JP2000311686A - Lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the cycle life characteristic of a battery by covering the grain surface of an amorphous carbon material as a negative electrode active material with an oxide film. SOLUTION: The grain surface of an amorphous carbon material is covered with an oxide film in a lithium secondary battery using the amorphous carbon material as a negative electrode active material and lithium manganate as a positive electrode active material. This oxide film suppresses the formation of a film inactive for the intrusion/desorption of lithium and does not cause extreme deterioration at a high temperature, and it improves the cycle life characteristic of the lithium secondary battery. When the oxygen/carbon weight ratio (O/C ratio) of the oxide film is below 0.02, the covering effect on the grain surface is small. When the 0/C ratio exceeds 0.07, the oxide film itself impairs the intrusion/desorption of lithium or causes a side reaction with an electrolyte, deterioration occurs on other performance, and the O/C ratio is preferably set to 0.02-0.07.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a lithium secondary battery, and more particularly to a lithium secondary battery using an amorphous carbon material as a negative electrode active material and lithium manganate as a positive electrode active material.

[0002]

2. Description of the Related Art In the field of rechargeable secondary batteries, aqueous batteries such as lead batteries, nickel-cadmium batteries, and nickel-hydrogen batteries have hitherto been the mainstream. However, due to global warming and exhausted fuel, electric vehicles (E
V) and hybrid vehicles, in which a part of the drive is assisted by an electric motor, have attracted attention, and batteries with higher capacity and higher output have been demanded for the batteries used for the power sources. As a power source meeting such a demand, a non-aqueous solution type lithium secondary battery having a high voltage has attracted attention.

[0003] A carbon material is generally used as a negative electrode material of a lithium secondary battery, and this carbon material is a graphite material such as artificial graphite such as natural graphite, scaly or massive, and mesophase pitch graphite. And an amorphous carbon material obtained by firing furan resin such as furfuryl alcohol. Graphite-based materials are characterized by low irreversible capacity, flat voltage characteristics and high capacity, but have the problem of poor cycle life characteristics. Amorphous carbon obtained by calcining a synthetic resin has the characteristic that it has a capacity greater than the theoretical capacity of graphite and has excellent cycle life characteristics, but has the disadvantage that it has a large irreversible capacity and it is difficult to increase the capacity in batteries. is there.

[0004] Lithium transition metal oxide is used for the positive electrode material. Among them, lithium cobalt oxide is used because of its balance in capacity and cycle characteristics. However, the amount of resources of cobalt as a raw material is small and the cost is high. Therefore, lithium manganate is promising as a material for batteries for electric vehicles and hybrid vehicles, and is being developed.

[0005]

However, the cycle life and storage life of a secondary battery using lithium manganate as a positive electrode material at a high temperature are not always sufficient when used for an electric vehicle. I can't say. That is, the cycle life becomes extremely short even at a high temperature of about 50 ° C. as compared with the normal temperature.

On the other hand, various proposals have been made to improve the cycle life characteristics by replacing a part of the manganese atoms in the lithium manganate crystal with a dissimilar metal such as cobalt or chromium. Although recognized, this is not enough.

The present invention has been made in view of the above circumstances, and has as its object to provide a lithium secondary battery using amorphous carbon as a negative electrode and lithium manganate as a positive electrode, the lithium secondary battery having improved cycle life characteristics. .

[0008]

To achieve the above object, the present invention provides a lithium secondary battery comprising an amorphous carbon material as a negative electrode active material and lithium manganate as a positive electrode active material. The carbon material is characterized in that the particle surface is coated with an oxide film. In the present invention, amorphous carbon having a particle surface coated with an oxide film is used as a negative electrode active material. This oxide film suppresses the formation of a film that is inactive against lithium insertion / desorption, and does not cause extreme deterioration even at high temperatures, so that the cycle life characteristics of the lithium secondary battery can be improved. Can be. At this time, if the weight ratio (O / C ratio) of oxygen to carbon (O / C ratio) of the oxygen film is less than 0.02, the effect of coating the particle surface is small, and if the O / C ratio exceeds 0.07, the oxygen coating itself becomes lithium. The O / C ratio is preferably set to 0.02 or more and 0.07 or less, since the insertion / desorption is prevented or a side reaction occurs with an electrolytic solution or the like to cause deterioration in other performance aspects.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of a lithium secondary battery to which the present invention is applied will be described in detail while comparing with a comparative example manufactured to confirm the effects of the embodiments.

(Example 1) <Method of Manufacturing Battery> [Negative Electrode] First, in order to manufacture a negative electrode, amorphous carbon particles were dispersed in a dry air stream at 80 ° C. for a predetermined time. An oxide film was formed on the surface of the particles. The weight ratio (O / C ratio) of this oxide film to oxygen was set to 0.01. In addition,
O / C ratio is XPS (Xray Photoelectron Spectroscop)
y, X-ray photoelectron spectroscopy).

To 90 parts by weight of the amorphous carbon powder as the negative electrode active material having an oxide film formed on the particle surface, 10 parts by weight of polyvinylidene fluoride as a binder was added to the negative electrode active material, and the dispersion solvent was added thereto. The slurry obtained by adding and kneading N-methylpyrrolidone was applied to both sides of a rolled copper foil having a thickness of 10 μm, and then dried, pressed and cut to a thickness of 7 μm.
A negative electrode of 0 μm was obtained.

[Positive Electrode] Lithium manganate (Li / Mn composition ratio = 0.58) as a positive electrode active material, 10 parts by weight of flake graphite with respect to 100 parts by weight of a positive electrode active material as a conductive material, and a binder 5 parts by weight of polyvinylidene fluoride, N-methylpyrrolidone as a dispersing solvent was added thereto, and the kneaded slurry was applied to both sides of a 20-μm-thick aluminum foil, and then dried, pressed and cut to a thickness of 70 μm. Was obtained.

[Preparation of Battery] The positive and negative electrodes prepared as described above are wound through a 25 μm-thick polyethylene separator to form an electrode group. The electrode group is inserted into a cylindrical battery container, and the electrolytic solution is discharged. After pouring a predetermined amount, the upper lid was swaged and sealed to obtain a cylindrical lithium secondary battery. As the electrolytic solution, a solution prepared by dissolving 1 mol / liter of lithium hexafluorophosphate (LiPF ₆ ) in a mixed solution of ethylene carbonate and dimethyl carbonate was used. The capacity of this lithium secondary battery is 4.0 Ah.

Examples 2 to 8 The negative electrodes used in Examples 2 to 8 were changed by changing the O / C ratio of the oxygen film formed on the surface of the amorphous carbon particles as shown in Table 1 below. Produced. A lithium secondary battery was assembled by the same method using the same positive electrode, separator, and electrolyte as in Example 1 except for the negative electrode.

[0015]

[Table 1]

(Comparative Example 1) Amorphous carbon (O / C ratio = 0) not subjected to an oxide film forming process was used. A lithium secondary battery was assembled in the same manner.

<Test / Evaluation> [Test] Next, a high-temperature cycle life test and an output test were performed on the batteries of the examples and the comparative examples thus manufactured. In the high-temperature cycle life test, after the initial capacity stabilization operation, constant-current constant-voltage charging (upper limit voltage: 4.1 V) was performed for 4 hours at an hourly rate (1 C) in an atmosphere of 50 ° C.
The conditions were such that DOD = 40% (24 minutes) at 1 hour rate (1C). The life was determined to be 80% of the initial capacity.
In the output test, 1A, 3A, 6A
Discharge at each current value of, and measure the voltage at 5 seconds.
It was determined from the voltage characteristics. In addition, the output ratio of the battery of Comparative Example 1 using the amorphous carbon powder not subjected to the oxide film forming process for the negative electrode was set to 100, and the output ratio of the battery of each example was calculated.

[Test Results] Table 2, FIG. 1 and FIG. 2 show test results of a high-temperature cycle life test and an output test.

[0019]

[Table 2]

[Evaluation] The amount of oxygen on the surface of the amorphous carbon
Examples 2 to 7 where the / C ratio was 0.02 or more and 0.07 or less
All of the batteries exhibited a good cycle life of 150 cycles or more at a high temperature of 50 ° C. In addition, the batteries of these examples maintained an output of 85% or more as compared with the batteries of Comparative Example 1 in which the oxide film was not formed. In the battery of Example 1, although the cycle life was improved,
While the batteries of the other examples exhibited a cycle life more than twice that of the battery of Comparative Example 1, the improvement in the cycle life was not so large at about 30%. In the battery of Example 8, the cycle life was greatly improved, but the output was also greatly reduced. This is considered to be due to the fact that when the O / C ratio is 0.08, the electrode resistance is rapidly increased due to the formed film. On the other hand, the battery of Comparative Example 1 had a cycle life of about 70 cycles, and was significantly inferior to the batteries of Examples in which an oxide film was formed.

From the above results, in the lithium secondary battery using the amorphous carbon powder in which the particle surface is coated with the oxide film as the negative electrode, the O / C ratio is 0.1% as in the batteries of Examples 2 to 7. 02
It was found that when the value was 0.07 or less, the cycle life was significantly improved without lowering the output. Further, the oxide film suppresses the formation of a film that is inactive against insertion / desorption of lithium, so that the cycle life can be improved. In addition, since this oxide film does not undergo extreme deterioration even at a high temperature of 50 ° C., it is considered to be effective during storage.

In this embodiment, the electrolyte is ethylene.
Into a mixed solution of carbonate and dimethyl carbonate 6
1 mol / liter of lithium fluorophosphate dissolved
Was used, but there is no particular limitation on the electrolytic solution and it is usually used.
The same effect as this embodiment was confirmed with the electrolyte
I have. That is, a general lithium salt is used as the electrolyte,
The present invention is also applicable to the use of an electrolyte in which
It can control these lithium salts and organic solvents.
No limit. For example, as the electrolyte, LiClO ₄, Li
AsF₆, LiPF₆, LiBF₄, LiB (C
₆H₅)₄, CH₃SO₃Li, CF₃SO₃Li, etc.
These mixtures can be used. In addition, organic solvents
As the medium, propylene carbonate, ethylene carbonate
Nate, 1,2-dimethoxyethane, 1,2-diethoxy
Cietan, γ-butyrolactone, tetrahydrofuran,
1,3-dioxolan, 4-methyl-1,3-dioxo
Run, diethyl ether, sulfolane, methyl sulfora
, Acetonitrile, propionitrile, etc. or these 2
More than one type of mixed solvent can be used.

[0023]

As described above, according to the present invention,
Since the surface of the amorphous carbon particles is covered with an oxide film, which suppresses the formation of a film that is inactive against lithium insertion / desorption, it does not cause extreme deterioration even at high temperatures. Thus, the effect that the cycle life characteristics of the lithium secondary battery can be improved can be obtained.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the O / C ratio and the number of cycles in the high-temperature cycle life test results of the batteries of Examples and Comparative Examples.

FIG. 2 shows O / O of output test results of batteries of Examples and Comparative Examples.
It is a graph which shows the relationship between C ratio and output ratio.

Continuation of the front page (72) Inventor Kensuke Hironaka 2-7-7 Nihonbashi Honcho, Chuo-ku, Tokyo F-term in Shin-Kobe Electric Co., Ltd. 5H003 AA04 BB01 BC01 BC05 BD04 5H014 AA01 CC01 CC07 EE08 HH01 5H029 AJ05 AK03 AL06 AM03 AM05 AM07 BJ13 HJ01 HJ12

Claims (2)

[Claims] 1. A lithium secondary battery using an amorphous carbon material as a negative electrode active material and lithium manganate as a positive electrode active material, wherein the amorphous carbon material has a particle surface coated with an oxide film. Lithium secondary battery. 2. The lithium secondary battery according to claim 1, wherein a weight ratio (O / C ratio) of oxygen to carbon in the oxygen film is 0.02 or more and 0.07 or less.