JPH0883609A - Lithium secondary battery - Google Patents

Abstract

PURPOSE: To improve the utilization factor of a negative electrode by forming the carbonaceous material constituting a negative material with the mixture of two kinds of specific carbonaceous materials. CONSTITUTION: This lithium secondary battery is provided with a negative electrode made of a carbonaceous material storing or releasing lithium ions, and the carbonaceous material is constituted of the mixture of two kinds of carbonaceous materials. One carbonaceous material is the graphitized mesophase pitch carbon fiber powder having the average fiber length of 10-100μm, the average fiber diameter of 4-15μm, and the spacing less than 0.338nm of the (002) plane of the graphite structure by the X-ray diffraction method. The other carbonaceous material is the carbon powder of a resin baked body or the thermal decomposition vapor phase carbon having the particle size distribution less than 5μm at 70vol.% or above and the spacing of 0.338-0.380nm of the (002) plane by the X-ray diffraction method. The weight ratio between two carbonaceous materials is set to 80-95:20-5.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithium secondary battery, and more particularly to a lithium secondary battery having an improved utilization rate of a negative electrode.

[0002]

2. Description of the Related Art In recent years, by using a carbonaceous material that absorbs and releases lithium, such as coke, a resin fired body, carbon fiber, and pyrolytic vapor-phase carbon, as a negative electrode incorporated in a lithium secondary battery, It has been proposed to improve the reaction with the water electrolyte and further the deterioration of the negative electrode characteristics due to the dendrite deposition.

Conventionally, in a negative electrode made of carbonaceous material,
It is considered that charging / discharging is performed by lithium ions entering and leaving between layers in a structure (graphite structure) in which hexagonal net-surface layers mainly composed of carbon atoms are stacked. Therefore, it is necessary to use a carbonaceous material having a graphite structure developed to some extent for the negative electrode of the lithium secondary battery.

However, when a carbonaceous material obtained by pulverizing giant crystals having advanced graphitization is used as a negative electrode in a non-aqueous electrolytic solution, the non-aqueous electrolytic solution is decomposed, resulting in a decrease in battery capacity and charge / discharge efficiency. To do. Also, as the charge / discharge cycle progresses, the crystal structure or fine structure of the carbonaceous material collapses,
There is a problem that the occlusion / release capacity of lithium is deteriorated and the cycle life is shortened.

Further, since the graphitized powder is in the form of flakes, the area exposed to the electrolytic solution on the surface of the graphite crystallite in which lithium ions are inserted in the c-axis direction becomes smaller, so that in a high-rate charge / discharge cycle. Has a problem that the capacity is rapidly reduced. Therefore, although battery characteristics have been improved by adding a conductive agent such as carbon black, there is a problem that the negative electrode packing density decreases. as a result,
High-capacity lithium secondary batteries cannot be realized with conventional graphitized products. Further, even in the case of carbon fibers having a high degree of graphitization, the non-aqueous electrolytic solution decomposes when made into powder, and there is a problem that the performance as a negative electrode is significantly reduced, as in the case of using a huge crystal powder. .

On the other hand, in the case of carbonaceous materials such as coke and carbon fiber having a low degree of graphitization, although the decomposition of the solvent can be suppressed to some extent, the capacity and charge / discharge efficiency are low, and the overvoltage of charge / discharge is large, and the discharge voltage of the battery However, there were problems such as lack of flatness and low cycle life.

[0007]

SUMMARY OF THE INVENTION An object of the present invention is to provide a lithium secondary battery having high capacity and excellent battery characteristics such as charge / discharge efficiency, high rate discharge characteristics, discharge voltage flatness, and high charge / discharge life. Is.

[0008]

The lithium secondary battery of the present invention is a lithium secondary battery comprising a negative electrode composed of a carbonaceous material that absorbs and releases lithium ions, a positive electrode, and a non-aqueous electrolyte. The carbonaceous material is a mixture of two kinds of carbonaceous materials, and (A) one of the carbonaceous materials is a graphitized mesophase pitch carbon fiber powder having an average fiber length of 10 to 1
The average fiber diameter is 00 μm, the average fiber diameter is 4 to 15 μm, and the interplanar spacing d ₀₀₂ of the (002) plane of the graphite structure by X-ray diffraction is 0.3
A carbonaceous material having a particle size of less than 38 nm; (B) another carbonaceous material is a coke, a resin fired body, or a block-like, flake-like or granular carbon powder of pyrolytic vapor phase carbon, the particle size distribution of which is 15 μm. The following particles are 70% by volume or more, and the interplanar spacing d ₀₀₂ of the (002) plane by the X-ray diffraction method is 0.338 to 0.380.
Carbonaceous matter having a nm; The weight ratio of the carbonaceous matter (A) and (B) is 80 to 95:
20 to 5; A lithium secondary battery characterized by the above.

Hereinafter, the lithium secondary battery of the present invention (for example, a cylindrical lithium secondary battery) will be described in detail with reference to FIG.

The bottomed cylindrical container 1 has an insulator 2 arranged at the bottom. The electrode group 3 is housed in the container 1. The electrode group 3 has a structure in which a band-shaped material in which a positive electrode 4, a separator 5, and a negative electrode 6 are laminated in this order is spirally wound so that the negative electrode 6 is located outside.

An electrolytic solution is stored in the container 1. The insulating paper 7 having an opening in the center is placed above the electrode group 3 in the container 1. The insulating sealing plate 8 is disposed in the upper opening of the container 1, and the vicinity of the upper opening is caulked inward to form the sealing plate 8.
Is liquid-tightly fixed to the container 1. The positive electrode terminal 9 is
It is fitted in the center of the insulating sealing plate 8. One end of the positive electrode lead 10 is connected to the positive electrode 4, and the other end is connected to the positive electrode terminal 9. The negative electrode 6 is connected to the container 1, which is a negative electrode terminal, via a negative electrode lead (not shown).

The container 1 is made of, for example, stainless steel. The positive electrode 4 is produced by suspending a conductive material and a binder in a positive electrode active material in an appropriate solvent, applying the suspension to a current collector, and drying the suspension to form a thin plate.

As the positive electrode active material, various oxides,
For example, manganese dioxide, lithium manganese composite oxide,
Examples thereof include lithium nickel oxide, lithium cobalt compound, lithium nickel cobalt oxide, vanadium oxide containing lithium, and chalcogen compounds such as titanium disulfide and molybdenum disulfide. Above all,
Using lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), lithium manganese composite oxide (LiMn ₂ O ₄ , LiMnO ₂ ),
It is preferable because a high voltage can be obtained.

Examples of the conductive material include acetylene black, carbon black and graphite. Examples of the binder include polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVD).
F), ethylene-propylene-diene copolymer (EPD
M), styrene-butadiene rubber (SBR) and the like can be used.

The mixing ratio of the positive electrode active material, the conductive material and the binder is 80 to 95% by weight of the positive electrode active material and 3 to 2 of the conductive material.
The range of 0% by weight and 2 to 7% by weight of the binder is preferable.

As the current collector, for example, aluminum foil, stainless foil, nickel foil, titanium foil or the like can be used.

As the separator, for example, a synthetic resin non-woven fabric, a polyethylene porous film, or a polypropylene porous film can be used.

The carbonaceous material (A) for the negative electrode 6 is produced as follows. First, a mesophase pitch carbonaceous material is used as a main raw material and a fiber length of 200 to 3 is obtained by a melt blow method.
After spinning a short fiber of 00 μm, it is carbonized to such an extent that it can be made infusible and crushed. This carbonization heat treatment is 600 ~
It is desirable to carry out at 2,000 ° C., preferably 800 to 1,500 ° C. The interplanar spacing d of the (002) plane of the carbonized mesophase pitch carbon fiber measured by X-ray diffractometry.
₀₀₂ is less than 0.338. The carbonized fiber thus obtained was pulverized to obtain 2,000 carbon fibers.
The mesophase pitch carbon fiber powder (A) described above is produced by graphitizing at 0 ° C or higher, and more preferably at 2,500 to 3,200 ° C. At this time, the crushing and firing steps are extremely important, and the carbon fiber is less likely to be longitudinally cracked by using a ball mill, a jet mill or the like at the time of crushing, and the average fiber length is 10 to 100 by uniformly crushing.
μm, more preferably 30 to 60 μm in average fiber length, and 4 to 15 μm in average fiber diameter, more preferably 6 to 8 μm.
Is desirable. If the average fiber length is less than 10 μm, the carbon fibers are likely to be longitudinally cracked by crushing. On the other hand, if the average fiber length exceeds 100 μm, the current collector cannot be coated, which is not preferable. Further, if the average fiber diameter is less than 4 μm, the strength of the fiber becomes brittle, while if the average carbon fiber diameter exceeds 15 μm, it is not preferable because coating on the current collector cannot be performed.

As for the carbon fiber powder (A), good results such as lithium doping amount and high rate characteristics were obtained as a result of charge and discharge evaluation by a glass cell alone. When a coating liquid mixed with a binder was applied on the current collector, the carbonaceous carbon powder alone was used, so there was little contact with the current collector, and there were few contacts between the fibers, so the electrode It was easily peeled off from the current collector during rolling and did not reach a high-strength negative electrode. for that reason,
The results of charge / discharge evaluation of a single substance using a glass cell were slightly inferior to the properties of the carbon fibers, but for the purpose of compensating for voids between the carbon fibers, mixing with various carbonaceous materials was examined.

As a result, as the carbonaceous material to be mixed with the carbon fiber, carbon powder (B) having a block shape, a flake shape or a granular shape of coke, a resin fired body or pyrolysis vapor phase carbon is desirable. This has a (002) plane spacing d ₀₀₂ of 0.338 to 0.3 according to the X-ray diffraction method.
It is desirable that the particle size of 80 nm and the particle size of 15 μm or less is 70% by volume or more. More preferably, the content is 90% by volume or more.

By mixing the carbon fiber powder (A) with the carbon powder (B), it was possible to obtain a high-strength negative electrode without peeling during electrode rolling. It is considered that the reason for this is that by mixing the carbonaceous material having a small particle size, the voids of the carbon fibers are filled and the number of contacts is increased, so that separation from the current collector is eliminated. The compounding ratio of the carbon fiber powder (A) and the carbon powder (B) in the carbonaceous material is 80 to 95:20 to 5, and more preferably 87 to 95 by weight.
93: 13-7 is desirable. When the mixing ratio of the carbon powder (B) is less than 5% by weight, a satisfactory electrode strength cannot be obtained, while when it exceeds 20% by weight, the electrode performance deteriorates and there is a problem in battery capacity and discharge rate characteristics.

The negative electrode 6 containing the carbonaceous material obtained by the above method is specifically manufactured by the following method. A binder is suspended in a mixture of the carbon fiber powder (A) and the carbon powder (B) in a suitable solvent, and the suspension is applied to a current collector and dried to form a thin plate, thereby forming the negative electrode. Create.

Examples of the binder include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), ethylene-propylene-diene copolymer (EPDM), styrene-butadiene rubber (SBR),
Carboxymethyl cellulose (CMC) or the like can be used.

The mixing ratio of the carbonaceous material and the binder is
The binder is preferably 90 to 98% by weight and the binder is preferably 2 to 10% by weight. As the current collector, for example, copper foil, stainless steel foil, nickel foil, or the like can be used.

The non-aqueous electrolytic solution contained in the container 1 is prepared by dissolving an electrolyte in a non-aqueous solvent.

As the non-aqueous solvent, ethylene carbonate (EC), dimethyl carbonate (DMC), methyl ethyl carbonate (MEC), diethyl carbonate (DEC), propylene carbonate (PC), γ-
Butyrolacurone (γ-BL), acetonitrile (A
N), ethyl acetate (EA), toluene, xylene and the like.

The electrolyte contained in the non-aqueous electrolyte is, for example, lithium perchlorate (LiClO ₄ ), lithium hexafluorophosphate (LiPF ₆ ), lithium borofluoride (LiBF ₄ ), lithium trifluorometasulfonate. Examples thereof include lithium salts (electrolytes) such as (LiCF ₃ SO ₃ ).

[0028]

Embodiments of the present invention will now be described in detail with reference to FIG.

Example 1 First, lithium cobalt oxide [Li _x CoO ₂ (0.
8 ≦ x ≦ 1)] 91 parts by weight of powder are mixed with 3.5 parts by weight of acetylene black, 3.5 parts by weight of graphite and 2 parts by weight of ethylene propylene diene monomer powder and toluene, and mixed together to collect an aluminum foil (30 μm). A positive electrode was produced by pressing after applying on an electric body.
Also, after carbonization at 900 ° C., pulverization, and calcination at 3,000 ° C., average fiber length 40 μm, average fiber diameter 7 μm, specific surface area 4 m ² / g by N ₂ gas adsorption BET method, graphite by X-ray diffraction method The (002) plane spacing d _{002 of the} structure is 0.336.
4nm mesophase pitch carbon fiber powder (A) and 1
92.2% by volume of particles of 5 μm or less and d ₀₀₂ of 0.35
96 parts by weight of carbonaceous material, 2.5 parts by weight of styrene-butadiene rubber, and carboxymethyl cellulose were mixed with the coke powder (B) having a block shape having a surface area of 0,2 nm and a specific surface area of 8.2 m ² / g in a weight ratio of 90:10. A 1.5-weight part was mixed together, this was apply | coated to the copper foil which is a collector, and the negative electrode was produced by drying.

The positive electrode, the separator made of a polyethylene porous film, and the negative electrode were laminated in this order, and then spirally wound so that the negative electrode was located on the outer side to prepare an electrode group. Further, lithium hexafluorophosphate (LiPF ₆ ) was added to ethylene carbonate (E
A non-aqueous electrolytic solution was prepared by dissolving 1.0 mol / L in a mixed solvent of C) and diethyl carbonate (DEC) (mixing volume ratio 50:50). The electrode group and the electrolytic solution were respectively housed in a bottomed cylindrical container made of stainless steel to assemble the cylindrical lithium secondary battery shown in FIG. 1 described above.

Example 2 A negative electrode was produced in the same manner as in Example 1 by using the coke powder (B) having a particle size of 15 μm or less in an amount of 70.1% by volume. The cylindrical lithium secondary battery shown in FIG. 1 was assembled in the same manner as in Example 1 except that this negative electrode was used.

Comparative Example 1 A negative electrode was produced in the same manner as in Example 1 using the coke powder (B) having a particle size of 15 μm or less and 65.7% by volume. The cylindrical lithium secondary battery shown in FIG. 1 was assembled in the same manner as in Example 1 except that this negative electrode was used.

Comparative Example 2 A negative electrode was produced in the same manner as in Example 1 by using the coke powder (B) containing 52.5% by volume of particles having a particle size of 15 μm or less. The cylindrical lithium secondary battery shown in FIG. 1 was assembled in the same manner as in Example 1 except that this negative electrode was used.

Example 3 Example 3 was repeated except that a carbonaceous material prepared by mixing the carbon fiber powder (A) prepared in Example 1 and the coke powder (B) in a weight ratio of 80:20 was used. The aforementioned cylindrical lithium secondary battery shown in FIG. 1 was assembled.

Comparative Example 3 The procedure of Example 1 was repeated except that a carbonaceous material prepared by mixing the carbon fiber powder (A) prepared in Example 1 and the coke powder (B) in a weight ratio of 70:30 was used. The aforementioned cylindrical lithium secondary battery shown in FIG. 1 was assembled.

Comparative Example 4 The procedure of Example 1 was repeated except that a carbonaceous material prepared by mixing the carbon fiber powder (A) prepared in Example 1 and the coke powder (B) in a weight ratio of 60:40 was used. The aforementioned cylindrical lithium secondary battery shown in FIG. 1 was assembled.

The obtained Examples 1 to 3 and Comparative Examples 1 to 4
3. Regarding the lithium secondary battery of, 4.
The battery was charged to 2 V for 3 hours and discharged to 2.7 V at a high rate current of 1 A, the discharge capacity of each battery was measured, and the result of the capacity ratio when the discharge capacity of Example 1 was 100% is shown in Table 1.
In addition, FIG. 2 shows the charge / discharge cycle characteristics of each battery under the same charge / discharge conditions.

[0038]

[Table 1]

As is clear from Table 1 and FIG. 2, the lithium secondary batteries of Examples 1 and 2 are the same as Comparative Examples 1 and 2.
It can be seen that the high rate characteristic is good and a high charge / discharge life can be obtained as compared with the above. This is due to the difference in particle size distribution of the coke powder (B) and the carbon fiber powder (A).
It is thought that this is due to the improved adhesion. It is also understood that the lithium secondary batteries of Examples 1 and 3 also have better high rate characteristics and charge / discharge cycle characteristics than Comparative Examples 3 and 4. It is considered that this is because the difference in the mixing ratio between the carbon fiber powder (A) and the carbon powder (B) reduced the voids between the carbonaceous materials and maximized the characteristics of the carbon fiber powder. . As the carbonaceous material to be mixed with the carbon fiber powder, similar results were obtained with a resin fired body or a pyrolytic vapor phase carbon powder, in addition to the coke powder.

[0040]

According to the present invention, it is possible to provide a lithium secondary battery having a high capacity, excellent high rate characteristics and capable of maintaining a long charge / discharge life.

[Brief description of drawings]

FIG. 1 is a partial cross-sectional view showing a cylindrical lithium secondary battery according to the present invention.

FIG. 2 is a graph showing the relationship between the number of charge / discharge cycles and the capacity retention rate in the lithium secondary batteries of Examples 1 to 3 and Comparative Examples 1 to 4.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Container 3 ... Electrode group 4 ... Positive electrode 6 ... Negative electrode 8 ... Sealing plate

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Takahisa Osaki, No. 1 Komukai Toshiba Town, Saiwai-ku, Kawasaki-shi, Kanagawa, Ltd. Within the Corporate Research and Development Center, Toshiba Corporation (72) Norio Takami, Komukai Toshiba, Kawasaki-shi, Kanagawa Prefecture Town No. 1 Toshiba Corporation Research & Development Center

Claims (1)

[Claims] 1. A lithium secondary battery comprising a negative electrode made of a carbonaceous material that absorbs and releases lithium ions, a positive electrode, and a non-aqueous electrolyte, wherein the carbonaceous material is a mixture of two kinds of carbonaceous materials. A) The one carbonaceous material is a graphitized mesophase pitch carbon fiber powder having an average fiber length of 10 to 1
The average fiber diameter is 00 μm, the average fiber diameter is 4 to 15 μm, and the interplanar spacing d ₀₀₂ of the (002) plane of the graphite structure by X-ray diffraction is 0.3
A carbonaceous material having a particle size of less than 38 nm; (B) another carbonaceous material is a coke, a resin fired body, or a block-like, flake-like or granular carbon powder of pyrolytic vapor phase carbon, the particle size distribution of which is 15 μm. The following particles are 70% by volume or more, and the interplanar spacing d ₀₀₂ of the (002) plane by the X-ray diffraction method is 0.338 to 0.380.
Carbonaceous matter having a nm; The weight ratio of the carbonaceous matter (A) and (B) is 80 to 95:
20 to 5; A lithium secondary battery characterized by the above.