JPH05275077A - Negative electrode for lithium secondary battery - Google Patents

Abstract

PURPOSE:To enabel both use of a carbon material seving as a negative electrode and use of propylene carbonate serving as at least a part of an organic solvent in an electrolytic solution by coating the surface of the carbon material with a thin film of a lithium ion conductive solid electrolyte. CONSTITUTION:A lithium secondary battery is composed of a positive electrode body 1, a separator 2, a negative electrode body 3, a case 4, a sealing plate 5, an insulating packing 6, and an electrolytic solution. The negative electrode body 3 is manufactured as follows : Namely, 99 parts by weight of a sintered body of a mesophase pitch and one weight part of a dispersion of PTFE are uniformly agitated and dried in a liquid phase into a paste-like state. The obtained negative electrode substance is press-fitted onto a nickel mesh so as to form a carbon electrode, followed by vacuum-drying. Next, the obtained negative electrode body 3 is used as a working electrode, and a lithium electrode is used as its counter electrode, lithium is stored in the negative electrode body 3 until its potential becomes zero volts.

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 excellent in energy density, discharge characteristics, cycle characteristics and the like, and a negative electrode material used therein.

[0002]

2. Description of the Related Art Lithium is used as a negative electrode active material, metal chalcogenide or metal oxide is used as a positive electrode active material, and a solution prepared by dissolving various salts in an aprotic organic solvent is used as an electrolytic solution. BACKGROUND ART A lithium secondary battery has attracted attention as a high energy density secondary battery and has been actively studied.

In a conventional lithium battery, lithium as a negative electrode active material is often used as a foil-shaped simple substance,
When charging and discharging are repeated, dendritic lithium is deposited and both electrodes are short-circuited, which has a drawback that the cycle life of charging and discharging is short.

In order to prevent the deposition of dendritic lithium,
A method has been proposed in which a fusible alloy containing aluminum or lead, cadmium, and indium is used as a negative electrode active material, lithium is deposited as an alloy during charging, and lithium is dissolved from this alloy during discharging (US Pat. No. 4,002492). No.). However, according to such a method, although the deposition of dendritic lithium can be suppressed, the energy density of the battery is lowered.

Further, in order to improve the discharge capacity, it has been attempted to support lithium on a carbon material. For example, it has been proposed to support lithium on a fibrous or powdery carbon material (Japanese Patent Laid-Open No. 63-11).
No. 4056, JP-A No. 62-268056). However, in a lithium secondary battery that uses a carbon material as a lithium carrier, it is extremely difficult to obtain an optimum combination of the carbon material and the electrolytic solution. That is, as an organic solvent used alone, propylene carbonate (propylene carbonate) has excellent properties such as relative permittivity and operating temperature range, and an organic solvent that shows properties superior to propylene carbonate as a whole is: Not found. However, regarding propylene carbonate, when a highly crystalline carbon material is used as an electrode, the problem that decomposition of propylene carbonate occurs during reduction has been pointed out (J. Electrochem. Soc., 117 (2)). , 222 (1970)
). Highly crystalline carbon materials have a developed layered structure, and considering the intercalation of lithium ions, the amount of lithium that can be supported (corresponding to the capacity) is considered to be large. It is preferable that it can be used as a negative electrode of a lithium secondary battery in combination with a liquid.

[0006]

Accordingly, the present invention provides an improved lithium secondary battery that uses a carbon material as the negative electrode and propylene carbonate as at least a portion of the organic solvent of the electrolyte. The main purpose is.

[0007]

The inventors of the present invention have made extensive studies in view of the above-mentioned state of the art, and as a result, use a carbon material as a negative electrode and propylene carbonate as at least one of the organic solvents of the electrolytic solution. In the lithium secondary battery used as the part, by coating the surface of the carbon material used as a constituent element of the negative electrode with a thin film of a lithium-ion conductive solid electrolyte, the problems of the prior art are substantially eliminated or greatly reduced. It was found to be reduced to.

That is, the present invention provides the following lithium secondary battery and its negative electrode material: A negative electrode for a lithium secondary battery in which the surface of a carbon material used as a component of the negative electrode is coated with a thin film of a lithium ion conductive solid electrolyte.

2. Item 1. The crystallite size (L _c ) of the carbon material in the C-axis direction is 300 Å or more.
The negative electrode for the lithium secondary battery according to.

3. A lithium secondary battery in which a material obtained by coating the surface of a carbon material with a thin film of a lithium ion conductive solid electrolyte serves as a constituent element of a negative electrode.

4. Item 3. The carbon material has a crystallite size (L _c ) in the C-axis direction of 300 angstroms or more.
The lithium secondary battery described in.

5. Item 4. The lithium secondary battery according to Item 3, wherein propylene carbonate is used as at least a part of the organic solvent of the electrolytic solution.

In the present invention, the origin of the carbon material used as the basic constituent element of the negative electrode (pitch-based, petroleum-based, P-based
AN type, etc.), type (carbon fiber, graphitized carbon fiber, etc.), form (powder, fibrous form, pellet, molded body such as electrode, etc.) are not particularly limited, but use of propylene carbonate as the electrolytic solution. Considering the above, a highly crystalline carbon material graphitized at a high temperature of 2000 ° C. or higher is particularly preferable. In terms of crystallite size, a carbon material having a crystallite size (Lc) in the C-axis direction of at least 300 Å is particularly preferable. Graphitization temperature is 20
When the temperature is lower than 00 ° C., the crystallite size (L _c ) in the C-axis direction of the carbon material becomes smaller than 300 angstrom, and propylene carbonate used as a solvent during charging tends to be decomposed.

In the present invention, a thin film of lithium ion conductive solid electrolyte is formed on the surface of the above carbon material. Such solid electrolytes include polyethylene oxide (PEO), LiI, LI ₃ N, Li ₅ AlO ₄ , L
_{i 5 GaO 4, Li 5 FeO} 4, Li-Na-β- _{alumina, LiAlSiO 4, Li 4 Zn (} GeO 4) 4,
Li ₁₁ N ₃ Cl ₂ , Li ₆ NBr ₃ , Li ₁₃ N ₄ Br,
Examples include Li ₅ NI ₂ . From the viewpoint of the diffusion rate by ionic conduction, polyethylene oxide is most preferable.

The method for forming the thin film is not limited as long as such a thin film of a lithium ion conductive solid electrolyte is formed, but an adsorption method in which the solid electrolyte is mixed with a solvent and applied is suitable. Covalent bond method of covalently bonding a functional group of a polymer (PEO or the like) to which a functional group has been imparted by various surface treatments and a functional group on the surface of a carbon material through an amide bond, an ether bond or a siloxane bond; Examples thereof include an electrolytic deposition method and a gas vapor deposition method in which a polymer compound is vaporized by heating and vapor-deposited on the electrode surface.

The thickness of the thin film is not particularly limited, but is usually about 0.01 to 10 μm.

As described above, when propylene carbonate is used as the organic solvent of the electrolytic solution and a highly crystalline carbon material is used for the electrode, the propylene carbonate is easily decomposed during the reduction, so that it is used as it is. Is impossible. However, when a lithium ion conductive solid electrolyte thin film is applied to the surface of such a carbon material, direct contact between the carbon electrode and the electrolytic solution is avoided, so that the decomposition of propylene carbonate is suppressed. Meanwhile, it becomes possible to proceed only the electrochemical reaction. In addition, lithium ions are prevented from intercalating (co-intercalating) between carbon layers in a solvated state, and the effect of increasing the capacity per unit weight is also achieved.

The carbon material on which the thin film of the lithium ion conductive solid electrolyte according to the present invention is formed is provided with lithium according to a conventional method and used as a negative electrode active material of a lithium secondary battery.

[0019]

According to the present invention, it is possible to obtain a high-performance lithium secondary battery using propylene carbonate as at least a part of the organic solvent of the electrolytic solution and a highly crystalline carbon material as the negative electrode material. ..

[0020]

EXAMPLES Examples will be shown below to further clarify the features of the present invention.

Example 1 Production of Negative Electrode Fired body of mesophase pitch (heat treatment temperature 3000 ° C.)
99 parts by weight and 1 part by weight of PTFE dispersion (D-1, manufactured by Daikin Industries, Ltd.) (as a solid content) were uniformly mixed and stirred in a liquid phase and then dried to obtain a paste. 3 mg of the negative electrode material thus obtained was pressed onto a nickel mesh to prepare a carbon electrode, which was vacuum dried at 200 ° C. for 6 hours.

On the other hand, 10% by volume of propylene oxide was dissolved in propylene carbonate in which LiClO ₄ was dissolved at a concentration of 1 mol / l to prepare an electrolytic solution, and then the above carbon electrode was used as a working electrode. Using a platinum electrode as a counter electrode and a silver-silver chloride electrode as a reference electrode, constant potential electrolysis of 1.5 V was performed. At the end of the polymerization, the time when the current stopped flowing was obtained, and then the electrolyte solution used was washed three times.

Next, the obtained negative electrode body was used as a working electrode, and lithium electrodes were used as a counter electrode and a reference electrode, and lithium was absorbed in the negative electrode body until the potential became 0V. The conditions (electrolyte solution, current density, etc.) in this operation were the same as the measurement conditions of the battery characteristics performed later.

Preparation of Battery Next, a lithium secondary battery shown in a sectional view in FIG. 1 was prepared using the following constituent materials.

Positive electrode body 1 ... Electrolytic manganese dioxide separator 2 ... Polypropylene nonwoven fabric Negative electrode body 3 ... What was obtained above Electrolyte solution ... LiClO ₄ dissolved in propylene carbonate at a concentration of 1 mol / l Propylene carbonate In FIG. The battery includes a case 4, a sealing plate 5, and an insulating packing 6 as components other than the above.

Measurement of Battery Characteristics In order to investigate the discharge characteristics of the lithium secondary battery obtained above, charging / discharging was performed under a constant current condition of 50 mA / g (negative electrode carbon standard). Battery capacity is 2.0V
It was set as the capacity until it decreased.

As a control, a mesophase pitch sintered body (heat treatment temperature of 1000 ° C., 2000 ° C.) similar to the above, which does not form a thin film of a lithium ion conductive solid electrolyte, is formed.
The battery characteristics were measured under the same conditions for the conventional lithium secondary battery using the negative electrode body that uses a negative electrode body and a negative electrode body that uses 3000 ° C.).

The results are shown in Table 1.

Table 1 Heat treatment temperature L _c Discharge capacity (Ah / kg) (° C.) (angstrom) 1 cycle 10 cycles Example 1 3000 580 300 300 290 Control 1 1000 20 20 250 140 Control 2 2000 300 300 125 95 Control 3 3000 580 00 0 Note: Control 3 could not be measured due to solvent decomposition.

As is clear from the results shown in Table 1, a highly crystalline carbon material having a crystallite size (Lc) in the C-axis direction of 300 angstroms or more is used as a negative electrode material coated with a lithium ion conductive solid electrolyte. According to the present invention, unlike conventional lithium secondary batteries, propylene carbonate is used as at least a part of the organic solvent of the electrolytic solution to provide high performance lithium with unprecedented high discharge characteristics and cycle characteristics. A secondary battery can be obtained.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of the lithium secondary battery of the present invention obtained in Example 1.

[Explanation of symbols]

 1 ... Positive electrode 2 ... Separator 3 ... Negative electrode 4 ... Case 5 ... Sealing plate 6 ... Insulating packing

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[Procedure amendment]

[Submission date] April 1, 1992

[Procedure Amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0014

[Correction method] Change

[Correction content]

In the present invention, a thin film of lithium ion conductive solid electrolyte is formed on the surface of the above carbon material. Such solid electrolytes include polyethylene oxide (PEO), LiI, Li ₃ N , Li ₅ AlO ₄ , L
_{i 5 GaO 4, Li 5 FeO} 4, Li-Na-β- _{alumina, LiAlSiO 4, Li 4 Zn (} GeO 4) 4,
Li ₁₁ N ₃ Cl ₂ , Li ₆ NBr ₃ , Li ₁₃ N ₄ B
r, Li ₅ NI _{2 and the} like are exemplified. From the viewpoint of the diffusion rate by ionic conduction, polyethylene oxide is most preferable.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shunichi Higuchi 5-8-2, Makihama, Minoh City, Osaka Prefecture 2-212 (72) Inventor Akihiro Mabuchi 4-1-2, Hiranocho, Chuo-ku, Osaka City, Osaka Prefecture Osaka Gas Stock Inside the company (72) Inventor Yoshiteru Nakagawa 4 1-2 1-2 Hirano-cho, Chuo-ku, Osaka City, Osaka Prefecture Osaka Gas Co., Ltd.

Claims (5)

[Claims] 1. A negative electrode for a lithium secondary battery, wherein a surface of a carbon material used as a constituent element of the negative electrode is coated with a thin film of a lithium ion conductive solid electrolyte. 2. A crystallite size (L
The negative electrode for a lithium secondary battery according to claim 1, wherein _c ) is 300 angstroms or more. 3. A lithium secondary battery comprising a material obtained by coating the surface of a carbon material with a thin film of a lithium ion conductive solid electrolyte as a constituent element of a negative electrode. 4. A crystallite size (L
The lithium secondary battery according to claim 3, wherein _c ) is 300 angstroms or more. 5. The lithium secondary battery according to claim 3, wherein propylene carbonate is used as at least a part of the organic solvent of the electrolytic solution.