JPH07282812A - Carbon negative electrode material for lithium secondary batteries - Google Patents

Abstract

(57) [Summary] [Object] The present invention provides a crushed carbon fiber powder as a negative electrode material for a lithium secondary battery, which has a large discharge capacity, high charge-discharge efficiency from the initial stage of the cycle, and excellent cycle characteristics. provide. [Structure] Carbon fibers made from pitch as a raw material are crushed, and the probability that the adjacent carbon mesh surface layers are in a regular laminated arrangement state is controlled (P ₁ ≧ 0.6, P _ABA ≧ 0.3 , P
_ABC ≧ 0.15), further limits the degree of the graphite crystal defects mechanically introduced during fiber crushed (intensity ratio R for 1580 cm ^-1 band of 1360 cm ^-1 band in the Raman spectroscopy using an argon laser is 0 .4 or less), the amount of lithium doped into the carbon material, that is, the discharge capacity, the charging / discharging efficiency of lithium insertion / desorption, and the cycle stability against repeated charging / discharging. It is possible to provide a high powdery carbon material.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a carbon negative electrode material for lithium secondary batteries which utilizes an insertion / desorption reaction of lithium, and relates to an electrode material in which the form of the carbonaceous material is powder.

[0002]

2. Description of the Related Art With the recent miniaturization and weight reduction of electric and electronic devices, the demand for higher energy density of secondary batteries has been increasing. As a high energy density secondary battery satisfying this requirement, a lithium secondary battery has been attracting attention and its development is urgently needed.

The greatest problem in the development of this lithium secondary battery is cycle deterioration due to repeated charging and discharging of lithium metal used for the negative electrode.

In order to solve this problem, various alternative materials for lithium metal have been studied, and the most promising among them is the carbon material.

In a system using a carbon material for the negative electrode of a lithium secondary battery, lithium is inserted between the layers of the carbon mesh surface layer of the carbon material from the electrolytic solution in which a large amount of lithium salt is dissolved during charging, and lithium is inserted during discharging. The so-called intercalation-deintercalation (insertion-desorption) reaction in which the generated lithium is released as an ion is used.

Therefore, the point of development of the carbon negative electrode material having high electrode performance is the crystal structure of the carbon material. Due to the regularity of the crystal structure, there are two major types of non-graphitizable carbon materials and graphitizable carbon materials. Research is being done in one direction.

[0007] As an overview of the research reports so far, the graphitizable material does not develop a graphite structure, and the interlayer of the carbon net surface layer is much wider than the ideal graphite structure. It is reported that the intercalation reaction of lithium between layers is easier than that of the material, and the large intercalation amount provides a high discharge capacity (for example, JP-A-2-66856, 60th Electrochemical Convention). 2G08).

On the other hand, however, the graphite crystallite did not develop,
Since they are randomly oriented, and the diffusion of lithium inside the material is slow, the efficiency of lithium insertion / desorption at the early stage of the cycle is low, and the capacity is largely reduced with repeated charge / discharge. It has been pointed out that there is a defect that is fatal as an electrode material for secondary batteries (for example, the 34th battery discussion meeting 3A07).

Further, since the non-graphitizable material is not densely packed with carbon atoms due to its crystal structure,
The true specific gravity is small, and the discharge capacity per volume of the electrode body is smaller than that of the graphitizable material, which is not suitable for a compact lithium secondary battery.

On the other hand, in the graphitizable material, an intercalation compound with lithium is more likely to be stably formed as the graphite structure develops, so that a large amount of lithium is inserted between the layers of the carbon mesh surface layer, and the discharge capacity becomes large. Have been reported (eg, Electrochemistry and Industrial Physical Chemistry, 61 (2), 1383 (19).
93)).

Finally, an ideal graphite intercalation compound LiC ₆ of graphite and lithium is formed, and the discharge capacity (372 mAh / g) in this case is generally regarded as the theoretical capacity (limit value) of the carbon negative electrode material. .

[0012] The graphitizable material generally has a crystal structure in which the regular arrangement of the carbon network plane layers maintains a long-range order by the graphitization treatment in an ultrahigh temperature range exceeding 2000 ° C.

Therefore, there is a report that it has a feature that the diffusion coefficient of lithium inside the material is large even if the interlayer distance is small (for example, 34th Battery Discussion Group 3A08).

In addition, insertion of lithium into the carbon mesh surface layer
Compared to the fact that the electrode potential during the desorption reaction changes greatly with the progress of the reaction in the non-graphitizable material, the potential is very close to that of Li metal (0.2 to 0.1 V vs. Li / Li +) and almost at the end of the reaction. Since this is maintained, the battery voltage when combined with the positive electrode material is high, and a high energy density can be obtained.

Further, since such a material having a developed graphite structure generally has a large true specific gravity, the discharge capacity per volume of the electrode body is large, which is advantageous in that the battery can be made compact.

Studies have been conducted for a long time focusing on natural graphite having an ideal graphite crystal structure and artificial graphite having a crystal structure close to that of natural graphite (for example, J. Elec).
trochem. Soc. , 117, 222 (197)
0), Carbon, 13, 337 (1975), Japanese Patent Laid-Open No. 64-2258, etc.), but since these carbon structures have a preferential orientation of a carbon net surface layer in a specific direction, The diffusion direction of lithium is limited, and the diffusion distance is very long.

Therefore, it has been confirmed that a high discharge capacity can be obtained only under a very small charge / discharge current (for example, Electrochimica Acta, 38).
(9), 1179 (1993)).

However, from a practical point of view, the current density cannot be increased, so that the range of use is likely to be greatly restricted.

It was also pointed out that the mesocarbon microbeads prepared by collecting the optically anisotropic phase of mesophase pitch in a spherical form had a problem in cycle characteristics (for example, No. 34). 3rd round battery discussion meeting
07).

Pitch-based carbon fibers belong to the category of easily graphitizable materials, and the graphitization treatment in the ultra-high temperature region results in the interlayer distance of the carbon mesh surface layer close to that of natural graphite, and the diffusion direction of lithium is the fiber periphery. In multiple directions from the inside to the inside, and the diffusion distance is at most 10 μm from the fiber outer circumference to the fiber axis
Due to its shortness, the powder obtained by crushing this carbon fiber has a large diffusion coefficient compared to other graphitizable materials, and is expected to be strong against heavy loads.

Further, regarding this pulverization, as disclosed in, for example, Japanese Patent Application Laid-Open No. 5-325967, it is necessary to carry out the pulverization of the pulverized powder while maintaining the shape of the carbon fiber until the aspect ratio reaches a certain level. It is considered to be important for fully exerting the electrode performance of the fiber.

Conventionally, regarding powders obtained by crushing carbon fibers, the graphitization process of the carbon fibers used changes from a state (turbulent layer structure) in which the stacking order regularity of adjacent graphite layers is disturbed to a regular state (graphite structure). Despite the transition process, the degree of graphitization is defined by the interlayer distance d ₀₀₂ of the carbon network plane layer and the average value of the macroscopic structural factor of the graphite crystallite such as the crystallite size Lc in the C-axis direction. It was just that.

In fact, lithium diffuses into the graphite layer of the carbon fiber having a certain stacking order, and as a result of interaction with the carbon atoms of the graphite layer, in-plane ordering is formed in the intercalant layer.

Therefore, the microscopic structural factors such as the stacking order regularity of the graphite layer have a great influence on the diffusion of lithium between the graphite layers and the in-plane ordering regularity formed in the intercalant layer, resulting in a high current density. In this case, the charging / discharging efficiency of lithium to the carbon fiber, the charging / discharging cycle characteristics, and the electrode characteristics such as the doping amount of lithium, that is, the discharge capacity are deteriorated.

However, the investigation from the viewpoint of the stacking order regularity of the adjacent graphite layers has been reported so far on the graphitized mesocarbon microbeads (Proc. Electrochem. Soc. Sp.
ring Meeting93-8, 64 (199
3)), Report on pitch coke (Electro
chimica Acta, 38, 9, 1179 (19
93)) only, and no study on carbon fibers was made.

On the other hand, the carbon powder obtained by pulverizing the carbon fibers is mechanically introduced with a structural defect different from the stacking order regularity of the graphite layer of the original carbon fibers, or the carbon powder is crystallized. The local structure causes a phenomenon such as transition or disappearance from hexagonal symmetry to lower symmetry.

The introduction of structural defects into the stacking order regularity of the graphite layer affects the diffusion of lithium inserted between the graphite layers and the in-plane ordering regularity in the intercalant layer, and the lithium charge / discharge efficiency and This is a factor that significantly deteriorates the charge / discharge cycle characteristics.

Therefore, in order to bring out the original performance of the carbon fiber, it is desirable to reduce the crystal structure defects as much as possible. However, the degree of regularity of stacking arrangement of the graphite layer of the carbon powder after the crushing treatment has been, for example, resin. Reports on fired products (for example, JP-A-63-114056, JP-A-63-236259, etc.) and reports on pyrolytic carbon bodies of hydrocarbon gas (for example, JP-A-5-
No. 78909), no studies have been made on pulverized powder of carbon fiber, and the electrode performance originally possessed by the carbon fiber itself has not been sufficiently exhibited in the pulverized powder of carbon fiber.

[0029]

In view of the above problems, the present invention provides a negative electrode material for a lithium secondary battery, which has a large discharge capacity, high charge / discharge efficiency from the initial stage of the cycle, and excellent cycle characteristics. The purpose of the present invention is to provide a crushed carbon fiber powder.

[0030]

The inventors of the present invention have controlled the probability that a powdery carbon material obtained by crushing carbon fibers made of pitch as a raw material has a regular laminated arrangement state of the adjacent carbon net surface. , And by defining a microscopic crystal structure factor that limits the degree of graphite crystal defects that are mechanically introduced during fiber crushing, the lithium insertion amount, the initial charge / discharge efficiency, and the discharge capacity per volume of the electrode body are increased. I found it possible to do. The present invention has been completed based on such findings.

That is, the carbon negative electrode material for a lithium secondary battery of the present invention has a P ₁ value of 0.6, which is an index of the probability that two mesh planes adjacent to each other in the C-axis direction of the crystallite have a graphite-like arrangement. that's all,
Probability that adjacent three mesh planes have a regular array of ABA as hexagonal system P _ABA value is 0.3 or more, and three adjacent mesh planes are regular as rhombohedral system of ABC The probability P _ABC of having an array is 0.15 or more.

Further, 1580 cm of 1360 cm ^-1 band in Raman spectroscopic analysis using an argon laser.
^The intensity ratio (R = I ₁₃₆₀ / I ₁₅₈₀ ) to the ⁻¹ band is 0.
It is characterized by being 4 or less.

The methods of expressing various physical properties used in the definition of the carbon fiber powder of the present invention and the measuring methods are shown below.

(1) X-ray diffraction method: P ₁ , P _ABA , and P _ABC CuKα are used as X-ray sources, high-purity silicon is used as a standard substance, and asymmetric with respect to a carbon material (10) or (11) diffraction line By measuring the peaks and Fourier-analyzing the diffraction line intensity distribution in each peak figure, P ₁ which is an index of the probability that two mesh planes adjacent to each other in the C-axis direction of the crystallite have a graphite-like arrangement, As shown in 1, a structure in which the second layer is translated by (2/3, 1/3) with respect to the first layer, and the third layer exactly overlaps the first layer, belongs to the hexagonal system of ABAB ... That is, the three nets adjacent in the C-axis direction of the crystallite are A
Probability P _ABA of having a regular arrangement as a hexagonal system of BA,
As shown in FIG. 2, the second layer is (2/3, 1
/ 3) shift, the third layer shifts further 1/3, 2/3),
The structure in which the fourth layer exactly overlaps with the first layer belongs to the rhombohedral system of ABCABC ... That is, the three nets adjacent to each other in the C-axis direction of the crystallite have a regular arrangement as the rhombohedral system of ABC. Probability P _ABC is calculated.

Warren et al. Have shown that the degree of graphitization can be determined from Fourier analysis of the intensity distribution of the two-dimensional (hk) diffraction of carbon (J. Appl. Phys., 25, 150.
3 (1954)).

In a carbon material having a low degree of graphitization, extremely wide two-dimensional (hk) diffraction is observed in the X-ray diffraction pattern, which sharply rises on the low angle side and gradually decreases on the high angle side.

According to Warren et al., Such diffraction occurs when each network plane is randomly rotated in the normal direction in a parallel layer group in which hexagonal carbon network planes are arranged in parallel (turbulent layer structure). On the other hand, when graphitization starts and some net plane pairs in the parallel layer group are positioned in a graphitic relationship (graphite structure), the two-dimensional (hk) diffraction is deformed.

The deformation factor is represented by the Fourier series, and its
Fourier series of_n(Hk) is the intensity distribution of (hk) diffraction
M measured from _2θ(Hk) uses the following formula
Required.

[0039]

[Equation 1]

This Fourier series A _n (hk) and P ₁ value, P
_{What is ABA} value and P _ABC value?

[0041]

## EQU2 ## P ₁ = -2A ₁ (10) = A ₁ (11)

[0042]

## _EQU00003 ## P _ABA = 1/3 {2A ₂ (10) + A ₂ (11)}

[0043]

## EQU4 ## P _ABC = 2/3 {A ₂ (11) -A ₂ (10)}

Since it can be related to (1
Each P value is obtained from the intensity distributions of 0) and (11) diffraction.

(2) Raman spectroscopy ... R = I ₁₃₆₀ / I
₁₅₈₀ Raman spectrum near 1580 cm ^-1 corresponding to E _2g type vibration corresponding to in-plane lattice vibration among the 9 types of lattice vibrations possessed by graphite structure, and 1360 cm ^-1 reflecting disorder of crystal structure such as defects Is measured by Raman spectroscopy using an argon laser.

The intensity ratio R = I ₁₃₆₀ / I ₁₅₈₀ is calculated from the intensity value of each Raman spectrum.

The specific contents of the present invention will be described below.

As a result of various studies on carbon materials having high cycle stability against repeated charging / discharging, the present inventors have found that the structure and shape of the carbon materials are very important factors. It was found that the above-mentioned graphitizable carbon material was the most suitable and the fiber shape was the most suitable, and the application was filed first (Japanese Patent Laid-Open No. 2-8).
2466).

This is because the development of the graphite structure is essential for the diffusion of lithium in the carbon material when lithium is inserted into the carbon material to form a stable graphite intercalation compound, while the shape is The carbon mesh surface spacing expands and contracts as lithium is inserted and released, but if it is a fiber shape, macroscopic structural destruction of the material can be avoided, and the structure is stable even if charging and discharging are repeated many times. It is presumed that it is possible to be involved in the battery reaction while being held at.

In the structure in this case, attention is paid only to the macroscopic crystal structure factor such as the interlayer distance d of the carbon network plane layer and the size Lc of the crystallite in the C-axis direction in the graphite crystallite in the regular laminated arrangement state. Absent.

However, since the electrode reaction is a reaction in which lithium is inserted / desorbed between the layers of the carbon network layer of the carbon material, a microscopic crystal structure factor such as the stacking order regularity of the network layer is important. is there.

From this point of view, as a result of studying using carbon fibers having various laminated arrangement regularities, it was found that it is desirable that the stacked state is as regular as possible.

That is, P ₁ , P _ABA , and P _ABC , which are the indexes for defining the stacking state, are 0.6 or more, 0.
The carbon powder obtained by pulverizing the carbon fibers of 3 or more and 0.15 or more has high discharge capacity, high charge / discharge efficiency, and high cycle stability.

In each index of the stacking state, 1 represents the state of a graphite structure having an ideal regular arrangement, and the closer to 0, the more the stacked state is disturbed.

When the graphitizable carbon material is graphitized, P
It has been reported that there is a threshold for the degree of graphite structure development when ₁ is around 0.6 (carbon, 47, 14
(1966)).

In the region where P ₁ exceeds 0.6, ABCA
A phase transition from the rhombohedral stacking structure of BC ... to the hexagonal stacking structure of ABAB ... occurs, and P _ABA is 0.3.
As described above, it is considered that P _ABC becomes 0.15 or more, and the graphite structure is suitable for the insertion of lithium between the layers of the carbon network plane layer and the formation of the graphite intercalation compound with lithium.

On the other hand, in a carbon material in which P ₁ is less than 0.6, P _ABA is less than 0.3, and P _ABC is less than 0.15, the graphite structure is not sufficiently developed, so that lithium is not yet developed. The reaction of inserting the graphite structure between the layers does not proceed smoothly, and a graphite intercalation compound cannot be stably formed with lithium inserted between the layers. Therefore, the amount of lithium doped into the carbon material, that is, the discharge capacity cannot be increased.

Further, since diffusion of lithium between the layers of the undeveloped graphite structure is not easy, the inserted lithium cannot be efficiently desorbed, resulting in a decrease in charge / discharge efficiency and a decrease in cycle characteristics. turn into.

On the other hand, the carbon powder according to the present invention is a powder obtained by crushing pitch-based carbon fiber, and by this crushing process, defects are mechanically introduced into the regular arrangement of graphite layers of the original carbon fiber. Or, a part where the symmetry is broken occurs in a part of the crystal.

Conventionally, the average particle size and shape of carbon powder,
That is, although it was defined only by the aspect ratio, such macroscopic shape factor of the carbon powder is not sufficient, and the R value which is an index for defining the crystal defects in the carbon powder and the stacking arrangement regularity of the crystals. It is essential to consider.

From this point of view, as a result of preparing and examining carbon fiber ground carbon powder in which crystal defects are introduced into carbon fiber powder under various grinding conditions, as a result, high electrode performance is obtained in carbon fiber powder having R value of 0.4 or less. Found to get.

Regarding the index R value of the stacking order regularity of the adjacent carbon network plane layers, 0 represents the state of an ideal graphite crystal, and the closer it is to 1, the closer it is to the disordered layer structure crystal.

Usually, the pitch-based carbon fiber is subjected to a graphitization treatment at a temperature of over 2600 ° C. in an inert atmosphere to remove the functional groups introduced into the carbon mesh surface of the fiber surface, and the fiber itself. Due to the repairing effect of the crystal defects having a regular stacking arrangement, it has a regular graphite crystal having a stacking arrangement with an R value of 0.4 or less.

In the carbon powder prepared by crushing this carbon fiber, if the R value exceeds 0.4, many defects will be introduced into the graphite crystal due to the crushing process. The lithium insertion-elimination reaction does not proceed efficiently, and the current efficiency at the initial stage of the charge / discharge cycle decreases. Further, the doping amount of lithium, that is, the discharge capacity also becomes small.

The pitch-based carbon fiber used in the present invention has a graphite structure (regular arrangement regularity of graphite layers) most suitable as a carbon negative electrode material for lithium secondary batteries.

Regarding the pitch for spinning, which is a raw material of pitch-based carbon fiber, it is essentially important that graphite crystallinity is easily developed by firing, that is, so-called graphitization is easy (graphability). Although the raw material is not limited, examples include 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. Also, those pitches may be subjected to hydrogenation treatment or the like.

Here, as an index showing the graphitization property of pitch, for example, an optically anisotropic phase, so-called mesophase can be used, and as the raw material pitch used in the present invention, the volume content of this mesophase is Is 70% or more, preferably 80% or more, more preferably 90% or more.

The pitch-based carbon fiber used in the present invention preferably has a fiber diameter of 20 μm or less, more preferably 15 μm or less.

In order to increase the discharge capacity per unit weight of the carbon material, it is important that the lithium atoms reaching the surface of the fiber as the electrode diffuse into the inside of the fiber.
With carbon fibers having a fiber diameter of more than 20 μm, the distance that lithium diffuses in the fiber is large, and it becomes difficult to uniformly diffuse to the center of the fiber. As a result, the amount of lithium taken in per fiber decreases, resulting in a discharge capacity It gets smaller.

Further, when the fiber diameter is large, the surface area involved in the reaction becomes small, so that the region where lithium insertion reaction is possible is restricted, and as a result, the discharge capacity becomes small.

In the carbon fiber crushing method as described above, it is ideal to maintain the cylindrical shape of the fiber and shear it in the lengthwise direction, and when the R value in the Raman spectrum is 0.4 or less. There is no limitation to this as long as it is a method and a device that meet certain requirements. For example, friction milling type ball mill, impact compression milling type vibrating disk mill, vibrating ball mill, jet mill, shearing milling type cutting mill, etc. Can be used.

The carbon fiber powder thus pulverized has an average particle size (weight average value in particle size distribution measurement results using a light scattering method according to Mie's scattering theory) of 5 μm or more and 20 μm or less and an aspect ratio of 50 or less. As a result, the charge / discharge cycle characteristics when molded using the binder can be improved.

If the average particle size exceeds 20 μm, it becomes difficult to insert lithium into the inside of the carbon fiber, and the utilization factor of the carbon fiber decreases.

If the particle size is less than 5 μm, most of the fibers are composed of powdery material in which the shape of the fibers has collapsed, and the characteristics of the fibers themselves cannot be obtained.

Further, if there are fibers having an aspect ratio of more than 50, the moldability is low and the cycle characteristics with respect to repeated charging / discharging are deteriorated.

Regarding the heat treatment of the pulverized pitch-based carbon fiber used in the present invention, graphitization obtained by graphitizing at a heat treatment temperature of usually 2600 ° C. or higher, preferably 2800 ° C. or higher, more preferably 3000 ° C. or higher. It is a highly graphitizable carbon powder.

When the heat treatment temperature is lower than 2600 ° C., the graphite structure of the fiber itself does not sufficiently develop, and the diffusion of lithium inside the carbon powder obtained by crushing the fiber is delayed, so that the high temperature current is not increased. The discharge capacity is small and the current efficiency is low at the initial stage of the charge / discharge cycle.

Further, the doping amount of lithium into the carbon powder, that is, the discharge capacity becomes small.

Regarding the molding of the carbon fiber powder provided by the present invention, it is possible to mold the powdery battery active material used in a lithium battery by a method commonly used,
As long as the performance of the carbon fiber powder is sufficiently drawn out, the moldability to the powder is high, and it is chemically and electrochemically stable, the carbon fiber powder is not limited thereto. There is a method in which fluorinated resin powder such as polytetrafluoroethylene is used as a binder and isopropyl alcohol or the like is added, followed by dry mixing and kneading.

There is also a method in which resin powder such as polyethylene or polyvinyl alcohol is added to carbon fiber powder, and then the dry mixture is inserted into a mold and molded by hot pressing.

Further, a carbon fiber powder is used with a fluororesin powder such as polyvinylidene fluoride or a water-soluble binder such as carboxymethyl cellulose as a binder, and N-methylpyrrolidone, dimethylformamide or a solvent such as water or alcohol is used. A slurry can be prepared by mixing, coated on a current collector, and dried to form a slurry.

The carbon material of the present invention can be used in an appropriate combination with a positive electrode active material and an organic solvent-based electrolyte. These organic solvent-based electrolytes and positive electrode active materials are usually used in lithium secondary batteries. If it is possible, it is not particularly limited.

Examples of the positive electrode active material include lithium-containing transition metal oxide LiM (1) xO2 (where X is 0 ≦ X
It is a numerical value in the range of ≦ 1, where M (1) represents a transition metal, Co, Ni, Mn, Cr, Ti, V, Fe, Zn, A
l, In, or Sn) or L
iM (2) 2-yO4 (Y in the formula is a numerical value in the range of 0 ≦ Y ≦ 1, where M (1) and M (2) represent a transition metal and C
o, Ni, Mn, Cr, Ti, V, Fe, Zn, Al,
At least one of In and Sn), transition metal chalcogenide, vanadium oxide (V2O5, V6O1)
3, V2O4, V3O8, etc) and its Li compound, the general formula MxMo6S8-y (where X is 0 ≦ X ≦ 4,
Y is a numerical value in the range of 0 ≦ Y ≦ 1, and in the formula, M represents a Chevrel phase compound represented by a metal such as a transition metal, activated carbon, activated carbon fiber or the like can be used.

The organic solvent in the organic solvent-based electrolyte is not particularly limited, but for example, propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, 1, 1- and 1, 2
-Dimethoxyethane, 1,2-diethoxyethane, γ-
Butyrolactone, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, 4-methyl-
1,3-dioxolane, anisole, diethyl ether, sulfolane, methylsulfolane, acetonitrile,
Chloronitrile, propionitrile, trimethyl borate, tetramethyl silicate, nitromethane, dimethylformamide, N-methylpyrrolidone, ethyl acetate, trimethyl orthoformate, nitrobenzene, benzoyl chloride, benzoyl bromide, tetrahydrothiophene, dimethyl sulfoxide, 3 -Methyl-2-oxazolidone, ethylene glycol, sulfite, dimethyl sulfite and the like can be used alone or in combination of two or more kinds.

Any known electrolyte can be used as the electrolyte, for example, LiClO4, Li
BF4, LiPF6, LiAsF6, LiB (C6H
5), LiCl, LiBr, LiCF3SO3, LiC
Examples thereof include H3SO3 and the like, or a mixture of two or more kinds.

[0086]

In the lithium secondary battery of the present invention, the pitch-based carbon fiber pulverized powder, which defines the regular arrangement of the carbon net surface layers adjacent to each other as the negative electrode material, is the carbon net surface layer as lithium is inserted and removed. Although the spacing between the layers of the carbon expands and contracts, the stacking state of the carbon network layer of the fiber itself is regular, which allows smooth insertion and removal of lithium from the carbon network layer, which results in charge and discharge. The current efficiency at the initial stage of the cycle is improved, and the cycle stability against repeated charging / discharging is improved.

Further, the powder prepared by crushing carbon fibers should be crushed within a range defined by the R value of the Raman spectrum for crystal defects mechanically introduced by crushing, reduction in symmetry, etc. Thereby, the discharge capacity, that is, the amount of lithium doped can be improved, and the excellent electrode performance of the carbon fiber can be sufficiently brought out.

[0088]

【Example】

[0089]

Example 1 Mesophase content is 90% (volume fraction)
The carbon fiber (fiber diameter after spinning 13 μm) using the coal tar pitch as a raw material was heated at a heating rate of 10 ° C./min, and 2
A series of carbon fibers carbonized by holding at 000 ° C. for 1 hour were crushed under the following conditions using a vibrating disk mill.

The crushed carbon fiber was again heated at a rate of 10 ° C./min and graphitized at 3000 ° C. for 1 hour.

Table 1 shows the index of the stacking order regularity of the obtained carbon fiber powder, the index of the crystal defect of the powder, and the shape. The fiber diameter of the carbon fiber after graphitization is about 10 μm.
Met.

[0092]

[Table 1]

To the carbon fiber powder thus prepared,
Polytetrafluoroethylene powder 5 as a binder
Add% by weight, knead with isopropyl alcohol,
Create an electrode sheet with a thickness of about 0.1 mm, about 10.53 m
A negative electrode was prepared by cutting out into g (10 mg in terms of carbon material) and pressing it onto a Ni mesh that is a current collector.

In order to evaluate the electrode characteristics of the above-mentioned molded electrode with a single electrode, a so-called three-electrode cell using lithium metal for the counter electrode and the reference electrode was used. As the electrolytic solution, a solution obtained by dissolving LiPF6 in a mixed solvent of ethylene carbonate and diethyl carbonate (volume ratio of 1: 1) at a ratio of 1 mol / l was used.

In the charge / discharge test, both charging and discharging were carried out at a constant current (0.5 mA / cm ² ) under the potential regulation. The potential range was 0V to 1.0V (based on lithium metal). The results of the electrode characteristics are shown in Table 2.

In the initial lithium doping of the carbon material, a flat potential portion (plateau) near 0.8 V was not observed, and the initial charge / discharge efficiency was very high and remained stable at almost 100% after the fifth cycle. .

Also, the discharge capacity was high, the capacity decrease with charge / discharge cycles was small, and the electrode performance was excellent.

[0098]

[Table 2]

[0099]

Example 2 Mesophase content is 90% (volume fraction)
The carbon fiber (fiber diameter after spinning 10 μm) made from the coal tar pitch of No. 2 was heated at a heating rate of 10 ° C./min, and
A series of carbon fibers carbonized by holding at 000 ° C. for 1 hour were crushed by a vibrating ball mill for 1 minute.

The carbon fiber after pulverization was heated again at a rate of 10 ° C./min, and heated to 2600, 2800, 3000, 320.
Graphitization treatment was performed at 0 ° C. for 1 hour.

Table 3 shows the index of the stacking order regularity of the obtained carbon fiber powder, the index of the crystal defect of the powder, and the shape. The fiber diameter of the carbon fiber after graphitization was about 7 μm.

[0102]

[Table 3]

To the carbon fiber powder thus prepared,
Polytetrafluoroethylene powder 5 as a binder
Add% by weight, knead with isopropyl alcohol,
Create an electrode sheet with a thickness of about 0.1 mm, about 10.53 m
A negative electrode was prepared by cutting out into g (10 mg in terms of carbon material) and pressing it onto a Ni mesh that is a current collector.

The electrode performance was measured in the same manner as in Example 1 except that LiBF4 was dissolved in a mixed solvent of ethylene carbonate and diethyl carbonate (1: 1 by volume) at a ratio of 1 mol / l as the electrolytic solution. The results are shown in Table 4.

In the initial lithium doping of the carbon material, a flat potential portion (plateau) near 0.8 V was not observed, and the charging / discharging efficiency was very high, and it was almost 100% in the third cycle and 100% thereafter. Remained stable at.

Further, the discharge capacity was high, and the decrease in capacity with charge / discharge cycles was very small, and the electrode performance was excellent.

[0107]

[Table 4]

[0108]

Example 3 Mesophase content is 90% (volume fraction)
The carbon fiber (fiber diameter after spinning 13 μm) made from the coal tar pitch of No. 1 was heated at a heating rate of 10 ° C. per minute to
A series of carbon fibers carbonized by holding at 800 ° C. for 1 hour were crushed with a jet mill.

The carbon fiber after pulverization was again heated at a rate of 10 ° C./min and graphitized at 3200 ° C. for 1 hour.

Table 5 shows the index of the stacking order regularity of the obtained carbon fiber powder, the index of the crystal defect of the powder, and the shape. The fiber diameter of the carbon fiber after graphitization is about 10 μm.
Met.

[0111]

[Table 5]

To the carbon fiber powder thus prepared,
5% by weight of polyvinylidene fluoride powder as a binder
In addition, N-methylpyrrolidone was mixed to prepare a slurry, which was uniformly applied onto a Cu foil having a thickness of about 20 μm to a thickness of about 0.1 mm and dried to prepare an electrode sheet.

From this sheet, a negative electrode was prepared by cutting it out into a circle having a diameter of about 16 mmφ.

On the other hand, for the positive electrode, LiCoO 2 prepared from lithium carbonate and cobalt carbonate was used. To this powder, 5% by weight of polyvinylidene fluoride powder and 5% by weight of Ketjen black were added and mixed with N-methylpyrrolidone. To prepare a slurry, which was uniformly applied onto an Al foil and dried to prepare an electrode sheet. A positive electrode was prepared by cutting a circular piece having a diameter of about 16 mmφ from this sheet.

Using the positive electrode and the negative electrode described above, LiClO 4 dissolved at a concentration of 1 mol / liter in a mixed solvent of ethylene carbonate and dimethyl carbonate (volume ratio of 1: 1) was used as an electrolytic solution. A simple coin-shaped battery was prepared using a polypropylene non-woven fabric, and a charge / discharge test was conducted. The test results are shown in Table 6.

In the initial charge / discharge cycle of the coin-shaped battery, the charge / discharge efficiency was very high, and it was almost 100% in the fifth cycle, and remained 100% thereafter.

Also, the discharge capacity per weight of the carbon material is high, and the decrease in capacity with charge / discharge cycles is very small.
The electrode performance was excellent.

[0118]

[Table 6]

[0119]

[Comparative Example 1] PAN-based carbon fiber (M46B manufactured by Toray Industries, Inc.)
Was immersed in methyl ethyl ketone, and the surface treatment agent was washed by applying ultrasonic vibration, and then pulverized by a vibrating disk mill.

Table 7 shows the index of the stacking order regularity of the obtained carbon fiber powder, the index of the crystal defect of the powder, and the shape. The fiber diameter of the carbon fiber after graphitization was about 7 μm.

[0121]

[Table 7]

When molding the carbon fiber powder thus prepared, the same method as in Example 1 was used. Also,
The charge / discharge test was also performed according to Example 1, and the results are shown in Table 8.
Shown in.

In the initial lithium doping of the carbon powder, a long potential flat portion (plateau) of 0.9 V to 0.6 V was observed, and in addition, the voltage difference up to the open circuit state after lithium doping was large. Since the diffusion of lithium inside did not proceed easily, the charge / discharge efficiency was a very low value.

The charge-discharge reaction finally reached almost 100% after the 10th cycle, and then reached 100%.
Remained the same. However, the discharge capacity was low, and the capacity decreased with charge / discharge cycles.

[0125]

[Table 8]

[0126]

Comparative Example 2 The same carbon fiber as in Example 1 (fiber diameter after spinning 13 μm) was carbonized in the same manner as in Example 1,
It was crushed with a pin mill. The crushed carbon fiber was again heated at a rate of 10 ° C./min and heat-treated at 3000 ° C. for 1 hour.

Table 9 shows the index of the stacking order regularity of the obtained carbon fiber powder, the index of the crystal defect of the powder, and the shape. The fiber diameter of the carbon fiber after graphitization is about 10 μm.
Met.

[0128]

[Table 9]

To the carbon fiber powder thus prepared,
Polytetrafluoroethylene powder 5 as a binder
Add% by weight, knead with isopropyl alcohol,
An electrode sheet with a thickness of about 0.1 mm was prepared, but since the fibrous powder with a coarse particle size was mixed in, the contact between the binder and the powder was poor, and the surface of the molded product was partly fluffed and formed as a whole. The seat was worn poorly.

About 10.53 mg portion was cut out from this sheet (10 mg in terms of carbon material) to obtain N as a current collector.
A negative electrode was prepared by pressure bonding to the i mesh.

The electrode performance was measured in the same manner as in Example 1 except that LiClO4 was dissolved in a mixed solvent of ethylene carbonate and dimethyl carbonate (volume ratio of 1: 1) at a ratio of 1 mol / l as the electrolytic solution. The results are shown in Table 10.

In the initial lithium doping of the carbon material, almost no potential flat portion (plateau) near 0.8 V was observed, so that the charge / discharge efficiency became a high value and remained stable at almost 100% after the third cycle. did.

However, although the discharge capacity at the initial stage of charge / discharge was high, the capacity was remarkably reduced with the charge / discharge cycle, and the result was that the molding state of the electrode sheet and the binding state of the carbon powder were largely reflected.

[0134]

[Table 10]

[0135]

Comparative Example 3 Carbon fiber obtained by carbonizing the same carbon fiber as in Example 1 (fiber diameter after spinning 13 μm) in the same manner as in Example 1 was used.
It was crushed for one hour with a vibrating ball mill. The crushed carbon fiber was again heated at a rate of 10 ° C./min and heat-treated at 3000 ° C. for 1 hour.

The index of the stacking order regularity of the obtained carbon fiber powder, the index of the crystal defect of the powder, and the shape are as shown in Table 11, and a considerably large crystal defect was introduced into the carbon powder by pulverization for a long time. I can see that The fiber diameter of the carbon fiber after graphitization was about 10 μm.

[0137]

[Table 11]

Using the carbon fiber powder thus prepared, a negative electrode sheet was prepared in the same manner as in Example 3.

From this sheet, a negative electrode was prepared by cutting out approximately 10.53 mg (10 mg in terms of carbon material). The electrode performance was measured according to Example 1, and the results are shown in Table 12.

In the initial lithium doping of the carbon material, a potential flat portion (plateau) near 0.8 V was not observed, but the voltage difference up to the open circuit state after lithium doping was large, and the lithium diffusion in the material was large. However, the charging / discharging efficiency was very low.

The charge / discharge reaction finally reached almost 100% after the 10th cycle, and then reached 100%.
Remained the same.

Although the discharge capacity was relatively high at the beginning of the cycle, it significantly decreased with the charge / discharge cycle, resulting in a large reflection of many crystal defects introduced into the carbon powder due to excessive pulverization of the carbon fiber.

[0143]

[Table 12]

[0144]

As is apparent from the above description, the carbon negative electrode material for a lithium secondary battery of the present invention is obtained by crushing carbon fibers using pitch as a raw material, and the rule of the adjacent carbon net surface layer. It regulates the probability of a typical stacked arrangement state, and further regulates the microscopic crystal structure factor that limits the degree of graphite crystal defects that are mechanically introduced during fiber crushing. It is possible to provide a powdery carbon material having a high doping amount, that is, a discharge capacity, a charging / discharging efficiency of lithium insertion / desorption, and a high cycle stability against repeated charging / discharging.

[Brief description of drawings]

Figure 1: Three nets adjacent to the crystallite in the C-axis direction are AB
It is explanatory drawing which shows the state which has a regular arrangement | sequence as a hexagonal system, and the unit cell of a hexagonal system was shown by the thick line in the figure.

[Fig. 2] Three nets adjacent to the crystallite in the C-axis direction are AB.
It is explanatory drawing which shows the state which has the regular arrangement | sequence as a rhombohedral system of C, and the unit cell of a rhombohedral system is shown with a thick line in the figure, and the unit cell when it is a hexagonal system is shown with a double line. It was

Front Page Continuation (72) Inventor Koichiro Mukai 1618 Ida, Nakahara-ku, Kawasaki City Nippon Steel Corp. Advanced Technology Research Laboratories

Claims (2)

[Claims] 1. A carbon fiber powder having an aspect ratio of 50 or less, which is obtained by crushing pitch-based carbon fibers having a fiber diameter of 20 μm or less, wherein two adjacent net surfaces in a stacked state of carbon net planes have a graphite arrangement. P ₁ which is an index of the probability of taking
The value is 0.6 or more, the probability that three adjacent net planes have a regular arrangement of ABA as a hexagonal system is P _ABA value is 0.3 or more, and the three adjacent net planes are ABC rhombuses. A carbon negative electrode material for a lithium secondary battery, which has a probability P _ABC value of 0.15 or more having a regular array as a tetrahedron system. 2. A Raman spectroscopy using an argon laser, claims strength ratio 1580 cm ^-1 band of 1360 cm ^-1 band _{(R = I 1360 / I 1580} ) is equal to or less than 0.4 1. A carbon negative electrode material for lithium secondary batteries according to 1.