JPH09306488A - Negative electrode material for nonaqueous electrolyte secondary battery, manufacture of this negative electrode material and nonaqueous electrolyte secondary battery using this negative electrode material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a negative electrode material of high capacity having a good cycle characteristic, and provide a secondary battery of high reliability having high energy density. SOLUTION: A nonaqueous electrolyte secondary battery is formed by having a negative/ positive electrode formed of carbon material capable of doping/dedoping lithium and a nonaqueous electrolyte formed by dissolving an electrolyte in a nonaqueous solvent. The negative electrode is a fiber-shaped carbon material, its sectional higher order structure is a random radial type structure, and a value of fractal dimension in the carbon material section is 1.1 or more and 1.8 or less, a negative electrode material for the nonaqueous electrolyte secondary battery is formed. In the case of delivery spinning a precursor of the carbon material by applying a pressure to a soft pitch, a forming method performing delivery spinning while applying an ultrasonic wave to a delivery hole or a forming method performing delivery spinning while applying a magnetic field to the delivery hole is used, the negative electrode material for the nonaqueous electrolyte secondary battery is manufactured. Further, the negative electrode material for the nonaqueous electrolyte secondary battery is used, the nonaqueous electrolyte secondary battery is constituted.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a negative electrode material for a lithium ion secondary battery and a secondary battery using the same, and more particularly to using a fibrous carbon material as the negative electrode material.

[0002]

2. Description of the Related Art Recent remarkable advances in electronic technology have made electronic devices smaller and lighter one after another. Along with this, demands for batteries as portable power sources that are smaller, lighter, and have higher energy density are increasing.

Conventionally, an aqueous solution type battery such as a lead battery or a nickel-cadmium battery has been mainly used as a secondary battery for general use. These batteries can satisfy the cycle characteristics to some extent, but cannot be said to be satisfactory in terms of battery weight and energy density. On the other hand, research and development of non-aqueous electrolyte secondary batteries using lithium or a lithium alloy for the negative electrode have been actively conducted in recent years. This battery has high energy density, low self-discharge, and excellent characteristics such as light weight, but as the charge and discharge cycle progresses, lithium grows into dendrite-like crystals during charging and reaches the positive electrode, causing an internal short circuit. However, it was a major obstacle to practical use.

As a solution to such a problem, a non-aqueous electrolyte secondary battery using a carbon material for the negative electrode, that is, a so-called lithium ion secondary battery has been receiving attention. The lithium-ion secondary battery utilizes doping / dedoping of lithium between carbon layers for the negative electrode reaction, and no dendrite-like deposition is observed during charging even if the charging / discharging cycle progresses, and good charging / discharging It shows cycle characteristics.

By the way, although there are several carbon materials that can be used as the negative electrode, the first practical materials are coke and glassy carbon. These are materials having low crystallinity obtained by heat-treating an organic material at a relatively low temperature, but they have been commercialized as a practical battery using an electrolytic solution containing PC (propylene carbonate) as a main component. More recently, when PC was used as the main solvent, even graphites that could not be used as the negative electrode reached a level where they could be used by using an electrolytic solution containing EC (ethylene carbonate) as a main component.

[0006] Graphites, which are scaly, are relatively easily available and have been widely used as conductive materials for alkaline batteries. These graphites have higher crystallinity and higher true density than the non-graphitizable carbon material. Therefore, if the negative electrode is constituted by this, a high electrode filling property is obtained and the energy density of the battery is increased. From this, it can be said that graphites are promising materials as negative electrode materials.

However, most of the above-mentioned carbon materials are in the form of blocks having a particle size larger than that actually used in batteries, and are pulverized into powder to be used. Therefore, even if the carbon structure is controlled microscopically or macroscopically by physical or chemical treatment, the structure is disturbed by pulverization, and the effect cannot be sufficiently obtained.

On the other hand, fibrous carbon (carbon fiber) obtained by carbonizing fibrous organic matter
Is relatively easy to control the carbon structure, and has recently attracted attention. The structure largely reflects the structure of the precursor organic fiber. As the organic fibers, there are fibers made from polymers such as polyacrylonitrile, pitches such as petroleum pitch, and fibers made from further oriented mesophase pitch. The fibers are spun into fibers. However, in all cases, when heat-treated during carbonization, they were melted and destroyed the fiber structure.

Therefore, carbonization is usually performed after the fiber surface is infusibilized by oxidation or the like. The fibrous carbon thus obtained has a cross-sectional structure derived from an organic fiber structure, and has a higher-order structure such as a concentrically oriented onion skin type, a radially oriented radial type, or an isotropic random type. Show. Graphite fibers obtained by subjecting these to graphitization have high true density and relatively high crystallinity, and thus are promising as a negative electrode material for non-aqueous electrolyte secondary batteries.

However, it is known that in the above-mentioned fibrous carbon, an intercalation reaction is a main negative electrode reaction, and the capacity due to this increases as the crystallinity increases. In the fiber structure, a radially oriented radial type structure relatively easily develops a crystal structure, and a high capacity can be obtained. However, since the radial type suffers from high crystallinity, the strength of the fiber itself becomes weak, and during expansion / contraction during charging / discharging, cracks occur parallel to the fiber axis and the fiber structure is easily destroyed.

In order to improve this, a fibrous carbon having a random radial type structure has been proposed by the present inventors, and cracks generated in parallel with the fiber axis due to expansion and contraction during charging / discharging are greatly reduced, resulting in charge / discharge reversibility. It was possible to improve the sex. However, there is no physical property parameter to evaluate the cross-sectional structure of this fibrous carbon of random radial type structure,
It was difficult to obtain uniform fibrous carbon.

[0012]

Therefore, the object of the present invention is to correctly evaluate the cross-sectional structure of a fibrous carbon having a random radial type structure, and to enable the production of a highly practical negative electrode material with little industrial variation, Further, by using this as a negative electrode, it is intended to provide a highly reliable non-aqueous electrolyte secondary battery having high energy density.

[0013]

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and a negative electrode and a positive electrode made of a carbon material capable of being doped with lithium and dedoped, and a nonaqueous electrolysis in which an electrolyte is dissolved in a nonaqueous solvent. In a non-aqueous electrolyte secondary battery comprising a liquid, the negative electrode is a fibrous carbon material,
For a non-aqueous electrolyte secondary battery in which the higher order structure of the cross section of the carbon material is a random radial type structure and the value of the fractal dimension of the cross section of the carbon material is 1.1 or more and less than 1.8. Form the negative electrode material.

When the precursor of the carbon material is discharged and spun by applying a pressure to the softening pitch, a method of discharging and spinning while applying ultrasonic waves to the discharge hole, or a method of discharging and spinning while applying a magnetic field to the discharge hole A negative electrode material for a non-aqueous electrolyte secondary battery is manufactured using the method described above.

A non-aqueous electrolyte secondary battery comprising a negative electrode made of a carbon material capable of being doped and dedoped with lithium, a positive electrode, and a non-aqueous electrolyte solution in which an electrolyte is dissolved in a non-aqueous solvent. A non-aqueous electrolyte secondary battery is constructed using a negative electrode material for a secondary battery to solve the above problems.

A fractal dimension of a fibrous carbon having a random radial type cross-sectional structure can be controlled to a fractal dimension value determined by fractal analysis in an optimum range, and the fiber has cycle characteristics with little variation on an industrial scale. Form carbon can be realized. Further, by using this as a negative electrode, a secondary battery having high energy density, long cycle life and high reliability can be obtained.

[0017]

FIG. 1 shows an embodiment of the present invention.
It will be described with reference to FIGS. FIG. 1 is a cross section of a fibrous carbon according to the present invention, FIG. 2 is a partially enlarged view of FIG. 1, and FIG. 3 is a view obtained by performing image processing on FIG. FIG. 4 is a side sectional view of a tubular battery using fibrous carbon according to the present invention. FIG. 5 is a diagram showing the relationship between the interplanar spacing and the capacity of the fibrous carbon according to the present invention, and FIG. 6 is a view showing the relationship between the fractal dimension and the capacity retention rate of the fibrous carbon according to the present invention.

In order to solve the above-mentioned problems, the present inventors can use the value of the fractal dimension obtained by fractal analysis of the higher-order structure of the cross section of random radial type fibrous carbon as a physical property parameter for evaluating the cross section structure. By controlling the fractal dimension within a specific range and controlling the crystallinity within an appropriate range, there is little variation in charge / discharge performance, and high capacity fibrous carbon with good charge / discharge cycle reversibility can be obtained. I found it feasible.

The fractal dimension of the fibrous carbon cross section structure defined in the present invention is an index representing the structure of the carbon mesh plane in the fiber cross section as shown in FIG. To obtain this, first use an electron microscope (field emission scanning electron microscope, etc.) to make an image of the fiber cross section into a photograph, etc., and then take that image into a computer using a scanner, etc.
After image processing of this, fractal analysis is performed. The average value obtained for 10 fibers was used as the value of the fractal dimension (hereinafter, simply referred to as "FD value").

The fractal dimension indicates the degree of curve of the curve in the plane, and takes a value of 1 to 2, but approaches 2 as the curve becomes more complicated. That is, a complex fibrous carbon cross-sectional structure can be quantitatively evaluated by the FD value, and particularly as a parameter for evaluating the curved structure of the carbon net surface that affects the capacity as the negative electrode and the reversibility of the charge / discharge cycle. The FD value is important.

As this value approaches 2, the curved structure becomes more complicated, the fiber strength increases, and the reversibility of the charge / discharge cycle improves. On the other hand, however, as the curved structure becomes more complicated, graphitization becomes more difficult, the crystallinity does not improve, and the intercalation capacity decreases. Therefore, it is highly crystalline and the FD value is preferably 1.1 or more and less than 1.8.
It is more preferably 25 or more and less than 1.8. Further, as a parameter of the crystal structure, the (002) plane spacing d ₀₀₂ obtained by the X-ray analysis method (Gakushin method) serves as an index. Here d ₀₀₂
Is preferably less than 0.340 nm, more preferably 0.335 nm or more and 0.337 nm or less.

In order to control the crystallinity and the FD value, the starting material and the method of fiber formation are important. When the fibrous carbon of the present invention is produced, polymers such as polyacrylonitrile and rayon as starting materials, petroleum pitch, coal pitch, synthetic pitch, and these are kept at about 400 ° C. for any time. It is possible to use pitches such as mesophase pitches in which aromatic rings are condensed and polycyclic to have a laminated orientation by accelerating the polymerization by adding an acid or the like.

In particular, when the mesophase pitch is used, the mesophase content greatly affects the spinnability, the physical properties of fibrous carbon, and the electrical and chemical properties. The mesophase content is preferably 60% or more, and 95
% Or more is more preferable. If it is less than this range, the crystal orientation is poor and the capacity of the material itself is lowered, which is not preferable.

When the organic carbon which is the fibrous carbon precursor of the present invention is produced, the above-mentioned polymers and pitches are heated to be in a molten state and molded and spun by discharge or the like. In this case, the melting point varies depending on each organic substance, and the optimum spinning temperature can be appropriately selected for each.

In particular, in order to control the FD value, the spinning conditions, that is, extrusion molding, is greatly affected by the extrusion speed and the shape of the discharge hole. In addition, it is also possible to form a curved structure of the carbon net surface in the cross-sectional structure by making the flow of the pitch in the discharge holes turbulent at the time of discharging.
In this case, use a method in which a gas such as air is blown out by providing a fine hole in the discharge hole, a method in which a magnetic field is generated in the vicinity of the discharge hole to disturb the pitch alignment, or a method in which the discharge hole is vibrated by ultrasonic waves or the like. You can

The above-mentioned organic fiber, which is a precursor of fibrous carbon, is infusibilized after spinning and before heat treatment. Although the specific means is not limited, for example, a wet method using an aqueous solution of nitric acid, mixed acid, sulfuric acid, hypochlorous acid, or the like, or a dry method using an oxidizing gas (air, oxygen), sulfur, ammonia nitrate,
A reaction with a solid reagent such as ammonium persulfate or ferric chloride is used. Moreover, when performing the said process, you may draw or tension a fiber.

The above infusibilized organic fibers are heat-treated in a stream of inert gas such as nitrogen. The conditions are as follows: carbonization at 300 to 700 ° C. and then in a stream of inert gas at every heating rate. Min 1-100 ° C, ultimate temperature 900-15
Calcination is performed at a temperature of 00 ° C. for a holding time of about 0 to 30 hours, and in order to obtain a graphitized product, it is preferable to perform heat treatment at 2000 ° C. or higher, preferably 2500 ° C. or higher. Of course, in some cases, the carbonization and calcination operations may be omitted. The fibrous carbon of the present invention graphitized by heat treatment at a high temperature of 2500 ° C. or higher has a true density close to that of artificial graphite and a high electrode packing density is obtained, which is preferable.

The produced fibrous carbon is classified or ground and classified to be used as a negative electrode material. Grinding may be carried out before or after carbonization, calcination, or during the temperature rising process before graphitization. In this case, the heat treatment for graphitization is finally performed in the powder state.

As the graphitized fibrous carbon, those having the above physical properties are preferable, and the fiber diameter and the aspect ratio can be appropriately selected according to the above physical properties, but the fiber diameter is 5 μm or more and 100 μm or less. Is preferable, and the aspect ratio is preferably 20 or less. This is because the smaller the fiber diameter, the larger the specific surface area, and the larger the fiber diameter, the lower the effect of imparting the fiber shape.

Further, by satisfying the physical property values described below, a more practical negative electrode material can be obtained.

To obtain a higher electrode packing density, the true density of the graphitized fibrous carbon is preferably 2.1 g / cm ³ or more, more preferably 2.18 g / cm ³ or more. The true density of graphite material (Pycnometer method using butanol solvent) is
The crystal structure parameters such as the (002) plane distance and the C-axis crystallite thickness of the (002) plane, which are determined by the crystallinity and obtained by the X-ray diffraction method (Gakushin method), are used as an index. In order to obtain a material with high true density, it is preferable that the crystallinity is high, and the (002) plane spacing obtained by X-ray diffraction is preferably less than 0.340 nm, more preferably 0.335 nm or more and 0.337 nm or less. . The C-axis crystallite thickness of the (002) plane is preferably 14.0 nm or more and 30.0 nm.
The above is more preferred.

In order to obtain good cycle characteristics, it is preferable to use a material having a bulk density of 0.4 g / cm ³ or more. The negative electrode composed of a graphite material having a bulk density of 0.4 g / cm ³ or more has a good electrode structure, and it is unlikely that the graphite material peels off from the negative electrode mixture layer. Therefore, a long cycle life can be obtained.

The bulk density regulated by the present invention is JIS
It is the value determined by the method described in K-1469.
If a graphite material with this value of 0.4 g / cm ³ or more is used,
Although a sufficiently long cycle life can be obtained, the bulk density is preferably 0.5 g / cm ³ or more, and more preferably the bulk density is 0.1 g / cm ³ .
It is preferable to use a material of 7 g / cm ³ or more.

Bulk Density Measuring Method The bulk density measuring method will be described below. The graduated cylinder with a capacity of 100 cm ³ whose mass has been measured in advance is tilted, and 100 cm of the sample powder is placed on this.
Gradually add ³ using a spoon. Then, the total mass is measured with a minimum scale of 0.1 g, and the mass of the graduated cylinder is subtracted from the mass to obtain the sample powder M.

Next, a graduated cylinder charged with the sample powder is capped with a cork, and the graduated cylinder in this state is dropped 50 times from a height of about 5 cm with respect to the rubber plate. As a result, the sample powder in the graduated cylinder is compressed,
Read the volume V of the compressed sample powder. And
The bulk density (g / cm ³ ) is calculated by the following formula (1).

D = W / V (1) where D: bulk density (g / cm ³ ) W: mass of sample powder in graduated cylinder (g) V: sample powder in graduated cylinder after 50 drops Volume of (cm ³ )

Further, when the average value of the shape parameter x expressed by the equation (2) is 125 or less, the cycle characteristics are further improved. That is, a typical shape of the graphite material powder is a flat columnar shape or a rectangular parallelepiped shape. When the thickness of the thinnest part of this graphite material powder is T, the length of the longest part is L, and the length in the direction orthogonal to the major axis corresponding to the depth is W, L The product of the values obtained by dividing each W by T is the shape parameter x. The smaller the shape parameter x, the higher the height with respect to the bottom area and the smaller the flatness.

X = (W / T) × (L / T) (2) Here, x: shape parameter T: thickness of the thinnest part of the powder L: length of the powder in the major axis direction w: Length of powder in the direction orthogonal to the long axis

Further, the average shape parameter xave. Is obtained by the following actual measurement. First, the graphite sample powder is SE
Observe using M (scanning electron microscope), and select 10 powders in which the length of the longest part is ± 30% of the average particle size. Then, the shape parameter x is calculated from the equation (2) for each of the 10 selected powders, and the average thereof is calculated. The calculated average value is the average shape parameter xave. The above effect can be obtained when the average shape parameter xave. Of the graphite powder is 125 or less, but preferably 2 or more and 115 or less, more preferably 2 or more and 100 or less.

When a material having a specific surface area of 9 m ² / g or less is used, a longer cycle life can be obtained. It is considered that the submicron particles adhering to the graphite particles influence the decrease in bulk density, and the specific surface area increases when the particles adhere, so even if the particle size is similar, the specific surface area The use of small graphite powder is not affected by the fine particles, a high bulk density is obtained, and as a result, cycle characteristics are improved.

However, the specific surface area as used herein means that measured and determined by the BET method. If the specific surface area of the graphite powder is 9 m ² / g or less, the above effect is sufficiently obtained, but preferably 7 m ² / g or less, more preferably 5 m
² / g or less is preferable.

In order to obtain high safety and reliability as a practical battery, the cumulative 10% particle size is 3 μm or more in the particle size distribution obtained by the laser diffraction method,
And the cumulative 50% particle size is 10 μm or more, and the cumulative 9%
It is desirable to use graphite powder having a 0% particle size of 70 μm or less.

The graphite powder with which the electrodes are filled can be packed more efficiently if the particle size distribution has a width, and it is preferable that the graphite powder is closer to the normal distribution. However, the battery may generate heat in an abnormal situation such as overcharging, and if the number of particles with small particle size is large, the heat generation temperature tends to be high, which is not preferable.

When the battery is charged, the intercalated graphite helium ions cause the crystallite to expand by about 10%, compressing the positive electrode and the separator in the battery, and causing an initial failure such as an internal short circuit during the initial charging. Although it tends to occur, it is not preferable when the distribution of large particles is large, because the incidence of defects tends to increase.

Therefore, by using the graphite powder having a particle size distribution in which particles having a large particle size to particles having a small particle size are well-balanced, a practical battery having high reliability can be obtained. If the shape of the particle size distribution is closer to the normal distribution, the particles can be packed more efficiently, but in the particle size distribution obtained by the laser diffraction method, the cumulative 10% particle size is 3 μm or more, and the cumulative 50% particle size is 10 μm or more, In addition, it is desirable to use graphite powder having a cumulative 90% particle size of 70 μm or less, and especially when the cumulative 90% particle size is 60 μm or less, initial defects are greatly reduced.

In order to improve the heavy load characteristics of a practical battery, the average value of the breaking strength of graphite particles is 6.0.
It is desirable to be kgf / mm ² or more. The load characteristics are affected by the ease of movement of ions during discharge, but especially when there are many pores in the electrode, a sufficient amount of the electrolytic solution is present and good characteristics are exhibited. On the other hand, a graphite material with high crystallinity has a hexagonal graphite net surface developed in the a-axis direction, and c-axis crystals are formed by stacking them.
The carbon hexagonal mesh plane bonds are weak bonds called van der Waals forces, and are therefore easily deformed by stress. Therefore, when the graphite powder particles are compression-molded and filled in the electrode, carbon fired at a low temperature is used. It is easier to crush than quality materials, and it is difficult to secure pores. Therefore, the higher the fracture strength of the graphite powder particles, the less likely they are to be crushed, and the easier it is to create voids, which makes it possible to improve the load characteristics.

However, the average value of the breaking strength of the graphite particles as used herein means one obtained by the following actual measurement. A Shimadzu micro compression tester (MCTM-500) manufactured by Shimadzu Corporation is used as a breaking strength measuring device. First, the graphite sample powder is observed with an attached optical microscope, and 10 powders in which the length of the longest part is ± 10% of the average particle size are selected. Then, a load is applied to each of the 10 selected powders, the breaking strength of the particles is measured, and the average thereof is calculated. This calculated average value is the average value of the breaking strength of the graphite particles. In order to obtain good load characteristics, it is preferable that the average breaking strength of the graphite particles is 6 kgf / mm ² or more.

On the other hand, the positive electrode material used in combination with the negative electrode made of such fibrous carbon or graphitized fibrous carbon is not particularly limited, but preferably contains a sufficient amount of Li, for example, the general formula LiMO. ₂ (However, M
Represents at least one of Co, Ni, Mn, Fe, Al, V, and Ti. ), A composite metal oxide composed of lithium and a transition metal, an intercalation compound containing Li, and the like are preferable.

In particular, the present invention is aimed at achieving a high capacity, so that the positive electrode is 250 mAh or more per 1 g of the negative electrode carbonaceous material in a steady state (for example, after repeating charging and discharging about 5 times). It is necessary to include Li in an amount equivalent to the charge / discharge capacity, and L corresponding to an equivalent charge / discharge capacity of 300 mAh or more.
More preferably, i is included.

It is not always necessary that Li is supplied from the positive electrode material, and the point is that Li corresponding to a charge / discharge capacity of 250 mAh or more per 1 g of carbonaceous material should be present in the battery system. Further, the amount of Li will be determined by measuring the discharge capacity of the battery.

In the non-aqueous electrolytic solution used in the non-aqueous electrolytic solution secondary battery of the present invention, a non-aqueous electrolytic solution obtained by dissolving an electrolyte in a non-aqueous solvent is used as the electrolytic solution. Here, in the present invention, since the graphite material is used for the negative electrode, conventional PC cannot be used as the main solvent of the non-aqueous solvent, and it is premised that other solvents are used. EC is most preferred as the main solvent, but compounds having a structure in which the hydrogen element of EC is replaced with a halogen element are also suitable.

Although it is reactive with a graphite material such as PC, a small amount of EC is used as a main solvent or a compound having a structure in which a hydrogen atom of EC is replaced with a halogen element. Good properties are obtained by substituting with a binary solvent. As the second component solvent, PC, butylene carbonate, 1,2-dimethoxyethane, 1,
2-diethoxymethane, γ-butyrolactone, valerolactone, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, 4-methyl-1,3-
Dioxolane, sulfolane, methylsulfolane and the like can be used, and the addition amount thereof is preferably less than 10 Vol%.

To further complete the present invention, the third solvent is added to the main solvent or a mixed solvent of the main solvent and the second component solvent to improve the conductivity and suppress the decomposition of EC. The low temperature characteristics may be improved and the reactivity with lithium metal may be lowered to improve the safety.

As the solvent for the third component, first, DEC
A chain ester carbonate such as (diethyl carbonate) or DMC (dimethyl carbonate) is suitable. Also, ME
Asymmetric chain ester carbonate such as C (methyl ethyl carbonate) or MPC (methyl propyl carbonate) is preferable. The volume ratio of the main solvent or the mixed solvent of the main solvent and the second component solvent of the chain carbonate as the third component (main solvent or the mixed solvent of the main solvent and the second component solvent: the third component solvent) is the volume. The ratio is preferably 10:90 to 60:40, more preferably 15:85 to 40:60.

Further, as the solvent for the third component, MEC and D
It may be a mixed solvent with MC. The MEC-DMC mixing ratio is preferably in a range represented by 1/9 ≦ d / m ≦ 8/2, where MEC capacity is m and DMC capacity is d. Further, the MEC-DMC mixing ratio of the main solvent, or the mixed solvent of the main solvent and the second component solvent and the solvent of the third component, when the MEC capacity is m, the DMC capacity is d, and the total amount of the solvent is T, 3/10 ≦ (m + d) / T ≦ 7/1
The range shown by 0 is preferable.

As the electrolyte which can be dissolved in such a non-aqueous solvent, it is possible to use one or more kinds of electrolytes as long as they are used in this type of battery. For example, LiPF ₆ is preferable, but LiClO 4, LiAsF ₆ , Li
BF ₄ , LiB (C ₆ H ₅ ) ₄ , CH ₃ SO ₃ Li, C
_{F 3 SO 3 Li, LiN (} CF 3 SO 2) 2, LiC
(CF ₃ SO ₂ ) ₃ , LiCl, LiBr and the like can also be used.

The present invention will be described below with reference to specific examples, but the present invention is not limited to these examples, and other means for embodying the technical idea of the present invention may be used. I can't wait for the matter.

Example 1 First, a negative electrode material was produced as follows. Hold the coal pitch at 425 ° C in an inert gas atmosphere for 5 hours,
A coal-based mesophase pitch having a softening point of 220 ° C. was obtained. At this time, the mesophase content was 90%. The inner diameter of the obtained coal-based mesophase pitch was 20μ at 305 ° C.
m discharge holes were used, and while the ultrasonic waves were applied thereto, the extrusion pressure was changed in a pulsed manner and the discharge spinning was performed to obtain precursor fibers. After that, it was infusibilized at 260 ° C., and calcined at a temperature of 1000 ° C. in an inert atmosphere to obtain fibrous carbon. Further, heat treatment was carried out at a temperature of 3000 ° C. in an inert atmosphere, and pulverization by air classification was carried out to obtain a sample powder of graphitized fibrous carbon. When the (002) plane spacing d ₀₀₂ and the FD value of the obtained sample powder were determined, they were d ₀₀₂ = 0.3363 nm and FD = 1.1.

Measurement method of fractal dimension. Next, a method for measuring the FD value will be described. First, in order to obtain an image of a fibrous carbon cross-sectional structure, the cross section was observed under an accelerating voltage condition of 2 kv using a field emission scanning electron microscope, and this static image was loaded into a computer, and the obtained image is shown in FIG. As described above, the cross-sectional image is divided into five different portions including the fiber central portion of 4 μm square (consisting of 512 × 512 pixels). When the brightness of the image is not uniform, each image is binarized after being subjected to smoothing by Fourier transform or filtering. When it is necessary to further clarify the curved shape of the carbon net surface, image processing is performed so that the white part of the binarized image can be recognized as a thin line (FIG. 3).

The above five images are calculated by using the equation (3) to obtain the fractal dimension d. d = -ΔlogN (l) / Δlogl (3) where, l: total number of squares when divided into squares of a certain size N: overlapped with a carbon mesh surface curve when divided into squares of a certain size The number of squares. That is, the image is divided into a grid of squares, the number of squares overlapping the curve of the carbon mesh plane is calculated, and the same calculation is performed while changing the size of the squares, and the average of the five images is obtained. Further, this measurement was performed on 10 fibers, and an average value was obtained and used as an FD value. This method is described in, for example, "Carbon TANSO199"
5, No. 169, P.I. It can also be measured by the method described in "207-214".

Next, using the sample powder as a negative electrode material,
A cylindrical non-aqueous electrolyte secondary battery was actually manufactured. The structure of the battery is shown in FIG.

The negative electrode 1 was produced as follows. 90 parts by weight of the above graphite powder and 10 parts by weight of polyvinylidene fluoride (PVDF) as a binder are mixed to prepare a negative electrode mixture,
It was dispersed in N-methylpyrrolidone as a solvent to form a slurry (paste form). The negative electrode current collector 10 has a thickness of 10
A band-shaped negative electrode 1 was prepared by applying a negative electrode mixture slurry on both sides of this current collector using a band-shaped copper foil having a thickness of μm, drying it, and then compression molding it at a constant pressure.

The positive electrode 2 was manufactured as follows. First, a positive electrode active material was prepared as follows. Lithium carbonate 0.
5 mol and 1 mol of cobalt carbonate are mixed and the mixture is calcined in air at a temperature of 900 ° C. for 5 hours. As a result of X-ray diffraction measurement of the obtained material, it was in good agreement with the peak of LiCoO ₂ registered in the JCPDS file.

This LiCoO ₂ was crushed to obtain a LiCoO ₂ powder having a cumulative 50% particle size of 15 μm obtained by a laser diffraction method. Then, 95 parts by weight of this LiCoO ₂ powder and 5 parts by weight of lithium carbonate powder were mixed to obtain 91 parts of this mixture.
By weight, 6 parts by weight of graphite as a conductive agent and 3 parts by weight of polyvinylidene fluoride as a binder were mixed to prepare a positive electrode mixture, which was dispersed in N-methylpyrrolidone to form a slurry (paste form).

A strip-shaped aluminum foil having a thickness of 20 μm is used as the positive electrode current collector 11, and the positive electrode mixture slurry is uniformly applied to both surfaces of the current collector, dried, and then compression molded to obtain the strip-shaped positive electrode 2. Was produced.

Then, the strip-shaped negative electrode 1 and the strip-shaped positive electrode 2 produced as described above were made to have a thickness of 25 μm as shown in FIG.
A negative electrode 1, a separator 3, a positive electrode 2, and a separator 3 are laminated in this order through a separator 3 made of a microporous polypropylene film of m and then wound many times to have an outer diameter of 18 mm.
A spirally wound electrode body was prepared.

The spirally wound electrode body thus produced was
It was housed in a nickel-plated iron battery can 5. Then, the insulating plates 4 are arranged on the upper and lower surfaces of the spiral type electrode, and the positive electrode lead 13 made of aluminum is led out from the positive electrode current collector 11 to the battery lid 7, and the negative electrode lead 12 made of nickel is connected to the negative electrode current collector 10. Was welded to the battery can 5.

Into the battery can 5, an electrolytic solution in which LiPF ₆ was dissolved at a ratio of 1 mol / l in a mixed solvent of equal volume of EC and DMC was injected. Then, by caulking the battery can 5 through a sealing gasket 6 whose surface is coated with asphalt, a safety valve device 8 having a current interruption mechanism, P
The TC element 9 and the battery lid 7 were fixed, the airtightness inside the battery was maintained, and a cylindrical nonaqueous electrolyte secondary battery having a diameter of 18 mm and a height of 65 mm was produced.

Example 2 A cylindrical non-aqueous electrolyte secondary battery was prepared in the same manner as in Example 1 except that the precursor fiber was obtained by discharge spinning under different pulse conditions. The (002) plane spacing d of the obtained test powder
₀₀₂ was 0.3365 nm and FD was 1.2.

Example 3 A cylindrical non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that the precursor fiber was obtained by discharge spinning under different pulse conditions. The (002) plane spacing d of the obtained test powder
₀₀₂ was 0.3367 nm and FD was 1.3.

Example 4 A cylindrical non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that the precursor fibers were obtained by discharge spinning under different pulse conditions. The (002) plane spacing d of the obtained test powder
₀₀₂ was 0.3372 nm and FD was 1.5.

Example 5 A cylindrical non-aqueous electrolyte secondary battery was prepared in the same manner as in Example 1 except that the precursor fiber was obtained by discharge spinning under different pulse conditions. The (002) plane spacing d of the obtained test powder
₀₀₂ was 0.3363 nm and FD was 1.3.

Comparative Example 1 A cylindrical non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that the precursor was obtained by discharge spinning without applying ultrasonic waves to the discharge hole. The (002) plane spacing d of the obtained test powder
₀₀₂ was 0.3410 nm and FD was 1.8.

Comparative Example 2 A cylindrical non-aqueous electrolyte secondary battery was prepared in the same manner as in Example 1 except that the precursor was obtained by performing discharge spinning without applying ultrasonic waves and pulsed pressure to the discharge hole. did. The (002) plane spacing d ₀₀₂ of the obtained test powder was 0.3361 nm, and FD
Was 1.0.

Table 1 shows the results of measuring the charge / discharge capacity of the fibrous carbon used in each example and comparative example.

[Table 1] Further, the relationship between the (002) plane spacing d ₀₀₂ and the capacity is shown in FIG.

Charging / Discharging Capacity Measuring Method The charging / discharging capacity measuring method will be described below. The measurement was performed by making a test cell described below. In producing the test cell, first, the sample powder was preheated in an Ar atmosphere under the conditions of a temperature rising rate of about 30 ° C./minute, an ultimate temperature of 600 ° C., and an ultimate temperature holding time of 1 hour. Then, polyvinylidene fluoride in an amount equivalent to 10% by weight was added as a binder, dimethylformamide was mixed as a solvent, and dried to prepare a sample mix. The 37 mg was weighed, and the diameter was 15.5 with the Ni mesh as the current collector.
mm pellets were formed to prepare a working electrode.

The structure of the test cell is as follows. Cell shape: coin type cell (diameter 20 mm, thickness 2.5 m
m) Counter electrode: Li metal Separator: Polypropylene porous membrane Electrolyte: LiPF ₆ dissolved in a mixed solvent of EC and DEC (volume ratio 1: 1) at a concentration of 1 mol / l

Using the test cell having the above structure, the carbon material 1
The capacity per gram was measured. Doping of lithium into the working electrode (charging: strictly speaking, in this test method, discharging is not charging in the process of doping the carbon material with lithium, but this doping process is convenient for the actual battery. Is charged, and the dedoping process is called discharge.) Is a constant current of 1 mA per cell, 0 V (Li / Li
⁺ ) Constant current constant voltage method for charging and discharging (de-doping process)
Was carried out at a constant current of 1 mA per cell up to a terminal voltage of 1.5 V, and the capacity at this time was calculated.

With respect to the cylindrical batteries produced in the respective examples and comparative examples, the charging current was 1 A and the maximum charging voltage was 4.2 V.
5h constant current constant voltage charging, then discharge current 700
The charging / discharging cycle of discharging to 2.75 V at mA was repeated, and the ratio of the capacity at the 100th cycle to the capacity at the second cycle (capacity retention rate) was obtained. The results of the capacity retention ratio at the 100th cycle with respect to the second cycle are shown in Table 1 above.
It was shown to. Further, the relationship between the FD value and the capacity retention rate is shown in FIG.

From the above results, it was found that the fibrous carbon having an FD value controlled, which is an index of the cross-sectional structure of the present invention, is a negative electrode material having excellent cycle characteristics and charge / discharge ability as compared with Comparative Examples.

[0081]

As is clear from the above description, the present invention is a fractal obtained by fractal analysis of the curved structure of the carbon net surface of the cross-section higher order structure in the fibrous carbon having the random radial type cross-section structure. By controlling the dimension value and d _{00 2} which is an index of crystallinity within an optimum range, a fibrous carbon having cycle characteristics with little variation on an industrial scale is provided, and this is used as a negative electrode. By using it, a secondary battery with high energy density, long cycle life and high reliability can be realized.

[Brief description of drawings]

1 is a cross section of a fibrous carbon according to the present invention.

FIG. 2 is a partially enlarged view of a cross section of fibrous carbon according to the present invention.

FIG. 3 is a diagram in which FIG. 2 is image-processed.

FIG. 4 is a side sectional view of a tubular battery using fibrous carbon according to the present invention.

FIG. 5 is a diagram showing the relationship between the interplanar spacing and the capacity of fibrous carbon according to the present invention.

FIG. 6 is a diagram showing the relationship between the fractal dimension and the capacity retention rate of fibrous carbon according to the present invention.

[Explanation of symbols]

1 ... Negative electrode, 2 ... Positive electrode, 3 ... Separator, 4 ... Insulating plate, 5
... Battery can 6 ... Sealing gasket, 7 ... Battery lid, 8 ... Safety valve device, 9
... PTC element 10 ... Negative electrode current collector, 11 ... Positive electrode current collector, 12 ... Negative electrode lead, 13 ... Positive electrode lead 14 ... Center pin

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁶ Identification code Office reference number FI technical display location // C01B 31/02 101 C01B 31/02 101Z

Claims (4)

[Claims] 1. A non-aqueous electrolyte secondary battery comprising a negative electrode and a positive electrode made of a carbon material capable of being doped and dedoped with lithium, and a non-aqueous electrolytic solution in which an electrolyte is dissolved in a non-aqueous solvent. Is a fibrous carbon material, and the higher order structure of the cross section of the carbon material is a random radial type structure, and the value of the fractal dimension of the cross section of the carbon material is 1.1 or more and 1.8. A negative electrode material for a non-aqueous electrolyte secondary battery, which is less than 2. The carbonaceous material precursor is formed by performing discharge spinning while applying ultrasonic waves to a discharge hole when performing discharge spinning by applying pressure to a softening pitch. A method for producing a negative electrode material for a non-aqueous electrolyte secondary battery as described. 3. The precursor of the carbon material is formed by performing discharge spinning while applying a magnetic field to the discharge holes when performing discharge spinning by applying pressure to the softening pitch. 1. A method for producing a negative electrode material for a non-aqueous electrolyte secondary battery. 4. A non-aqueous electrolyte secondary battery comprising a negative electrode made of a carbon material capable of being doped and dedoped with lithium, a positive electrode, and a non-aqueous electrolytic solution in which an electrolyte is dissolved in a non-aqueous solvent. A non-aqueous electrolyte secondary battery comprising the negative electrode material for a non-aqueous electrolyte secondary battery as described in 1.