JPH0393162A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PURPOSE:To decrease the self-discharge rate of a nonaqueous electrolyte secondary battery using a carbonaceous material in a negative electrode by specifying the crystal size in the direction of c axis in the X-ray diffraction of the carbonaceous material. CONSTITUTION:In a nonaqueous electrolyte secondary battery having a negative electrode 2 comprising a carbonaceous material and a nonaqueous electrolyte in which a lithium salt is dissolved in a nonaqueous solvent, the carbonaceous material whose crystal size (Lc) in the direaction of c axis in the x-ray diffraction is in the range of 73-85Angstrom is used. Small Lc value means low graphitization and impurities are easily contained and self-discharge is increased. Large Lc value means high graphitization and doping-undoping of lithium ions is difficult and discharge capacity is decreased. By specifying the Lc value, self-discharge rate is remarkably decreased without large decrease in discharge capacity.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to improvement of a non-aqueous electrolyte secondary battery using a carbonaceous material as a negative electrode.

[Summary of the invention]

In a nonaqueous electrolyte secondary battery using a carbonaceous material for the negative electrode, by using a carbon material having a crystal thickness Lc in the C-axis direction in X-ray diffraction in the range of 73 Å to 85 Å as the carbonaceous material. The present invention provides a nonaqueous electrolyte secondary battery that has excellent charge/discharge cycle characteristics, a large discharge capacity, and a reduced self-discharge rate.

[Conventional technology]

In recent years, with the spread of portable devices such as video cameras and radio cassette recorders, there has been an increasing demand for reusable secondary batteries instead of disposable primary batteries. Most secondary batteries currently in use are nickel-cadmium batteries that use an alkaline electrolyte. However, since the voltage of this battery is as low as approximately 1.2V, it is difficult to improve the energy density of the battery. Another disadvantage of this battery is that its self-discharge rate at room temperature is more than 20% in one month. Therefore, a secondary battery with a high voltage of 3 V or more and a low self-discharge rate was investigated by using a light metal such as lithium for the negative electrode and a non-aqueous solvent for the electrolyte. However, in such a secondary battery, lithium or the like used in the negative electrode tends to form a dendrite due to repeated charging and discharging, and the negative electrode and positive electrode tend to melt, causing an internal short circuit in the battery.

For this reason, consideration has been given to a secondary battery in which lithium or the like is alloyed with other metals and this alloy is used for the negative electrode.

However, the alloy of the negative electrode of this battery also tends to disintegrate as charging and discharging progresses, making it difficult to put it into practical use. Therefore, a secondary battery using a carbonaceous material such as coke as a negative electrode was proposed, for example, as shown in Japanese Patent Application Laid-Open No. 62-90863. It has been found that this proposal makes it possible to avoid the problem of deterioration of the negative electrode due to repeated charging and discharging as described above. Here, the carbonaceous material is said to act as a negative electrode of a secondary battery by doping and dedoping of alkali metal ions through an electrolytic solution. As described above, secondary batteries with negative electrodes made of carbonaceous materials with excellent charge-discharge cycle characteristics, however, have extremely high self-discharge rates compared to batteries with metal lithium, etc. as negative electrodes, and for this reason have not been put into practical use. is the current situation.

[Problem to be solved by the invention]

In view of the above-mentioned current situation, an object of the present invention is to reduce the high self-discharge rate, which is a problem of non-aqueous electrolyte secondary batteries using carbonaceous materials for the negative electrode.

[Means to solve the problem]

The present invention provides a non-aqueous electrolyte secondary battery comprising a negative electrode having a carbonaceous material and a non-aqueous electrolyte in which a lithium salt is dissolved in a non-aqueous solvent as an electrolyte, wherein the carbonaceous material is
Crystal thickness Lc in the C-axis direction in line diffraction (hereinafter simply referred to as L
A carbonaceous material ranging from 73 Å to 85 Å is used.

If Lc is small, self-discharge tends to be large, and conversely, if LC is large, discharge capacity tends to be small. Considering the balance between the two, it is preferable that Lc is in the range of 73 to 85 people. A small Lc means that graphitization has not progressed very much, and in such a state, impurities are likely to be included, which is thought to increase self-discharge.

On the other hand, a large LC means that graphitization is progressing, and in such a state, it is difficult to dope and dedope lithium ions, which is thought to reduce the discharge capacity. As the positive electrode material used in the present invention, transition metal compounds such as manganese dioxide and vanadium pentoxide, and transition metal chalcogen compounds such as iron sulfide, which are generally used in lithium secondary batteries, can be used. As the electrolyte, a non-aqueous electrolyte in which a lithium salt is dissolved in a non-aqueous solvent can be used. Here, the nonaqueous solvent is not particularly limited, but includes, for example, propylene carbonate, ethylene carbonate, 1,2-dimethoxyethane, 1,2-diethoxyethane, γ-butyrolactone, tetrahydrofuran, 1.
3-dioxolane, 4-methyl-1,3-dioxolane,
Solvents such as diethyl ether, sulfolane, methylsulfolane, acetonitrile, and propionitrile may be used alone or in combination of two or more thereof. Any conventionally known electrolytes can be used, including LiCl04, LiA
sF. , LiPFa, LiBFn, Lid(CJs
)4, LiC1,1iBr, CH3SO3Li, C
There are F3SO1Li, etc.

[Effect]

In a non-aqueous electrolyte secondary battery using a carbonaceous material for the negative electrode, by selecting Lc of the carbonaceous material in the range of 73 Å to 85 Å, the discharge capacity can be reduced not much.
Self-discharge rate can be significantly reduced.

[Example] FIG. 1 is a cross-sectional view schematically showing the structure of a cylindrical non-aqueous electrolyte secondary battery applied in an example of the present invention. This battery is a spiral cylindrical non-aqueous electrolyte secondary battery having an electrode structure in which a positive electrode 1 and a negative electrode 2 are spirally wound with a separator 3 in between, as shown in FIG. An embodiment will be described below with reference to FIG.

Example 1 First, negative electrode 2 was produced as follows. The pulverized needle coke was treated in an electric furnace in an argon atmosphere at 1500° C. for 2 hours to produce a coke with an Lc of 73, which was used as a negative electrode material.

Here, Lc was determined by X-ray diffraction from the 002 diffraction line pattern according to the Japan Society for the Promotion of Science method (Lc was determined in the same manner in all the Examples and Comparative Examples below). The true density of this high temperature treated 21 dollar coke is 2.16
g/cs”, the spacing of the 002 plane in X-ray diffraction is d
o02 (hereinafter simply referred to as 6002) was 3.44λ.

10 parts by weight of polyvinylidene fluoride as a binder was added to 90 parts by weight of this twenty-one dollar coke and mixed to form a negative electrode mixture. This negative electrode mixture was then dispersed in a solvent N-methylpyrrolidone to form a slurry (paste). next,
This negative electrode mixture slurry is used as a negative electrode current collector to a thickness of lO
It was applied to both sides of μm copper foil and dried. After drying, it is compressed using a roller press machine and then compressed for 34.5s+m.
A strip-shaped negative electrode 2 was made by cutting it to a width of .

In this strip-shaped negative electrode 2, is the negative electrode mixture the negative electrode mixture? The film was formed on both sides of the body with approximately the same thickness, and the sum of these film thicknesses was approximately 175 μm.

Next, the positive electrode 1 was produced as follows.

1 mole of lithium carbonate and cobalt carbonate were mixed and burned in air at 900°C for 5 hours to obtain LiCoOz, which was used as a positive electrode active material, and 91 parts by weight of this LiCoO was added with an amount of graphite}Sfi as a conductive material. and 3M parts of polyvinylidene fluoride as a binder were added and mixed to prepare a positive electrode mixture. Then, this positive electrode mixture was dispersed in a solvent N-methylpyrrolidone to form a slurry (paste). Next, this positive electrode mixture slurry was applied to a thickness of 2 as a positive electrode current collector.
It was applied to both sides of a 0 μm strip of aluminum foil and dried. After drying, it was compressed into a shape using a roller press and cut into a width of 3.5+u+ to produce a strip-shaped positive electrode 1.

In this strip-shaped positive electrode 1, the positive electrode mixture was formed on both sides of the positive electrode current collector with approximately the same film thickness, and the sum of these film thicknesses was about 175 μm. and a strip-shaped positive electrode l;
A strip-shaped negative electrode 2 and a separator 3 made of a microporous polypropylene film with a thickness of 25 μm are laminated in the order of negative electrode 2, separator 3, positive electrode 1, and separator 3, and then this laminate is spirally wound many times. A rolled body was created by doing this. As shown in FIG. 1, the volume produced as described above was housed in a nickel-plated iron battery can with an inner diameter of 13.3+ am. Then, in order to collect current from the positive electrode l,
A positive electrode board 8 made of aluminum was attached to the positive electrode 1, led out from the positive electrode 1, and welded to the battery lid 7. In addition, in order to collect current from the negative electrode 2, a nickel negative electrode lead 9 is attached to the negative electrode 2, and this is led out from the negative electrode 2, and the battery can 5
Welded to. An electrolytic solution prepared by mixing propylene carbonate and 1,2-dimethoxyethane in which lithium hexafluorophosphate was dissolved at a concentration of 1 mol/liter was poured into the battery can. Next, an insulating plate 4 was placed inside the battery can 5 so as to face the top and bottom surfaces of the first body. Further, the battery can 5 and the battery lid 7 were caulked together with an insulating sealing gasket 6 interposed therebetween to seal the battery 7 attached thereto.

As above, diameter 13.8a+a+, height 42II
A cylindrical non-aqueous electrolyte secondary battery of Im was manufactured.

Example 2 Pulverized $21 coke was treated in an electric furnace in an argon atmosphere at 1600° C. for 2 hours to produce a coke with an Lc of 85, which was used as a negative electrode material. The true density of this high temperature treated 21 dollar coke is 2.17g/cm', do
02 was 3.43 in. A battery was produced in the same manner as in Example 1 except for this.

Comparative Example 1 Crushed 21-dollar coke was used as a negative electrode material without being heat-treated. The Lc of this 21 dollar coke is 59 people, the true density is 2.12 g/cm', and the do02 is 3.44.
It was λ. A battery was produced in the same manner as in Example 1 except for this.

Comparative Example 2 Pulverized 21 dollar coke was treated at 1200''C in an argon atmosphere for 2 hours to produce a coke with an Lc of 60, which was used as a negative electrode material. The true density of coke is 2.12g/cm", d
002 had 3.44 people. A battery was produced in the same manner as in the example except for this.

Comparative Example 3 Pulverized 21-dollar coke was treated at 1300° C. for 2 hours in an electric furnace in an argon atmosphere to produce a coke with Lc of 63, which was used as a negative electrode material. The true density of this high temperature treated 21 dollar coke is 2.13g/c+w3, d0
02 was 3.44 people. A battery was produced in the same manner as in Example 1 except for this.

Comparative Example 4 Pulverized 21 dollar coke was treated at 1400''C in an electric furnace in an argon atmosphere for 2 hours to produce a coke with an Lc of 65, which was used as a negative electrode material.This high temperature treated 21 dollar coke The true density of coke is 2.15g/cm3, d0
02 was 3.44 people. A battery was produced in the same manner as in Example 1 except for this.

Comparative Example 5 Crushed 21-dollar coke was treated in an electric furnace in an argon atmosphere at 1700° C. for 2 hours to produce a coke with an Lc of 98, which was used as a negative electrode material. The true density of this high temperature treated 21 dollar coke is 2.17g/ca+', d
o02 was 3.42 in. A battery was produced in the same manner as in Example 1 except for this.

The batteries shown in the examples and comparative examples were charged at a charging current of 190
Constant current charging was performed for 3 hours at sA with an upper limit voltage of 4.1 V, and then 20 charge/discharge cycles were performed in which discharge was performed at 160 ohms to a final voltage of 2.9 V, and the discharge capacity at the 20th time was measured. Next, after charging these batteries again under the above-mentioned charging conditions, the batteries were left at a temperature of 24°C for 720 hours, and then a discharge test was conducted, and the discharge capacity was also measured.
The self-discharge rate was calculated by comparing with the 20th discharge capacity.

The results of the above measurements are shown in Table 1.

Table 1 From Table 1, 21-dollar coke with a small Lc has a larger discharge capacity, and when Lc is 85 or less, a discharge capacity of 320 mA or more can be obtained, but when Lc is 85 or more, the discharge capacity suddenly decreases. A trend can be seen. In addition, the larger Lc is, the lower the self-discharge rate becomes. If Lc is 73 or more, a low self-discharge rate of 6% or less can be obtained, but if Lc is less than 73, the self-discharge rate tends to suddenly increase. Can be seen. in this way,
Considering both self-discharge and discharge capacity, Lc is 7.
Results were shown for 21 dollar coke ranging from 3 Å to 85 people, but similar results were obtained for pitch coke.

〔Effect of the invention〕

The present invention has solved the problem of large self-discharge, which was a conventional drawback of non-aqueous electrolyte secondary batteries that use carbonaceous materials such as coke for the negative electrode. This makes it possible to provide a secondary battery with high energy density, excellent cycle characteristics, and excellent storage stability, which has great industrial value.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view schematically showing the structure of a cylindrical non-aqueous electrolyte secondary battery applied in an example of the present invention. In the symbols used in the diagram, 1 - - - 1 - - 1 - - 1 positive electrode 2 - - - - - - - 1 negative electrode

Claims (1)

[Claims] In a nonaqueous electrolyte secondary battery having a negative electrode having a carbonaceous material and a nonaqueous electrolyte dissolved in a nonaqueous solvent using a lithium salt as an electrolyte, the crystal thickness in the C-axis direction in X-ray diffraction of the carbonaceous material A nonaqueous electrolyte secondary battery characterized in that Lc is in the range of 73 Å to 85 Å.