JPH09283145A - Carbon material for lithium secondary battery and manufacture thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To increase the doping amount of lithium, enhance charging/ discharging efficiency, and make high rate charging/discharging possible by forming the specified material into fibers, milling the fibers, then graphitizing the milled fibers, under the specified condition. SOLUTION: Graphite fibers, produced from meso-phase pitch, for a lithium secondary battery negative electrode material has a thermal conductivity of 400W/m.K or more, a specific resistance in the temperature range of 6-300K of 2×10<-4> ωcm or less. Milled graphite fibers are produced y forming infusible fibers from meso-phase pitch, carbonizing the infusible fibers at 40-1200 deg.C, forming into milled fibers having a mean partical size of 5-50μm, then graphitizing at 2000 deg.C or hither. Alternatively hydrocarbon having an aromatic carbon ratio of of 0.6 or more and a number average molecular weight of 100-350 is polymerized under the existence of a catalyst to form meso-phase pitch having a softening point of 250 deg.C or higher, and 100% meso-phase pitch is used as the raw material.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a graphite fiber suitable for a negative electrode material for a lithium-based non-aqueous secondary battery and a method for producing the same. More specifically, the lithium-based non-aqueous secondary battery using the graphite fiber for negative electrode material obtained by the present invention has a large charge / discharge capacity, a high energy density, and excellent charge / discharge cycle characteristics. Have.

[0002]

2. Description of the Related Art In recent years, electronic devices have undergone rapid technological development aiming at miniaturization, light weight, and high performance, and as a result, portable electronic devices such as cellular phones, PHSs, camcorders, and personal computers have become more popular. . With the development of this new equipment, nickel-hydrogen batteries and lithium-based non-aqueous secondary batteries have emerged as new secondary batteries. In particular, the lithium-based non-aqueous secondary battery has high energy density and high electromotive force, and since it uses a non-aqueous electrolyte, it has a wide operating temperature range, is excellent in long-term storage, and is lightweight and compact. Have advantages. Therefore, such a lithium-based non-aqueous 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.

Improvement of the performance and safety of the lithium-based non-aqueous secondary battery has been realized by using a carbon-based material instead of metallic lithium for the negative electrode. That is, when a carbon-based material is used for the negative electrode, lithium dendrites are not formed because lithium ions are incorporated into the carbon structure.
This solved the problem caused by dendrite precipitation and dramatically improved safety. There are two types of carbon materials used for this negative electrode, a low crystalline carbon material (hereinafter simply referred to as "carbon material") such as a fired product of furfuryl alcohol, and mesocarbon microbeads (MCM).
Graphitized products of B) and graphitized materials such as natural graphite (hereinafter simply referred to as "graphite material") have been put to practical use.

The carbon material is characterized by a large amount of lithium taken in per weight, that is, a large capacity per weight. Various carbon materials have been reported which have a discharge capacity far exceeding 372 mAh / g, which is the theoretical capacity of graphite-based C ₆ Li. However, these carbon materials have problems such as low initial charge / discharge efficiency (initial discharge capacity / initial charge capacity), poor cycle characteristics, or low density of the carbon material itself, and thus they are not suitable for small secondary batteries. In such cases, there are various problems to be solved.

On the other hand, in the case of a graphite material, the reaction mechanism consists of a simple reaction of intercalation / deintercalation of lithium between graphite layers, and the first charge / discharge efficiency is relatively high accordingly. Expectations are rising because high energy per volume can be obtained due to the high density of the graphite material itself. However, when the graphitized MCMB is used, it is difficult to withstand long-term use due to poor conductivity, which is considered to be caused by repeated expansion and contraction of the graphite material due to repeated charging and discharging.

Further, 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 naturally taken out is small, and the charge / discharge at high current density is also large. However, there is a problem that the charging / discharging efficiency decreases. It is necessary to extract a large current from such a material, and it is desirable to use it for the negative electrode of a high-load power supply, such as a power supply for equipment having a drive motor, for which it is desirable to charge at a high current density in order to shorten the charging time. Is not suitable.

As one of such graphite materials, Japanese Patent Laid-Open No.
As disclosed in Japanese Patent No. 325967, it has been pointed out that a carbon fiber obtained by graphitizing carbon fibers using a pitch having a mesophase volume content of 70% or more is excellent from the measurement results of various battery characteristics. There is. According to this publication, the mesophase pitch used as the raw material is not particularly limited to the raw material as long as it has a high graphitizable property, and the spinning conditions and the infusibilization conditions are not specified. However, in the current situation where a higher performance negative electrode material is required, unless a specific raw material is manufactured under specific manufacturing conditions,
It is not possible to manufacture a high-performance negative electrode material with stable quality.

[0008]

DISCLOSURE OF THE INVENTION According to the present invention, when the conventional lithium non-aqueous secondary battery has a small charge / discharge capacity, a low initial charge / discharge efficiency, a low charge / discharge rate, and a short cycle life. It is an object of the present invention to provide a method for manufacturing a negative electrode material that solves the problem.

[0009]

Means for Solving the Problems The present inventor has made detailed investigations on the steps of spinning, infusibilizing, and carbonizing from raw materials in relation to the production of mesophase pitch carbon fibers, and as a result, uses specific raw materials and specific production conditions. Based on the above, it was found that it is possible to produce carbon fibers for a negative electrode material of a lithium-based non-aqueous secondary battery having excellent performance, and the present invention has been completed. That is, the present invention is as follows: 1) The specific resistance is 2 × 10 ⁻⁴ Ω in the temperature range of 400 W / m · K or more and 6 K or more and 300 K or less.
Provide a mesophase pitch-based graphite fiber for a negative electrode material of a lithium-based non-aqueous secondary battery, which has a size of cm or less. 2) Mesophase pitch infusibilized fiber 400 ℃ or more 1
Carbonized in a temperature range of 200 ° C or less, and then an average particle size of 5
After milling so that the thickness is from μm to 50 μm, 2
Graphitized at 200 ℃ or higher, thermal conductivity is 400W / m ・
Provided is a mesophase pitch-based graphite fiber mill for a lithium-based non-aqueous secondary battery negative electrode material, which has a specific resistance of 2 × 10 ⁻⁴ Ω · cm or less in a temperature range of K or higher and 6K or higher and 300K or lower. Also

3) When the aromatic carbon ratio fa is 0.6 or more,
Provided is a graphite fiber milled from a mesophase pitch having a softening point of 250 ° C. or higher and a mesophase component of substantially 100%, which is obtained by polymerizing a hydrocarbon having a number average molecular weight of 100 to 350 in the presence of a catalyst. To do. Furthermore, 4) a softening point of 250 ° C. or higher obtained by polymerizing a hydrocarbon having an aromatic carbon ratio fa of 0.6 or more and a number average molecular weight of 100 or more and 350 or less in the presence of hydrogen fluoride and boron fluoride. Of mesophase component of which the mesophase component is substantially 100% as a raw material, spinning is carried out in a viscosity range of 5 poise or more and 100 poise or less, and then the oxygen content is 4 wt% in the temperature range of 150 ° C to 350 ° C. % To 10 wt% or less, then carbonized in an inert gas in the temperature range of 400 ° C. to 1200 ° C., and then milled so that the average particle size is 5 μm to 50 μm, Next, there is also provided a method for producing a graphite fiber mill, which comprises performing graphitization at a temperature of 2200 ° C. or higher.

The present invention will be specifically described below. (1) Raw material pitch In the present invention, carbon fibers heat-treated (graphitized) at 2000 ° C. or higher are referred to as graphite fibers, and those milled (ground) as described below are referred to as graphite fiber milled. . <Pitch raw material> In order to produce a graphite fiber for a negative electrode material of a high performance lithium-based non-aqueous secondary battery, it is important to select a hydrocarbon source as a raw material of pitch. If the starting raw material is selected incorrectly, there is a limit to how devised in the subsequent manufacturing method, and the final desired high performance carbon material for negative electrode cannot be obtained. As a raw material for pitch production, it is essential that the aromatic carbon ratio fa is 0.6 or more. It is preferably 0.7 or more.

The aromatic carbon ratio is a ratio of carbon atoms of an aromatic ring structure to all carbon atoms, and fa = 1 in all cases of aromatic carbon such as naphthalene. fa is 0.
When it is less than 6, the planar structure of the aromatic molecules constituting the mesophase is low, and the graphite fiber obtained using such a pitch as a raw material is difficult to graphitize and the capacity as a negative electrode does not increase. Further, as a raw material for pitch production, it is essential that the number average molecular weight is 100 or more and less than 350, preferably 120 or more and less than 330.

When the number average molecular weight is 100 or less, the pitch yield obtained is extremely low, which is not preferable. 3
When it is 50 or more, the number of molecular species of the raw material hydrocarbon is unnecessarily increased, various reactions proceed, the uniformity of the pitch produced is impaired, and the quality of the final product varies greatly, which is not preferable.

<Production of Pitch> High molecular weight hydrocarbons are gradually polymerized by dehydrothermal polycondensation reaction by heat treatment in an inert gas at a temperature of 300 ° C. or higher to form pitch. On the other hand, as a pitching reaction using a low molecular weight hydrocarbon as a raw material, JP-A-61-83317 discloses a method of heat treatment in the presence of a Lewis acid catalyst such as aluminum chloride. Also, JP-A-63-1469
Japanese Patent Publication No. 20 discloses a method of polymerizing a substance containing a condensed polycyclic hydrocarbon in the presence of hydrogen fluoride / boron trifluoride.

According to the present invention, a hydrocarbon having an aromatic carbon ratio and a number average molecular weight, that is, having a fa of 0.6 or more and a number average molecular weight of 100 or more and 350 or less is used as a raw material, and 1 mol of this hydrocarbon is used. Pitch production in which 0.1 mol to 20 mol of hydrogen fluoride and boron trifluoride are added at a ratio of 0.05 mol to 1 mol to perform thermal polymerization is preferable. The reaction temperature at this time is 250 ° C to 350 ° C. Even if the amounts of hydrogen fluoride and boron trifluoride are used in excess of 20 mol and 1 mol, respectively, the reaction rate is not increased and it is not advantageous.
On the other hand, the amounts of hydrogen fluoride and boron trifluoride were each 0.
When it is less than 1 mol and 0.05 mol, substantially 100% of mesophase pitch cannot be obtained.

Also, when the reaction temperature is 250 ° C. or lower, the reaction rate becomes remarkably slow, which is not preferable. On the other hand, when the reaction temperature is higher than 350 ° C., the reaction rate becomes runaway and it is difficult to obtain a uniform pitch, which is not preferable. The softening point of the pitch produced is 250 ° C. or higher, preferably 260 ° C.
That is all. If the softening point is less than 250 ° C., the reactivity with the oxidizing gas is poor in the infusibilization process, and the infusibilization reaction time is long, and oxygen diffuses into the fibers to disturb the orientation of the graphite layer surface, resulting in the progress of graphitization. This is difficult to do and is not preferable. Further, if the softening point is unnecessarily high, spinning becomes difficult, which is not preferable. The temperature is preferably 310 ° C or lower, more preferably 300 ° C or lower. The softening point here is a so-called METTLER softening point.

Further, an essential condition for the raw material pitch is that the pitch is substantially 100% mesophase. When an optically isotropic component is contained, graphitization of that portion is difficult to proceed, which is not preferable. The mesophase content can be determined by observing with a polarization microscope.

(2) Spinning The essential condition for spinning the raw material pitch is that the spinning viscosity is 5 poises or more and 100 poises or less. This spinning viscosity can be set from the viscosity-temperature chart of the raw material pitch and the nozzle temperature at the time of spinning which have been obtained in advance. If the spinning viscosity is less than 5 poise, shots during spinning will be extremely generated, the yarn diameter will be greatly varied, and the nozzle will be heavily soiled, which is not preferable from the viewpoint of stable production. Extreme variations in yarn diameter cause a difference in the degree of graphitization of individual fibers, which is not preferable. In spinning, it is necessary to equalize the discharge amount of each spinning hole of the nozzle in order to keep the yarn diameter and yarn quality constant, but if the spinning viscosity is less than 5 poise, the back pressure of the spinning nozzle will be low. Since it is easily affected by external factors, the pressure in each hole becomes uneven as a result.

That is, the discharge amount of the pitch from each hole varies, and the yarn diameter varies. It is desirable to adjust the conditions so that the variation in yarn diameter is 20 or less in terms of the coefficient of variation. Further, in order to reduce the spinning viscosity, the spinning temperature must be increased,
It is not preferable to unnecessarily reduce the spinning viscosity because the temperature range is likely to cause thermal deterioration of the pitch.

When the spinning viscosity is high such that it exceeds 100 poise, the degree of freedom of movement of the mesophase contained in the raw material pitch is small, and therefore, the shearing occurs during the drawing process when passing through the spinning hole and after the hole. Even under stress, the orientation in the fiber axis direction was not sufficiently performed, the orientation of the graphite layer was inferior even after the final graphitization firing, and the graphitization development at the same graphitization temperature was inferior to that of spinning at 100 poise or less. Not preferable.

The spinning method is not particularly limited, and various methods such as melt spinning, melt blowing, centrifugal spinning, and vortex spinning can be used.
From the viewpoint of low viscosity spinning of 00 poise, productivity during spinning and quality of the obtained fiber, the melt blow method is preferable. At this time, the size of the spinning hole is 0.1 mmφ or more and 0.5 mm.
φ or less, preferably 0.15 mmφ or more and 0.3 mmφ or less. The spinning speed is 1000 m / min or more, preferably 1500 m / min or more, more preferably 20 min / min.
It is more than 00m.

Although the spinning temperature varies somewhat depending on the raw material pitch, it may be at a temperature not lower than the softening point of the raw material pitch and not deteriorating the pitch, and usually 300 ° C. or higher and 400 ° C. or lower, preferably 380 ° C. or lower. Further, the yarn diameter is not particularly limited, and in the melt-blowing method, fibers having a diameter of about 1 μm to yarn diameters of several tens μm can be spun. When the diameter of the fiber becomes extremely thick, the surface area of the fiber per unit weight becomes relatively small, so it takes time to infusibilize in the next step, and the brittleness of the fiber causes fiber cracking during electrode sheet production. Is not preferred.

On the other hand, if the fiber diameter is too small, oxygen permeates into the inside of the fiber during infusibilization, and as a result, the degree of graphitization is difficult to increase, which is not preferable. Therefore, the diameter of the fiber is preferably about 3 μm to 30 μm. More preferably, it is 5 μm to 20 μm. Further, the cooling rate of the fiber after spinning is also an important factor. That is, it is necessary to cool and solidify the fibers which have been controlled so as to have a predetermined graphite orientation and which have come out of the nozzle, while maintaining their structure. Unnecessarily, if the cooling rate is slow, the internal structure of the fiber whose orientation is controlled during spinning is relaxed, and the orientation aligned in the fiber axis direction is disturbed, and the degree of graphitization is suppressed even when the final graphitizing treatment is performed.

The preferred average cooling rate is 1 × 10 ⁴ ° C / sec or more, preferably 1 × 10 ⁵ ° C / sec or more. As described above, the orientation state of the graphite layer surface inside the fiber is substantially determined in the spinning process. Therefore, control of the spinning conditions is an important factor in determining the graphitization degree of the fiber as well as the selection of the raw material pitch. Using the above specific raw material pitch, by graphitizing the spun under specific spinning conditions,
Basically, it becomes possible to form a skeleton of mesophase-based carbon fiber having a fiber structure in which the graphite layer surface is open on the fiber surface.

(3) Insolubilization As the infusibilization method, 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 of nitric acid or chromic acid, and further A polymerization treatment method using light, γ-rays or the like is also possible, but a simpler infusibilizing method is a method of heat treatment in air. The infusibilizing step is a step of polymerizing the thermoplastic pitch to a non-melting state by oxidative polycondensation by the reaction of hydrocarbon and oxygen.

If the oxidation reaction in the infusibilizing step is insufficient, the fibers are melted in the subsequent carbonizing step and the fibers are fused to each other, or the fiber form cannot be maintained, which is not preferable. The reaction between oxygen and the aromatic hydrocarbons in the pitch in this infusibilization step also promotes the reaction for opening the double bond of the aromatic compound. This reaction promotes the disorder of the orientation of the graphite layer formed in the spinning process and oriented regularly in the fiber axis direction, and as a result, inhibits the progress of graphitization. That is, if the oxidation reaction proceeds too much, graphitization becomes difficult to proceed, which is not preferable.

Therefore, an appropriate infusibilizing condition exists. The oxygen content in the infusibilized yarn is 4 wt% or more and 10 wt% or less, preferably 5 wt% or more and 8 wt% or less. If it is less than 4 wt%, the orientation of the graphitized layer in the fiber is re-melted in the carbonization step and the orientation is disturbed, and graphitization is difficult to proceed. On the other hand, when it exceeds 10 wt%, oxygen penetrates into the inside of the fiber, causing disorder of the graphite layer that is regularly oriented in the fiber axis direction generated in the spinning process,
Also in this case, development of graphitization becomes difficult.

If more specific manufacturing conditions that meet such infusibilizing conditions are shown, the infusibilizing temperature range in the air atmosphere is gradually raised by heating from 150 ° C to 350 ° C. It is effective to do. When the maximum treatment temperature is set to 350 ° C. or higher, the thermal reaction with oxygen becomes extreme, resulting in a decrease in oxidized weight, which is not preferable. Further, if the temperature is 150 ° C. or lower, it will be unnecessarily time-consuming for infusibilization, which is not preferable. More preferably, a temperature range of 170 ° C. to 300 ° C. is average heating rate 4 ° C. to 10 ° C., and further preferably 5
It is to be 8 ° C to 8 ° C.

(4) Carbonization, milling and graphitization (i) Carbonization The infusibilized fiber can be converted into carbon fiber (and graphite fiber) by heat treatment in an inert gas atmosphere. . The heating rate and the holding time at this time are not particularly limited. The degree of progress of the degree of graphitization depends most on the maximum processing temperature and then on the processing time.

(Ii) Milling As a general method for manufacturing the current negative electrode, a method is used in which a powdery carbonaceous material is slurried with a binder and various additives together with a solvent and coated on a copper foil. Has been. In this case, it becomes necessary to grind the carbon-based material into a particle size within a specific range. In the present invention, pulverizing the fibers to have a particle size within a specific range is referred to as "milling". Generally, fiber milling means pulverizing the fiber length to 1 mm or less. In the case of pitch carbon fiber milling, the average fiber length is 200 μm or less and the aspect ratio is 100 or less. Also, from the viewpoint of battery performance, it is preferable that the particle size of the milled fiber is smaller, because the fiber cross-sectional area per fiber volume (or weight) is larger, but on the other hand, if it is finely pulverized, the active graphite layer is exposed. Also increases, and there is a drawback that the capacity is reduced by reacting with the electrolytic solution.

Therefore, in the present invention, it is an essential condition to mill the fibers so that the average particle diameter of the fibers is 5 to 50 μm, preferably 10 to 30 μm. The particle size of the fiber is measured by a laser diffraction particle size measuring device. When milling the mesophase pitch-based carbon fiber of the present invention, it is an essential condition to mill the mesophase pitch carbon fiber at the time when it receives a certain temperature history. That is, an appropriate carbonization treatment temperature after infusibilization treatment and before milling is 400 ° C. or higher and 12
00 ° C or lower, preferably 500 ° C or higher and 1000 ° C or lower,
More preferably, it is 600 ° C. or higher and 800 ° C. or lower.

When milled at 1200 ° C. or higher, the number of active surfaces increases, so that the number of active surfaces increases, which is not preferable as a negative electrode from the viewpoint of decomposition of the electrolytic solution, and the hardness of the fibers increases and the crushing device increases. The wear of the contact portion with the carbon fiber is increased, which is not preferable. Further, since the structure formation of the fiber is insufficient at 400 ° C or lower, the mechanical properties of the fiber are not sufficiently developed, and the powder is pulverized by the impact during crushing until the fiber shape is lost, resulting in a negative electrode material with low initial charge / discharge efficiency. Not preferable.

As a method for milling the carbon fiber, it is effective to mill it with a Victory mill, a jet mill, a cross flow mill, a turbo mill or the like. In order to efficiently carry out milling of the carbon fibers, it is appropriate to cut the carbon fibers in a direction perpendicular to the fiber axis by, for example, rotating a rotor equipped with a plate at a high speed. The fiber length (particle diameter) of the milled fiber can be controlled by adjusting the number of rotations of the rotor, the angle of the plate, the size of the mesh of the filter attached around the rotor, and the like.

(Iii) Graphitization In the production of a graphite fiber mill, it is essential to carry out mild carbonization in the above temperature range, then carry out milling, and then perform graphitization. By processing such two stage heat,
In addition to preventing vertical cracking of the fiber during milling, the graphite layer surface newly exposed on the surface during milling may undergo polycondensation / cyclization reaction during the secondary heat treatment at a higher temperature, resulting in a decrease in surface activity. , Effective in preventing decomposition of the electrolytic solution.

The temperature for graphitization is 2500
C. or higher, preferably 2700.degree. C. or higher, more preferably 2800.degree. C. or higher. However, when graphitizing after adding a boron compound such as boron carbide or boron oxide, a sufficient graphitization degree can be obtained if the graphitizing temperature is 2200 ° C. or higher. The amount of the boron compound added at this time is 2 wt% or more and 7 wt% or less as the effective boron amount with respect to the object to be treated. When the addition amount is 2 wt% or less, the addition effect of the boron compound is small, and when the addition amount is 7 wt% or more, the effect does not increase so much even if it is added more, and the cost is not positive at all.

(Iv) Characteristics of Graphite Fiber Milled In this way, the graphite fiber milled using the specific raw material according to the present invention under the specific conditions is the most suitable graphite for the non-aqueous secondary battery having the developed graphite crystal structure. It becomes a chemical material. The graphite fiber milled product of the present invention has a lattice spacing (d ₀₀₂ ) of 0.34 nm or less, more preferably 0.338 nm or less, and a crystallite size in the C-axis direction, as shown by crystallite parameters by X-ray diffraction. (Lc) is 20 nm or more, preferably 3
0 nm or more. However, in the region where graphitization is developed to this extent, it is no longer possible to express a subtle difference in the graphite material only by the X-ray diffraction data.

According to the present inventor, one of the essential conditions for a graphite fiber mill suitable for a negative electrode material for a lithium-based non-aqueous secondary battery is the direction of the fiber axis when graphitized at a temperature of 2200 ° C. or higher. By selecting the raw material pitch and the manufacturing method so that the specific resistance is 2 × 10 ⁻⁴ Ω · cm or less in the temperature range of 400 W / m · K or more and 6 K or more and 300 K or less. is there. In the present invention, in the case of a graphite fiber milled, since it is difficult to measure the thermal conductivity and the specific resistance, these measurements use a carbon fiber having a length suitable for the measurement before milling, and the graphite fiber The measurement result was taken as the measurement value of the milled product that was graphitized under the same temperature condition.

In the temperature range where the thermal conductivity is 400 W / m · K or less, or 6 K or more and 300 K or less, the specific resistance is 2
When it is not less than × 10 ⁻⁴ Ω · cm, even if the X-ray diffraction data satisfies the above range, graphitization does not proceed and the capacity of the negative electrode becomes small, which is not preferable. In addition, in the present invention, the thermal conductivity was measured by an optical alternating current method using a monofilament. In addition, the measurement of the resistivity is JISR7.
It measured based on 601.

(5) Battery When the carbon material according to the present invention is used for the negative electrode to prepare a lithium ion secondary battery, the electrolyte may be any one capable of dissolving a lithium salt, but especially an aprotic acid. Organic solvents having a high dielectric constant are preferred. Further, the negative electrode made of the carbon material thus produced has a large capacity per unit volume and is suitable for downsizing of a battery.

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. These solvents can be used alone or in a suitable mixture.
As the electrolyte, a lithium salt that produces a stable anion, for example, lithium perchlorate, lithium borofluoride,
Lithium hexachloroantimonate, lithium hexafluoroantimonate (LiPF ₆ ) and the like are preferable.

Examples of the positive electrode of the lithium ion secondary battery include metal oxides such as chromium oxide, titanium oxide, cobalt oxide, vanadium pentoxide, lithium manganese oxide (LiMn ₂ O ₄ ), lithium cobalt oxide. (LiCoO ₂ ), lithium nickel oxide (LiNi
Lithium metal oxides such as O ₂ ); chalcogen compounds of transition metals such as titanium sulfide and molybdenum sulfide, and conjugated polymer substances having conductivity such as polyacetylene, polyparaphenylene, and polypyrrole.

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. The secondary battery of the present invention, the separator,
By using the battery constituent elements such as the current collector, the gasket, the sealing plate, and the case, and the specific negative electrode of the present invention, a lithium ion secondary battery of a cylindrical type, a square type, a button type or the like can be assembled according to a conventional method. .

[0043]

The graphite fiber used as the negative electrode material for a non-aqueous secondary battery according to the present invention is produced by using a specific hydrocarbon compound as a raw material to produce a raw material pitch under specific conditions, and further by performing specific spinning, infusibilization, and carbonization. -By graphitizing, higher capacity and higher charge / discharge efficiency can be obtained. When this graphite fiber is used for the negative electrode, it is possible to manufacture a negative electrode having a larger electric capacity than ever before, high charge / discharge efficiency, particularly high initial charge / discharge efficiency, and further excellent cycle characteristics. Particularly noteworthy is that when the graphite fiber according to the present invention is used, the charge / discharge speed can be increased several times faster than the conventional ones.

[0044]

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) A petroleum-based catalytically cracked oil having a number average molecular weight of 280 and fa of 0.68 was used as a raw material, and hydrogen fluoride was added to an autoclave in an amount of 5 mol per mol of the petroleum-based catalytic cracked oil.
After supplying 0.5 mol of boron fluoride, heat treatment was performed at 300 ° C. for 3 hours. The obtained pitch was a pitch having a 100% mesophase component as observed by a polarization microscope, and the Mettler softening point was 286 ° C. The pitch yield based on the raw material hydrocarbon was 69%. Using this mesophase pitch as a raw material, a spinneret having 400 spinning holes with a diameter of 0.2 mmφ in a row in a slit with a width of 1.5 mm was used, heated air was ejected from the slit, and the melt pitch was pulled to obtain an average diameter of 13 μm. Pitch fibers were produced. At this time, the temperature of the spinning nozzle was 355 ° C., and the discharge amount was 0.6 g / hole · minute. The spinning viscosity at this time was determined to be 12 poise from the viscosity-temperature of the raw material pitch, which was previously measured with a Rhodovisco viscometer manufactured by Haake.

The spun fiber was collected on the belt while sucking it from the back surface of the belt made of a stainless steel wire mesh having a 15 mesh collecting portion, and then continuously raised in air from 170 ° C to 300 ° C on average. The infusibilizing treatment was performed by raising the temperature at a temperature rate of 7 ° C / min. As a result of elemental analysis, the oxygen content of the obtained infusible yarn was 6.2 wt%.
After primary carbonization at 0 ° C., it was milled to obtain a carbon fiber mill having an average particle size of 21 μm (according to a laser diffraction formula). This was heated in argon gas to 3000 ° C. at a rate of 3 ° C./min, and further held at 3000 ° C. for 1 hour to obtain a graphite fiber mill.

The graphite fiber having a long fiber length obtained by subjecting the primary carbonized yarn obtained in the same manner to the graphitization treatment in the same manner as above without milling it has a thermal conductivity of 1060 W / mK. It was considered that similar values were obtained for the thermal conductivity of the fiber mill. In addition, FIG. 1 shows the result of the measurement of the specific resistance similarly performed using a fiber having a long fiber length. Approximately 1.6 × over the entire temperature range from 6K to 300K
It was a constant value of 10 ⁻⁴ Ω · cm. Results of X-ray diffraction measurement of this graphite fiber milled d ₀₀₂ = 0.3367, L
c ₀₀₂ = 59 nm.

To 5 g of this graphite fiber mill, 0.15 g of polytetrafluoroethylene powder was added and kneaded to prepare a negative electrode sheet, and a charge / discharge test was conducted in a 3-electrode cell. The test was carried out by using metallic lithium as the anode, mixed carbonate solvent prepared by adjusting ethylene carbonate (EC) / dimethyl carbonate (DMC) to 1/1, and lithium perchlorate (LiCl0 ₄ ) as an electrolyte at a concentration of 1 mol. It carried out in the dissolved electrolyte and measured the charge / discharge capacity characteristic. The charge / discharge capacity characteristics were measured under constant current discharge of 100 mAh / g, and the discharge capacity was defined as the capacity until the battery voltage dropped to 2V. The first discharge capacity is 310 mAh / g, and the charge / discharge efficiency is 92.
%, The second discharge capacity was 305 mAh / g, and the charge / discharge efficiency was about 100%, which were high values.

(Embodiment 2) 300 times during Embodiment 1 and graphitization
A graphite fiber milled product was obtained in the same manner except that it was held at 0 ° C for 10 hours. Using the obtained graphite fiber mill, the physical properties were measured and the electrodes were evaluated in the same manner as in Example 1. d
₀₀₂ = 0.3364, Lc ₀₀₂ = 65 nm, showing a crystallinity, a thermal conductivity of 1250 W / mK, and a specific resistivity of Fig. 1.
As shown in FIG. 3, the value was 1.3 × 10 ⁻⁴ Ω · cm, which was a substantially constant value. In addition, the initial discharge capacity is 315 mAh / g
The charging / discharging efficiency showed a high value of 93%.

(Example 3) The mesophase pitch fiber obtained in the same manner as in Example 1 was heated in the air at an average speed of 5 ° C to be infusibilized. The oxygen content of the obtained infusible yarn was 8.7% by weight, and this infusible yarn was mildly carbonized at 450 ° C. and then milled, and then 5% by weight was added to the fiber milled mildly carbonized with boron carbide. And 1 at 2200 ℃
Graphite treatment was carried out by holding for a time to obtain a graphite fiber mill. Using the obtained graphite fiber mill, the physical properties were measured and the electrodes were evaluated in the same manner as in Example 1. As a result, d ₀₀₂ = 0.3
366, Lc ₀₀₂ = 45 nm, the thermal conductivity was 550 W / m · K, and the specific resistance was 1.9 × 10 ⁻⁴ Ω · cm, which was a substantially constant value as shown in FIG. It was The initial discharge capacity was 305 mAh / g, and the charge / discharge efficiency was 91%, which was a high value.

(Comparative Example 1) Physical properties were measured and electrodes were evaluated in the same manner as in Example 1 by using two kinds of mesophase pitch carbon fibers commercially available, Sonel P100 and Sonel P120. As for the physical properties of each fiber, Sonel P100 is d
₀₀₂ = 0.3389, shows a crystalline Lc ₀₀₂ = 29 nm, the thermal conductivity of 580W / m · K. Sonel P1
20 showed crystallinity of d ₀₀₂ = 0.3376 and Lc ₀₀₂ = 32 nm, and the thermal conductivity was 720 W / m · K. As shown in FIG. 1, the specific resistivities tended to decrease as the temperature increased from 6 K to 300 K, and even the lowest value exceeded 2 × 10 ⁻⁴ Ω · cm. In addition, as a result of the three-electrode cell evaluation, the discharge capacity of the Sonel P100 is 258.
mAh / g, the initial charge / discharge efficiency was 82%, the discharge capacity of Sonel P120 was 275 mAh / g, and the initial charge / discharge efficiency was 83%, which were both low values. The measurement results of Examples 1, 2, and 3 and Comparative Example 1 are summarized in Table 1.

Example 4 Using the mesophase pitch obtained in Example 1, spinning temperature = 334 ° C., discharge rate = 0.
Spinning was carried out at 4 g / hole · minute and spinning viscosity = 45 poise. Using this fiber, infusibilization, carbonization, milling and graphitization were carried out in the same manner as in Example 1 to obtain a graphite fiber milled. When physical properties were measured and negative electrode characteristics were evaluated in the same manner as in Example 1, the thermal conductivity was 802 W / m · K and the specific resistance (at 6K to 300K) was 1.7 × 10 ⁻⁴ Ω ·.
The value was almost constant as cm. The initial discharge capacity =
It was 300 mAh / g and the initial charge / discharge efficiency was 91%.

(Comparative Example 2) The physical properties were measured in the same manner as in Example 1 except that the spinning viscosity was changed to 110 poise (spinning temperature = 318 ° C.), and the spinning was carried out in the same manner as in Example 4. When the negative electrode characteristics were evaluated, the thermal conductivity was 380 W / mK and the initial discharge capacity was 275 mAh / g.
And showed a low value.

Comparative Example 3 Using the mesophase pitch of Example 4 as a raw material, spinning viscosity = 3 poise (spinning temperature = 37).
When spinning was performed at 0 ° C., the nozzle was soiled due to the pitch and spinning became impossible 10 minutes after the spinning was started.

(Comparative Example 4) A mesophase pitch fiber obtained in the same manner as in Example 1 was heated in the air from 170 ° C to 260 ° C.
The infusibilizing treatment was carried out by raising the temperature to 7 ° C at an average rate of 7 ° C / min.
The oxygen content of the obtained infusible fiber was 3.7 wt%, and fusion was observed between the fibers, resulting in poor flexibility. This infusible fiber was carbonized, milled and graphitized in the same manner as in Example 1 to obtain a graphite fiber milled. When physical properties were measured and negative electrode characteristics were evaluated in the same manner as in Example 1, the thermal conductivity was 1100 W / m · K, and the specific resistivity was
1.6 × 10 ^-4 Ω · cm (from 6K to 300K)
The value is almost constant and the initial discharge amount is 310 mAh.
However, the initial charge / discharge efficiency was as low as 82%, and the discharge capacity at the 10th time was 270 mAh / g.

(Comparative Example 5) Mesophase pitch fibers obtained in the same manner as in Example 1 were heated in the air at 170 ° C to 300 ° C.
And an infusibilizing treatment was performed at an average rate of 1 ° C./min. The oxygen content of the obtained infusible fiber was 11.0 wt%, and this infusible fiber was carbonized, milled and graphitized in the same manner as in Example 1 to obtain a graphite fiber milled. When physical properties were measured and negative electrode characteristics were evaluated in the same manner as in Example 1, the thermal conductivity was
320 W / mK, the initial discharge amount is 260 mAh /
The initial charge / discharge efficiency was also as low as 86% in g.

(Example 5) 0.5 mol per 1 mol of industrial naphthalene (number average molecular weight = 132, fa = 0.97)
Molar hydrogen fluoride and 0.2 mol of boron fluoride were charged into an autoclave and reacted at 280 ° C for 3 hours. The obtained pitch has a yield of 72% and has a METTLER softening point of 260.
It was a 100 ° C., 100% mesophase component. This raw material pitch was spun in the same manner as in Example 1 to obtain pitch fibers.
The spinning viscosity at this time was 35 poise. After infusibilizing this fiber to an oxygen content of 4.5 wt%,
It was carbonized at 600 ° C., milled, and then graphitized in argon gas at 2800 ° C. to obtain a graphite fiber milled. When physical properties were measured and negative electrode characteristics were evaluated in the same manner as in Example 1, the thermal conductivity was 420 W / m · K and the specific resistance (at 6K to 300K) was 1.9 × 10 ⁻⁴ Ω.
The value was almost constant as cm. The initial discharge capacity =
It was 302 mAh / g, and the initial charge / discharge efficiency was 90%.

(Comparative Example 6) Number average molecular weight = 380, fa
Using a petroleum fraction of 0.72 as a raw material, heat treatment was performed in the presence of hydrogen fluoride and boron fluoride in the same manner as in Example 1. The obtained pitch is a pitch having a mesophase component of 100% by observation with a polarization microscope, and the Mettler softening point is 295 ° C.
And Using this raw material pitch as in Example 1,
After spinning at a spinning temperature such that the spinning viscosity was 35 poise, infusibilization was carried out so that the oxygen content was 8.5 wt%. This infusible fiber was carbonized and milled in the same manner as in Example 1 to give 30
Graphitization treatment was carried out at 00 ° C to obtain a graphite fiber mill. Example 1
Similarly to the above, when the physical properties were measured and the negative electrode characteristics were evaluated, the thermal conductivity was 410 W / m · K, and the specific resistance (at 6K to 300K) was 2.0 × 10 ⁻⁴ Ω · cm. However, the initial discharge capacity was as low as 280 mAh / g.

(Comparative Example 7) Number average molecular weight = 120, fa
Heat treatment was performed in the presence of hydrogen fluoride and boron fluoride in the same manner as in Example 1 using a petroleum fraction of 0.52 as a raw material. The obtained pitch is a pitch having a mesophase component of 100% by observation with a polarization microscope and has a Mettler softening point of 273.
It was ℃. After spinning this raw material pitch in the same manner as in Example 1 at a spinning temperature at which the spinning viscosity is 40 poise,
Infusibilization was performed so that the oxygen content rate was 6.9 wt%.
This infusible fiber was carbonized and milled in the same manner as in Example 1,
Graphitization treatment was performed at 3000 ° C. to obtain a graphite fiber mill. Physical properties were measured and negative electrode characteristics were evaluated in the same manner as in Example 1. The thermal conductivity was 440 W / m · K and the specific resistance was 2.0 × 10 ⁻⁴ Ω · cm (at 6K to 300K). However, the initial discharge capacity was 262 mAh / g, which was a low value.

Comparative Example 8 A pitch having a mesophase content of 98% and a softening point of 277 ° C. was produced in the same manner as in Example 1 except that the heat treatment time at 300 ° C. was 2.5 hours.
This pitch was spun at a spinning viscosity of 40 poise in the same manner as in Example 1, and then a graphite fiber milled was obtained in the same manner as in Example 1. When physical properties were measured and negative electrode characteristics were evaluated in the same manner as in Example 1, the thermal conductivity was 340 W / m · K,
The specific resistance (at 6K to 300K) is also 2.2 × 10 ⁻⁴
Ω · cm, and the initial discharge capacity is 275 mAh /
The value was as low as g.

[0060]

[Table 1]

[0061]

EFFECT OF THE INVENTION The graphite fiber mill for lithium-based non-aqueous secondary battery according to the present invention is made by forming a specific raw material into fiber under specific conditions,
It is formed into a milled or graphitized form, has a large doping capacity for lithium, has a high charge / discharge efficiency, and is capable of high-speed charge / discharge. When the carbon fiber according to the present invention is used for the negative electrode, it is possible to manufacture a lithium-based non-aqueous secondary battery which has excellent capacity and cycle characteristics and produces less lithium dendrite than ever before.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between thermal conductivity and specific resistance of graphite fibers obtained in Examples and Comparative Examples.

Claims (4)

[Claims] 1. The specific resistance is 2 in the temperature range of 400 W / m · K or more and 6 K or more and 300 K or less.
A mesophase pitch-based graphite fiber for a negative electrode material of a lithium-based non-aqueous secondary battery, which is characterized by having a ^{density of} × 10 ^-4 Ω · cm or less. 2. An infusible fiber obtained by a conventional method using mesophase pitch as a raw material is carbonized in a temperature range of 400 ° C. to 1200 ° C., and then has an average particle size of 5 μm to 5 μm.
After being milled to 0 μm or less, graphitized at 2200 ° C. or higher, the thermal conductivity is 400 W / m · K or higher,
A mesophase pitch-based graphite fiber mill for a lithium-based non-aqueous secondary battery negative electrode material, which has a specific resistance of 2 × 10 ⁻⁴ Ω · cm or less in a temperature range of 6 K or more and 300 K or less. 3. A mesophase component having a softening point of 250 ° C. or higher obtained by polymerizing a hydrocarbon having an aromatic carbon ratio fa of 0.6 or more and a number average molecular weight of 100 or more and 350 or less in the presence of a catalyst. 3. The graphite fiber milled product according to claim 2, wherein the raw material is substantially 100% mesophase pitch. 4. The softening point obtained by polymerizing a hydrocarbon having an aromatic carbon ratio fa of 0.6 or more and a number average molecular weight of 100 or more and 350 or less in the presence of hydrogen fluoride and boron fluoride.
The mesophase component having a mesophase component of 50 ° C or higher is substantially 100% as a raw material, spinning is performed in a viscosity range of 5 poises to 100 poises, and then oxygen content is contained in a temperature range of 150 ° C to 350 ° C. So that the rate is 4 wt% or more and 10 wt% or less, then carbonized in an inert gas in the temperature range of 400 ° C. or more and 1200 ° C. or less, and then so that the average particle size is 5 μm or more and 50 μm or less. The method for producing a graphite fiber milled product for a lithium-based secondary battery according to claim 2, characterized in that it is made into a mild form and then graphitized at a temperature of 2200 ° C or higher.