JP2001185142A - Positive electrode active material for lithium ion secondary battery and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a positive electrode active material for a lithium ion secondary battery having superior rate characteristics and cyclic characteristics, and a manufacturing method therefor. SOLUTION: Lithium carbonate having a particle diameter of 1 to 40 μm, a mean particle diameter of 5 to 15 μm, and a single peak on the Rosin- Rammler grain size distribution chart is mixed with cobalt oxide (Co3O4) having a mean particle diameter and the maximum particle diameter smaller than those of the lithium carbonate. The resulting mixture is mixed with a medium for granulation, drying and burning at the given temperature to obtain powdered lithium cobalite of a general formula LiCoO2 having a single peak in its grain size distribution on the Rosin-Rammler grain size distribution chart, and preferably, the particle diameter of most of the powdered lithium cobaltite in the range of 1 to 40 μm, and a mean particle diameter of 5 to 15 μm.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a positive electrode active material for a lithium ion secondary battery using lithium metal, lithium alloy or carbon for a negative electrode.

[0002]

2. Description of the Related Art In recent years, with the spread of mobile devices such as mobile phones and notebook computers, there is a strong demand for the development of small, lightweight, high capacity secondary batteries having a high energy density. As such a device, there is a lithium ion secondary battery using lithium, a lithium alloy or carbon as a negative electrode, and research and development have been actively conducted.

A lithium ion secondary battery using lithium cobaltate (LiCoO ₂ ) as a positive electrode active material can obtain a high voltage of 4V class, and is expected to be a battery having a high energy density, and is being put to practical use. In addition, in order to meet the recent demand for higher rates and longer life, the amount of conductive agent such as carbon mixed with the positive electrode active material is adjusted to increase the conductivity of the electrode, and to reduce the size of the positive electrode active material by charging and discharging. Measures such as prevention are also being studied.

[0004]

In general, lithium cobaltate is prepared by mixing a lithium salt, for example, lithium carbonate, and a cobalt compound, for example, cobalt carbonate at a predetermined ratio and firing at a temperature of 600 to 1100 ° C. -30
No. 4664), or lithium carbonate and cobalt tetroxide having an average particle size of 2 to 25 μm are mixed at a predetermined ratio and fired at 800 to 900 ° C. (Japanese Patent Laid-Open No. 9-28314).
4). However, the conventional lithium cobalt oxide has problems such as a decrease in the rate characteristics when the amount of the conductive agent is reduced in order to increase the battery capacity, and a decrease in the battery capacity due to repeated charging and discharging.

[0005] An object of the present invention is to solve the above-mentioned problems associated with the conventional positive electrode, and to provide a positive electrode active material for a lithium ion secondary battery having excellent rate characteristics and cycle characteristics, and a method for producing the same.

[0006]

According to the present invention for solving the above-mentioned problems, in a lithium cobalt oxide powder represented by the general formula LiCoO ₂ , the particle size distribution of the lithium cobalt oxide powder is represented by a rosin-Rammler particle size distribution diagram. In this case, the positive electrode active material for a lithium ion secondary battery has a single peak. Preferably, most of the particle diameter is in the range of 1 to 40 μm and the average particle diameter is 5 to 15 μm.

[0007] Such a method for producing lithium cobaltate powder comprises lithium carbonate, cobalt oxide (Co ₃
O ₄ ), a medium is mixed with the obtained mixture, granulated, dried and calcined at a predetermined temperature to obtain lithium cobalt oxide.
0 μm and an average particle size of 5 to 15 μm, and a single peak in a rosin-Rammler particle size distribution diagram, and the average particle size and the maximum particle size of A material smaller than lithium is used.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION As a result of various studies, the present inventors have found that the particle size distribution pattern of lithium cobalt oxide is extremely important, and that the limitation of the particle size and the average particle size will have a further effect. I found it. For example, it has been found that the presence of fine particles increases the specific surface area of the entire positive electrode active material, and requires a large amount of a conductive agent such as carbon to be added to impart conductivity.

That is, it has been found that, when lithium cobalt oxide in the electrode is increased and the amount of the conductive agent is decreased in order to increase the battery capacity, the conductivity of the electrode is reduced and the rate characteristics are deteriorated. Coarse particles are formed by charging and discharging.
As a result of the expansion and contraction in the axial direction, it was also found that the particles were cracked or cracked, whereby the contact with the conductive agent or the current collector could not be maintained, and the cycle characteristics were lowered. It has been found that when these fine particles and coarse particles are mixed, the load that the coarse particles that have failed to function due to cracks and cracks originally satisfactorily concentrate on the fine particles, and the cycle characteristics deteriorate at an accelerated rate.

Since the positive electrode active material according to the present invention has a single peak in the rosin-Rammler particle size distribution diagram, a homogeneous electrolytic reaction is obtained, and a sharp decrease in capacity due to charging and discharging can be prevented. Further, when the particle diameter is in the range of 1 to 40 μm and the average particle diameter is 5 to 15 μm, the specific surface area can be suppressed to be small, and a high rate characteristic can be realized even with a small amount of the conductive agent. Deterioration of cycle characteristics due to cracking can also be suppressed.

In order to produce the lithium cobaltate of the present invention, for example, the following method is used. That is, as shown in the rosin-Rammler diagram of FIG. 1, lithium carbonate (1) having a particle size in the range of 1 to 40 μm and an average particle size of 5 to 15 μm, and a single peak in the rosin-Rammler particle size distribution diagram. Li ₂ CO ₃ ) and cobalt oxide (Co ₃ O ₄ ) having an average particle size and a maximum particle size each smaller than the above-mentioned lithium carbonate as shown in FIG.
The mixture is precisely weighed and mixed to obtain a mixed powder. A solution obtained by dissolving polyvinyl alcohol (PVA) in water was added so that PVA was about 1.4 parts by weight with respect to 100 parts by weight of the mixed powder, and a mixture provided with a stainless steel stirring blade and an agitator was used. Mix with a granulator and granulate to a diameter of 3-5 mm. Then, after the obtained granules are dried at 120 ° C. for 5 hours, they are baked at 950 ° C. for 20 hours.

The composition of lithium cobaltate obtained in this way is almost the same as the charged composition. In addition, in the identification of the generated phase by powder X-ray diffraction using Cu Kα ray, L of JCPDS file number 16-427 was used.
1CoO ₂ and traces of unreacted Li ₂ CO ₃ , Co ₃ O ₄
Only are detected.

Further, if the particles are sieved with a sieve having an opening of 40 μm, as shown in FIG. 3, the rosin-Rammler particle size distribution diagram is a single peak lithium cobaltate powder having a particle diameter of 1 to 40 μm, Those having an average particle size of 5 to 15 μm can be obtained.

[0014]

Next, the present invention will be further described with reference to examples. (Example 1) FIG. 3 shows a rosin-Rammler particle size distribution chart produced as described above, which has a single peak, most of the particle diameter is 1 to 40 μm, and the average particle diameter is 5 to 15 μm. The following tests were performed using lithium cobaltate powder.

The above-mentioned lithium cobaltate is used as a positive electrode active material, this is mixed with acetylene black and polytetrafluoroethylene resin (PTFE) in a weight ratio of 80: 15: 5 to prepare a mixture, and 50 mg of the mixture is prepared. It was weighed and press-molded at a pressure of 200 MPa into a disk having a diameter of 10 mm. The disc is placed in a vacuum dryer at 120 ° C, 1
After drying overnight, a positive electrode was obtained.

The negative electrode is made of Li metal having a diameter of 16 mm and a thickness of 1 mm, and an electrolytic solution is a mixed solution of an equal amount of ethylene carbonate (EC) and 1,2-dimethoxyethane (DME) using 1 mol of LiPF ₆ as a supporting salt. Using. A 2032 type coin battery was assembled in a glove box in which the dew point was controlled at −80 ° C. in an Ar atmosphere using a 25 μm-thick polyethylene porous film as a separator.

The coin-type battery was left for about 10 hours after assembling. After the open circuit voltage (OCV) was stabilized, the battery was charged to a cut-off voltage of 4.3 V at a charging current density of 1.0 mA / cm 2 and then left for 2 hours. And a discharge current density of 1.0 mA /
A discharge test was performed to 3.0 V at cm 2. Table 1 shows the discharge capacity.

The charge / discharge test was repeated under the same conditions as described above, and the 100th discharge capacity retention rate was determined according to Equation 1.
The results are shown in Table 1.

Equation 1 Capacity maintenance rate (cycle characteristic%) = 100th discharge capacity / First discharge capacity × 100

Next, a battery was assembled again in the same manner as described above. The rate characteristics of the discharge capacity were measured as follows.

The coin type battery is left for about 10 hours after assembly,
After the OCV is stabilized, the charging current density is 1.0 mA / cm ²
After the battery was charged to a cutoff voltage of 4.3 V and left for 2 hours, a discharge test was performed at a discharge current density of 1.0 mA / cm ² to a cutoff voltage of 3.0 V to obtain a discharge capacity (1).
After leaving the discharge test for 2 hours, the battery was charged again to a cut-off voltage of 4.3 V at a charge current density of 1.0 mA / cm ² , and then left for 2 hours to reduce the discharge current density to 8.0 mA / c.
cut-off voltage 3 in the m ^2. A discharge test was performed to OV to determine a discharge capacity (2). The rate characteristic is obtained from Equation 2,
The results are shown in Table 1.

Equation 2 Rate characteristic (%) = discharge capacity (2) / discharge capacity (1) ×
100

Comparative Example 1 Lithium carbonate showing a rosin-Rammler particle size distribution diagram shown in FIG. 4 and a lithium carbonate shown in FIG.
Lithium cobalt oxide was obtained in the same manner as in Example 1, except that cobalt oxide having an average particle size and a maximum particle size showing a rosin-Rammler particle size distribution diagram larger than the lithium carbonate shown in FIG. 4 was used. In the rosin-Rammler particle size distribution diagram of the obtained lithium cobaltate, the maximum peak was found at 20 μm, and was not a single peak. In addition, coarse particles having a particle size of 40 μm or more existed in a large amount of 3% of the whole.

Using this lithium cobalt oxide as the positive electrode active material, a coin-type battery was manufactured in the same manner as in Example 1, and the rate characteristics were similarly obtained. Table 2 shows the obtained results.

Table 1 Discharge capacity (mAh / g) Capacity maintenance rate (%) Rate characteristics (%) 1st time 152 100th time 148 97.4 --- 1 mA / cm2 1508 mA / cm2 143 --- 95. 3 Table 2 Discharge capacity (mAh / g) Capacity maintenance rate (%) Rate characteristic (%) 1st 149 100th 129 86.6 --- 1 mA / cm2 150 8 mA / cm2 107 --- 71.3

From Table 1, it can be seen that the use of the lithium cobaltate of the present invention, which has a single peak in the rosin-Rammler particle size distribution diagram, can realize high rate characteristics and high cycle characteristics even with a small amount of the conductive agent. It is considered that the concentration of the load on the specific particles is alleviated, thereby preventing a rapid decrease in capacity due to charge and discharge.

It is considered that the reason why the high rate characteristic is obtained is that the specific surface area is reduced because fine particles (particle diameter of 1 μm or less) are not contained. Further, the reason why the high cycle characteristics are obtained is presumably because the number of coarse particles (40 μm or more) is small, so that the deterioration of the current collecting function due to cracks and cracks of lithium cobalt oxide is prevented.

On the other hand, as is clear from Table 2, the lithium cobalt oxide synthesized without considering the particle size of the raw material has a large specific surface area due to the presence of many fine particles, and the amount of the conductive agent is the same as in Example 1. In this case, the discharge capacity is low or the rate characteristics deteriorate. In addition, when there are a plurality of peaks in the rosin-Rammler particle size distribution diagram and there are coarse particles of 40 μm or more, repetition of charge and discharge causes fine primary particles to be refined and electrical contact cannot be obtained,
The capacity retention rate deteriorates due to dropping from the current collector,
The load is concentrated on other particles, and the particles are destroyed.

[0029]

The use of the lithium cobalt oxide according to the present invention as a positive electrode active material of a lithium ion secondary battery can improve the cycle characteristics and rate characteristics of the secondary battery, and produce an excellent secondary battery. it can.

[Brief description of the drawings]

FIG. 1 shows the particle size distribution of lithium carbonate used in Examples.

FIG. 2 shows the particle size distribution of cobalt oxide used in the examples.

FIG. 3 shows the particle size distribution of lithium cobaltate prepared in the examples.

FIG. 4 is a particle size distribution of lithium carbonate used in a comparative example.

FIG. 5 is a particle size distribution of cobalt oxide used in a comparative example.

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 4G048 AA04 AB01 AC06 AD04 AE05 5H003 AA01 AA04 BA01 BA03 BB05 BC01 BD00 BD02 5H014 AA02 BB01 BB06 EE10

Claims (3)

[Claims] 1. A lithium cobalt oxide powder represented by the general formula LiCoO ₂ , wherein the lithium cobalt oxide powder has a single peak when represented by a rosin-Rammler particle size distribution diagram. Cathode active material for ion secondary batteries. 2. The positive electrode active material for a lithium ion secondary battery according to claim 1, wherein most of the particle diameter is in the range of 1 to 40 μm and the average particle diameter is 5 to 15 μm. 3. A lithium carbonate and a cobalt oxide (Co)
₃ O ₄ ), a medium is mixed with the obtained mixture, granulated, dried and calcined at a predetermined temperature to obtain lithium cobaltate.
0 μm and an average particle size of 5 to 15 μm, and a single peak in a rosin-Rammler particle size distribution diagram, and the average particle size and the maximum particle size of A method for producing a positive electrode active material for a lithium ion secondary battery, comprising using a material smaller than lithium.