JPH05144440A - Carbon for negative electrode of lithium secondary battery - Google Patents

Abstract

PURPOSE:To enhance the cycle characteristics by using carbon with the oxygen content reduced to a negative electrode of lithium secondary battery. CONSTITUTION:Carbon having undergone adjustment through such a process as to reduce the oxygen content is used to a negative electrode of a lithium secondary battery. This type of carbon is prepared by subjecting the starting material to a processing in a reductive atmosphere in a temp. range between 400-3000 deg.C. Such a processing draws out, oxygen atoms of such an oxygen- containing radical existing at the surface of the carbon as carbonyl, carboxyl, hydroxyl, and ether radical, which enables to obtain carbon which oxygen atoms do not exist at the surface substantially. This permits decreasing remarkably the amount of lithium which does not commit itself in the battery reactions. Thus the lithium as battery active substance can be utilized effectively.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to carbon for a negative electrode of a lithium secondary battery, and particularly to carbon used for the negative electrode to obtain a lithium secondary battery having excellent energy density, discharge characteristics, cycle characteristics and the like.

[0002]

2. Description of the Related Art So-called lithium secondary batteries using lithium as a negative electrode active material, metal chalcogenides or metal oxides as a positive electrode active material, and aprotic organic solvent solutions of various salts as an electrolyte have a high energy density. As a type of rechargeable battery, it has attracted attention and is being actively researched.

However, in a conventional lithium secondary battery, lithium as a negative electrode active material is often used as a simple substance such as a foil, and dendritic lithium is deposited and both electrodes are short-circuited during repeated charging and discharging. Therefore, it has a drawback that the cycle life of charging and discharging is short.

Therefore, aluminum, lead, cadmium,
A method has been proposed in which a fusible alloy appropriately containing indium or the like is used to deposit lithium as an alloy during charging and dissolve lithium from the alloy during discharging.
[US Patent No. 400249]. However, with such a method, the deposition of dendritic lithium can be suppressed, but the fusible alloy is present, so the energy density is reduced.

[0005] Attempts have also been made to support lithium on carbon for the purpose of improving discharge capacity while avoiding reduction in energy density. For example, various fibrous or powdery carbon-supporting materials are used. Used as [Japanese Patent Application Laid-Open No. 2-66856, Japanese Patent Application Laid-Open No. 63-1
14056, etc.]. It is speculated that this is because lithium ions are intercalated between carbon layers to store lithium in carbon.

However, even in such a lithium secondary battery, there is a problem due to dirt and impurities on the surface of carbon used as a carrier. That is, there arises a problem that the discharge capacity is insufficiently increased due to an irreversible chemical reaction on the carbon surface, and the charge / discharge efficiency in the initial cycle is not good.

[0007]

The present invention has been made in order to overcome the above-mentioned drawbacks of lithium secondary batteries containing carbon as a supporting material.

[0008]

As a result of intensive studies, the present inventors have found that the above problems can be solved by reducing the amount of oxygen contained in carbon,
The present invention has been completed based on this finding.

That is, the finding that the present inventors have found by chance is the fact that the discharge characteristics and cycle characteristics of the obtained lithium secondary battery are improved as the oxygen content of carbon used as a lithium supporting material is reduced. However, this will be easily understood from the examples described later.

Therefore, the gist of the present invention is to incorporate carbon for a lithium secondary battery negative electrode prepared through a process for reducing oxygen content, a lithium secondary battery negative electrode using the carbon, and the negative electrode. Exists in lithium secondary batteries.

The carbon of the present invention is not particularly limited as long as it has undergone a process or history that reduces the oxygen content as described above, but normally the oxygen content is 0.015.
% (Weight) or less. Such carbon is prepared by, for example, treating in a temperature range of 400 to 3000 ° C. under a reducing atmosphere. That is, carbonization or graphitization may be performed in an atmosphere containing a gas that can react with oxygen such as hydrogen or carbon monoxide. This gas may be only a gas capable of reacting with oxygen such as hydrogen or carbon monoxide, or a gas in which an inert gas such as nitrogen, argon or helium is mixed in an appropriate ratio. When the above-mentioned treatment is performed, the oxygen atoms of the oxygen-containing groups such as carbonyl group, carboxyl group, hydroxyl group, and ether group existing on the carbon surface are extracted, resulting in the carbon having almost no oxygen atoms on the surface. Will be obtained.

The oxygen-containing groups as described above existing on the carbon surface react with the oxygen atoms that enter the lithium ions during charging to form lithium oxide, and are no longer intercalated between the carbon layers. It is considered that if a treatment for reducing oxygen-containing groups on the carbon surface is performed, the amount of lithium that does not participate in such battery reactions can be significantly reduced, and it is possible to effectively use lithium as a battery active material. Be done. The reduction of oxygen-containing groups on the carbon surface also has the effect of drastically reducing the amount of water strongly adsorbed to the surface by hydrogen bonding and shortening the time required for the carbon drying step. This effect is a great merit in actually manufacturing a battery.

[0013]

EXAMPLES The present invention will be described in more detail with reference to the following examples. [Preparation of carbon material] 500 g of coal tar pitch [softening point (Mettler method) = 100 ° C., quinoline insoluble matter (QI component) = 0%, benzene insoluble matter (BI component) = 18.3%] in 1 L
Charge the autoclave and add 1% of nitrogen gas containing 2% oxygen.
70% at 380 ° C with stirring while blowing at a rate of 0 L / min.
Heat treated for minutes. The properties of the obtained heat-treated pitch are as follows: softening point (Mettler method) = 262.4 ° C., QI component = 15.5%,
BI component = 72.2%.

Optically anisotropic pitch-based fibers were obtained by spinning this pitch from a nozzle. Next, the pitch fiber was heated to 300 ° C. at a rate of 3 ° C./min in air, and then held at the same temperature for 2 hours to be infusibilized. Then, it heated and baked in carbon monoxide gas at 1000 degreeC, and obtained the pitch type | system | group carbon fiber. The oxygen content of this pitch-based carbon fiber was 0.015% (weight) or less.

[Preparation of Negative Electrode Body] The pitch-based carbon fiber thus obtained was used as a cathode, and lithium metal was used as an anode.
A negative electrode was prepared by carrying out cathode reduction using propylene carbonate in which LiClO ₄ was dissolved at a concentration of mol / L under conditions of a current density of 0.5 mA / cm ² and an electrolysis time of 12 hours to support lithium.

[Preparation of Battery] In addition to the negative electrode body obtained above, electrolytic manganese dioxide is used as the positive electrode body, and propylene carbonate in which LiClO ₄ is dissolved at a concentration of 1 mol / L is used as the electrolytic solution. A lithium secondary battery having a size of 2 mm × 20 mmφ was prepared by using the battery constituent element of. The cross-sectional view is shown in the attached FIG. Figure 1
Inside, 1 is a positive electrode body, 2 is a separator, 3 is a negative electrode body, 4 is a case, 5 is a sealing plate, and 6 is an insulating packing.

[Measurement of Battery Characteristics] The discharge characteristics of the lithium secondary battery obtained in this experiment were measured. The measurement is usually
It was carried out under a constant current charge / discharge of 0.4 mA / cm ² . The discharge capacity was the capacity until the battery voltage dropped to 2.0V.

As a control, pitch-based carbon fibers (oxygen content 0.020%) carbonized by heating and firing in argon gas are used.
A lithium secondary battery using (weight) was also measured under the same conditions. The results are shown in Table 1.

[0019]

[Table 1] The discharge capacity is a value of 1ST cycle, and the number of cycles is shown as a value until the discharge capacity is reduced to 60% of the discharge capacity of 1ST cycle.

As is clear from Table 1, the oxygen content is 0.0
Compared with a lithium secondary battery using a carbon material of 20%, a lithium secondary battery using a carbon material having an oxygen content of 0.015% or less is excellent in discharge capacity and cycle characteristics.

[0021]

EFFECTS OF THE INVENTION The present invention provides an electrode for a secondary battery in which the discharge capacity per unit volume (weight) is increased and the cycle characteristics are improved.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of a lithium secondary battery created in an example.

[Explanation of symbols]

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

Claims (4)

[Claims] 1. A carbon for a lithium secondary battery negative electrode, which is prepared through a process of reducing an oxygen content. 2. A negative electrode for a lithium secondary battery using the carbon according to claim 1. 3. A lithium secondary battery incorporating the negative electrode according to claim 2. 4. Carbon according to claim 1, having an oxygen content not exceeding 0.015% by weight.