JPH10172570A - Lithium secondary battery and its positive electrode active material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium secondary battery and its positive electrode active material, with which deterioration of the discharge capacity is less in the initial cycle and for a long-term cycle and also a high initial capacity is assured. 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 LiXMnYOZ having spinel structure, a separator interposed between the two electrodes, and an organic electrolytic solution. In the given chemical formula, X, Y, and Z satisfy the condition 0<Z-(X+Y)×(4/3)<=0.15, and the analytical value (m) Mn valence for LiXMnYOZ is related as 3.5<m<=3.75.

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 positive electrode active material thereof.

[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, the above-mentioned conventional lithium secondary battery has a problem that the discharge capacity is rapidly reduced when the charge / discharge cycle is repeated. On the other hand, a lithium secondary battery as disclosed in, for example, Japanese Patent Application Laid-Open No. 2-37665 has been proposed to prevent a decrease in discharge capacity during charging and discharging.

[0004] This conventional lithium secondary battery is a lithium secondary battery using lithium manganese oxide (Li _x Mn _Y O _Z ) as a positive electrode active material.
The atomic ratio between i and Mn and the Mn valence analysis value are specified to specific values. However, in this conventional lithium secondary battery, although a certain effect can be obtained with respect to a decrease in the discharge capacity due to a long charge / discharge cycle after about 10 cycles, the discharge capacity in the initial cycle up to about 10 cycles is reduced. Is still severely degraded.

The present invention has been made in view of such conventional problems, and provides a lithium secondary battery and a positive electrode active material having a high initial capacity with little deterioration in discharge capacity in an initial cycle and a long cycle. What you want to do.

[0006]

According to the first aspect of the present invention, there is provided 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 made of Li _x Mn _Y O _Z having a spinel structure. , A lithium secondary battery having a separator interposed therebetween and an organic electrolyte,
_X Mn _Y O _Z in X, Y, Z is, 0 <Z- (X +
Y) satisfy the relationship × (4/3) ≦ 0.15, and the Li _X Mn _Y O Mn valence analysis m in _Z is 3.
A lithium secondary battery is characterized in that 5 <m ≦ 3.75.

[0007] Most notably in the present invention, the Li _X Mn _Y O _Z in X, Y, Z is 0 <Z-
The relationship of (X + Y) × (4/3) ≦ 0.15 is satisfied, and the Mn valence analysis value m in the above Li _x Mn _Y O _Z
Is that 3.5 <m ≦ 3.75.

The value of Z− (X + Y) × (4/3) (hereinafter referred to as “A value” as appropriate) is the value of Li _{x in} the spinel structure.
Is a value representing the excess oxygen in the Mn _Y O _Z. When the A value is 0 or less, there is a problem that the oxygen amount in Li _x Mn _Y O _Z does not become excessive and the capacity deterioration rate cannot be improved. On the other hand, if it exceeds 0.15, there is a problem that oxygen becomes excessively large, and conversely, the capacity deterioration rate becomes worse.

The Mn valence analysis value m (hereinafter, appropriately referred to as m value) is an index indicating the average ionic valence of Mn, and can be measured by the following iodometry method. That is, KI is added to a sample, dissolved in hydrochloric acid, all Mn is reduced to divalent by iodine ion (I ⁻ ), and the amount of I ₂ generated thereby is determined by redox titration with Na ₂ S ₂ O _3. I do. Since the amount of I ₂ is equal to the amount of electron transfer required for Mn reduction,
The average valence is obtained by dividing by the Mn concentration in the sample obtained by P emission spectroscopy and adding the valence of reduced Mn to 2. This average valence is the above-mentioned Mn valence analysis value m.

The above-mentioned m value is characterized in that the smaller the value is, the larger the obtained initial capacity is. However, when the value is 3.5 or less, there is a problem that the capacity deterioration rate becomes large.
On the other hand, when it exceeds 3.75, there is a problem that the initial capacity becomes too small.

Next, as a method for preparing a positive electrode active material in which the A value and the m value are regulated to the above specific values, for example, as described later, there is a method of rapid synthesis by a thermal spray decomposition method. In this method, oxygen deficiency is suppressed and oxygen excess is adjusted by controlling the oxygen partial pressure in the spray atmosphere, and Li _x Mn _Y O having a desired composition is adjusted.
_Z can be obtained.

In addition, Li _X M which already has oxygen vacancies
Prepare the n _Y O _Z, which is also a method of adding oxygen by heat treatment in air or in an oxygen atmosphere. On the other hand, Li _x Mn, which already has too much oxygen,
There is also a method in which _Y O _Z is prepared and heat-treated in a vacuum or the like to obtain Li _X Mn _Y O _Z with an appropriate excess amount of oxygen.

Next, in forming a positive electrode using the above-mentioned positive electrode active material, for example, acetylene black or the like is used as a conductive material, and PTFE or the like is used as a binder. In addition, as the negative electrode active material used for the negative electrode,
It is necessary to use a substance capable of occluding and releasing lithium ions, and lithium (metallic lithium), a lithium alloy, a carbon body, or the like can be used.

As the 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 organic solvent, one or more solvents selected from ethylene carbonate, diethyl carbonate, propylene carbonate, butylene carbonate, tetrahydrofuran, 2-methyltetrahydrofuran, dimethoxyethane, dioxolane and γ-butyrolactone are preferred. As the electrolyte, LiPF ₆ , LiClO ₄ , LiBF ₄ ,
LiAsF _{6 and the} like.

Next, the operation of the present invention will be described. The lithium secondary battery of the present invention has an A value and m in the above specific range.
Li _X Mn _Y O _Z having a certain value is used as the positive electrode active material. Therefore, the initial discharge capacity (initial capacity) is high,
Further, even if the charge / discharge cycle is repeated, the capacity deterioration can be reduced over a long period from the initial stage.

The reason is considered as follows. That is, in the present invention, by maintaining the value A in a specific range, Li _X Mn _Y O _Z as a positive electrode active material can be moderately excessive in oxygen. The oxygen-excess state, Li _X Mn _Y O _Z Li spinel structure in, Mn
Is vacant (deleted). Therefore, the presence of the vacancies facilitates the repeated diffusion of Li ions during charge and discharge.

In particular, by forming vacancies at the 8a site and the 16d site in the spinel structure, diffusion of Li ions can be made very easy. Therefore, damage to the spinel structure during charging and discharging is reduced, and deterioration of the positive electrode is greatly suppressed.

On the other hand, the initial discharge capacity can be adjusted by the above-mentioned Mn valence analysis value m. By adjusting the value within the above range, a high discharge capacity can be maintained. Here, in the prior art, the higher the initial capacity, the greater the damage to the positive electrode, and the more the capacity deterioration rate tends to increase rapidly. On the other hand, in the present invention, diffusion of Li ions can be facilitated as described above by regulating the A value.
Therefore, damage to the positive electrode during charge and discharge is reduced, and capacity deterioration can be significantly improved even in the initial stage where the discharge capacity is very large.

Next, there is the following invention as a positive electrode active material used for the above-mentioned excellent lithium secondary battery. That is, as in the invention of claim 2, Li _x Mn _Y having a spinel structure
Consists O _Z, X in said _{Li X Mn Y O Z, Y} , Z
Is, 0 <Z- (X + Y ) satisfy the relationship × (4/3) ≦ 0.15, and the Li _X Mn _Y O Mn valence analysis m in _Z is, 3.5 <m ≦ 3. 75. A positive electrode active material for a lithium secondary battery having a relationship of 75
A raw material solution obtained by dissolving a raw material of _X Mn _Y O _{Z 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 to form the solvent in the droplets. There is a positive electrode active material for a lithium secondary battery, which is obtained by evaporating.

Most notably, in the present invention, in order to obtain the positive electrode active material used in the first aspect of the present invention, the above-mentioned raw material is sprayed on a raw material solution in a solvent, and this is heated to remove the solvent. That is, a manufacturing process in which the above-described raw materials are reacted together with the evaporation is selected. The reason for limiting the range of the value (A value) of [Z− (X + Y) × (4/3)] and the Mn valence analysis value m is the same as above.

The above-mentioned heat treatment means that a droplet having substantially the same composition as the raw material solution is heated to be thermally decomposed to cause the raw material components in the droplet to undergo a solution reaction and to evaporate the solvent in the droplet. Refers to operations. As a result, a powdery positive electrode active material and a solvent vapor generated by the reaction are generated. The two are separated, for example, in a collector, and a positive electrode active material can be obtained.

In the heating treatment, the raw material can be heated in a container for spraying the raw material in the form of droplets by various methods described later. 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. and so on.

It is preferable that the atmosphere in the heat treatment is controlled to have an oxygen partial pressure of 0.2 to 1 atm. If the pressure is less than 0.2 atm, oxygen deficiency is likely to occur in the spinel structure, and the A value becomes too low. On the other hand, when the pressure exceeds 1 atm, there is a problem that oxygen becomes excessive.

Next, in the present invention, the following operation is obtained. That is, in the present invention, the raw material is dissolved in a solvent to form a raw material solution. Therefore, the raw materials can be made uniform. Further, such a uniform raw material solution is sprayed on droplets, and then the raw materials are reacted as described above. Therefore, the raw material reacts instantaneously in the droplet by the heat during the heat treatment to become a positive electrode active material. On the other hand, the solvent in the droplets evaporates as unnecessary portions due to heat during the heat treatment, and is released outside the reaction system.

Therefore, when this manufacturing process is selected,
By controlling the raw material composition, the atmosphere during the heat treatment, and the like, it is possible to obtain a positive electrode active material having a uniform composition having the A value and the m value in the specific range. Also,
Compared to the case where other known manufacturing processes are selected, it is very easy to adjust the structure to have the A value and m value in the above specific range, and a more uniform active material can be obtained. . Therefore, the positive electrode active material obtained according to the present invention can exhibit excellent functions when used in a lithium secondary battery.

Next, according to the third aspect of the present invention, the Li
The raw material of _X Mn _Y O _Z comprises a lithium compound and manganese or a manganese compound, and the lithium compound is one of oxides, hydroxides, carbonates, nitrates, acetates, and oxalates of Li. It is preferable that it is above. These are relatively easy to ionize in a solution and are excellent in improving uniformity.

Also, as in the invention of claim 4, the manganese compound is preferably at least one of oxides, hydroxides, carbonates, nitrates, acetates and oxalates of Mn. These are relatively easy to ionize in a solution and are excellent in improving uniformity.

Further, as in the invention of claim 5, as the solvent, for example, one or more of water, an aqueous acid solution, an aqueous alkali solution, and an organic solvent are used.

Further, as in the invention of claim 6, the heat treatment includes heating from the outside of the container for spraying the raw material, heating by injecting a heating gas into the container, and heating by combustion heat of the solvent. Any one or more of them can be used.

In the case of heating from outside 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.

[0031]

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiment 1 A lithium secondary battery according to an embodiment of the present invention will be described. The lithium secondary battery of the present embodiment includes a negative electrode containing lithium, a positive electrode using a positive electrode active material made of Li _X Mn _Y O _Z having a spinel structure, and an organic electrolytic solution.

[0032] X in the above _{Li X Mn Y O Z, Y} , Z
Is, 0 <Z- (X + Y ) satisfy the relationship × (4/3) ≦ 0.15, and the Li _X Mn _Y O Mn valence analysis m in _Z is, 3.5 <m ≦ 3. There is a relationship of 75. Hereinafter, the method for producing the positive electrode active material will be described.

The production of the positive electrode active material in the present example
A raw material solution obtained by dissolving the raw material of i _x Mn _Y O _{Z 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 in the droplets are reacted. The method was performed by evaporating the solvent to produce the powdered positive electrode active material.

Specifically, first, lithium nitrate and manganese nitrate were weighed so that the atomic ratio of Li to Mn was 1: 2, and a 10% by weight solution was prepared using water as a solvent. It is a liquid mixture. Next, the atmosphere and temperature in a container (heat treatment furnace) for spraying the mixture are set to predetermined conditions.

Next, the above mixture is sprayed into a heat treatment furnace. The sprayed mixture is rapidly heated in a heat treatment furnace, and the organic solvent is burned and powdery Li _X Mn _Y O _Z is rapidly synthesized. The synthesis time was adjusted by changing the spray speed. Next, the obtained powdery Li _X Mn _Y O _Z is collected by a collector connected to a heat treatment furnace.

In this example, the sample N
o. As shown in E1 to E4, the A value (Z− (X + Y)) of the Li _x Mn _Y O _Z is controlled by controlling conditions such as the atmosphere, temperature, and synthesis time in the heat treatment furnace.
× (4/3) value) and the Mn valence analysis value m were various and powders of the positive electrode active material within the range of the present invention were obtained. The sample No. E5 is the sample No. E4 Li _X
It is obtained by heat treatment for 48 hours at Mn _Y O _Z powder further temperature of 700 degrees in the air.

For comparison, a sample prepared by the same spray pyrolysis method as above and having the above-mentioned m value outside the range of the present invention (sample Nos. C1 and C2) and the above-mentioned A value were compared with those of the present invention. Samples out of the range (Sample Nos. C3 and C4) were prepared. Further, for comparison, samples prepared by a solid phase method in which the raw materials were reacted in a solid phase state and the A value was out of the range of the present invention (sample Nos. C5 to C8) were also prepared. The active material obtained by the solid phase method was obtained by mixing lithium nitrate and electrolytic manganese dioxide, firing at a temperature of 700 ° C. for 120 hours, and then pulverizing.

Next, in this example, a lithium secondary battery was constructed using each of the above-mentioned positive electrode active materials, and the performance thereof was tested. In manufacturing a lithium secondary battery, first, a positive electrode was manufactured using the positive electrode active material, acetylene black as a conductive material, and PTFE (polytetrafluoroethylene) as a binder.

The negative electrode was composed of Li metal as a negative electrode active material. Further, as the organic electrolyte, EC (ethylene carbonate) and DEC (diethyl carbonate) were used in a ratio of 1: 1.
A solution obtained by dissolving 1M LiPF ₆ in a solution mixed at a ratio of 1% was used.

Next, a charge / discharge test was performed using a lithium secondary battery having a positive electrode composed of the positive electrode active materials having various compositions described above. The charge and discharge conditions are cut-off voltage 3.5 to
The voltage was 4.5 V, and the current density was 1.0 to 4.0 mA / cm ² . The method for measuring the Mn valence is as described above.

The test results are shown in Table 1 and FIGS. In FIG. 1, the horizontal axis represents the Mn valence analysis value m, and the vertical axis represents the initial capacity (mAh / g). In FIG. 2, the horizontal axis shows the initial capacity (mAh / g), and the vertical axis shows the capacity deterioration rate (% / cycle). The capacity deterioration rate is [｛(initial capacity)-(5
0th cycle capacity)｝ / ｛(initial capacity) × (50−
1)｝] × 100.

As can be seen from FIG. 1, the initial capacity is M
The lower the n-valency analysis value m, the higher the value, and no clear difference was observed between the product of the present invention and the comparative product. And m
When the value was more than 3.5 and not more than 3.75, good initial capacities were obtained in all the test products. However, regarding the capacity deterioration rate, as is known from FIG.
It was found that the manner of deterioration was clearly divided into two groups.

That is, the first group is a group mainly including the product of the present invention, and the capacity deterioration rate is relatively low even if the initial capacity is high. On the other hand, the second group is Mn valence,
This is a group of comparative products in which one of the A values is out of the range of the present invention, and the capacity deterioration rate is large. As can be seen from FIGS. 1 and 2 and Table 1, when the product of the present invention is compared with the comparative product, even when the Mn valence is almost the same and the initial capacity is almost the same, the product of the present invention has a capacity deterioration rate. Is suppressed to a low level, which is very good.

[0044]

[Table 1]

[0045]

As described above, according to the present invention, it is possible to provide a lithium secondary battery and its positive electrode active material, which have a small deterioration in discharge capacity in an initial cycle and a long cycle and a high initial capacity. .

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing a relationship between an Mn valence analysis value m and an initial capacity in the first embodiment.

FIG. 2 is an explanatory diagram showing a relationship between an initial capacity and a capacity deterioration rate in the first embodiment.

Claims (6)

[Claims] And 1. A negative electrode using lithium ions capable of absorbing and releasing the negative electrode active material, Li _X M having a spinel structure
n _Y O and a positive electrode using the positive electrode active material consisting of _Z, a separator interposed between them, a lithium secondary battery having an organic electrolyte solution, X in the _{Li X Mn Y O Z, Y} , Z Is 0 <Z− (X + Y) × (4/3) ≦ 0.
15. A lithium secondary battery which satisfies the relationship of No. 15 and wherein the Mn valence analysis value m of Li _X Mn _Y O _Z satisfies the relationship of 3.5 <m ≦ 3.75. 2. Li _X Mn _Y O _Z having a spinel structure
More becomes, X in said _{Li X Mn Y O Z, Y} , Z is
0 <Z- (X + Y) satisfies the × (4/3) ≦ 0.15 relationships, and the Li _X Mn _Y O Mn valence analysis m in _Z is, 3.5 <m ≦ 3.75 in a positive electrode active material for lithium secondary batteries have a relationship, the Li _X M
The material of the n _Y O _Z spraying the raw material solution in a solvent in droplet form, then heating the droplets, the solvent in the liquid droplets with reacting the material in the droplets A positive electrode active material for a lithium secondary battery, which is obtained by evaporating. 3. The method according to claim 2, wherein the Li _x Mn _Y O
The raw material for _Z consists of a lithium compound and manganese or a manganese compound, and the lithium compound is at least one of oxides, hydroxides, carbonates, nitrates, acetates, and oxalates of Li. A positive electrode active material for a lithium secondary battery, comprising: 4. The lithium secondary battery according to claim 3, wherein the manganese compound is at least one of oxides, hydroxides, carbonates, nitrates, acetates and oxalates of Mn. Positive electrode active material. 5. The method according to claim 2, wherein
The positive electrode active material for a lithium secondary battery, wherein the solvent is at least one of water, an aqueous acid solution, an aqueous alkali solution, and an organic solvent. 6. The method according to claim 2, wherein:
The heat treatment is performed by using at least one of heating from the outside of the container for spraying the raw material, heating by injection of a heating gas into the spray container, and heating by combustion heat of a solvent. A positive electrode active material for secondary batteries.