JP2001110415A - Graphite material for negative electrode of lithium ion secondary battery, manufacturing method of the same, negative electrode for lithium ion secondary battery using the graphite material, and lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a graphite material for the negative electrode of a lithium ion secondary battery, there a strength ratio I101/I100 of a plane 101 to a plane 100 is not less than 1.2 in X-ray diffraction measurement, and initial charging/ discharging efficiency and initial charging/discharging capacity, when soaked in electrolyte solution then charged at 30 to 50 degrees centigrade, are higher than those when soaked in electrolyte solution then charged at 25 degrees centigrade. SOLUTION: By using a graphite material for a negative electrode which contains a high ratio of graphite and has specific charging/discharging characteristic at high temperatures, a lithium ion secondary battery which shows high charging/discharging efficiency and has high discharging capacity after being charged or discharged especially at high temperatures can be provided.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a graphite material suitable as a material for a negative electrode used in a non-aqueous lithium ion secondary battery, a method for producing the graphite material, a negative electrode for a lithium ion secondary battery using the same, and It relates to a lithium ion secondary battery.

[0002]

BACKGROUND OF THE INVENTION Generally, a battery using an alkali metal as a negative electrode active material has a wide operating temperature range because it uses a high energy density, a high electromotive force, and a non-aqueous electrolyte, has excellent long-term storage properties, and is lightweight. Due to its many features, such as its small size, it is expected to be put to practical use as a high-performance battery for powering portable electronic devices, electric vehicles and power storage.

However, current prototype batteries do not sufficiently exhibit the characteristics inherent in lithium secondary batteries, and have incomplete cycle life, charge / discharge capacity, and energy density. One of the major reasons is the negative electrode used. For example, when metallic lithium is used for the negative electrode, lithium precipitated during charging forms needle-like dendrites, which tends to cause a short circuit between the positive electrode and the negative electrode, which is problematic in terms of cycle life and safety.

Further, since the reactivity of lithium is very high, the surface of the negative electrode is denatured by the decomposition reaction of the electrolyte, and there is a problem that the battery capacity is reduced by repeated use. In order to solve these problems in lithium secondary batteries, various negative electrode materials have been studied. For example, the use of lithium-aluminum, wood alloy, or the like for the negative electrode as an alloy containing lithium has been studied. However, there are problems such as a change in the crystal structure due to a difference in operating temperature or charge / discharge conditions.

In recent development trends, the mainstream study has been to use exclusively carbon-based materials (which are distinguished from carbon materials and graphite materials by the degree of graphitization) as the negative electrode active material. That is, it is an attempt to prevent the generation of dendrite by taking in (intercalating) lithium ions generated during charging between graphite layers to form a so-called interlayer compound.

The carbon materials include coal, coke, and P which are heat-treated at a relatively low temperature (generally, 2000 ° C. or lower).
AN-based carbon fibers, pitch-based carbon fibers, and the like have been studied. However, these carbon materials have a small graphite crystallite size and disordered crystal arrangement, so their charge / discharge capacity is inadequate. There were many problems such as a shortened life.

On the other hand, at present, graphite materials such as natural graphite and artificial graphite have received the most attention and are being studied as one of the carbon-based materials for the negative electrode of a lithium ion secondary battery. 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. There is a problem that the efficiency is reduced.

The artificial graphite as the graphite material has been heat-treated at a relatively high temperature (generally 2,000 ° C. or higher). In particular, JP-A-6-168725 discloses that the artificial graphite is obtained by spinning a mesophase pitch. It has been pointed out from the measurement results of various battery characteristics that the mesophase pitch-based graphite fiber obtained by infusing the melted pitch fiber and further carbonizing and graphitizing it is excellent as a negative electrode material for a lithium ion secondary battery.

It is known that the degree of graphitization of a graphite material is closely related to the charge / discharge capacity of a lithium ion secondary battery using the same as a negative electrode material. Therefore, how to increase the degree of graphitization is an important issue in manufacturing graphite materials for negative electrodes. For mesophase pitch-based graphite fibers, for example, a method of reducing the degree of infusibilization of pitch fibers, then carbonizing and graphitizing the obtained infusibilized fibers, and improving the degree of graphitization of the obtained graphite material is known. .

However, in the graphite fiber thus obtained, the charge / discharge capacity of a lithium ion secondary battery using the same as a negative electrode material is not so large as expected from the degree of graphitization, and the initial charge / discharge efficiency is low. There was a problem. Boron has long been known as an element for promoting graphitization, and is disclosed in Japanese Patent Application Laid-Open Nos.
JP-A-306359 discloses a method in which boron is added to carbon powder or pitch-based carbon fiber powder and graphitized at 2500 ° C. or higher to increase the degree of graphitization and improve the charge / discharge capacity.

However, in such a method, boron compounds such as boron nitride are formed on the surface of the graphite material, and these compounds inhibit lithium ion intercalation and have poor conductivity. There was a problem that the charge / discharge capacity was not improved as much as expected. The present inventors have conducted various studies and studies to solve the problems of the prior art associated with such a graphite material having a high degree of graphitization, and as a result, an infusible pitch fiber having a specific degree of infusibility,
In particular, graphite fibers obtained using infusible pitch fibers having specific IR analysis characteristics and a specific oxygen / carbon atom ratio have a high degree of graphitization, and are immersed in an electrolyte to obtain
Charging or charging / discharging at 50 ° C has a new and unique characteristic of higher initial charging / discharging efficiency and charging / discharging capacity than charging / discharging at 25 ° C, and once charging or charging at such a high temperature. It has been found that the discharge and discharge can maintain the high charge and discharge capacity even when the charge and discharge are performed under normal temperature conditions, for example, about 20 to 25 ° C., and the present invention has been completed.

[0012]

SUMMARY OF THE INVENTION The present invention has been made to solve the problems associated with the prior art as described above, and when used as a negative electrode material of a lithium ion secondary battery, the initial charge / discharge efficiency is high. Provided is a graphite material for a negative electrode having a high degree of graphitization, a method for producing the graphite material, and a negative electrode for a lithium ion secondary battery and a lithium ion secondary battery using the same, which can provide a secondary battery having a large discharge capacity. It is intended to be.

[0013]

SUMMARY OF THE INVENTION Graphite material for a negative electrode of a lithium ion secondary battery according to the present invention has 101 surfaces and 10 surfaces in X-ray diffraction measurement.
The zero surface intensity ratio I ₁₀₁ / I ₁₀₀ is 1.2 or more, and the battery is charged at 30 to 50 ° C. as compared with the initial charge / discharge efficiency and initial discharge capacity when immersed in an electrolytic solution and charged at 25 ° C. In this case, the initial charge / discharge efficiency and the initial discharge capacity are higher.

The graphite material for a negative electrode of the lithium ion secondary battery according to the present invention has an initial charge / discharge efficiency and an initial discharge capacity of 30 to 5 times when charged and discharged at 25 ° C.
The initial charge / discharge efficiency and the initial discharge capacity when charged and discharged at 0 ° C. are higher. The method for producing a graphite material for a negative electrode of a lithium ion secondary battery according to the present invention comprises an FT-IR
(Fourier transform infrared) 1670 cm
Peak area of the carbonyl group appears at ^-1 to 1830 cm ^-1 or less: S of (CO), 1530 cm ^-1 167
Peak area for ethylene bond appearing below 0 cm ^-1 : ratio of S (CO) / S (C = C) to S (C = C) is 0.40 to 0.5
8, characterized in that in the elemental analysis, the mesophase pitch-based infusible fiber having an atomic ratio of oxygen to carbon O / C of 0.045 to 0.060 is carbonized and then graphitized.

In the method for producing a graphite material for a negative electrode according to the present invention, it is desirable that the carbon fiber obtained in the carbonization step be milled and then graphitized. Also, after mixing the milled carbon fiber obtained by milling with a boron compound,
The degree of graphitization may be improved by graphitization. The graphite material having a high degree of graphitization obtained in this way has the above-mentioned X-ray diffraction characteristics and the above-mentioned high-temperature charge / discharge characteristics.

Further, in the method for producing a graphite material for a negative electrode according to the present invention, the graphite material obtained in the graphitization step may be subjected to a treatment of charging or discharging at 30 to 50 ° C. in an electrolytic solution. Good. By performing the treatment at such a high temperature, a high charge / discharge capacity can be maintained even after the charge / discharge is performed under normal temperature conditions, for example, about 25 ° C. The negative electrode for a lithium ion secondary battery according to the present invention is characterized by containing the graphite material or the graphite material obtained by the method for producing the graphite material for a negative electrode.

The lithium ion secondary battery according to the present invention comprises:
It is characterized by having such a negative electrode.

[0018]

DETAILED DESCRIPTION OF THE INVENTION The graphite material for a negative electrode of a lithium ion secondary battery according to the present invention has a high degree of graphitization, and in particular, the intensity ratio I ₁₀ between the 101 plane diffraction peak and the 100 plane diffraction peak in X-ray diffraction. ₁ / I ₁₀₀ is 1.2 or more, preferably 1.2
To 3.0, more preferably 1.5 to 2.5.

The peak intensity ratio I ₁₀₁ / I ₁₀₀ is an index indicating the degree of graphitization of the graphite material. By using a graphite material having a peak intensity ratio I ₁₀₁ / I ₁₀₀ in such a range, a high initial intensity can be obtained. A negative electrode material capable of realizing charge / discharge efficiency and charge / discharge capacity, particularly high efficiency and high capacity at a high temperature, can be obtained. The graphite material for a negative electrode of the present invention has a graphite interlayer distance (d _{00 2} ) of 0.3380 nm or less, preferably 0.3360 nm or less, as measured from X-ray diffraction, and a crystallite size (Lc) in the C-axis direction. ) Is 35 nm or more, preferably 48 n
m or more, and the size (La) of the crystallite in the a-axis direction is 50 nm.
Above, preferably 60 nm or more.

These are also indexes indicating the degree of graphitization of the graphite material, and it is desirable that the graphite material also satisfies these indexes in order to improve the performance of the battery. Here, various Xs used to define the structure of the graphite material in this specification are used.
The line parameters will be briefly described. X-ray diffraction method refers to Cu
kα is an X-ray source, high-purity silicon is used as a standard substance, and a diffraction pattern is measured on carbon fibers and the like. Then, the graphite interlayer distance d ₍₀₀₂₎ , the crystallite size Lc ₍₀₀₂₎ in the c-axis direction from the peak position and half width of the 002 plane diffraction pattern, and the a-axis direction from the half width of the 110 plane diffraction pattern Are calculated, respectively, of the crystallite size La ₍₁₁₀₎ . The calculation method is based on the Gakushin method. For the measurement of the peak intensity ratio I ₁₀₁ / I ₁₀₀ , a base line is drawn on the obtained diffraction chart, and the 101 plane (2θ ≒ 4) is drawn from the base line.
The height of each peak of 4.5) and 100 planes (2θ ≒ 42.5) is measured, and the height of 101 diffraction peak is divided by the height of 100 diffraction peak.

The graphite material for a negative electrode of a lithium ion secondary battery according to the present invention has such characteristics in X-ray diffraction and, when immersed in an electrolytic solution and charged or charged / discharged, has excellent chargeability at high temperatures. It has a new and unique characteristic of exhibiting discharge characteristics. In this specification,
The initial charge / discharge efficiency is the ratio of the discharge capacity to the charge capacity in the first charge / discharge, and the initial discharge capacity refers to the discharge capacity in the first charge / discharge. These charge and discharge characteristics
For example, graphite material as it is or an auxiliary material such as artificial or natural graphite is added, kneaded in an N-methylpyrrolidone solution of PVDF (polyvinylidene fluoride), and then coated on a copper foil to form a negative electrode. It can be evaluated by a three-electrode cell using lithium metal thin film.

As an electrolytic solution used in such a triode cell, an organic solvent, particularly preferably ethylene carbonate,
It is desirable to use an organic electrolytic solution in which an electrolyte such as lithium borofluoride, lithium hexafluorophosphate, or lithium perchlorate is dissolved in dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, or the like, alone or in a suitable mixture.

The charge / discharge characteristics are measured, for example, by setting the electrolyte of the triode cell to a predetermined temperature,
Specifically, in the range of 100 to 600 mA / g,
0 mA / g constant current density and constant voltage,
Charging at a constant voltage in the range of 02 to 0.001 V, particularly 0.01 V, and then changing the temperature of the electrolyte as needed,
At a constant current density, specifically in the range of 100 to 600 mA / g, especially at a constant current density of 100 mA / g, 1.5 V (v
s. Li / Li ⁺ ).

In the charge / discharge characteristic test, the graphite material for a negative electrode according to the present invention has an initial charge / discharge efficiency and an initial discharge capacity of 30 to 50 ° C. when charged at room temperature of 25 ° C. The initial charge / discharge efficiency and the initial discharge capacity when charged at an arbitrary temperature, particularly at a high temperature of 40 ° C., are higher. More specifically, the graphite material for a negative electrode according to the present invention has an initial charge / discharge efficiency at room temperature of 100 and a ratio of the initial charge / discharge efficiency at high temperature to 102/100 to 130/100. 100, especially 103/10
It is as high as 0 to 120/100. In the graphite material for a negative electrode according to the present invention, the ratio of the discharge capacity after charging at a high temperature to the initial discharge capacity at normal temperature is 100, for example, 101/100 to 125/100, particularly 102/1.
It is as high as 00 to 120/100.

In the measurement of the charge / discharge characteristics of the graphite material for a negative electrode according to the present invention, the temperature condition at the time of discharge is as follows:
There is no particular limitation as long as it is the same for the case of normal temperature charging and the case of high temperature charging. For example, in particular, the graphite material for a negative electrode according to the present invention has an initial charge / discharge efficiency and an initial discharge capacity when charged and discharged at a normal temperature of 25 ° C., and 30%.
When comparing the initial charge / discharge efficiency and the initial discharge capacity when charging and discharging are performed at an arbitrary temperature of up to 50 ° C., particularly at a high temperature of 40 ° C., the latter is higher as in the case described above.

In the graphite material for a negative electrode according to the present invention, high charge / discharge efficiency and discharge capacity can be obtained even if charge / discharge at room temperature is repeated after high temperature charge or charge / discharge under the above-described conditions. Will be maintained. More specifically, the graphite material for a negative electrode according to the present invention, the initial charge and discharge efficiency at normal temperature charge and discharge after performing the normal temperature charge and discharge for the first time as 100, the charge or charge and discharge at a high temperature for the first time to this. The ratio of the initial charge / discharge efficiency in the normal temperature charge / discharge after the execution is, for example, 102/1.
00 to 130/100, especially 103/100 to 120 /
It will be as high as 100. Further, in the graphite material for a negative electrode according to the present invention, the discharge capacity in normal temperature charge / discharge after the first charge / discharge at room temperature is set to 100, and the normal temperature after charge / charge / discharge at the first high temperature for this. The ratio of the discharge capacity in charge / discharge is, for example, 101/100 to 125/100, particularly 10
2/100 to 120/100.

Although the specific charge / discharge characteristics of the graphite material for a negative electrode according to the present invention are not clear, the surface of the graphite material serving as the negative electrode during the first charge (or the interface between the graphite material and the electrolyte) is not clear. It is considered that the protective film formed by the decomposition is involved. That is, when a graphite material is used as a negative electrode of a lithium ion secondary battery and charged using a propion carbonate (PC) electrolyte, PC is decomposed on the surface of the graphite material and lithium ions are intercalated into the graphite material. It is known to inhibit. Therefore, an electrolytic solution that is unlikely to decompose on the graphite material surface, for example, an ethylene carbonate (EC) -based electrolytic solution has been developed.

However, even when an EC-based electrolyte is used, the amount of lithium ions electrochemically intercalated at the time of charging may be larger than the amount of lithium ions taken out at the time of charging in the first charge / discharge. Has been observed. This is because at the time of lithium intercalation, a compound in which the lithium ion in the electrolyte and the solvent component of the electrolyte are reductively decomposed precipitates as a coating on the surface of the graphite material, and this reaction is an irreversible reaction. . This protective film was confirmed by SEM observation, and SEI (Soli
d Electrolyte Interphase) The coating is called.

The SEI coating is formed in several charge / discharge cycles, especially in the first charge / discharge, is permeable to lithium ions, and inhibits further reductive decomposition of the solvent component. Therefore, after the SEI film is formed, stable lithium intercalation and de-intercalation are performed through this film, and the charge capacity and the discharge capacity become equal.

The conventional graphite material for a negative electrode cannot form a stable SEI film during charging even with a high degree of graphitization, so that the initial charge / discharge efficiency and initial discharge capacity do not increase as expected. It is considered. On the contrary, in the graphite material for a negative electrode according to the present invention, the initial charge / discharge efficiency and the initial discharge capacity in charge / discharge at room temperature are as low as those of the conventional graphite material, but the SEI in charge at a high temperature of 30 to 50 ° C. It is considered that the film has a unique charge / discharge characteristic in that the film is formed well and the initial charge / discharge efficiency and the initial discharge capacity are high.

Further, the graphite material for a negative electrode of the present invention maintains high charge / discharge efficiency and discharge capacity even after charging / discharging at room temperature after charging at a high temperature. After that, it can be explained from the fact that stable lithium ion intercalation and deintercalation can be performed. A graphite material for a negative electrode having such a charge / discharge characteristic or a SEI film forming characteristic and having a high degree of graphitization has not yet been announced.

The graphite material for a negative electrode according to the present invention can be manufactured, for example, by the method described below, that is, the method for manufacturing a graphite material for a negative electrode of a lithium ion secondary battery according to the present invention. The method for producing a graphite material for a negative electrode according to the present invention,
It can be produced by carbonizing a mesophase pitch-based infusibilized fiber having specific infrared spectral characteristics and an oxygen / carbon atom ratio, and then graphitizing it.

The mesophase pitch-based infusible fiber used in the method of the present invention can be produced by spinning a mesophase pitch raw material and then infusifying the obtained pitch fiber. The mesophase pitch raw material used for spinning may be any of a petroleum-based, coal-based and synthetic pitch, but a pitch having a mesophase content of 100% is preferred.

The viscosity of the mesophase pitch raw material is not particularly limited as long as it can be spun, but a material having a low softening point is advantageous in terms of production cost and stability. For example, 230 ° C to 350 ° C, preferably Is 250 ° C. to 310
It is desirable to have a softening point of ° C. The method of spinning such a pitch raw material is not particularly limited, but various methods such as a melt spinning method, a melt blow method, a centrifugal spinning method, and a vortex spinning method can be applied. Of these,
Particularly, a melt blow method is preferable.

According to the melt blow method, the pitch raw material is
Spinning and cooling at high speed with low viscosity of tens of poise or less at high speed, high pitch fiber productivity, and pitch fiber, in particular, fiber shape that can provide milled fiber with desired aspect ratio give. Further, when the mesophase pitch is spun by the melt blow method, the graphitization treatment is advantageous in that the graphite layer surface is arranged in parallel to the fiber axis and a surface where lithium ions can be easily absorbed can be formed.

In such a melt blow method, the spinning hole is usually 0.1 mmФ to 0.5 mmФ, preferably 0.1 mmФ.
15 mmФ to 0.3 mmФ. Spinning speed is 5 per minute
It is desirably at least 00 m, preferably at least 1500 m / min, more preferably at least 2000 m / min. The spinning temperature varies somewhat depending on the raw material pitch, but is usually 300 ° C to 400 ° C, preferably 300 ° C to 380 ° C.
It is.

In the method for producing a graphite material for a negative electrode according to the present invention, a mesophase pitch system having specific infrared spectral characteristics and an oxygen / carbon atomic ratio obtained by infusibilizing the pitch fibers thus obtained. Uses fused fiber. That is, the infusible fiber used in the production method of the present invention is FT-I
In R (Fourier analysis infrared absorption) measurement, 1670
area of the peak appearing in cm ^-1 or 1830 cm ^-1 or less:
S of (CO), the area of the peak appearing below 1530 cm ^-1 or 1670 cm ^-1: (sometimes abbreviated as follows carbonyl intensity ratio) S (C = C) for the ratio S (CO) / S (C = C ) Is 0.40
０．５0.58, preferably 0.42 to 0.56.

When the carbonyl intensity ratio S (CO) / S (C = C) is 0.
When it is less than 40, infusibilization is insufficient, so that problems such as fusion of fibers during carbonization later occur. Also, 0.5
If it exceeds 8, the desired degree of graphitization cannot be obtained after the graphitization treatment. The mesophase pitch-based infusible fiber used in the method for producing a graphite material for a negative electrode according to the present invention has an atomic ratio O / C of oxygen to carbon in elemental analysis of 0.045 to 0.045.
0.060, preferably 0.047 to 0.057.

This oxygen / carbon atom ratio O / C is 0.045
If less than, infusibilization is insufficient, during the subsequent carbonization,
Problems such as fusion between fibers occur. Also, 0.06
If it exceeds 0, a desired degree of graphitization cannot be obtained after the graphitization treatment. In the production method according to the present invention, the method for producing such infusible fibers is not particularly limited. The infusibilization of the pitch fibers can be performed by, for example, a method of performing heat treatment in an atmosphere of an oxidizing gas such as nitrogen dioxide or oxygen, or a method of performing treatment in an oxidizing aqueous solution such as nitric acid or chromic acid.

A simpler infusibilizing method is a method of performing a heat treatment in air, and it is particularly preferable to divide the heating rate into two stages. That is, the infusibilization of the pitch fibers slightly varies depending on the raw material, but in the air, the average heating rate is 4 to 12.
° C / min, preferably 5-10 ° C / min, 190-260
C., especially 200 to 250.degree. C., and then at an average heating rate of 10 to 22.degree. C./min, preferably 15 to 20.degree. C./min, to 280 to 350.degree. C., especially 300 to 330.degree. Is desirable for obtaining the infusible fiber having the above-mentioned physical properties.

In the present invention, such infusible pitch fibers are graphitized after being carbonized. The carbonization of the infusible pitch fiber is performed, for example, under an inert gas atmosphere at 400 ° C. to 15 ° C.
It is desirable to carry out at a temperature of 00C, preferably 500C to 1000C. When the pitch-based carbon fiber is milled, setting the carbonization temperature to 500 ° C. to 900 ° C. is advantageous for preventing longitudinal cracking of the milled fiber.

The pitch-based carbon fiber thus obtained may be used as it is in a graphitization treatment described below, but is preferably milled prior to this treatment.
In order to efficiently produce milled carbon fiber suitable for the present invention, for example, a rotor equipped with a plate is rotated at high speed, and a device that cuts the fiber in a direction perpendicular to the fiber axis, for example, a Victory mill, a cloth It is effective to use a high-speed rotation mill such as a flow mill or a jet mill.

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. For milling pitch-based carbon fiber, use a Henschel mixer, ball mill,
Although there are methods using a grinder or the like, these methods are not preferable because a pressing force acts on the fiber in a direction perpendicular to the fiber and longitudinal cracks increase in the fiber axis direction. In addition, this method requires a long time for milling, and is not an appropriate milling method.

The milled pitch-based carbon fiber thus obtained preferably has an average particle size of 10 to 50 μm and an aspect ratio of 1 to 30, especially 1 to 20 or less.
The average particle size and the aspect ratio do not change significantly even if the final product is a graphite fiber. In the manufacturing method of the present invention, the pitch-based carbon fibers described above are graphitized. This graphitization treatment is, for example, preferably 2200
C. or more, more preferably 2500 C. or more, especially 280
It can be performed at a temperature of 0 ° C. or more and 3300 ° C. or less.

In the method for producing a graphite material of the present invention, a boron compound may coexist during such a graphitization treatment.
As such a 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. The addition of such a boron compound is usually limited by a method in which a solid boron compound is directly added and uniformly mixed as necessary, and a method in which the boron compound is made into a solution and the raw material is immersed in the solution. Not something.

When the boron compound is used as a solid,
In order to mix uniformly with milled carbon fiber, the average particle size is 5
It is desirable to use the powder after pulverizing it to not more than 00 μm, preferably not more than 200 μm. Examples of the solvent for preparing the boron compound solution include water, methanol, glycerin, acetone, and the like, and may be appropriately selected according to the type of the boron compound to be used. It is also possible to add a boron compound at the stage of the raw material pitch.

Such a boron compound is added to the above-mentioned material,
The obtained carbon material has a boron content of 1000 ppm or more and 30 or more.
It is added in an amount of 000 ppm or less. When such a boron compound is allowed to coexist, the graphitization treatment is performed at a relatively low temperature, for example, preferably 2,200 ° C. or more, more preferably 2,400 ° C. or more, and particularly 2,400 ° C. or more, It can be performed at a temperature of 100 ° C. or less.

Such a graphitization treatment is preferably carried out in the absence of oxygen, for example, in an atmosphere of an inert gas such as nitrogen 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. In such a graphitization treatment, most of impurities such as nitrogen, oxygen, sulfur, and metal contained in the carbon material raw material are discharged out of the system during the graphitization treatment. However, in order to obtain a carbon material having a higher purity, a halogen element (gas) such as chlorine is introduced during the carbonization or graphitization treatment to react with the impurities in the carbon material, and the impurities are removed out of the system as halides. A method, that is, a purification treatment may be performed.

The negative electrode according to the present invention can be manufactured by a usual method using the graphite material for a negative electrode according to the present invention described above or the graphite material for a negative electrode prepared by the method described above. Further, the negative electrode according to the present invention may have a current collector made of a metal plate or a metal foil of copper, nickel, or the like.

Such a negative electrode can be produced, for example, by the following method. (1) A graphite material is mixed with an appropriate amount of a binder such as polyethylene, polytetrafluoroethylene, polyvinylidene fluoride, SBR (styrene butadiene rubber), and formed into a sheet having a thickness of about 10 to 100 μm by a press roller. Pressure bonding to one or both sides of a metal foil made of copper, nickel, etc. of about 50 μm, and a thickness of 50 to 50 μm
The sheet is about 200 μm. (2) A graphite material is mixed with an appropriate amount of a binder such as polyethylene, polytetrafluoroethylene, and polyvinylidene fluoride, and is slurried using an organic solvent or an aqueous solvent. The sheet is about 50 to 200 μm thick.

The lithium ion secondary battery according to the present invention
The negative electrode described above is provided, and the negative electrode can be manufactured in combination with a known solid electrolyte or electrolyte and a positive electrode. Among these, an organic electrolyte in which an electrolyte is dissolved in an aprotic organic solvent having a high dielectric constant is particularly preferable. Examples of such an organic solvent include, for example, propylene carbonate, ethylene carbonate, tetrahydrofuran, 2-methyltetrahydrofuran, dioxolan, 4-methyl-dioxolan, acetonitrile, dimethyl carbonate,
Methyl ethyl carbonate, diethyl carbonate and the like can be mentioned. These solvents may be used singly or in a suitable mixture.

Examples of the electrolyte include lithium salts that generate stable anions, such as lithium perchlorate, lithium borofluoride, lithium antimonate hexachloride, lithium antimonate hexafluoride, and lithium hexafluorophosphate. These electrolytes are suitable, and these electrolytes may be used alone or in an appropriate mixture. Examples of the positive electrode material include metal oxides such as chromium oxide, titanium oxide, cobalt oxide, and vanadium pentoxide, and lithium manganese oxide (LiMn oxide).
Lithium oxide such as ₂ O ₄ ), lithium cobalt oxide (LiCoO ₂ ), and lithium nickel oxide (LiNiO ₂ ).

A non-woven fabric made of synthetic fiber or glass fiber, a woven fabric, a polyolefin-based porous membrane, a non-woven fabric of polytetrafluoroethylene, or the like is provided between the negative electrode and the positive electrode as a separator. Using the positive electrode, the electrolyte, the separator, the current collector, the gasket, the sealing plate, the components such as the case and the negative electrode obtained according to the present invention, a lithium ion in a form of a cylinder, a square, or a button according to a usual method, Can be assembled into a secondary battery.

[0054]

According to the graphite material for a negative electrode of a lithium ion secondary battery according to the present invention, it has a high degree of graphitization and has a unique charge / discharge characteristic at a high temperature. A secondary battery having high charge / discharge efficiency and discharge capacity after charging can be provided. In the method for producing a graphite material for a negative electrode of a lithium ion secondary battery according to the present invention, a graphite material for a negative electrode having a high degree of graphitization and having unique high-temperature charge / discharge characteristics can be efficiently produced.

Further, according to the negative electrode for a lithium ion secondary battery and the lithium ion secondary battery according to the present invention, since the graphite material for a negative electrode is used, the charge / discharge efficiency particularly after charging or charging / discharging at a high temperature. In addition, a secondary battery having a large discharge capacity can be provided.

[0056]

The present invention will be described in more detail with reference to the following Examples and Comparative Examples, which do not limit the scope of the present invention.

[0057]

Example 1 (a) Spinning Using a petroleum-based mesophase pitch (mesophase 100%) having an optical anisotropy and a specific gravity of 1.25 as a raw material, a row of spinning holes having a diameter of 0.2 mmφ is arranged in a slit having a width of 3 mm. 500
Using a die having individual pieces, heated air was blown out from a slit to draw and spin a 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 obtained pitch fibers were collected on a belt made of a stainless steel wire mesh having a collecting portion of 20 mesh while sucking from the back of the belt. (B) Infusibilization In this manner, the pitch fibers collected on the belt and formed into a mat are heated in the air from room temperature to 240 ° C. at an average heating rate of 8 ° C./min, and subsequently to 320 ° C. Average heating rate 1
The temperature was raised at a rate of 7 ° C./min to perform the infusibilization treatment.

Using the obtained infusibilized fiber, the carbonyl intensity ratio S (CO) / S (C = C) by FT-IR measurement and the oxygen / carbon atomic ratio O / C by elemental analysis were determined. Each 0.
54 and 0.052. (C) Carbonization / graphitization The infusible fiber was carbonized at 650 ° C. in a nitrogen atmosphere, and then pulverized with a cross flow mill to obtain a milled carbon fiber having an average particle size of 18 μm.

The obtained milled carbon fiber was graphitized by raising the temperature to 3000 ° C. at a rate of 3 ° C./min in a nitrogen atmosphere, and further maintaining the same temperature for 1 hour. The obtained graphite fibers were used to measure crystal parameters by an X-ray diffraction method. As a result, the intensity ratio I ₁₀₁ / I _{100 of} the 101 plane diffraction peak and the 100 plane diffraction peak was 1.9, and the graphite interlayer distance (d
₀₀₂ ) was 0.3357 nm, and the crystallite size (Lc) in the C-axis direction was 100 nm. (D) Electrode sheet 51.9 g of the obtained milled graphite fiber was added to a solution (binder solution) of 4.1 g of polyvinylidene fluoride in 45.0 g of N-methylpyrrolidone and kneaded to obtain a coating liquid. Next, this coating solution was applied to a copper foil having a thickness of 18 μm with an electrode weight of 100 g /
m ² , dried and roll-pressed to produce an electrode sheet having an electrode density of 1.4 g / cm ³ . (E) Charge / discharge test A triode cell having a test electrode (length 20 mm, width 20 mm) punched out from the electrode sheet as a working electrode, metallic lithium as a counter electrode and a reference electrode was assembled, and this was connected to a charge / discharge tester. The test was performed. The electrolyte is ethylene carbonate (EC)
And dimethyl carbonate (DMC) in a volume ratio of 1/1
Lithium perchlorate (Li
ClO ₄ ) at 1 molar concentration was used.

In the charge / discharge test, (a) 25 ° C. and (b)
Initial charge / discharge efficiency, charge capacity and discharge capacity at the 10th cycle in charge / discharge at 40 ° C., and (c) 4
After charging at 0 ° C., discharging at 25 ° C., and thereafter discharging and charging at 25 ° C. were repeated. The discharge capacity at the 10th cycle was measured, and the results are shown in Table 1. The charge / discharge conditions at this time are constant current-constant voltage charge (100 mA / g-10 mV) in (a) and (b).
The test was performed at a cut-off voltage of 1.5 V (vs. Li / Li ⁺ ) with a constant current discharge (100 mA / g) for 8 hours, a pause of 10 minutes, and the like. Moreover, in (b), it carried out similarly to (a) except having set 120 minutes of pause.

Table 1 shows the obtained results.

[0063]

Example 2 A mat-shaped pitch fiber was heated in the air from room temperature to 220 ° C. at an average heating rate of 6 ° C./min, kept at that temperature for 5 minutes, and then heated to 320 ° C. at an average heating rate of 17 ° C./min. An infusibilized pitch fiber was manufactured in the same manner as in Example 1 except that the infusibilizing treatment was performed by raising the temperature in minutes.

Using the obtained infusible fiber, the carbonyl intensity ratio S (CO) / S (C = C) in FT-IR measurement and the oxygen / carbon atom number ratio O / C by elemental analysis were determined. Each,
0.55 and 0.055. Using this infusibilized fiber, a graphite fiber was produced in the same manner as in Example 1. The obtained graphite fibers were used to measure crystal parameters by an X-ray diffraction method.
The intensity ratio I ₁₀₁ / I _{100 of the} plane diffraction peak was 1.7, the graphite interlayer distance (d ₀₀₂ ) was 0.3358 nm, and the crystallite size (Lc) in the C-axis direction was 90 nm.

Using this graphite fiber, a negative electrode was manufactured in the same manner as in Example 1, and a charge / discharge test was performed using the negative electrode. Table 1 shows the obtained results.

[0066]

Example 3 A mat-like pitch fiber was heated in air from room temperature to 240 ° C. at an average rate of 6 ° C./min.
An infusibilized pitch fiber was produced in the same manner as in Example 1 except that the infusibilizing treatment was performed by increasing the temperature to 20 ° C at an average temperature increasing rate of 15 ° C / min. Using the obtained infusibilized fiber, FT-
The carbonyl intensity ratio S (CO) / S (C = C) by IR measurement and the oxygen / carbon atom number ratio O / C by elemental analysis were determined to be 0.57 and 0.058, respectively.

Using this infusible fiber, a graphite fiber was produced in the same manner as in Example 1. Using the obtained graphite fiber,
When the crystal parameters were measured by the X-ray diffraction method, the intensity ratio I ₁₀₁ / I ₁₀₀ between the 101 plane diffraction peak and the 100 plane diffraction peak was 1.6, and the graphite interlayer distance (d ₀₀₂ )
Was 0.3358 nm, and the crystallite size (Lc) in the C-axis direction was 90 nm.

Using this graphite fiber, a negative electrode was manufactured in the same manner as in Example 1, and a charge / discharge test was performed using the negative electrode. Table 1 shows the obtained results.

[0069]

Example 4 Milled carbon fiber had an average particle size of 2% by weight.
Graphite fibers were produced in the same manner as in Example 1 except that 0 μm of boron carbide was added, mixed uniformly, and then graphitized.
Using the obtained graphite fiber, the crystal parameter was measured by an X-ray diffraction method. As a result, the intensity ratio I ₁₀₁ / I _{100 of the} 101 plane diffraction peak and the 100 plane diffraction peak was 2.1, and the graphite interlayer distance (d ₀₀₂ ) Was 0.3355 nm, and the crystallite size (Lc) in the C-axis direction was 100 nm.

Using this graphite fiber, a negative electrode was manufactured in the same manner as in Example 1, and a charge / discharge test was performed using the negative electrode. Table 1 shows the obtained results.

[0071]

Comparative Example 1 The procedure of Example 1 was repeated, except that the mat-shaped pitch fiber was heated in the air from room temperature to 300 ° C. at an average heating rate of 6 ° C./min to perform infusibility treatment. Infusible pitch fibers were produced. Using the obtained infusibilized fiber, FT
When the carbonyl intensity ratio S (CO) / S (C = C) in the IR measurement and the oxygen / carbon atom ratio O / C by elemental analysis were determined,
They were 0.63 and 0.073, respectively.

Using this infusible fiber, a graphite fiber was produced in the same manner as in Example 1. Using the obtained graphite fiber,
When the crystal parameters were measured by the X-ray diffraction method, the intensity ratio I ₁₀₁ / I ₁₀₀ between the 101 plane diffraction peak and the 100 plane diffraction peak was 1.1, and the graphite interlayer distance (d ₀₀₂ )
Was 0.3365 nm, and the crystallite size (Lc) in the C-axis direction was 40 nm.

Using this graphite fiber, a negative electrode was manufactured in the same manner as in Example 1, and a charge / discharge test was performed using the negative electrode. Table 1 shows the obtained results.

[0074]

[Table 1]

 ──────────────────────────────────────────────────続 き Continued on the front page F-term (reference) 4G046 CA00 CA07 CB01 CB09 5H003 AA02 BA01 BA03 BA04 BB02 BC02 BC05 BD01 BD03 BD05 5H014 AA02 BB01 BB04 BB06 BB11 EE08 HH01 HH04 HH06 HH08 5H029 AJ02 AM03 A03 AM07 AM16 BJ04 BJ12 CJ01 CJ04 CJ08 CJ16 DJ17 EJ04 HJ00 HJ01 HJ04 HJ07 HJ13 HJ14 HJ16 HJ19

Claims (8)

[Claims] 1. 101 planes and 100 planes in X-ray diffraction measurement
The surface intensity ratio I ₁₀₁ / I ₁₀₀ was 1.2 or more, and the battery was charged at 30 to 50 ° C. as compared with the initial charge / discharge efficiency and initial discharge capacity when immersed in an electrolytic solution and charged at 25 ° C. A graphite material for a negative electrode of a lithium ion secondary battery, wherein the graphite material has higher initial charge / discharge efficiency and initial discharge capacity in such a case. 2. The initial charge / discharge efficiency and the initial discharge capacity when charged and discharged at 30 to 50 ° C. are higher than the initial charge and discharge efficiency and the initial discharge capacity when charged and discharged at 25 ° C. The graphite material for a negative electrode of the lithium ion secondary battery according to the above. 3. In an FT-IR spectroscopic measurement, 1670
cm ^-1 or 1830 cm ^-1 peak area for the carbonyl groups appearing in the following: S in (CO), 1530cm ^-1 or 1
Peak area for ethylene bond appearing below 670 cm ^-1 : ratio of S (CO) / S (C = C) to S (C = C) is 0.40
Lithium ion characterized by carbonizing a mesophase pitch-based infusible fiber having an atomic ratio O / C of oxygen to carbon of 0.045 to 0.060 in elemental analysis and then graphitizing the lithium ion. A method for producing a graphite material for a negative electrode of a secondary battery. 4. The method according to claim 3, wherein the carbon fiber obtained in the carbonization step is milled and then graphitized. 5. The production method according to claim 3, wherein the carbon fibers obtained in the carbonization step are milled, and the obtained milled carbon fibers are mixed with a boron compound and then graphitized. 6. The production method according to claim 3, wherein the graphite material obtained in the graphitization step is subjected to a treatment of charging or discharging at 30 to 50 ° C. in an electrolytic solution. . 7. A lithium ion secondary battery comprising the graphite material according to claim 1 or 2, or the graphite material obtained by the method according to any one of claims 3 to 6. Negative electrode. 8. A lithium ion secondary battery comprising the negative electrode according to claim 7.