JPH0722022A - Negative electrode for lithium secondary battery and manufacture thereof - Google Patents

Abstract

PURPOSE:To provide an electrode of high energy density by using a sheet-like carbon fiber-containing carbon material in which carbon fibers are oriented in parallel to a sheet surface and most of the carbon fibers are exposed on the sheet surface. CONSTITUTION:Pitch group carbon fibers of which fiber axes are arranged in parallel are impregnated with a phenol resin, preliminarily cured at 80-100 deg.C to obtain a semi-hardened prepreg sheet. Two of these sheets are placed in a metal die with their fiber axes arranged, left as they are under pressure of 100kg weight and at 110 deg.C in the atmosphere for 20 minutes, and further processed at 150 deg.C for 20 minutes, and taken out from the metal die to obtain a hardened mold product. Furthermore, this hardened mold product is increased in temperature at a rate of 100 deg.C per hour in argon atmosphere, processed for 5 hours at 1000 deg.C for carbonization, and a sheet-like carbon fiber containing carbon material having high energy density and high electric conductivity for an electrode mold body can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a negative electrode for a lithium secondary battery using carbon fiber and a method for producing the same.

[0002]

2. Description of the Related Art Conventionally, a lithium secondary battery using lithium metal as a negative electrode can be expected to have high energy density, but
There are problems that the charge and discharge efficiency is low and the discharge capacity is significantly reduced with the charge and discharge cycle. This is due to peeling of dendrites of lithium called dendrites deposited on the negative electrode during charging, or short circuit between the grown dendrites and the positive electrode. Examples of methods for preventing the generation of dendrites include a method using an alloy containing lithium in the negative electrode, a method using a carbon material, and a method using a conductive polymer.

The method using an alloy prevents the formation of dendrites by alloying the lithium deposited during charging through a diffusion process and taking it into the electrode. However, because the diffusion rate of lithium inside the alloy electrode is low, the current density cannot be increased, and because the crystal is distorted due to the change in the lithium composition in the alloy, deep charging and discharging cannot be performed and the energy density of the negative electrode decreases. However, there are problems such as deterioration in cycle stability due to structural destruction due to crystal distortion.

The method using a conductive polymer prevents the formation of dendrites by using lithium doping / undoping in the negative electrode reaction, but the polymer itself is electrochemically stable at a base reaction potential. However, there is a problem in chemical stability against lithium metal, and at present, it is difficult to apply it to a negative electrode of a lithium secondary battery.

On the other hand, the carbon material has high electrochemical stability and chemical stability to lithium, and theoretically the amount of lithium that can be incorporated into the carbon material is C ₆ Li = 3.
Since it is as large as 72 mAh / g, it has attracted attention as a negative electrode material to replace lithium metal, and various carbon materials have been studied (for example, JP-A-62-268,058 and JP-A-63-213,258). , JP-A-64-645, JP-A-1
-221,859, JP-A-2-82,466, etc.).

The intercalation reaction of lithium into the carbon material in the organic electrolytic solution differs depending on the micro- and macro-structure of the carbon material as the base material, and as a result of diligent studies by the present inventors, pitch was used as the raw material. It was found that the above-mentioned carbon fiber is very excellent in the negative electrode characteristics such as lithium composition that can be inserted / removed, that is, discharge capacity and stability against charge / discharge cycles.
82,466).

The essence of the high performance of the above-mentioned carbon fiber lies in the stability of the macro structure against expansion and contraction between the carbon layers, and this stability is maintained by the form of the fiber. That is, the form of the fiber has an essential importance for the negative electrode intercalation reaction and the cycle stability, and therefore, it is most desirable to form the electrode while maintaining the fiber shape. However, since the conventional electrode molding method mainly targets powder, it is not a method of molding carbon fiber as it is.

[0008]

DISCLOSURE OF THE INVENTION The present invention is to solve the above problems, and in a negative electrode of a lithium secondary battery using carbon fiber as a negative electrode, while maintaining the shape of the carbon fiber, a high packing density. It is intended to provide an electrode having a new structure molded into

[0009]

By controlling the arrangement of carbon fibers, the present inventor can obtain an electrode having a high energy density without impairing the characteristics of carbon fibers as a negative electrode for a lithium secondary battery. I found what I could do.
It was also found that it is important to carbonize or graphitize the binder component as a manufacturing method thereof. The present invention has been completed based on such findings.

That is, the present invention is a sheet-shaped carbon fiber-containing carbon material, wherein the carbon fibers are oriented parallel to the surface of the sheet, and most of the carbon fibers are exposed on the surface of the sheet. And a negative electrode for a lithium secondary battery,
The manufacturing method is characterized in that after a sheet-shaped carbon fiber aggregate in which carbon fibers are oriented parallel to the surface thereof is molded using a binder, the binder used for molding is carbonized or graphitized. is there. According to the invention,
It has become possible to provide a battery for a lithium secondary battery having a high packing density of carbon fibers without impairing the original negative electrode characteristics of carbon fibers.

The contents of the present invention will be described in detail below. The features of the present invention are as follows. The first feature is that the carbon fiber is formed into an electrode without breaking it. Generally, carbon fiber has the highest initial charge / discharge efficiency and cycle stability in the shape of the fiber. For example, the pitch-based carbon fiber has an initial charge / discharge efficiency of 90% or more, and a performance exceeding 95% when an electrolytic solution is selected [31st
The 3rd battery discussion meeting 3B09 (1990) and the 32nd battery discussion meeting 2B10 (1991)] have been obtained. Since the sheet-shaped molded body of carbon fiber obtained by the present invention is molded without breaking the fiber, as shown in Examples, it is possible to exhibit good performance while maintaining the performance of carbon fiber.

The carbon fiber of the present invention is not particularly limited as long as it is suitable for a lithium secondary battery negative electrode reaction, that is, a lithium insertion / desorption reaction, but a pitch-based carbon fiber is preferable. It is more preferable that the pitch-based carbon fiber has a carbon spacing (d _{00 2} ) determined by X-ray diffraction of 0.35 nm or less. The above pitch-based carbon fiber,
Since it has a large discharge capacity and a large initial charge / discharge efficiency, it is suitable for the present invention.

The second feature is to form a sheet-like carbon fiber aggregate in which carbon fibers are oriented parallel to the surface of the sheet by orienting the carbon fibers in one direction or in two directions orthogonal to each other. Is. By aligning the fiber axes in one direction, it is possible to increase the packing density of carbon fibers in the carbon fiber aggregate, and the carbonized or graphitized sheet-shaped molded product can be used in a direction perpendicular to the fiber axis. It is characterized by having flexibility.

The sheet-like carbon fiber aggregates oriented in two directions orthogonal to each other are, for example, a woven fabric of carbon fibers such as plain weave, or a sheet-like carbon fiber aggregates in which carbon fibers are oriented in one direction. Represents a stack with an orientation angle of 90 degrees. The characteristics of the sheet-shaped carbon fiber aggregates oriented in two directions orthogonal to each other are that the packing density of carbon fibers can be increased and that the carbonized or graphitized sheet-shaped molded body is strong. .

The thickness of the sheet-shaped carbon fiber molded body of the present invention is not particularly limited as long as it does not prevent all of the carbon fibers constituting the molded body from participating in the electrode reaction. It is preferably 5 mm or less, more preferably 1 mm or less. In a molded product having a thickness of more than 5 mm, the carbon fibers inside the molded product cannot participate in the electrode reaction, and the energy density of the molded product is reduced.

For molding the negative electrode used in the present invention, for example,
Molding with a resin or pitch can be preferably applied, and more preferably, a thermosetting resin can be used. By using a thermosetting resin or pitch and thermosetting it under pressure, a molded body suitable for the present invention can be prepared. As the thermosetting resin, for example, phenol resin, furan resin, epoxy resin or the like can be used.

The third feature is that the effect of the binder on the electrode reaction is removed and conductivity is imparted by carbonizing or graphitizing the binder used for molding the carbon fiber. When a thermosetting resin is used as a binder, in the state after curing, the cured resin dissolves in the electrolytic solution, which induces deterioration of the electrode performance due to modification of the electrolytic solution, and because a resin with low electrical conductivity is used. However, the electric conductivity of the molded body may decrease. By carbonizing or graphitizing a resin using a binder to obtain a carbon fiber-containing carbon material, dissolution of the resin in the electrolytic solution is eliminated, and since the carbonized resin has electrical conductivity, The electrical conductivity can be increased.

The fourth characteristic is that most of the carbon fibers are exposed on the surface of the sheet-shaped molded body by carbonizing or graphitizing the binder used for molding the carbon fibers. In the present invention, the surface of the carbon fiber participates in the electrode reaction, that is, the surface of the carbon fiber serves as a place for lithium insertion / desorption reaction. Therefore, in the sheet-shaped carbon fiber molded body, it is important that the carbon fibers are exposed on the surface of the sheet involved in the electrode reaction. In the molded body in which the binder is carbonized or graphitized in the present invention, the binder covers only a very small part of the surface of the carbon fiber and most of the carbon fiber is exposed. There is no loss.

The carbonization of the molded article in the present invention is preferably carried out at 800 ° C. or higher in an inert atmosphere, more preferably 1,000 ° C. or higher. There is no particular upper limit to the carbonization temperature, and the resin itself is a non-graphitizable material, and the graphitization treatment does not affect the electrode characteristics and shapeability. 80
The molded product carbonized at a temperature of less than 0 ° C. does not smoothly proceed with the lithium insertion reaction, has a large decrease in capacity with charge / discharge cycles, and has a large deterioration such as an increase in overvoltage, and is suitable for use in the present invention. Absent.

As the thermosetting resin used in the molding of the present invention, for example, phenol resin, furan resin, epoxy resin and the like can be preferably used, but the shapeability after carbonization is important and the carbonization yield is high. If it is high and the shapeability is high,
It is not limited to the above-mentioned phenol resin, furan resin and epoxy resin.

The carbon material of the present invention is composed of a positive electrode and an organic solvent-based electrode.
It can be used in combination with the lysate as appropriate.
These organic solvent-based electrolytes and positive electrodes are passed through a lithium secondary battery.
If it can always be used, limit this
Not something to do. As the positive electrode material, for example, lithium
Metal oxide containing (Li_xMO₂M = Co, Ni,
One or more of Mn), transition metal chalcogenide, vana
Dium oxide (V₂O_Five, V₆O₁₃), And the ammo
Rufus compound, general formula M_xMo₆S _8-y(M = metal)
Chevrel phase compound represented, or activated carbon, conductive
A polymer or the like can be used.

As the organic solvent, for example, propylene carbonate, ethylene 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, pyropionitrile, anisole, diethyl carbonate, dimethyl carbonate,
Dimethyl sulfoxide and the like can be used, and one of them can be used alone, or two or more can be used as a mixed solvent.

As the electrolyte, LiClO ₄ , LiBF _4
4 , LiPF ₆ , LiAsF ₆ , LiCF ₃ SO ₃ , L
Examples thereof include iBr, LiCl, LiB (C ₆ H ₅ ) ₄ .

[0024]

The carbon fiber aggregate according to the present invention is formed without destroying the carbon fiber, so that the electrode characteristics of the carbon fiber are not impaired. Further, by orienting the transition axis parallel to the surface of the sheet, the packing density of the fibers in the carbon fiber aggregate is increased, and by carbonizing or graphitizing the binder component used for molding, the binder component exhibits conductivity. The carbon fiber aggregate according to the present invention is an electrode molded body having high energy density and high electric conductivity. Further, the carbon fiber molded body has flexibility in a direction perpendicular to the fiber axis by, for example, being oriented in one direction, and becomes a rigid molded body by being oriented in two directions perpendicular to each other. That is, the mechanical strength can be designed by controlling the orientation of the fibers.

[0025]

EXAMPLES The present invention will be specifically described below based on Examples and Comparative Examples.

Example 1 Pitch-based carbon fibers whose transition axes are aligned in parallel (fiber diameter of about 1
0 μm, d ₀₀₂ = 0.342 nm) was impregnated with a phenol resin and pre-cured at 80 ° C. to 100 ° C. to prepare a semi-cured prepreg sheet. The two sheets are placed in a mold with their fiber axes aligned, and left at 110 ° C for 20 minutes in the air under a pressure of 100 kg, and then at 150 ° C for 20 minutes.
A cured molded product was produced by processing for minutes. The molded body that had undergone the curing process was taken out of the mold, and further heated in an argon atmosphere at 100 ° C./hour and treated at 1,000 ° C. for 5 hours to perform carbonization. The size of the molded body after carbonization was a square having a side of 150 mm, and the thickness was 0.12 mm. The bulk density of the molded body after carbonization is 1.20 g.
/ Cm ³ , the shapeability was high, and a flexible, blind-shaped molded body could be produced. The surface of this molded body is
Observation with a device such as an electron beam scanning microscope revealed that the carbon fibers were exposed and were not covered with the binder.

This molded body was cut into a square thin plate having a side of 10 mm and wrapped with a Ni net to collect current, which was tested as a negative electrode of a lithium secondary battery. In the performance test, a triode cell having lithium metal as the counter electrode and the reference electrode was used. A solution prepared by dissolving LiClO ₄ at a concentration of 1 mol in 1 liter of a propylene carbonate solvent was used as an electrolytic solution. Using this three-electrode cell, the potential of the test electrode is 0 V or more with respect to the reference electrode 1
Charging and discharging were repeated at a constant current in the range of V or less to evaluate the performance. That is, the lithium insertion reaction, which is the reduction reaction of the test electrode, was terminated at 0 V, and the lithium release reaction, which was the oxidation reaction, was terminated at 1 V. The charge / discharge current density was 30 mA per 1 g of the carbon fiber molded body.

The discharge capacity of the first cycle is about 180 mA.
The h / g and charge / discharge efficiency were about 90%. Also, the third
The charge / discharge efficiency after the cycle was 98% or more and 100% or less. FIG. 1 shows the cycle change of the discharge capacity with repeated charging and discharging. The capacity decreased linearly in the initial 30 cycles, and the capacity thereafter became stable. The discharge capacity at the 100th cycle was 155 mAh / g.

Example 2 The molded body after carbonization used in Example 1 was further subjected to an argon atmosphere at 2,800 ° C., 3,000 ° C. and 3,200 ° C.
Were graphitized for 1 hour each. The thickness of the molded body after graphitization is about 0.13 mm, and the bulk density is 1.
It was 08 g / cm ³ . The shapeability due to the binder did not change due to graphitization, and it was possible to prepare a blind-shaped electrode sheet having the same flexibility as in Example 1. The electrolyte solution was prepared by dissolving electrolyte LiClO ₄ at a concentration of 1M in a mixed solvent of ethylene carbonate and γ-butyrolactone at a volume ratio of 1: 1 and was tested under the same conditions as in Example 1.

Table 1 shows the carbon spacing (d
₀₀₂ ), discharge capacity and charging / discharging efficiency. By increasing the graphitization temperature, the capacity of the molded body increases. This is because the degree of graphitization of the carbon fiber itself is improved by the graphitization treatment, and the capacity is increased accordingly. Figure 1
Shows the cycle change of the discharge capacity of 3,200 ° C. treatment. The characteristics regarding the charge / discharge cycle showed the same tendency as in Example 1.

[0031]

[Table 1]

Example 3 A prepreg sheet similar to that of Example 1 was prepared, two sheets were laminated so that the fiber axis directions thereof were orthogonal to each other, and the subsequent curing and carbonization treatments were performed under the same conditions as in Example 1. Further, it was graphitized at 3,200 ° C. for 1 hour to prepare an electrode molded body. The size of the molded body is a square with a side of 10 mm, the thickness after carbonization is 0.15 mm, and the thickness after graphitization is 0.16 m.
m, the bulk density after carbonization was 1.15 g / cm ³ , and the bulk density after graphitization was 1.05 g / cm ³ . The molded body is
The carbon fibers were exposed on the surface and were not covered with the binder. In addition, there was a slight expansion in the thickness direction during the graphitization process, which reduced the bulk density, but the shapeability was high in any state after carbonization and after graphitization.

Electrolyte L was added to a mixed solvent of propylene carbonate and ethylene carbonate in a volume ratio of 1: 1.
An electrode evaluation test was conducted on this carbon fiber molded body under the same conditions as in Example 1 except that iClO ₄ dissolved in 1 M was used. Table 2 shows the carbon plane spacing (d ₀₀₂ ) by X-ray diffraction, the discharge capacity, and the charge / discharge efficiency. By the graphitization treatment as in Example 2, the capacity of the molded body was significantly increased. There is a difference in the absolute value of the capacitance for both electrodes,
It showed very stable cycle characteristics and was found to be a good electrode.

[0034]

[Table 2]

Example 4 A sheet-shaped electrode was produced by impregnating a biaxially woven cloth of carbon fiber bundles with phenol and carrying out the same curing and carbonization treatment steps as in Example 1. Molded body is 100 mm x 70 m
It has a size of m × 0.32 mm and a bulk density of 1.00.
It was g / cm ³ . Further, the carbonized product was fired at 3,200 ° C. for 1 hour in an argon atmosphere. The bulk density after graphitization was 1.00 g / cm ³ , and there was no change in the bulk density due to graphitization. Both of the two kinds of electrode samples had high shapeability. The electrodes were evaluated under the same conditions as in Example 3. The results are shown in Table 3. It showed very stable cycle characteristics and was found to be a good electrode.

[0036]

[Table 3]

Comparative Example 1 The electrode characteristics of a molded body which had been cured under the same conditions as in Example 1 but had not been carbonized were examined. An electrode having the same size as in Example 1 was cut out and wrapped with a Ni net to give a negative electrode.
The test cell and test conditions were the same as in Example 1. The profile of the charge / discharge curve was very similar to that of Example 1, but the overvoltage was large, the discharge capacity in the first cycle was 120 mAh / g, and the charge / discharge efficiency was 78%.
Charge / discharge efficiency is 98% or more and 100% after the 5th cycle
It was below. The discharge capacity monotonously decreased in the initial 40 cycles and then remained stable, but the discharge capacity at the 100th cycle was 80 mAh / g.

Comparative Example 2 The same carbon fiber as in Example 1 was used, and the carbon fiber bundled with Ni wire without molding was used as a negative electrode and tested in the same test cell and test conditions as in Example 1. The discharge capacity of the first cycle is 185
The charge / discharge efficiency was 93% at mAh / g. The profile of the charge / discharge curve in the first cycle was the same as in Example 1. The cycle change of charge / discharge efficiency and discharge capacity is shown in Example 1.
Was equivalent to. The discharge capacity at the 100th cycle is 16
It was 0 mAh / g. From the results of Comparative Example 2 and the results of Example 1, it can be seen that the molded body of Example 1 sufficiently exhibits the electrode performance of the constituent carbon fibers.

[0039]

According to the present invention, by controlling the arrangement of the carbon fibers in the negative electrode of a lithium secondary battery using carbon fibers, the characteristics of the carbon fibers as a negative electrode for lithium secondary batteries are not impaired. Therefore, an electrode having a high energy density can be obtained, and an electrode having a new structure molded to a high packing density can be provided. Further, according to the method of the present invention, the negative electrode of the lithium secondary battery using such carbon fiber can be easily manufactured.

[Brief description of drawings]

FIG. 1 shows a molded body obtained by curing with a resin and then carbonizing in Example 1, and 3, 20 in Example 2.
It is explanatory drawing which shows the cycle change of the discharge capacity of the molded object obtained by baking at 0 degreeC.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Kojin Suzuki, Inventor 1618 Ida, Nakahara-ku, Kawasaki City, Kanagawa Prefecture, inside the Advanced Technology Research Laboratories, Nippon Steel Corporation (72) Koichiro Mukai, Ida, Nakahara-ku, Kawasaki City, Kanagawa Prefecture 1618, Nippon Steel Corporation Advanced Technology Research Laboratories (72) Inventor Yoetsu Yoshihisa 6-6 Josaimachi, Takatsuki City, Osaka Prefecture, Yuasa Corporation (72) Inventor Kazuya Kuriyama Takatsuki City, Osaka Prefecture 6-6 Josaimachi, Yuasa Corporation

Claims (3)

[Claims] 1. A negative electrode for a lithium secondary battery, which is a sheet-shaped carbon fiber-containing carbon material, wherein the carbon fibers are oriented parallel to the surface of the sheet. 2. The negative electrode for a lithium secondary battery according to claim 1, wherein most of the carbon fibers are exposed on the surface of the sheet. 3. The lithium carbon according to claim 1, wherein the sheet-shaped carbon fiber aggregate in which carbon fibers are oriented parallel to the surface thereof is molded using a binder, and then the binder is carbonized or graphitized. Manufacturing method of negative electrode for secondary battery.