JPH11219705A - Positive electrode active material for lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve a heavy load characteristic and a charging/discharging cycle characteristic at a high temperature by using a positive electrode active material mainly made of a spinel Li-Mn(SLM) composite oxide containing fine grains having the specific surface area and average grain size within specific ranges respectively and containing fine grains having the average grain size of a specific value or below at a specific wt.% or below. SOLUTION: This positive electrode active material for a lithium secondary battery is mainly made of an SLM composite oxide containing fine grains having the specific surface area of 0.1-5 m<2> /g and the average grain size of 5-30 μm and containing fine grains having the average grain size of 1 μm or below at 3 wt.% or below. When the positive electrode active material with this composition is used, the reaction to an electrolyte is made proper by its function, and the deterioration of a battery life, various battery performance, a charging/ discharging cycle characteristic and a high-load characteristic can be suppressed. Raw amaterial resources are stable and inexpensive, and high electromotive force can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a spinel type Li ·
The present invention relates to a positive electrode active material for a lithium secondary battery containing a Mn-based composite oxide as a main component.

[0002]

2. Description of the Related Art Lithium secondary batteries are generally receiving more and more attention because they are superior in terms of electromotive force and energy density. In the art, various improvements have been studied in order to develop more practical products. Has been keen. Research on improvement of the positive electrode active material is also one of the important issues, and a rock salt-based lithium transition metal-based composite oxide capable of obtaining a high electromotive force, for example, Li
CoO ₂ , LiNiO _2, etc. have received particular attention. Various proposals have also been made to improve the discharge capacity and charge / discharge cycle characteristics of a lithium secondary battery using the rock salt-based composite oxide as a positive electrode active material.

For example, Japanese Patent Laid-Open No. 4-56064 discloses that
LiCoO_TwoOf low discharge capacity of battery using lithium as positive electrode active material
LiCoO to improve the title_TwoUsed in the manufacture of
Li _TwoCO_ThreeIs less than 10% by weight and specific surface area
Is 2m^Two/ G or less of LiCoO_TwoA proposal was made to use
ing.

Japanese Patent Application Laid-Open No. Hei 4-249073 discloses a Li _x MO having a specific surface area of 0.01 to 0.30 m ² / g in order to improve the charge / discharge cycle characteristics of a lithium secondary battery.
₂ (0.05 ≦ x ≦ 1.10, M is a transition metal) has been proposed.

Further, Japanese Patent No. 2615854 discloses that
In order to improve the problem of low discharge capacity of the battery, 30% by volume of particles having an average particle size of 10 to 150 μm and 5 μm or less are used.
The invention using less than Li _x MO ₂ (0.05 ≦ x ≦ 1.10, M is a transition metal) is described.

Recently, however, the above-mentioned LiCo
Instead of O ₂ , LiNiO _2, etc., Li · Mn-based composite oxides which are inexpensive due to their stable resources and which are expected to have a higher electromotive force, especially rock salt-based Li A spinel-type Li · Mn-based composite oxide, which is expected to have a higher electromotive force than the Mn-based composite oxide (hereinafter, the spinel-type composite oxide is abbreviated as an SLM-based composite oxide) as a positive electrode active material Attention has been paid. However, recently, a secondary battery capable of withstanding a large current discharge, that is, a secondary battery having excellent heavy load characteristics and charge / discharge cycle characteristics, particularly, 60 ° C.
Despite the growing demand for secondary batteries having excellent charge / discharge cycle characteristics before and after high temperatures, batteries using SLM-based composite oxides have not yet been able to meet the above requirements. In addition, despite the various proposals and inventions described above for improving the discharge characteristics and charge / discharge characteristics of rock salt-based lithium transition metal-based composite oxides, research on the improvement of SLM-based composite oxides has not yet been made. It is in.

[0007]

SUMMARY OF THE INVENTION Accordingly, the present invention provides an S
An object of the present invention is to propose a positive electrode active material for a lithium secondary battery having a LM-based composite oxide as a main component and having improved heavy load characteristics and charge / discharge cycle characteristics at high temperatures.

[0008]

The above-mentioned problems can be solved by the following positive electrode active material. (1) Specific surface area of 0.1 to 5 m ² / g, average particle size of 5 to 5
Spinel type Li · Mn having a content of fine particles having a particle size of 30 μm and an average particle size of 1 μm or less of 3% by weight or less
A positive electrode active material for a lithium secondary battery, characterized by containing a composite oxide as a main component.

[0009]

The specific surface area of the SLM-based composite oxide is set to 0.1.
1 to 5 m ² / g, the average particle diameter is 5 to 30 μm, and the content of fine particles having an average particle diameter of 1 μm or less is 3% by weight or less. Can be solved.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, SLM-based composite oxides include those in which Mn is not substituted with another element, those in which Mn is partially substituted with another element, and those in which Mn is partially substituted with another element. Various SLM-based composite oxides, such as those substituted with two or more elements among those substituted with, can be used, for example, those represented by the following general formula (1). _{Li A Mn 2-x Me x} O 4 (1) General formula (1), A is from 0.05 to 2.3, preferably 0.5 to 1.5, X is 0 to 0.5 is there. X
Is 0, it means that Mn is not replaced by another element. On the other hand, when X is not 0, it means that Mn is replaced by the element Me, and in this case, X is 0.01 to 0.5,
It is particularly preferably 0.02 to 0.2. Element Me
As elements of Group 3 to 10 of the new periodic table, for example, Zr,
V, Cr, Mo, Fe, Co, Ni, etc., or 13 to
Group 15 element, for example, B, Al, Ge, Pb, Sn, Sb
And so on. In the case of an SLM-based composite oxide in which Mn is substituted by two or more of these elements, the total amount of the two or more elements may be within the above range of X.

In the present invention, the specific surface area of the SLM-based composite oxide can be determined by the constant pressure one-point method of the BET method using nitrogen gas as an adsorption gas (for example, Yasuo Arai, “Powder Material Chemistry,”
(See pages 78-184, Baifukan (Tokyo), 1995)
It is a value measured in.

In the present invention, the average particle size of the SLM-based composite oxide is determined by scattering a laser beam using a well-known Microtrac particle size analyzer and the number (n) of particles in the visual field and the diameter (d) of each particle (d) ( Note, according to the Microtrac particle size analyzer, the sphere equivalent diameter is automatically measured for each particle having various shapes.) The number of particles (n) and the diameter (d) of each particle are It is a value (average volume diameter or volume-weighted average particle diameter) calculated by equation (2). (Σnd ³ / Σn) ^1/3 (2) Not only SLM-based composite oxides but also various Li-Mn-based composite oxides have a structure in which many small particles generally called primary particles are aggregated. And the aggregate of the primary particles is called a secondary particle. The values obtained from the above measurement methods and calculations correspond to the average particle size of the secondary particles.

When the SLM-based composite oxide of the present invention functions as a positive electrode active material for a lithium secondary battery, it is necessary that the reactivity with an electrolytic solution is appropriate. In general, if the reactivity is excessive, the deterioration of the positive electrode active material and the electrolytic solution is quick, and the battery life is shortened. Conversely, if the reactivity is too small, various battery performances tend to decrease. Especially the specific surface area is 5m
² / g and an average particle diameter of less than 5 μm,
In other words, when small particles having a large surface area are used, such particles have excessive reactivity with the electrolytic solution, so that a side reaction between the particles and the electrolytic solution is increased and both are deteriorated,
The charge / discharge cycle characteristics of the battery at high temperatures are likely to deteriorate.

On the other hand, a large particle having a specific surface area of less than 0.1 m ² / g and an average particle diameter of more than 30 μm, in other words, a large particle having a very small surface area has a small reaction area with an electrolytic solution, And the passage of lithium ions becomes longer, making it difficult to extract a large current. As a result, the high-load characteristics of the lithium secondary battery deteriorate.

The positive electrode active material layer is usually made of S
A composition comprising an LM-based composite oxide, a conductive agent, and a binder is applied to a current collector, and is formed by drying if necessary. Large particles having an average particle diameter of greater than 30 μm were used. In such a case, regardless of the specific surface area, the applicability of the composition is poor and it is difficult to form a uniform positive electrode active material layer.

The object of the present invention cannot be achieved even if the content of fine particles having an average particle size of 1 μm or less in the SLM-based composite oxide is more than 3% by weight. Although the reason is not clear at present, the SLM-based composite oxide generally has a large specific surface area, and therefore, the fine particles are poorly compatible with the binder and easily fall off from the positive electrode active material layer, so that they fall off. This is probably because fine particles short-circuit the positive and negative electrodes.

Thus, in the present invention, the SLM complex acid
Specific surface area of 0.2 to 4m ^Two/ G, especially 0.2
5-3.5m^Two/ G, and the average particle size is 8 to 25 μm.
m, especially 10 to 20 μm, and the average particle diameter is 1
The content of fine particles having a particle size of 1 μm or less is 1% by weight or less, and
It is preferably at most 5% by weight.

The SLM-based composite oxide used in the present invention, for example, one represented by the above general formula (1) is an oxide (oxide) or a hydroxide of an element (excluding oxygen) contained in the general formula (1).
Carbonates or nitrates are mixed so that the atomic ratio of each element becomes the ratio represented by the general formula (1), and the resulting mixture is heated and fired at 500 to 1000 ° C. for 1 to 50 hours in the air. After cooling, it can be obtained by crushing and classifying. The fine particles having an average particle diameter of 1 μm or less are
It can be removed or reduced by increasing the degree of classification.

The cathode active material of the present invention can be prepared by a method known in the field of a nonaqueous electrolyte lithium secondary battery or a solid electrolyte lithium secondary battery using a conventional SLM-based composite oxide as a cathode active material. It can be used in the same way. Hereinafter, some examples of typical or preferable practical methods will be described.

Examples of the binder for the positive electrode active material include polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, and ethylene-propylene-diene-based polymers. Examples of the conductive agent include various types of conductive graphite and conductive carbon. Black is exemplified.

The amount of the positive electrode active material used is 80 to 9 per 100 parts by weight of the total amount of the positive electrode active material, the binder, and the conductive agent.
It is about 5 parts by weight, and the amount of the binder used is 10
The amount is about 1 to 10 parts by weight per 0 parts by weight, and the amount of the conductive agent is about 3 to 15 parts by weight per 100 parts by weight of the positive electrode active material.

The positive electrode sheet can be formed by applying a mixed composition comprising a positive electrode active material, a binder, and a conductive agent to one or both surfaces of a positive electrode current collector, sufficiently drying the resultant, and rolling. One having a positive electrode active material layer having a thickness of about 20 to 500 μm, particularly about 50 to 200 μm on one or both sides is exemplified.

Preferred examples of the negative electrode active material used in common with the positive electrode active material of the present invention include various natural graphites and artificial graphites, for example, graphites such as fibrous graphite, flake graphite, and spherical graphite. Examples of the binder include polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, and an ethylene-propylene-diene-based polymer. The amount of the negative electrode active material used is about 80 to 96 parts by weight per 100 parts by weight of the total amount of the negative electrode active material and the binder.

As the positive electrode current collector, a conductive metal such as aluminum, an aluminum alloy,
About 100 μm, especially about 15 to 50 μm foil or perforated foil, about 25 to 300 μm thickness, especially about 30 to 150 μm
A certain degree of expanded metal is preferred.

As the negative electrode current collector, copper, nickel, silver,
About 5-100 μm in thickness of conductive metal such as SUS,
Especially about 8 to 50 μm foil or perforated foil, thickness 20 to 30
Expanded metal having a thickness of about 0 μm, particularly about 25 to 100 μm is preferable.

Examples of the non-aqueous electrolyte include an electrolyte in which salts are dissolved in an organic solvent. The salts include LiC
10 ₄ , LiBF ₄ , LiPF ₆ , LiAsF ₆ , Li
Examples thereof include AlCl ₄ and Li (CF ₃ SO ₂ ) ₂ N, and one or a mixture of two or more thereof is used.

As the organic solvent, ethylene carbonate, propylene carbonate, dimethyl carbonate,
Diethyl carbonate, ethyl methyl carbonate, dimethyl sulfoxide, sulfolane, γ-butyrolactone, 1,2-dimethoxyethane, N, N-dimethylformamide, tetrahydrofuran, 1,3-dioxolan, 2-methyltetrahydrofuran, diethyl ether, and the like, One or a mixture of two or more thereof is used. The concentration of the above salts in the electrolyte is suitably about 0.1 to 3 mol / l.

[0028]

EXAMPLES The present invention will be described in more detail with reference to the following examples, and comparative examples will also be described to show the remarkable effects of the present invention.

[0029] Examples 1-5, about a homogeneous mixture of Comparative Examples 1 to 4 electrolytic MnO ₂ and Li ₂ CO ₃ 75
Bake at 0 ° C. for about 15 hours, then pulverize to obtain LiMn ₂ O
A powder having the chemical formula of ₄ was obtained. The powder thus obtained was classified under various conditions using an MDS-1 type airflow classifier manufactured by Nippon Pneumatic Co., Ltd., and as shown in Table 1, specific surface area, average particle diameter, and fine particles having an average particle diameter of 1 μm or less were obtained. Nine kinds of SLM-based composite oxides as positive electrode active materials of Examples 1 to 5 and Comparative Examples 1 to 4 having different contents were obtained.

Examples 6 to 9 and Comparative Example 5 Electrolytic MnO_Two, Li_TwoCO_Three, Co_TwoO_Three, And Ge
_TwoO_ThreeBaking at about 750 ° C for about 15 hours
And then pulverized into LiMn_1.9OCo_0.05Ge _0.05O_Four
A powder having the following chemical formula was obtained. Powder obtained in Japan
Using a Pneumatic MDS-1 airflow classifier
Classified under various conditions, specific surface area and average as shown in Table 1.
Including fine particles having a particle diameter and an average particle diameter of 1 μm or less
Five types of positive electrode active materials of Examples 6 to 9 and Comparative Example 5 having different weights
An SLM-based composite oxide as a substance was obtained.

Using the positive electrode active materials of Examples 1 to 9 and Comparative Examples 1 to 5, 92 parts by weight of the positive electrode active material, 3 parts by weight of acetylene black, 5 parts by weight of polyvinylidene fluoride, and 70 parts by weight of N-methyl-2-pyrrolidone 70 Parts by weight were mixed to form a slurry. This slurry was applied on an aluminum foil and dried to prepare a positive electrode sheet having a positive electrode active material of 20 mg / cm ² . Each positive electrode sheet thus obtained and the Li foil were closely adhered to each other via a porous polyethylene separator, and 1 mol of LiPF ₆ per liter of a mixed solvent of ethylene carbonate and ethyl methyl carbonate (mixing volume ratio was 1: 1) was added. The dissolved solution was used as an electrolytic solution, which was impregnated between the positive electrode sheet and the Li foil to produce a sealed coin-type lithium secondary battery. For each lithium secondary battery, the charge / discharge cycle characteristics at high temperature (60 ° C.) and room temperature (2
(0 ° C.).

[Test method for charge / discharge cycle characteristics at high temperature] The positive electrode sheet was charged at a constant current of 1 mA per 1 cm ² area and a constant voltage of 4.3 V for 5 hours, and then charged per 1 cm ² area of the positive electrode sheet. Discharge is performed under a constant current of 0.4 mA until the terminal voltage becomes 3 V, and then charging and discharging are paused for one hour. The above charge / discharge and pause are repeated as one cycle for 20 times, but only at the first time at room temperature (20 ° C.)
Performed at 60 ° C. for the second and subsequent cycles. The discharge capacity in each cycle is calculated by calculating the quantity of electricity (mA · H) from the discharge current value and the discharge time, and calculating the discharge capacity (mA · H) from the weight (g) of the positive electrode active material contained in the lithium secondary battery.
/ G). Table 1 shows that the 20
The ratio is shown as the ratio of the discharge capacity at the cycle, that is, the discharge capacity maintenance ratio (%).

[Test Method for Heavy Load Characteristics] After the first charge / discharge and pause in the above test method for charge / discharge cycle characteristics at high temperature, the area of the positive electrode sheet was 1 cm ² at the same temperature at the second cycle. Constant current of 1 mA per unit and 4.3
The battery was charged at a constant voltage of 5 V for 5 hours, and then a terminal voltage of 3 V was applied under a constant current of 6 mA per 1 cm ² of the area of the positive electrode sheet.
Discharge until the point where. Table 1 shows the ratio of the discharge capacity in the second cycle to the first discharge capacity, that is, the discharge capacity maintenance ratio (%).

From Table 1, it can be seen that each of the batteries using the positive electrode active materials of Examples 1 to 9 exhibited good performance in both charge-discharge cycle characteristics at high temperatures and heavy load characteristics at room temperature. On the other hand, it is understood that each of the batteries using the positive electrode active materials of Comparative Examples 1 to 5 is inferior in both of the above characteristics.

[0035]

[Table 1]

The above Examples 1 to 9 and Comparative Examples 1 to 5
Of the positive electrode active materials described above, those of Example 1 and Comparative Example 1 were selected as representatives, and a more detailed comparison of both positive electrode active materials was performed as shown in FIG.
3 to FIG.

FIG. 1 shows the particle size distribution of both positive electrode active materials.
It is a graph which shows the relative particle amount (%) for every particle diameter (micrometer). As can be seen from the figure, in Example 1, the maximum relative particle amount exists around a particle diameter of 10 μm.
It can be seen that the maximum relative particle amount exists around 0 μm.

[0038] Figure 2, instead of the second cycle of the discharge current (6 mA / cm ²⁾ in the test method of the above-described heavy load characteristics, each of the case where various varied from 0.4 mA / cm ² until 6 mA / cm ² 4 is a graph showing a change in discharge capacity with respect to a discharge current. The vertical axis in the figure indicates the discharge capacity under the initial discharge current (0.4 mA / cm ² ) of 100%.
Shows the discharge capacity maintenance ratio (%) in the second cycle in the case of. According to the figure, no difference is observed between Example 1 and Comparative Example 1 in a region where the discharge current in the second cycle is small (light load region), but in a heavy load region of 3 mA / cm ² or more,
It can be seen that there is a clear difference between the two.

FIG. 3 is a graph showing a discharge capacity retention ratio (%) for each cycle in the course of the charge / discharge cycle characteristics test at a high temperature (60 ° C.) up to 20 cycles. From the figure, it can be seen that as the number of cycles increases, the difference in the maintenance ratio between the two gradually increases.

[0040]

The positive electrode active material of the present invention is inexpensive due to stable raw material resources, and is excellent in charge / discharge cycle characteristics and heavy load characteristics at high temperatures with high electromotive force. It is suitable for the production of long-life lithium secondary batteries for various electric devices, especially portable products.

[Brief description of the drawings]

FIG. 1 is a graph comparing the particle size distribution of both positive electrode active materials of Example 1 and Comparative Example 1.

FIG. 2 is a graph comparing discharge capacity retention ratios with respect to discharge current of both lithium secondary batteries using each positive electrode active material of Example 1 and Comparative Example 1.

FIG. 3 is a graph showing a change in a discharge capacity retention ratio for each cycle in a charge / discharge cycle test at 60 ° C. of both lithium secondary batteries using each of the positive electrode active materials of Example 1 and Comparative Example 1.

[Explanation of symbols]

 A Example 1 B Comparative Example 1

Claims (1)

[Claims] 1. Spinel-type Li having a specific surface area of 0.1 to 5 m ² / g, an average particle diameter of 5 to 30 μm, and a content of fine particles having an average particle diameter of 1 μm or less of 3% by weight or less. A positive electrode active material for a lithium secondary battery, comprising a Mn-based composite oxide as a main component.