JPH08306359A - Negative electrode material for lithium secondary battery and manufacturing method thereof - Google Patents

Abstract

(57) [Summary] [Object] The present invention relates to a carbon material for a negative electrode of a lithium secondary battery having a high capacity and a high initial charge / discharge efficiency, which uses a pitch-based carbon fiber, and provides a product and a manufacturing method thereof. [Structure] By mixing a pitch-based carbon fiber powder with a boron compound in an amount of 1 to 20% by weight in terms of boron and firing the mixture at a temperature of 2500 ° C. or higher, the graphitization degree is improved by the catalytic effect of boron, and The discharge capacity can be increased without impairing the high initial charge / discharge efficiency of the pitch-based carbon fiber.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a negative electrode material for a lithium secondary battery utilizing a doping / dedoping reaction of lithium and a method for producing the same. More specifically, it is formed by pulverizing pitch-based carbon fibers. A crushed carbon fiber product,
The present invention relates to a negative electrode material containing carbon and boron as main components.

[0002]

2. Description of the Related Art With the recent miniaturization and weight reduction of electric devices, the demand for higher energy density of secondary batteries has become stronger.

From the viewpoint of environmental protection, an electric vehicle is being developed as a pollution-free vehicle, and it is strongly desired to improve the performance of a secondary battery as a power source for driving the motor.

As a high energy density secondary battery satisfying these requirements, a lithium secondary battery has been attracting attention and its development is urgently needed.

The biggest problem in the development of this lithium secondary battery is that the lithium metal used for the negative electrode undergoes cycle deterioration with repeated charging and discharging.

This is due to the deposition form of lithium metal that is deposited on the negative electrode during charging. For example, dendrites, which are dendrites, peel off from the negative electrode plate or
This will cause a short circuit with the counter electrode.

In order to solve these problems, various lithium alloys and negative electrode materials such as conductive polymers and carbonaceous materials which are doped and dedoped with lithium have been proposed and studied.

However, lithium alloys still have problems such as difficulty in deep charge / discharge and unsuitability for charge / discharge with high current density, and have not yet developed a high energy battery.

On the other hand, carbon materials and conductive polymers have been proposed as lithium doping materials, but the conductive polymers have a small doping amount and are chemically and electrochemically unstable with respect to lithium. However, there are still limits to increasing the energy density.

At present, as a negative electrode material for a lithium secondary battery, a carbonaceous material is most attracting attention as an alternative material to lithium metal.

When a carbonaceous material is used as the negative electrode of a lithium secondary battery, lithium is inserted between the layers of the carbonaceous material from the electrolytic solution during charging, forming a so-called graphite intercalation compound.

During discharge, lithium between layers is released into the electrolytic solution. Therefore, deterioration of the negative electrode due to charge / discharge cycles such as dendrite that occurs when lithium metal is used for the negative electrode may be theoretically eliminated by using the carbonaceous material.

As a result of studying various carbon materials as such a carbonaceous material for the negative electrode of a lithium secondary battery, carbon fibers made of pitch as a raw material have a large discharge capacity and a high initial charge / discharge efficiency.

Therefore, it has been found that the amount of lithium lost due to repeated charge and discharge (referred to as lithium loss) is small, and that it is very effective in achieving high energy density, and a patent application has been filed in advance (Japanese Patent Laid-Open No. Hei 10-1999) 5-3259
67 publication).

However, although the initial charge and discharge efficiency is high, the discharge capacity reaches up to about 80% of the theoretical capacity.
There was a problem with the discharge capacity.

C6Li which is the theoretically expected maximum capacity (corresponding to a capacity of 372 mAh per 1 g of carbon)
The carbon material containing boron (El) with the aim of achieving a high capacity comparable to
electrochimica Acta 38 (1993)
1179, JP-A-5-251080, JP-A-5-
266880, Japanese Patent Application Laid-Open No. 5-290843, etc.) have been reported to be applied to a negative electrode.

Electrochimica Acta
38 (1993) 1179, a carbon material containing boron (boron content in carbon is 5 atom.%) Is prepared by utilizing a CVD technique to study negative electrode characteristics.

The discharge capacity is about 94% of the theoretical capacity (up to 3
50 mAh / g), but the graphite crystallinity is low (90
Therefore, since the true specific gravity of the carbon material is about 1.9 at most, the discharge capacity per unit volume is 3
The problem remains that it will be less than 00 mAh / g.

Further, the material must be prepared by using CVD, which is problematic in terms of mass industrial production and material production cost.

Japanese Unexamined Patent Publication (Kokai) No. 5-290843 reports an example in which a compound in which a part of carbon atoms forming a skeleton of a graphite crystal is replaced with a boron atom is applied to a negative electrode.

There, it is said that the compound having the composition represented by BC3 is suitable for the negative electrode, but as a result of additional tests by the present inventor, although the capacity improvement is recognized, the charge / discharge efficiency of the first cycle is at most 85%. It was found that the problem remains in the initial charge and discharge efficiency.

Further, Japanese Unexamined Patent Publication No. 5-251080 reports that a carbon material containing boron is applied to the negative electrode.

There, the boron content is 0.1-10 w
t. %, The carbon material is said to be suitable for the negative electrode, but the initial charge / discharge efficiency is at most 85%, which also leaves a problem in the initial charge / discharge efficiency.

In JP-A-5-266880, 0.1-10 wt. % Carbon compound containing a boron compound is carbonized and graphitized, and a method for producing a carbon material for a negative electrode containing boron is reported.

However, as a result of additional tests conducted by the present inventor with respect to the coal-based pitch, the results as exemplified are not obtained, and particularly regarding the charge / discharge efficiency, the first cycle is 84
%, The improvement in charge and discharge efficiency was small even after that, and was only 95% even after 10 cycles.

As described above, none of the studied examples has succeeded in satisfying both the capacity and the initial charge / discharge efficiency at the same time, and in particular, the charge / discharge efficiency cannot exceed 85%. It was the current situation.

[0027]

Although the pitch-based carbon fiber has a very high charge / discharge efficiency, it has a problem that the capacity is smaller than the theoretical capacity and therefore the energy density cannot be increased.

The boron-containing carbon material exhibits a discharge capacity that is considerably close to the theoretical capacity, but since lithium loss is also large at the same time, a lithium source for compensating for that loss, for example, L
A large amount of positive electrode material such as iCoO ₂ is required.

That is, there is a problem that the energy density of the battery is lowered without utilizing the high capacity of the negative electrode because the initial charge / discharge efficiency of the negative electrode is low.

Therefore, an object of the present invention is to develop a negative electrode material having a large discharge capacity and a high charge / discharge efficiency, that is, a small lithium loss, and a method for producing the same.

Thus, it is intended to provide a negative electrode material for a lithium secondary battery having a high energy density and excellent cycle life characteristics.

[0032]

Means for Solving the Problems As a result of intensive studies for achieving the above-mentioned object, the present inventor has found that the negative electrode characteristics of a carbon material formed by adding boron show the morphology and texture of the carbon material used as a base material. It was found that the structure is dominated by the pitch-based carbon fiber to which boron is added, by controlling the amount of boron added and the crystal structure, a high discharge capacity is expressed while maintaining high initial charge / discharge efficiency. The inventors have found that they exhibit excellent characteristics as a negative electrode material, and have completed the present invention.

That is, the negative electrode material of the present invention is a crushed carbon fiber product obtained by crushing pitch-based carbon fiber, which contains carbon and boron as main components, and the content of boron is 0.3 to 10.
10% by weight, the (002) plane spacing (d ₀₀₂ ) is 0.337 nm or less, and the size of the crystallite in the C-axis direction (L
c) is 40 nm or more.

In the production method of the present invention, 1 to 20% by weight of a boron compound in terms of boron is added to and mixed with a pulverized carbon fiber product obtained by pulverizing pitch-based carbon fibers, and the mixture is 2,500 in an inert atmosphere. It is characterized by firing at a temperature of ℃ or more.

The essence of the function of boron addition to a carbon material is
It is to promote the graphite structure development during firing in the presence of boron, that is, to act as a graphitization catalyst for the boron compound. During the firing, boron is solid-solved in the graphite crystal, which has the effect of removing the strain of the crystal, and as a result, the graphite structure is developed (Otani, Carbon, No. 102 (19).
80) 118).

The development of the graphite structure leads to an increase in the position of the doped lithium in the carbon material, that is, an increase in the discharge capacity.

However, there is a limit to the amount of solid solution of boron in graphite, and the excessive coexisting boron exceeding the limit remains as boron carbide (B ₄ C) (CE Lowell). , J. Am. Ceram. So
c. , 50 (1967) 142).

Since boron carbide does not participate in the lithium doping reaction at all, the coexistence of excess boron results in a decrease in discharge capacity.

The present invention was devised on the basis of the above-mentioned idea, in which the content of boron exhibiting optimum characteristics as a negative electrode is 0.3 to 10% by weight, and the degree of graphitization of X is shown.
The crystal structure parameter by line diffraction was "d ₀₀₂ was 0.
337 nm or less, Lc is 40 nm or more ”.

When the boron content is less than 0.3% by weight, the degree of development of the graphite crystal structure is not sufficient and the discharge capacity becomes small.

On the other hand, if the boron content exceeds 10% by weight, the strain of the graphite structure becomes large because the solid solution amount of boron is too large, and the amount of lithium that can be incorporated into the structure becomes small. The storage capacity is lowered due to two causes, namely, the deposition of electrochemically inactive boron carbide.

Further, even when either d ₀₀₂ or Lc deviates from the above range, the degree of development of the graphite crystal structure is not sufficient and the discharge capacity also becomes small.

Discharge capacity of 320 mAh / g or more and 90%
In order to achieve the above initial charge / discharge efficiency, it is preferable that the boron content is 0.4 wt% to 8 wt%, the d ₀₀₂ is 0.3365 nm or less, and the Lc is 50 nm or more.

Here, the method of calculating the parameter of the graphitization degree by the X-ray wide-angle diffraction method is described, for example, in "Carbon Fiber" (Kindai Shosha, published in March 1986), pages 733 to 742.

The shape of the carbon fiber powder is such that the weight average particle diameter is 5 μm or more from the viewpoint of improving the bulk density during electrode molding.
It is desirable that the thickness is 0 μm or less and the aspect ratio is 50 or less. For measuring the weight average particle diameter, for example, a particle size distribution measuring device utilizing laser light scattering by fine particles in a liquid can be applied.

The improvement of the bulk density of the molded electrode also leads to the improvement of the electrical conductivity of the molded product and the charge / discharge cycle characteristics.

The weight average particle diameter is equivalent to the average fiber length in the case of a fiber shape, and the weight average particle diameter is 20.
If it exceeds μm, the bulk density of the molded electrode decreases, and as a result, the energy density of the electrode decreases.

If it is less than 5 μm, the specific surface area becomes large and the high initial charge / discharge efficiency of the fiber cannot be utilized.

Further, if there is a fiber having an aspect ratio of more than 50, the moldability and the cycle stability are degraded.

The fiber diameter (diameter) of the carbon fiber used in the present invention is preferably 3 μm or more and 15 μm or less. When the fiber diameter exceeds 15 μm, defects are likely to occur in the crushing process such as destruction of the fiber shape, and the capacity decreases due to the defects.

When the fiber diameter is less than 3 μm, the fiber diameter is too small and the graphite structure does not develop sufficiently, resulting in a smaller discharge capacity.

The method for producing a negative electrode material of the present invention comprises firing a mixture of carbon fiber powder and a boron compound in an inert atmosphere.

The boron compound is not particularly limited as long as it has a function as a graphitization catalyst. For example, metallic boron, boron carbide (B
₄ C), boron oxide (B ₂ O ₃ ), H ₃ BO ₃ etc.

The amount of the boron compound mixed with the carbon fiber powder is preferably 1 to 20% by weight, and more preferably 3 to 15% by weight in terms of boron.

The amount of the boron compound mixed is optimized from the viewpoint of exhibiting the action of the graphite catalyst most effectively.

When the amount of boron added is less than 1% by weight, the effect as a catalyst is small, the degree of graphitization is not improved, and the capacity is small.
Further, if it exceeds 20 wt%, boron is part without any solid solution remaining, B ₄ C is the capacitance correspondingly to an electrochemically inert decreases as B ₄ C.

When a mixture of carbon fiber powder and a boron compound is fired, part of boron disappears due to evaporation or the like. Therefore, in order to obtain a carbon material having a boron content of a target composition, It is necessary to mix more boron-converted boron compounds into the carbon fiber powder in advance.

It is important to increase the contact area with the carbon fiber in order to accelerate solid solution of boron into the carbon fiber during firing.

Therefore, the shape of the boron compound to be mixed is
A powder having a particle diameter as small as possible is suitable, preferably 50 μm or less, more preferably 30 μm or less.

The carbon fiber suitable for the present invention is preferably a powder obtained by crushing carbon fiber using pitch as a raw material. It is essentially important for the spinning raw material pitch that the graphite structure easily develops by firing, that is, high so-called graphitization property is essential, and the raw material pitch is not particularly limited.

For example, petroleum pitch, asphalt pitch, coal tar pitch, crude oil cracking pitch, petroleum sludge pitch, pitch obtained by thermal decomposition of a high molecular polymer and the like can be used. In addition, the above pitch may be subjected to hydrogenation treatment or the like.

Optical anisotropy, so-called mesophase, can be used as an index showing the graphitization property of pitch, and the pitch used in the present invention has a mesophase volume content of 70% or more, preferably, It is preferably 80% or more, and more preferably 90% or more.

It is essentially important that the structure of the carbon fiber to be crushed is not destroyed by crushing, and there is no particular limitation on the heat treatment temperature before crushing.

As an example, 500 ° C. in an inert atmosphere
The one fired at the above temperature is desirable, and preferably 6
A firing temperature of 00 ° C or higher is desirable.

When carbon fibers having a firing temperature of less than 500 ° C. are crushed, the fiber shape is significantly destroyed by the crushing, and as a result, the lithium loss, which is a feature of the fibers, is impaired.

The crushing device used for crushing the carbon fiber is ideally one that maintains the cylindrical shape of the fiber and cuts it in the length direction, and any device that satisfies this is not limited.

For example, a friction crushing type ball mill, a vibrating ball mill which gives an impact to the ball mill, an impact compression crushing type oscillating disc mill, a jet mill, a shear crushing type cutting mill and the like can be used.

The carbon fiber and the boron compound may be mixed by mixing the carbon fiber powder and the boron compound powder, which have been ground beforehand, or by adding the boron compound at the same time when the carbon fiber is ground, You may mix a boron compound, crushing carbon fiber.

The firing temperature is desirably as high as possible for the purpose of developing a graphite structure, and for the purpose of the present invention, 2500 ° C. or higher is appropriate, and preferably 260.
It is preferably 0 ° C or higher, more preferably 2800 ° C or higher.

At a firing temperature of less than 2500 ° C., the graphite structure is not sufficiently developed, so that the discharge capacity becomes small.

The carbon fiber powder provided by the present invention can be molded by a method usually used for powdery battery active materials used in lithium batteries.

For example, a method of mixing with polytetrafluoroethylene, adding isopropyl alcohol, kneading and molding, mixing dimethylformamide with polyvinylidene fluoride as a solvent to prepare a slurry, coating on a current collector and drying A pressing method or the like can be used.

However, if the binder itself does not participate in the negative electrode reaction, is electrochemically and chemically stable, and has a high shapeability for powder, other materials can be used as the binder.

The non-aqueous electrolytic solution is prepared by appropriately combining an organic solvent and a lithium salt which is an electrolyte. The organic solvent and the electrolyte are not particularly limited as long as they can be usually used in a lithium battery. Not something to do.

For example, as the organic solvent, propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, methyl ethyl carbonate, 1,2-dimethoxyethane, 1,2-diethoxyethane, γ-butyrolactone, tetrahydrofuran, 2 -Methyltetrahydrofuran, 1,3-dioxolane, 4-methyl-1,3-dioxolane, diethyl ether, sulfolane, methylsulfolane, acetonitrile,
Examples thereof include propionitrile, anisole, acetic acid ester, butyric acid ester, and propionic acid ester, which may be used alone or in admixture of two or more. Further, as the electrolyte, LiClO ₄ , LiAsF ₆ , Li
PF ₆ , LiBF ₄ , LiB (C ₆ H ₅ ), LiCH ₃ S
O ₃ , LiCF ₃ SO ₃ , LiBr, LiCl or the like can be used.

Examples of the positive electrode include transition metal oxides such as manganese dioxide and vanadium pentoxide, transition metal chalcogenides such as iron sulfide and titanium sulfide, and complex oxides of these with lithium and Chevrel phase compounds. , Activated carbon, activated carbon fiber and the like can be used.

[0077]

When the pitch-based carbon fiber powder is used as the negative electrode active material in the lithium secondary battery using the non-aqueous electrolyte, the initial charge / discharge efficiency is high, but the discharge capacity is small because the graphite structure is not sufficiently developed. Therefore, the energy density of the battery cannot be increased.

The boron-containing pitch-based carbon fiber powder satisfying the predetermined conditions in the present invention has a graphite structure which promotes the development of the graphite structure due to the graphitization catalytic action of boron, and has excellent electrode characteristics of the carbon fiber. Because we are maintaining
The discharge capacity is large and the initial charge / discharge efficiency can be increased.

By using the negative electrode material of the present invention for the negative electrode, a lithium secondary battery having a high energy density can be obtained.

[0080]

【Example】

[0081]

Example 1 Mesophase content is 90% (volume fraction)
The carbon fiber (fiber diameter after spinning 13 μm) made from the coal tar pitch of No. 3 was heated at a temperature rising rate of 10 ° C./min under a nitrogen atmosphere to 500 ° C., 800 ° C., 1200 ° C., 15 ° C.
It was carbonized by holding it at 00 ° C. for 1 hour.

Carbon dioxide powder obtained by crushing a series of these carbon fibers with a vibrating disk mill was mixed with boron oxide (B ₂ O ₃ ) powder (8% by weight), and the resulting mixture was uniformly mixed. The temperature was raised to 2900 ° C. at a heating rate of 10 ° C./min and graphitized at 2900 ° C. for 1 hour.

The fiber diameter of the carbon fiber powder after graphitization is about 10
The average particle size of the powder was about 12 μm. Table 1 shows the crystal parameters (d ₀₀₂ , Lc) of a series of materials by wide-angle X-ray diffraction and the boron content (% by weight) in the materials.

[0084]

[Table 1]

The carbon fiber powder material thus obtained was
90 parts by weight of this was used as a negative electrode active material, and 10 parts by weight of polyvinylidene fluoride (PVDF) was mixed as a binder to prepare a negative electrode mixture. This negative electrode mixture was dispersed in a solvent N-methyl-2-pyrrolidone to form a slurry.

Further, this negative electrode mixture slurry was applied on a copper foil having a thickness of 20 μm, which is a negative electrode current collector, and the solvent was dried, followed by compression molding with a roller press machine to prepare a negative electrode molded body.

This negative electrode molded body was cut into 1 cm square pieces, and a polypropylene microporous membrane was used as a separator to face a 0.5 mm thick lithium metal sheet used for the counter electrode, and the whole was sandwiched with a Teflon plate for compression bonding. .

The bipolar cell thus prepared was immersed in an electrolytic solution, and lithium metal was used as a reference electrode.
It was a triode cell.

For the electrolytic solution, L was added to a solvent prepared by mixing ethylene carbonate and diethyl carbonate in a volume ratio of 1: 1.
A solution in which iClO ₄ was dissolved at a concentration of 1 mol / l was used.

Next, with respect to the three-electrode cell thus prepared, the charging current and the discharging current were both 0.5 mA, and the constant current charging / discharging was repeated in the potential range of 0 V to 1.0 V to obtain a carbon fiber powder negative electrode. The characteristics were examined.

Table 2 shows the charge and discharge efficiency and discharge capacity in the first cycle, and lithium loss due to repeated charge and discharge. After the second cycle, the charging / discharging efficiency was stable at almost 100% at any carbonization temperature.

[0092]

[Table 2]

[0093]

Comparative Example 1 The carbonization temperature before grinding in Example 1 was 80.
Using carbon fiber powder before graphitization at 0 ° C, boron oxide (B
₂ O ₃ ) The powder was graphitized as it was without adding powder.

The characteristics of the negative electrode active material were evaluated in the same manner as in Example 1 under the conditions such as graphitization conditions, electrode molding and evaluation.

Table 3 shows the crystal parameters (d ₀₀₂ , Lc) by wide-angle X-ray diffraction, and Table 4 shows the charge and discharge efficiency and discharge capacity in the first cycle, and the lithium loss due to repeated charge and discharge.

From the comparison of the results of Example 1 and Comparative Example 1,
The effect of accelerating graphitization by boron oxide is clear, and in Example 1, the discharge capacity is greatly improved as the graphitization degree is improved.

Further, it can be seen that the decrease in the initial charge / discharge efficiency due to the addition of boron is hardly recognized.

Further, both the materials of Example 1 and Comparative Example 1 had high cycle stability of discharge capacity, and almost no decrease in capacity was observed even after 50 cycles.

[0099]

[Table 3]

[0100]

[Table 4]

[0101]

Comparative Example 2 The same carbon fiber as in Example 1 was used, the carbonization temperature before grinding was adjusted to 400 ° C., and the other conditions were exactly the same as in Example 1, and the characteristics as a negative electrode active material were examined. Table 5 shows the crystal parameters (d ₀₀₂ , by wide-angle X-ray diffraction).
Lc), the boron content (% by weight) in the material, and Table 6 shows the charge and discharge efficiency and discharge capacity in the first cycle and the lithium loss due to repeated charge and discharge.

[0102]

[Table 5]

[0103]

[Table 6]

In Comparative Example 2, the initial charge / discharge efficiency is lower than in Example 1. In Comparative Example 2, the charge / discharge efficiency remained at about 100% after the fourth cycle. The discharge capacity was stable even after repeated cycles.

[0105]

Example 2 The carbonization temperature before grinding in Example 1 was 80.
Boron oxide (B
₂ O ₃ ) powder was added (8% by weight) and mixed so as to be sufficiently uniform, and then the temperature was raised at a heating rate of 10 ° C./min under an argon atmosphere to 2500 ° C., 2800 ° C., 3000 ° C. In 1
A series of samples that had been subjected to time graphitization treatment were prepared, and the negative electrode characteristics were evaluated in the same manner as in Example 1.

As in Example 1, the carbon fiber powder after graphitization had a fiber diameter of about 10 μm, and the average particle diameter of the powder was about 12 μm.
μm.

Table 7 shows the crystal parameters (d ₀₀₂ , Lc) of a series of materials by wide-angle X-ray diffraction and the boron content (% by weight) in the materials.

Table 8 shows the charge and discharge efficiency and discharge capacity in the first cycle and the lithium loss due to repeated charge and discharge.

The material graphitized at any temperature had a high initial charge / discharge efficiency of 92% or more, and remained at about 100% after the second cycle.

Neither the initial charge / discharge efficiency nor the discharge capacity depended on the graphitization temperature. The discharge capacity of each material did not decrease after 50 cycles.

[0111]

[Table 7]

[0112]

[Table 8]

[0113]

Comparative Example 3 A sample was prepared under the same conditions as in Example 2 except that the graphitization temperature was 1500 ° C., 2000 ° C. and 2400 ° C. The evaluation of the negative electrode characteristics of a series of samples was also performed under the same conditions as in Example 2.

Table 9 shows the crystal parameters (d ₀₀₂ , Lc) of a series of materials by wide-angle X-ray diffraction and the boron content (% by weight) in the materials. Table 10 shows the charge-discharge efficiency of the first cycle. And discharge capacity, and lithium loss due to repeated charging and discharging.

In graphitization at 1500 ° C. and 2000 ° C., since the diffusion reaction of boron into the carbon material is not sufficient,
The graphitization catalytic effect of boron addition has not been revealed. That is,
The discharge capacity is small, and the initial charge / discharge efficiency is as low as 85% or less.

The material treated at 2400 ° C. is in a state where the catalytic graphitization effect of boron is insufficient, that is, the graphitization degree is relatively low, as shown in the crystal parameters (d ₀₀₂ , Lc).

Therefore, the discharge capacity was smaller than that in Example 2, and the initial charge / discharge efficiency was smaller than that in Example 2.

[0118]

[Table 9]

[0119]

[Table 10]

[0120]

Example 3 In Example 2, boron oxide (B ₂ O ₃ )
Only the amount of powder added is 3, 6, 9, 1 in terms of boron
The sample was prepared under the same conditions as in Example 2 except that the content was changed to 2, 15, and 18% by weight. The negative electrode characteristics of a series of samples were evaluated under the same evaluation conditions as in Example 2.

Table 11 shows the crystal parameters (d ₀₀₂ , Lc) of a series of materials by wide-angle X-ray diffraction and the boron content (% by weight) in the materials. Table 12 shows the charge and discharge efficiency of the first cycle. And discharge capacity, and lithium loss due to repeated charging and discharging.

High discharge capacity was exhibited at any mixing ratio of boron oxide (B ₂ O ₃ ). Also, the initial charge / discharge efficiency decreased slightly with the increase of the mixing amount, but in all cases, 9
A high value of 0% or more was maintained. The charge / discharge efficiency remained at about 100% after the second cycle, and the discharge capacity hardly decreased even after 50 cycles.

[0123]

[Table 11]

[0124]

[Table 12]

[0125]

Example 4 Boron oxide (B ₂ O ₃ ) in Example 3
A sample was prepared under the same conditions as in Example 2 except that the powder was changed to metallic boron. The mixing amount of metallic boron was 4, 6, 8, 10, 12 wt% in terms of boron. The evaluation of the negative electrode characteristics of a series of samples was also performed under the same conditions as in Example 2.

Table 13 shows the crystal parameters (d ₀₀₂ , Lc) of a series of materials by wide-angle X-ray diffraction and the boron content (% by weight) in the materials. Table 14 shows the charge and discharge efficiency of the first cycle. And discharge capacity, and lithium loss due to repeated charging and discharging.

The charging / discharging efficiency is approximately 10 after the second cycle.
The discharge capacity remained unchanged at 0%, and the discharge capacity hardly decreased even after 50 cycles.

[0128]

[Table 13]

[0129]

[Table 14]

[0130]

Example 5 In Example 3, boron oxide (B ₂ O ₃ )
The powder was changed to boron carbide (B ₄ C), and the sample was prepared under the same conditions as in Example 2 except for above. The mixing amount of boron carbide was 4, 6, 8, 10, 12 wt% in terms of boron. In addition, evaluation of the negative electrode characteristics of a series of samples was also performed in Example 2
The same conditions were used.

Table 15 shows the crystal parameters (d ₀₀₂ , Lc) of a series of materials by wide-angle X-ray diffraction and the boron content (% by weight) in the materials. Table 16 shows the charge and discharge efficiency of the first cycle. And discharge capacity, and lithium loss due to repeated charging and discharging.

The charging / discharging efficiency is approximately 10 after the second cycle.
The discharge capacity remained unchanged at 0%, and the discharge capacity hardly decreased even after 50 cycles.

[0133]

[Table 15]

[0134]

[Table 16]

[0135]

Comparative Example 4 A sample was prepared under the same conditions as in Example 2, except that the amount of metallic boron powder added was changed to 0.5 and 25% by weight in terms of boron in Example 3. The negative electrode characteristics of the two samples were also evaluated under the same conditions as in Example 2.

Table 17 shows the crystal parameters (d ₀₀₂ , Lc) of the two materials by wide-angle X-ray diffraction and the boron content (% by weight) in the materials. Table 18 shows the charge and discharge of the first cycle. The efficiency and the discharge capacity, and the lithium loss due to repeated charging and discharging are shown.

[0137]

[Table 17]

[0138]

[Table 18]

[0139]

Example 6 The amount of boron oxide added in Example 3 is 10
A coin-type battery (outer diameter 20 mm, thickness 2.5 mm) was prepared using the material of wt% and its cycle characteristics were examined.

First, the amount of boron oxide added in Example 3 was 10
A negative electrode molded body having a diameter of 15 mm was prepared in the same manner as in Example 1 by using the material in an amount of wt%.

The positive electrode was made of LiCoO ₂ as follows. 5% by weight of polyvinylidene fluoride powder, 5% by weight of Ketjen black and 5% by weight of N-methylpyrrolidone were added to LiCoO ₂ and kneaded to prepare a slurry.
A positive electrode molded body was prepared by uniformly applying it onto the 1-foil and drying it. This was cut into a positive electrode having a diameter of 15 mm, which was the same as the negative electrode.

Using the above negative electrode and positive electrode, the electrolytic solution was a solution prepared by dissolving LiClO ₄ at a concentration of 1 mol / l in a solvent in which ethylene carbonate and diethyl carbonate were mixed at a volume ratio of 1: 1 and a separator was used. For the above, a simple coin-shaped battery was prepared using a polypropylene porous film, and a charge / discharge test was conducted.

The amount of the positive electrode active material is such that the lithium equivalent to the capacity of the negative electrode is added to the total lithium loss of the negative electrode and the total lithium loss of the positive electrode to supply the weight of the positive electrode active material. Decided.

The charging / discharging test was carried out at a constant current (0.5 mA / cm ² ) for both charging and discharging, and charging / discharging was repeated between battery voltages of 3V to 4.2V.

As a result, the discharge capacity in the first cycle is 1
It was 2.3 mAh, and the charge / discharge efficiency was 92.2%. In the subsequent cycles, the capacity was maintained, and the charge / discharge efficiency remained at 100% after the second cycle.

The charging / discharging was repeated up to 100 cycles, but after 100 cycles, the discharge capacity did not decrease, and stable cycling was possible.

[0147]

[Comparative Example 5] Using coal-based pitch coke powder as a raw material, 10% by weight of boron carbide was added and thoroughly mixed in a mortar, and then 60 times per hour in a nitrogen gas atmosphere.
The temperature is raised at a rate of 0 ° C., 2400 ° C., 2600 ° C., 280
A boron-containing carbon material was prepared by holding each temperature of 0 ° C. for 16 hours. The average particle size of the powder after firing was about 12 μm.

The characteristics of this carbon material as a negative electrode active material were evaluated in the same manner as in Example 1, except that the conditions for electrode molding, evaluation and the like were the same.

Table 19 shows the results of discharge capacity and charge / discharge efficiency in the first cycle. The discharge capacity remains stable after the second cycle, and the charging / discharging efficiency is almost 10 after the third cycle.
It remained at 0%.

[0150]

[Table 19]

[0151]

Comparative Example 6 Petroleum pitch was used as a raw material and H ₃ B
After adding 2 wt% of O ₃ , boron carbide, and metallic boron in terms of boron, and thoroughly mixing them in a mortar, the temperature was raised at a rate of 100 ° C. for 1 hour in a nitrogen gas atmosphere to obtain 100
Hold at 0 ° C. for 10 hours to prepare a boron-containing carbon material. The average particle size of the powder after firing was about 12 μm.

The characteristics of this carbon material as a negative electrode active material were evaluated in the same manner as in Example 1, except that the conditions for electrode molding, evaluation and the like were the same.

Table 20 shows the results of the discharge capacity and charge / discharge efficiency in the first cycle. The discharge capacity remains stable after the 2nd cycle, and the charge / discharge efficiency is almost 1 after the 10th cycle.
It remained at 00%.

[0154]

[Table 20]

[0155]

Comparative Example 7 Petroleum pitch was used as a raw material and H ₃ B
After adding 2 wt% of O ₃ , boron carbide, and metallic boron in terms of boron, and thoroughly mixing them in a mortar, the temperature was raised at a rate of 100 ° C. for 1 hour in a nitrogen gas atmosphere to obtain 100
After being kept at 0 ° C. for 10 hours, the temperature was further raised at a rate of 50 ° C. for 1 hour and kept at 2000 ° C. for 20 hours to prepare a boron-containing carbon material. The average particle size of the powder after firing is about 12
μm.

The characteristics of this carbon material as a negative electrode active material were evaluated in the same manner as in Example 1 except that the conditions for electrode molding, evaluation and the like were the same.

Table 21 shows the results of the discharge capacity and charge / discharge efficiency in the first cycle. The discharge capacity remained stable after the second cycle, and the charge / discharge efficiency was almost 10 after the fifth cycle.
It remained at 0%.

[0158]

[Table 21]

[0159]

As is clear from the above description, the pitch-based carbon fiber pulverized product to which the boron of the present invention is added, while maintaining the characteristic of the pitch-based carbon fiber powder that the lithium loss is small, and It is possible to increase the discharge capacity. Further, it can be stably cycled, and is very effective as a negative electrode material for a high energy density lithium secondary battery.

Further, according to the manufacturing method of the present invention, a negative electrode material having excellent characteristics can be manufactured by a simple operation, and a lithium secondary battery can be manufactured without increasing the manufacturing cost or deteriorating the productivity. A suitable negative electrode material can be manufactured.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Maki Sato 1618 Ida, Nakahara-ku, Kawasaki City Nippon Steel Corp. Advanced Technology Research Laboratories

Claims (2)

[Claims] 1. A crushed carbon fiber product obtained by crushing pitch-based carbon fibers, wherein the crushed product contains 0.3 to 10% by weight of boron, and the interplanar spacing (d ₀₀₂ ) of (002) faces is 0.3
37 nm or less, the crystallite size (Lc) in the C-axis direction is 4
A negative electrode material for a lithium secondary battery, which has a thickness of 0 nm or more. 2. A carbon fiber crushed product obtained by crushing pitch-based carbon fibers, mixed with 1 to 20% by weight of a boron compound in terms of boron, and fired at a temperature of 2500 ° C. or higher in an inert atmosphere. A method for producing a negative electrode material for a lithium secondary battery, comprising: