization treatment, and the boron carbide inhibits the negative electrode characteristics. Therefore, as a result of various studies on methods for efficiently removing boron carbide from the graphite material and exhibiting the performance expected from the high degree of graphitization, (B) further heat treatment at 1,800 ° C or higher is required. Then, the performance of the negative electrode is improved by performing (C) heat treatment at a specified relatively low temperature in an atmosphere containing oxidizing gas, or (C) 'heat treatment at a temperature of 2,000 ° C or higher under reduced pressure. I found that

Next, these processing conditions will be described. (i) Heat treatment (B) The graphite material graphitized as described above is further subjected to a nitrogen gas atmosphere or an argon gas atmosphere for further 1,
Heat treatment is performed at 800 ° C. or higher. Further, heat treatment using a so-called Acheson type furnace is also possible. The heat treatment temperature is not particularly limited as long as it is a temperature at which the boron carbide remaining in the graphite material can be removed.
It is carried out at a temperature of 800 ° C. or higher, preferably 2,100 ° C. or higher. When the heat treatment is performed in a nitrogen gas-containing atmosphere, boron nitride is formed on the surface of the graphite material during the heat treatment to impair the negative electrode characteristics. It is desirable to carry out at 3000 ° C. or higher. The heat treatment time is not particularly limited as long as boron carbide does not remain in the graphite material, but is usually 3
It is carried out for 0 minutes or longer, preferably 60 minutes or longer. By performing this heat treatment, most of the boron carbide in the graphite material can be removed.

(Ii) Low temperature heat treatment (C) It is preferable that the graphite material obtained by the above heat treatment is then heat treated at a relatively low temperature in an atmosphere containing oxygen gas. The oxygen concentration is not particularly limited as long as it is an atmosphere containing oxygen gas, but the oxygen concentration is usually 5% by volume or more, preferably 10% by volume or more. A simpler method is a method of heat treatment in air. The heat treatment temperature is preferably 500 ° C. or higher and 900 ° C. or lower, more preferably 600 ° C. or higher 8
20 minutes to 5 hours, preferably 30 minutes to 3 at 00 ° C or lower
Do on time. If the temperature is lower than 500 ° C, the effect of the present invention may not be obtained, which is not preferable. A temperature above 900 ° C. is not preferable because the oxidation reduction of the graphite material is promoted. By carrying out such low temperature heat treatment, it is possible to increase the specific surface area of the graphite material and facilitate the entry and exit of lithium ions, and to develop the high charge / discharge capacity expected from the graphitization degree of the graphite material, The discharge efficiency is also high.

(Iii) Heat treatment under reduced pressure (C) 'As another method in the present invention, after the heat treatment of the above (i), heat treatment at a relatively high temperature under reduced pressure is preferably performed. Heat treatment conditions under reduced pressure include 10t
under reduced pressure of not more than orr, preferably not more than 1 torr;
The temperature is 2,000 ° C or higher, preferably 2,200 ° 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 degree of depressurization and the heat treatment temperature, it cannot be unambiguously determined, but it is desirable to hold it for usually 30 minutes or longer, preferably 30 minutes to 20 hours. In this case, the higher the degree of pressure reduction and the higher the heat treatment temperature, the more easily the boron compound remaining in the graphite material is removed by sublimation or the like, and the pressure is higher than 10 torr or 20
A heat treatment temperature of less than 00 ° C. is not preferable because sublimation of the boron compound hardly occurs. Further, the upper limit of the temperature is not particularly limited, but it is preferable that the temperature is not higher than the heat treatment temperature for graphitization in view of cost and physical properties of the graphite material.

(5) Negative Electrode Material for Lithium Ion Secondary Battery The graphite material obtained by the present invention has a shape suitable for forming a negative electrode by adding a binder such as polyethylene, polyvinylidene fluoride or polytetrafluoroethylene, For example, it is obtained by pressure roll forming into a sheet or plate. The negative electrode thus produced has a large capacity per unit volume and is suitable for downsizing of batteries. Further, when the graphite material according to the present invention is used for the negative electrode to prepare a lithium ion secondary battery, the electrolytic solution may be any one capable of dissolving a lithium salt, but particularly has an aprotic dielectric constant. Large organic solvents are preferred. Examples of the organic solvent include propylene carbonate, ethylene carbonate, tetrahydrofuran, 2
-Methyltetrahydrofuran, dioxolane, 4-methyl-dioxolane, acetonitrile, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate and the like can be mentioned. These solvents can be used alone or in an appropriate mixture.

As the electrolyte, a lithium salt which produces a stable anion, for example, lithium perchlorate, lithium borofluoride, lithium hexamonate antimonate, lithium hexafluorophosphate (LiPF ₆ ) or the like is suitable. Further, as the positive electrode of the lithium ion secondary battery, for example, metal oxides such as chromium oxide, titanium oxide, cobalt oxide, vanadium pentoxide, lithium manganese oxide (LiMn ₂ O ₄ ), lithium cobalt oxide ( LiCo
O ₂ ), lithium metal oxides such as lithium nickel oxide (LiNiO ₂ ), chalcogen compounds of transition metals such as titanium sulfide and molybdenum sulfide, and conjugated polymer having conductivity such as polyacetylene, polyparaphenylene, polypyrrole A substance or the like can be used.

A separator made of synthetic fiber or glass fiber non-woven fabric, woven fabric, polyolefin-based porous membrane, polytetrafluoroethylene non-woven fabric, or the like is provided between the positive electrode and the negative electrode. -Also, a current collector can be used like a conventional battery. As the negative electrode current collector, a conductor that is electrochemically inactive in the electrode, the electrolytic solution, or the like, for example, a metal such as copper, nickel, titanium, or stainless steel can be used in the form of a plate, foil, or rod. -The secondary battery of the present invention uses a battery component such as the separator, current collector, gasket, sealing plate, and case, and a specific negative electrode of the present invention, and has a cylindrical shape, a rectangular shape, a button shape, or the like according to a conventional method. It can be assembled into a form of lithium-ion secondary battery.

[0028]

The graphite material for lithium ion secondary battery of the present invention is
(A) A carbon material such as carbon fiber milled is graphitized in the presence of a boron compound, and (B) heat-treated, if necessary, and further (C) in an oxygen gas atmosphere at a temperature of 500 ° C or higher 9
Graphite for negative electrode of lithium secondary battery, which has a large charge / discharge capacity and excellent charge / discharge efficiency by heat treatment at a temperature of 00 ° C or lower or (C) 'heat treatment at a temperature of 2,000 ° C or higher under reduced pressure The material can be obtained, and the performance for a lithium ion secondary battery can be further improved.

[0029]

EXAMPLES The present invention will be described in more detail with reference to the following examples.
These do not limit the scope of the invention. (Example 1) <Production of carbon fiber milled> Optically anisotropic and specific gravity of 1.2
Width 3 mm, using the petroleum mesophase pitch of No. 5 as the raw material
5 rows of spinning holes with a diameter of 0.2 mm in the slit
Using a spinneret having 00 pieces, heated air was ejected from 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 · minute. The spun fiber was collected on the belt while suctioning from the back surface of the belt made of a stainless steel wire mesh having a repaired portion of 20 mesh. The collected mat was heated in the air from room temperature to 300 ° C. at an average temperature rising rate of 6 ° C./min for infusibilization treatment. Subsequently, this infusible yarn was lightly carbonized at 650 ° C. and then pulverized with a cross flow mill to obtain a carbon fiber mill having an average particle diameter of 18 μm.

<Graphitization> 5% by weight of boron carbide having an average particle diameter of 10 μm was added to the carbon fiber mill obtained above, and the mixture was stirred and mixed so as to be uniform and then heated to 3,000 ° C. in an argon atmosphere. The temperature was raised at a rate of 3 ° C./min, and the temperature was maintained for 10 hours. When the degree of graphitization after graphitization is measured by X-ray diffraction, the graphite interlayer distance (d ₀₀₂ ) = 0.3355
nm, crystallite size in the C-axis direction (Lc) = 100 nm
As described above, the crystallite size in the a-axis direction (La) = 100 nm
As described above, the peak ratio (P ₁₀₁ / P ₁₀₀ ) of the (101) diffraction peak and the (100) diffraction peak was 2.4. Furthermore, from the measurement result of X-ray diffraction of this material, it was confirmed that boron carbide remained in the material.

<Post-Treatment> The obtained graphitized carbon fiber mill was further heat-treated at 3,000 ° C. for 1 hour in a nitrogen atmosphere. Next, this heat-treated graphitized carbon fiber mill was heat-treated in air at 700 ° C. for 90 minutes. When the degree of graphitization after the post-treatment is measured by X-ray diffraction, the graphite interlayer distance (d002) = 0.3355 nm, the crystallite size in the C-axis direction (Lc) = 100 nm or more, and the crystallite size in the a-axis direction. (La) = 100 nm or more, the peak ratio of the (101) diffraction peak to the (100) diffraction peak (P101 / P1
00) = 2.4. In addition, boron carbide was not found from the measurement results of the X-ray diffraction of the material.

(Charge / Discharge Test) A negative electrode was prepared using the graphitized carbon fiber mill obtained by the above post-treatment. N-methylpyrrolidinone solution of polyvinylidene fluoride was added to 93 parts by weight of graphitized carbon fiber milled so as to be 7 parts by weight of polyvinylidene fluoride to form a slurry having a thickness of 18 μm.
It was applied to a copper foil of m to obtain a negative electrode. Using this negative electrode, a charge / discharge test was conducted using a 3-electrode cell. That is, using metallic lithium for the counter electrode and the reference electrode, ethylene carbonate (EC)
/ Diethyl carbonate (DEC) in a mixed carbonate ester solvent prepared in a volume ratio of 1/1, lithium perchlorate (LiClO ₄ ) as an electrolyte was dissolved in a concentration of 1 mol / l The charge / discharge capacity characteristics were measured.
Charging / discharging was carried out at a constant current of 100 mA / g-10 mV-constant voltage for 8 hours, and discharging was performed at a constant current of 100 mA / g (2.
The electric potential was set to 0 V / Li / Li ⁺ ) and the measurement was repeated 10 times. As a result, the initial discharge capacity of 354 mAh /
g, charge and discharge efficiency was 92.8%, which were all excellent values.

(Example 2) <Oxidation heat treatment> Graphitization was carried out in the same manner as in Example 1 and, after further heat treatment, heat treatment was carried out in air at 700 ° C. for 150 minutes. The obtained graphite material was subjected to X-ray diffraction measurement. As a result, the graphite interlayer distance (d ₀₀₂ ) = 0.3355 nm,
C-axis crystallite size (Lc) = 100 nm or more,
Crystallite size in the a-axis direction (La) = 100 nm or more,
The peak ratio of the (101) diffraction peak to the (100) diffraction peak was (P ₁₀₁ / P ₁₀₀ ) = 2.4. Also, as a result of similarly measuring the charge / discharge characteristics, the discharge capacity was 356 mAh /
g, and the charge / discharge efficiency was 93.0%.

(Example 3) <Heat treatment under reduced pressure> Graphitization was carried out in the same manner as in Example 1,
The graphitized carbon fiber milled obtained by further heat treatment,
Then, under a reduced pressure of 0.1 torr in a nitrogen atmosphere, 2,30
Heat treatment was performed at 0 ° C. for 1 hour. The obtained graphite material is X
As a result of line diffraction measurement, the graphite interlayer distance (d ₀₀₂ ) = 0.3
355 nm, C-axis crystallite size (Lc) = 10
0 nm, crystallite size in the a-axis direction (La) = 100 n
The peak ratio (P ₁₀₁ / P ₁₀₀ ) = 2.4 of the (101) diffraction peak and the (100) diffraction peak was 2.4 or more. Also,
From the measurement results of X-ray diffraction, no boron carbide was detected. Similarly, as a result of measuring charge / discharge characteristics, the initial discharge capacity was 356 mAh / g and the charge / discharge efficiency was 93.2%.

(Comparative Example 1) With respect to the graphitized fiber mill obtained by performing only the graphitization treatment in Example 1, the charge and discharge characteristics were measured. As a result, the discharge capacity was 342 mAh / g and the charge and discharge efficiency was 90.1. %, A relatively high value was exhibited, but the performance expected from the graphitization degree was not exhibited.

[0036]

According to the present invention, a carbon material is graphitized in the presence of a boron compound to improve the graphitization, and a graphite material is subjected to a specific post-treatment, that is, a heat treatment (B). Further, by heat treatment (C) in an oxygen gas-containing atmosphere or heat treatment (C) 'under reduced pressure, as described above, the boron compound, which is considered to impede the performance improvement of the battery, is not substantially contained, and is high. (EN) It is possible to provide a graphite material suitable for a negative electrode for a lithium-ion secondary battery, which has a stable discharge capacity.

   ─────────────────────────────────────────────────── ─── Continued front page    (71) Applicant 301054313             Petka Materials Co., Ltd.             2-10-1 Toranomon, Minato-ku, Tokyo (72) Inventor Norimune Yamazaki             Kashima Stone, 4 Towada, Kamisu Town, Kashima District, Ibaraki Prefecture             Oil Kashima Refinery Co., Ltd. (72) Inventor Toshifumi Kawamura             Kashima Stone, 4 Towada, Kamisu Town, Kashima District, Ibaraki Prefecture             Oil Kashima Refinery Co., Ltd. (72) Inventor Toshio Tamaki             Kashima Stone, 4 Towada, Kamisu Town, Kashima District, Ibaraki Prefecture             Oil Kashima Refinery Co., Ltd. F term (reference) 4G046 EA05 EB02 EB04 EC01 EC06                 4L037 AT05 CS04 FA02 FA05 PA31                       PC10 PF12 PF23 PG04 PP03                       PP38 PS12 UA04 UA20                 5H029 AJ02 AJ03 AK02 AK03 AK16                       AL07 AM02 AM03 AM05 AM07                       CJ02 CJ28 DJ16 DJ17 HJ13                       HJ14 HJ15                 5H050 AA02 AA08 BA17 CA02 CA08                       CA09 CA11 CA20 CA21 CA22                       CB08 FA17 FA19 GA02 HA13                       HA14 HA15

Claims (8)

[Claims] 1. A graphite material obtained by graphitizing a carbon material (A) in the presence of a boron compound, having an inter-graphite distance (d ₀₀₂ ) of 0.338 nm or less by X-ray diffraction and an intensity of a (101) diffraction peak. Intensity ratio of (100) diffraction peak (P ₁₀₁ /
A graphite material for a negative electrode of a lithium-ion secondary battery, characterized in that (B) a graphite material having P ₁₀₀ ) of 2.0 or more and containing B ₄ C is heat-treated at a temperature of 1,800 ° C. or more. Manufacturing method. 2. (C) Further, in an atmosphere in which oxygen gas is present, 50
The method for producing a graphite material for a negative electrode of a lithium ion secondary battery according to claim 1, wherein the heat treatment is performed at a temperature of 0 ° C. or higher and 900 ° C. or lower. 3. The method for producing a graphite material for a negative electrode of a lithium ion secondary battery according to claim 1, wherein (C) ′ is further heat-treated at a temperature of 2,000 ° C. or higher under reduced pressure. 4. The graphite material for a negative electrode of a lithium ion secondary battery according to claim 1, wherein the graphitization treatment of the carbon material in the presence of a boron compound is performed in an argon atmosphere. Manufacturing method. 5. The carbon material (A) to be graphitized in the presence of a boron compound is a carbon fiber obtained by milling mesophase pitch as a raw material.
5. The method for producing a graphite material for a lithium ion secondary battery negative electrode according to any one of 4 above. 6. A graphite material for a negative electrode of a lithium ion secondary battery, which is obtained by the method according to any one of claims 1 to 5. 7. A lithium ion secondary battery negative electrode comprising the graphite material according to claim 6. 8. A lithium-ion secondary battery comprising the negative electrode according to claim 7.