JPH10172569A - Lithium secondary battery and manufacture of its positive electrode active material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing a lithium secondary battery and its positive electrode active material, with which a high initial capacity per unit volume of positive electrode can be obtained along with an excellent cyclic characteristics. SOLUTION: A lithium secondary battery is composed of a negative electrode using an active material capable of occluding and emitting lithium ions, a positive electrode using active material consisting of lithium-manganese oxide having spinel structure, a separator interposed between the two electrodes, and an organic electrolytic solution. The lithium-manganese oxide consists of secondary particles having a mean particle size between 20 and 100μm produced through coaguration of primary particles having a mean particle size between 0.01 and 3.0μm.

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 lithium secondary battery using a positive electrode active material having excellent characteristics, and a method for producing the positive electrode active material.

[0002]

2. Description of the Related Art Among various secondary batteries, lithium secondary batteries, in particular, have high voltage, low self-discharge, and excellent storage stability. Therefore, it is expected as a promising secondary battery in many fields. As a conventional lithium secondary battery, there is one using a metal oxide such as LiMn ₂ O ₄ having a spinel structure as a positive electrode active material.

[0003]

However, the above-mentioned conventional lithium secondary battery has the following problems. That is, a secondary battery is required to have a high initial discharge capacity (initial capacity) and a small deterioration in discharge capacity (cycle characteristics) due to repeated charge / discharge cycles.
Further, as for the initial capacity, a battery having a large discharge capacity per unit volume of the positive electrode is required from the viewpoint of miniaturization of the battery. In this respect, the conventional lithium secondary battery cannot be said to have both high initial capacity characteristics per unit volume of the positive electrode and excellent cycle characteristics.

In order to solve this problem, for example, Japanese Patent Application Laid-Open
As disclosed in Japanese Unexamined Patent Publication No. 3-25791, an active material composed of secondary particles having an average particle size of 0.1 to 15 μm formed by aggregating primary particles having an average particle size of 0.01 to 5 μm is used as a positive electrode active material. A non-aqueous secondary battery used has been proposed.

However, in the non-aqueous secondary battery, the positive electrode composed of the positive electrode active material including the secondary particles has a low apparent density. Therefore, the absolute amount of insertion and extraction of Li ions per unit volume of the positive electrode decreases, and it is difficult to secure a high initial capacity unless the volume of the positive electrode is increased.

The present invention has been made in view of such conventional problems, and has a high initial capacity per positive electrode volume, and
An object of the present invention is to provide a method for producing a lithium secondary battery and its positive electrode active material having excellent cycle characteristics.

[0007]

The invention of claim 1 comprises a negative electrode using a negative electrode active material capable of inserting and extracting lithium ions and a positive electrode using a positive electrode active material comprising lithium manganese oxide having a spinel structure. In a lithium secondary battery having a separator interposed and an organic electrolyte,
The lithium manganese oxide has an average particle size of 0.01 to
Average particle diameter of 20 to 1 in which primary particles of 3.0 μm are aggregated.
A lithium secondary battery comprising secondary particles of 00 μm.

The most remarkable point in the present invention is that a lithium manganese oxide having a spinel structure is used as the positive electrode active material, and the lithium manganese oxide has an average particle size of 0.01 to 3.0 μm. It consists of secondary particles having an average particle diameter of 20 to 100 μm formed by agglomeration of primary particles.

When the average particle size of the primary particles is less than 0.01 μm, the apparent density (bulk density) of the primary particles becomes low, and a positive electrode having a sufficient apparent density is obtained when forming a positive electrode by agglomeration. On the other hand, 3.0 μm
If it exceeds m, there is a problem that the effect of increasing the specific surface area when the particles are aggregated into secondary particles is reduced.

The average particle size of the secondary particles is 20 μm.
If it is less than 50%, there is a problem that the apparent density at the time of forming the positive electrode by collecting the secondary particles is low.
If the thickness exceeds μm, there is a problem that particles may become larger than the thickness of the electrode.

Next, in producing a positive electrode using the positive electrode active material composed of the secondary particles, a conductive material such as acetylene black and the like
A binder such as TFE is used. The negative electrode is formed using a negative electrode active material capable of inserting and extracting lithium ions. As the negative electrode active material, for example, Li, Li
Use alloys, carbon bodies, etc.

As a separator interposed between the two electrodes, for example, a porous film of polypropylene or a glass filter is used. As the organic electrolyte to be impregnated in the separator, there is a solution obtained by dissolving an appropriate amount of electrolyte in an organic solvent. As the above organic solvent, one or more solvents selected from ethylene carbonate, diethyl carbonate, propylene carbonate, butylene carbonate, tetrahydrofuran, 2-methyltetrahydrofuran, dimethoxyethane, dioxolan and γ-butyrolactone are preferred. As the electrolyte, LiPF ₆ , LiClO ₄ , LiBF ₄ ,
LiAsF _{6 and the} like. Among them, especially EC + DEC
/ 1M LiPF ₆ or PC + DMC / 1M Li
A combination of ClO ₄ is preferred.

Next, the operation of the present invention will be described. In the lithium secondary battery of the present invention, the lithium manganese oxide composed of the above secondary particles is used as a positive electrode active material constituting a positive electrode. That is, the active material is not formed of ordinary single particles, but is formed by aggregating primary particles of the specific particle size into secondary particles of the specific particle size.

Therefore, the positive electrode constituted by collecting the secondary particles has a large specific surface area, and can easily occlude and release Li ions. Therefore, damage to the positive electrode during charging and discharging is reduced, and cycle characteristics can be improved. Also, since the positive electrode active material is composed of secondary particles having the above-mentioned size, the apparent density can be increased, and a large amount of the active material can be filled in the same volume, so that a high discharge capacity can be obtained. it can. Therefore,
The lithium secondary battery of the present invention can exhibit high initial capacity and excellent cycle characteristics.

Next, as in the second aspect of the present invention, it is preferable that the bulk density of the positive electrode active material is 1.5 to 3.5 g / cm ³ . Here, the bulk density is a so-called apparent density, which is obtained by simply dividing the mass of the positive electrode active material by its apparent volume. Therefore, the above-mentioned volume includes voids and the like remaining in the aggregate of the positive electrode active materials. When the bulk density is less than 1.5 g / cm ³ , there is a problem that a sufficient capacity cannot be obtained when constituting a battery. On the other hand, when the bulk density exceeds 3.5 g / cm ³ , However, there is a problem in that a space through which the water penetrates cannot be sufficiently obtained.

Next, there are the following inventions as methods for producing the positive electrode active material for a lithium secondary battery having the above excellent characteristics. That is, a method for producing a positive electrode active material for a lithium secondary battery comprising lithium manganese oxide having a spinel structure and secondary particles in which primary particles are aggregated as in the invention of claim 3. Then, a raw material solution obtained by dissolving the raw material of the lithium manganese oxide in a solvent is sprayed in the form of droplets, and then the droplets are subjected to a heat treatment so that the raw materials in the droplets are reacted and the droplets are reacted. Evaporating the solvent to produce primary particles, and then mixing the primary particles with the raw material solution and subjecting the mixture to heat treatment to produce secondary particles formed by agglomeration of the primary particles. There is a method for producing a positive electrode active material for a lithium secondary battery.

The most remarkable point in this production method is that the primary particles are first synthesized by spraying the above-mentioned raw material solution and the above-mentioned heat treatment, and then the primary particles are mixed with the above-mentioned raw material solution and heat-treated again. To produce secondary particles which are aggregates of primary particles.

The raw material of the lithium manganese oxide comprises a lithium compound and manganese or a manganese compound. The lithium compound is selected from the group consisting of oxides, hydroxides, carbonates, nitrates, acetates and oxalates of Li. It is preferable that at least one of the following is used. These are relatively easy to ionize in a solution and are excellent in improving uniformity.

The manganese compound is preferably at least one of oxides, hydroxides, carbonates, nitrates, acetates and oxalates of Mn. These are
As in the above case, it is relatively easily ionized in the solution, and is excellent in improving the uniformity. The above solvent is
One or more of water, an aqueous acid solution, an aqueous alkali solution, and an organic solvent can be used.

The heat treatment for synthesizing the above-mentioned primary particles means that the sprayed droplets are thermally decomposed by heating to cause a solution reaction of the raw material components in the droplets and a solvent in the droplets. Refers to the operation of evaporating. As a result, powdery primary particles generated by the reaction and solvent vapor are generated.
The two can be separated, for example, in a collector to obtain primary particles.

In the heat treatment, the raw material can be heated in the following various methods in a container for spraying the raw material in the form of droplets. That is, the heat treatment for producing the primary particles is performed by heating the raw material from outside the spray container,
One or more of heating by injection of a heated gas into the spray container and heating by combustion heat of a solvent can be used.

In the case of external heating of the spray container, the heating temperature can be easily adjusted. Also,
In the case of injecting the heating gas into the container, the heating efficiency can be improved. In addition, in the case of heating by burning the solvent, an effect is obtained that primary particles can be synthesized instantaneously, and the uniformity of the composition is improved.

The method of spraying the raw material solution in the form of droplets includes a method of applying ultrasonic vibration to the raw material solution, a method of spraying a compressed solution from a nozzle, and a method of spraying a solution and gas using a two-fluid nozzle. There is a method of spraying.

Next, in the heat treatment for producing the secondary particles, the primary particles and the raw material solution are sufficiently mixed.
This is performed in a state where the raw material solution is interposed between the primary particles. That is,
The reaction of the raw material solution is performed in the gap between the adjacent primary particles. As a result, lithium manganese oxide newly synthesized by reacting between primary particles becomes
Connects multiple primary particles. Thereby, secondary particles in which many primary particles are aggregated are formed.

As described above, in the present production method, since the primary particles are produced by the spray pyrolysis method as described above, primary particles having a uniform composition and a relatively uniform particle size can be obtained. Can be. Further, in producing the secondary particles, heat treatment is performed in a state where the primary particles and the raw material solution are mixed. Therefore, between the primary particles, a lithium manganese oxide having the same composition is synthesized as a binder, and the primary particles are aggregated. Therefore, secondary particles having a uniform composition can be easily obtained.

Further, as described above, the obtained secondary particles exhibit an excellent effect as a positive electrode active material in a lithium secondary battery.

[0027]

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiment 1 A method for producing a lithium secondary battery and a cathode active material thereof according to an embodiment of the present invention will be described. In this example, as shown in Table 1, various positive electrode active materials were manufactured by the manufacturing method of the present invention and the conventional manufacturing method, and then, a lithium secondary battery using these positive electrode active materials was manufactured. Tested.

First, three types of positive electrode active materials (Sample Nos. E1 to E3) were manufactured by the manufacturing method of the present invention. That is, a raw material solution obtained by dissolving the raw material of the lithium manganese oxide in a solvent is sprayed in the form of droplets, and then the droplets are subjected to a heat treatment so that the raw materials in the droplets react and the droplets are reacted. The solvent was evaporated to produce primary particles.

Next, the primary particles and the raw material solution were mixed and subjected to a heat treatment to prepare secondary particles formed by agglomeration of the primary particles. Note that the lithium manganese oxide constituting the secondary particles has a spinel structure. Hereinafter, a specific description will be given.

First, lithium nitrate and manganese nitrate are
An aqueous solution mixed so that i: Mn was 1: 2 was prepared and used as a raw material solution. The solvent in this case is water. Next, the raw material solution is sprayed into a heating furnace in the form of droplets in a manner similar to the method disclosed in Japanese Patent Application Laid-Open No. 8-236112, and is heated. An electric furnace was used as the heating furnace.

The droplets of the raw material solution were sprayed into the electric furnace while ultrasonically vibrating the raw material solution. Specifically, the raw material solution is put into a solution tank connected to an electric furnace, ultrasonic waves are operated under the condition of 1.63 MHz, and the raw material solution is sucked into the heating furnace in the form of droplets by operating a decompression device. did.

At this time, the temperature of the electric furnace was 800 ° C. As a result, the droplets are rapidly heated in the heating furnace, and primary particles are synthesized. In addition, about 8
The synthesized primary particles were collected by a repairer maintained at 0 to 220 ° C. FIG. 1 shows a drawing substitute photograph of the obtained primary particles. FIG. 1 is an SEM photograph at a magnification of 1000 times. As can be seen from the figure, the primary particles obtained in this example are particles having an average particle size of about 2 μm.

Next, the obtained primary particles and the raw material solution are mixed. The mixing ratio at this time was changed to three types. That is, based on the amount of Li contained in each of the primary particles and the raw material solution, a sample No. having the amount of 10: 1 between the primary particles and the raw material solution was used. E1, 10:
Sample No. 2 E2, 10: 3 sample N
o. It was set to E3.

The heat treatment of the mixture of the primary particles and the raw material solution was performed by holding the mixture in a furnace heated to 700 ° C. for 24 hours. As a result, secondary particles in which the primary particles were aggregated were obtained. As an example, the above sample No. FIG. 2 is a drawing substitute photograph of the secondary particles of E1. FIG. 2 is an SEM photograph at a magnification of 1000 times. As is known from the figure, the secondary particles of this example are formed by aggregating a large number of primary particles.

Further, as shown in Table 1, the average particle size of the obtained secondary particles was as follows: E1 is about 30 μm, sample N
o. E2 is about 50 μm, sample No. E3 was about 70 μm. This is thought to be due to the difference in the mixing ratio between the primary particles and the raw material solution. As shown in Table 1, the apparent density (bulk density) of each secondary particle was 1.9.
２2.2 g / cm ³ , which was a relatively high value. In addition, the larger the average particle size, the higher the apparent density.

Next, for comparison, various cathode active materials were produced by a method other than the production method of the present invention. First, the first comparative material uses the primary particles obtained by the above manufacturing method as an active material as it is (Sample No. C).
4). The apparent density in this case is as shown in Table 1.
The value was as small as ³ g / cm ³ .

The second comparative material was manufactured by a solid phase method in which a raw material of lithium manganese oxide was reacted in a solid state. Specifically, manganese dioxide powder and lithium carbonate were mixed so that Mn: Li contained in them was 2: 1 and kept in a furnace heated to 700 ° C. for 48 hours. Thus, a lithium manganese oxide as a positive electrode active material was obtained.

As the comparative material, those having an average particle size of the manganese dioxide particles of 40 μm and 10 μm were used. Thus, as shown in Table 1, the obtained lithium manganese oxide was also 40 μm (sample N
o. C5) and 10 μm (sample No. C6) were obtained. The apparent density at this time is 40 μm above.
m is a relatively large value of 2.1 g / cm ³ ,
On the other hand, that of the above 10 μm was 1.6 g / cm ³ .

Next, using the above six kinds of positive electrode active materials,
Each produced a lithium secondary battery. The positive electrode is composed of the above positive electrode active materials, acetylene black as a conductive material,
It was produced using PTFE as a binder.

For the negative electrode, Li metal was used as a negative electrode active material. A glass filter was used as a separator. As an organic electrolytic solution to be impregnated therein, a solution obtained by dissolving 1M LiPF ₆ as an electrolyte in a solvent in which EC (ethylene carbonate) and DEC (diethyl carbonate) were mixed at a ratio of 1: 1 was used. The obtained six types of lithium secondary batteries are respectively the same as the positive electrode active material sample Nos. The same sample No.
And

Next, a charge / discharge test was performed using the above-mentioned six types of lithium secondary batteries. The charge and discharge conditions are cut and
OFF voltage 3.5 to 4.5 V, current density 1.0 to 4.0 m
A / cm ² . Table 1 shows the test results.

As can be seen from Table 1, the initial discharge capacity of the sample No. All about 130mA except C5
It showed a high value of h / g. On the other hand, the charge / discharge cycle was repeated. E1 to E3 and C4 are all 110
A high discharge capacity exceeding mAh / g was maintained. However, sample no. For C5 and C6, the discharge capacity was significantly reduced, and the cycle characteristics were poor.

When the above results were converted and evaluated per unit volume of the positive electrode active material used for the positive electrode, as shown in Table 1, Sample No. C4 has a very low initial capacity and a very low capacity at the 100th cycle. C6 also had a poor capacity at the 100th cycle. This corresponds to sample no. C4
Is because the apparent density of the positive electrode active material is low.
Sample No. It is considered that the cycle deterioration rate of C6 is inferior due to the difference in the manufacturing method from the present invention.

The sample No. C5 has the same apparent density as that of the product of the present invention and the positive electrode active material, but has a slightly inferior result to the product of the present invention because the capacity per unit weight of the active material is slightly lower. Thus, the comparative material sample No. C4
-C6 showed a defect in any of the evaluation items, however, Sample No. It can be seen that E1 to E3 show excellent characteristics in any of the initial discharge capacity and cycle characteristics, the initial discharge capacity per unit volume of the positive electrode active material used for the positive electrode, and the discharge capacity at the 100th cycle.

[0045]

[Table 1]

[0046]

As described above, according to the present invention, it is possible to provide a lithium secondary battery having a high initial capacity per positive electrode volume and excellent cycle characteristics, and a method for producing the positive electrode active material thereof. .

[Brief description of the drawings]

FIG. 1 is a photograph as a drawing showing the particle structure of primary particles in the first embodiment.

FIG. 2 is a drawing-substitute photograph showing the particle structure of secondary particles in the first embodiment.

Claims (3)

[Claims] 1. A negative electrode using a negative electrode active material capable of inserting and extracting lithium ions, a positive electrode using a positive electrode active material made of lithium manganese oxide having a spinel structure, and a separator interposed therebetween. And a lithium secondary battery having an organic electrolyte, the lithium manganese oxide is formed from secondary particles having an average particle size of 20 to 100 μm, which are formed by aggregating primary particles having an average particle size of 0.01 to 3.0 μm. A rechargeable lithium battery. 2. The lithium secondary battery according to claim 1, wherein the positive electrode active material has a bulk density of 1.5 to 3.5 g / cm ³ . 3. A method for producing a positive electrode active material for a lithium secondary battery, comprising a lithium manganese oxide having a spinel structure and secondary particles in which primary particles are aggregated, comprising the steps of: The raw material solution obtained by dissolving the raw material of the product in a solvent is sprayed in the form of droplets, and then the droplets are heat-treated to react the raw materials in the droplets and evaporate the solvent in the droplets. Make primary particles,
Next, a method for producing a positive electrode active material for a lithium secondary battery, comprising mixing the primary particles and the raw material solution and subjecting the mixture to heat treatment to produce secondary particles formed by agglomeration of the primary particles. .