JPH04329263A - Lithium secondary battery - Google Patents

Abstract

PURPOSE:To provide the positive electrode active material for lithium battery having the high operation voltage and a high active material utilization factor. CONSTITUTION:LiCoO2 obtained by immediately cooling after the burning is used as the positive electrode active material of a lithium secondary battery to improve the active material utilization factor. As a result, discharge voltage and discharge capacity are enlarged to improve the energy density of the battery remarkably.

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, and more particularly to a positive electrode thereof.

0002

2. Description of the Related Art Lithium batteries using lithium as a negative electrode active material have come to be widely used because of their characteristics of high voltage, high energy density, and high reliability, and these are primary batteries. Recently, research on secondary batteries has been actively conducted, and some have even been put into practical use. However, the characteristics of these batteries are still not sufficient.

[0003] Recently, a battery with an operating voltage of around 4V has been announced and is attracting attention as a battery having high voltage, which is one of the characteristics of lithium batteries. As an active material whose operating voltage is around 4V, K. Mat. by Mizushima et al. Res. Bull. , Vol. 15, pp. 7
83, 1980. LiCoO2, which was reported in

0004

[Problem to be solved by the invention] Therefore, the inventors of the present invention
After conducting intensive research on iCoO2, we found that Mi
It has been found that LiCoO2 obtained by firing and slowly cooling to room temperature according to the method of Zushima et al. has a problem of low utilization and relatively low energy density despite high voltage.

[0005] In order to solve the above-mentioned problems, the present invention aims to obtain LiCoO2 with a high utilization rate.

[0006]

[Means for Solving the Problems] The present invention is a lithium secondary battery using LiCoO2 obtained by rapid cooling after firing as a positive electrode active material.

[0007]

[Operation] Unlike LiCoO2 obtained by slow cooling, LiCoO2 obtained by rapid cooling after firing has high crystallinity, but is soft and easy to crush, and the particle size and shape tend to be uniform. It also exhibits an order of magnitude higher electronic conductivity. This change is thought to be due to the fact that many small particles with high crystallinity grew instead of growing into large particles due to rapid cooling. LiCoO produced when slow cooling
2 is also considered to have high crystallinity, but it is thought that the large force applied during crushing reduces the crystallinity and deteriorates the properties.

[0008]

EXAMPLES The details of the present invention will be explained below based on examples.

(Example) In preparing the positive electrode active material, 37 g of lithium carbonate, a commercially available special grade reagent, and 1 cobalt carbonate were used.
The mixture was thoroughly mixed while being crushed with a ball mill, and the mixture was placed in an alumina crucible and calcined in air at 650°C for 5 hours, and then at 950°C for 20 hours. After firing, the crucible was taken out into the air, rapidly cooled to room temperature, and the resulting material was pulverized, which was used as positive electrode active material A.

For comparison, a material baked in the same manner as above was slowly cooled to room temperature and then pulverized to obtain positive electrode active material B.

The X-ray diffraction patterns of the obtained positive electrode active materials A and B are shown in FIG. From FIG. 1, it can be seen that the obtained positive electrode active material is LiCoO2.

Table 1 shows the measurement results of the electronic conductivity of the active materials A and B thus obtained. From Table 1, it can be seen that the positive electrode active material A obtained by rapid cooling has an electronic conductivity one order of magnitude higher than that of B obtained by slow cooling, and electrons flow easily.

[0013]

A button-type lithium battery was prototyped using the positive electrode active materials A and B in the following manner. The positive electrode active material, acetylene black, and polytetrafluoroethylene powder were mixed at a weight ratio of 85:10:5, toluene was added, and the mixture was sufficiently kneaded. This was molded into a sheet with a thickness of 0.8 mm using a roller press. Next, add this to 16m
A positive electrode was obtained by punching out a circular shape with a diameter of m and heat-treating it under reduced pressure at 200° C. for 15 hours. The negative electrode was used by punching out a lithium foil with a thickness of 0.3 mm into a circular shape with a diameter of 15 mm, and press-fitting it to the negative electrode can via a current collector. The non-aqueous electrolyte contains γ-butyrolactone and 1
A solution containing mol/1 LiBF4 was used, and a microporous thin film made of polypropylene was used as a separator. Using the above positive electrode, negative electrode, electrolyte and separator, the diameter is 20 mm.
A button-shaped lithium battery with a thickness of 1.6 mm was manufactured.

[0015] Among the batteries thus produced, one using positive electrode active material A was used as a battery of the present invention, and one using positive electrode active material B was used as a comparison battery. A charge/discharge test was conducted using the battery of the present invention and the comparative battery. The test conditions were: charging current 3mA;
The final charge voltage was 4.5 V, the discharge current was 3 mA, and the final discharge voltage was 3.0 V. FIG. 2 shows the relationship between discharge voltage and discharge capacity obtained in this test. It can be seen from FIG. 2 that the battery of the present invention has a larger discharge capacity than the comparative battery. This shows that the positive electrode active material A obtained according to the present invention has a high utilization rate.

[0016]

As described above, a positive electrode using LiCoO2 as an active material obtained by rapid cooling after firing exhibits a higher utilization rate than a positive electrode using LiCoO2 as an active material obtained by slow cooling. As a result, a lithium secondary battery equipped with a positive electrode, a negative electrode, and an electrolyte according to the present invention has a higher discharge voltage and a larger discharge capacity than conventional lithium secondary batteries, and has a significantly improved energy density. Can be done.

[0017] The present invention is not limited to the active material manufacturing method, positive electrode, negative electrode, electrolyte, separator, battery shape, etc. described in the examples, and the rapid cooling process may be divided into two stages or It can also be applied to control the quenching rate. It is also applicable to those that use an organic fired body for the negative electrode, those that use a solid electrolyte instead of an electrolyte, or a separator.

[Brief explanation of drawings]

FIG. 1 is a diagram showing X-ray diffraction patterns of positive electrode active materials A and B.

FIG. 2 is a relationship diagram between discharge capacity and discharge voltage.

Claims (1)

[Claims] 1. A lithium secondary battery characterized in that LiCoO2 obtained by rapid cooling after firing is used as a positive electrode active material.