JPH10172571A - 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 is assured along with an excellent cyclic characteristic. SOLUTION: A lithium secondary battery is composed of a negative electrode, positive electrode, and organic electrolytic solution, wherein the negative electrode contains an active material capable of occluding and emitting lithium ions, while the positive electrode contains active material consisting of a powder of LiXMnYOZ or LiX(Mn+M)YOZ having spinel structure, (where M is one or more among Co, Ni, Cr, Fe, Ti, V, Mo, B, C), and the powder is of two-layer structure consisting of a center layer and facial layer having different Mn valency.

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. Conventional lithium secondary batteries are disclosed in, for example, JP-A-63-114065, JP-A-63-274059, JP-A-2-37665, JP-A-2-139860, and JP-A-2-27.
As disclosed in Japanese Patent Application Laid-Open Nos. 069, 3-250563 and 4-240117, there is a material using a metal oxide such as LiMn2O4 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.
In this regard, the conventional lithium secondary battery cannot be said to have both high initial capacity characteristics and excellent cycle characteristics.

The present invention has been made in view of the above-mentioned conventional problems, and an object of the present invention is to provide a lithium secondary battery having a high initial capacity and excellent cycle characteristics, and a method for producing the positive electrode active material thereof. It is.

[0005]

According to a first aspect of the present invention, there is provided a lithium secondary battery having a negative electrode, a positive electrode, a separator interposed between the two electrodes, and an organic electrolyte, wherein the negative electrode stores and stores lithium ions. It contains a releasable negative electrode active material, while the positive electrode has a spinel Li _x M
n _Y O _Z or _{Li X (Mn + M) Y} O Z ( wherein, M is Co, Ni, Cr, Fe, Ti, V, Mo, B, in which one or more of C) positive electrode made of powder It contains an active material, and the powder has a two-layer structure composed of a center layer and a surface layer, and its Mn valence is different between the center layer and the surface layer. It is in a lithium secondary battery.

The most remarkable point in the present invention is that the positive electrode active material has a two-layer structure, and the composition of the positive electrode active material in the central layer and the surface layer constituting the positive electrode active material is different. The valences are different. However, any layer also having a spinel structure Li _X Mn _Y O _Z or Li
Is _{X (Mn + M) Y O} Z.

The above Mn in the center layer and the surface layer
The difference in the valence is represented by X / M in the above Li _x Mn _Y O _Z.
It can be realized by changing one or more values of Y, X / Z and Y / Z. Li _x Mn with spinel structure
The Mn valence of _Y O _Z is 3.5 in stoichiometric composition (hereinafter referred to as stoichiometric value).

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 Mn valence of the positive electrode active material is Li
In order to secure the amount, it is necessary that the Mn valence (3.5) is larger than the stoichiometric value. And
The positive electrode active material has such characteristics that the smaller the Mn valence in the above range (closer to 3.5), the higher the discharge capacity, and the larger the expansion / contraction accompanying occlusion / release of Li ions. On the other hand, although the discharge capacity is lower as the Mn valence number is larger, expansion and contraction due to occlusion and release of Li ions are reduced, and the cycle characteristics are improved.

Further, as the positive electrode active material, Li _X M
n _Y O _Z or Li _X (Mn + M) can be used _Y O _Z. M in the Li _x (Mn + M) _Y O _Z is Co, Ni, Cr, Fe, Ti,
V, Mo, B, or C. The addition of these elements can maintain the same effect as in the case of Mn alone.

In particular, when Co is added, an effect is obtained that is effective in improving the cycle deterioration rate of the capacity. In addition, not only Co but also low-valent ions such as Fe, Ni, and Mg having an ionic radius close to Mn ³⁺ can obtain the effect of improving the cycle characteristics.

Next, in producing a positive electrode using the positive electrode active material comprising the two-layered particles, in addition to the positive electrode active material, for example, acetylene black as a conductive material and PTFE as a binder are used. Is used. As the active material used for the negative electrode, it is necessary to use a material capable of occluding and releasing lithium ions. For example, lithium, a lithium alloy, a carbon body, or the like can be used.

Further, examples of the separator interposed between the two electrodes include a porous film of polypropylene and a glass filter. 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. Examples of the organic solvent include ethylene carbonate, diethyl carbonate, propylene carbonate, butylene carbonate, tetrahydrofuran,
One or more solvents selected from -methyltetrahydrofuran, dimethoxyethane, dioxolane and γ-butyrolactone are preferred. Further, as the electrolyte, LiPF ₆ , LiClO ₄ , LiBF ₄ , LiAs
There is F ₆ and the like.

Next, the operation of the present invention will be described. The lithium secondary battery of the present invention has a positive electrode active material comprising the above-mentioned two-layer structure particles, and the Mn valence of the center layer and the surface layer constituting the two-layer structure is different as described above. ing.

Therefore, for example, when the Mn valence of the center layer is small and the Mn valence of the surface layer is large, the center layer has a high discharge capacity structure and the surface layer has a high cycle characteristic structure. In other words, in this case, the central layer exhibits high discharge capacity characteristics, while the surface layer exhibits excellent cycle characteristics, and thus both high initial capacity and excellent cycle characteristics can be achieved in the whole particles. .

Conversely, increasing the Mn valence of the central layer,
When the Mn valence of the surface layer is reduced, the central layer exhibits a small expansion / contraction characteristic, and the surface layer exhibits a high discharge capacity and a large expansion / contraction characteristic. Therefore, the expansion / contraction characteristics during the charge / discharge cycle are reduced in the center layer, and excellent durability is exhibited. In addition, the surface layer exhibits a high discharge capacity as described above. Therefore, also in this case, both high initial capacity and excellent cycle characteristics can be achieved.

As described above, in the present invention, the positive electrode active material has a two-layer structure and the Mn valence of each layer is different, so that both high discharge capacity characteristics and excellent cycle characteristics can be achieved. it can. Therefore, a lithium secondary battery using such a positive electrode active material can exhibit high initial capacity and excellent cycle characteristics.

Next, as the invention of claim 2, the above core layer and the surface layer, while is Li _X Mn _Y O _Z, the other is _{Li X (Mn + M) Y} O Z ( wherein , M is C
o, Ni, Cr, Fe, Ti, V, Mo, B, and C). That is the central layer of Li _X Mn _Y O _Z, whereas the surface layer is Li
_X (Mn + M) may be used as the _Y O _Z, conversely, the central layer is a _{Li X (Mn + M) Y} O Z, whereas the surface layer may be a Li _X Mn _Y O _Z. In this case, the same effect as the first aspect can be obtained.

Next, there is the following invention as a method for producing the positive electrode active material in the excellent lithium secondary battery. That is, according to the third aspect of the present invention, there is provided a method for producing a positive electrode active material for a lithium secondary battery comprising a powder having a two-layer structure of a center layer and a surface layer.
Prepare a powder for the central layer consisting of _X Mn _Y O _Z ,
The powder for the center layer is added to a solution containing Li ions and Mn ions and mixed to prepare a mixed raw material, and then the mixed raw material is heated to obtain a powder having a two-layer structure. There is a method for producing a positive electrode active material for a lithium secondary battery.

The most remarkable point in the present production method is that the mixed raw material is prepared from the center layer powder and the solution, and then the mixed raw material is heated. The powder for the center layer can be obtained, for example, by various conventional production methods. For example, it can be prepared by a solid phase method in which raw materials are reacted in a solid state, or a spray pyrolysis method using a raw material solution in a liquid state as shown in JP-A-8-236112.

As described above, the solution is a solution containing Li ions and Mn ions, and can be prepared using, for example, various compounds and various solvents described below. Further, this solution is prepared so that Li _X Mn _Y O _Z having a composition different from the composition of the particles for the center layer is synthesized.

The mixed raw materials are mixed so that the particles for the central layer are sufficiently covered with the solution. Further, the heat treatment is preferably performed in an oxygen-excess atmosphere. As a result, an effect is obtained in which part of Li and Mn in the spinel structure of Li _x Mn _Y O _Z is vacant (deleted) to facilitate diffusion of Li ions.

The temperature of the heat treatment is 300 ° C.
It is preferably performed at 1200 ° C. If the temperature is lower than 300 ° C., the heat treatment is insufficient.
If the number exceeds the limit, there is a problem that stable operation of the apparatus becomes difficult.

According to the present production method, heat treatment of the mixed raw material causes a new amount of L around the center layer particles.
i _X Mn _Y O _Z are combined surface layer and Naruru. This Li
The composition of the _X Mn _Y O _Z can be easily changed by the preparation of the solution. Therefore, M between the central layer and the surface layer
A positive electrode active material composed of particles having a two-layer structure having different n valences can be easily produced.

Next, as in the invention of claim 4, it is preferable that the heat treatment of the mixed raw material is performed in a state of being sprayed in the form of droplets. That is, it is preferable to perform spray pyrolysis using the mixed raw material obtained by mixing the particles for the center layer and the solution.
In this case, the formation of the droplets makes it possible to easily obtain two-layer particles having a uniform surface layer composition and a relatively uniform particle size.

According to a fifth aspect of the present invention, the heating of the mixed raw material is performed by heating the container from which the raw material is sprayed from the outside, heating by injecting a heated gas into the spray container, and removing the solvent. Any one or more of heating by combustion heat 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 the case of heating by burning of the solvent, the primary particles can be synthesized instantaneously, and the effect of improving the uniformity of the composition can be obtained.

Further, according to the present invention, the Li
The solution containing ions and Mn ions comprises a lithium compound and manganese or a manganese compound, wherein the lithium compound is one of the oxides, hydroxides, carbonates, nitrates, acetates or oxalates of Li. The compounds are preferably the above compounds. These are relatively easy to ionize in a solution and are excellent in improving uniformity.

Further, as in the invention of claim 7, the manganese compound is preferably at least one of oxides, hydroxides, carbonates, nitrates, acetates and oxalates of Mn. As described above, these are relatively easily ionized in a solution and are excellent in improving uniformity.

Further, as in the invention of claim 8, as the solvent of the solution, for example, one or more of water, an aqueous acid solution, an aqueous alkali solution, and an organic solvent can be used.

[0031]

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, as shown in Table 1, four types of positive electrode active materials as the product of the present invention were produced (Sample Nos. E1 to E).
4). Sample No. E1 and E2 are L values of the spinel structure in advance.
i _X Mn _Y O _Z , a powder for the center layer is prepared, and then the powder for the center layer is added to and mixed with a solution containing Li ions and Mn ions to produce a mixed raw material. Was sprayed in the form of droplets and heat-treated. That is, the mixed raw material was treated by the spray pyrolysis method.

Specifically, the sample No. In preparing the positive electrode active material of E1, a powder having a Mn valence of 3.56 was prepared as the powder for the center layer.
Lithium nitrate and manganese nitrate have a Li: Mn of 0.53
An aqueous solution prepared to be 6: 1 was prepared. The powder for the center layer and the solution were mixed to obtain a mixed raw material. The solvent in this case is water.

Next, the mixed raw material 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 droplet spray of the raw material solution was performed by sucking the raw material solution into an electric furnace while ultrasonically vibrating. 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 droplet is rapidly heated in the heating furnace, a surface layer is synthesized around the powder for the center layer, and a positive electrode active material composed of particles having a two-layer structure is obtained. Then, the positive electrode active material was collected by a repairer connected to an electric furnace and maintained at about 80 to 220 ° C.

The obtained positive electrode active material (sample No. E1)
As shown in Table 1, the Mn valence of the central layer is 3.56,
The Mn valence of the surface layer was 3.51. In addition, Li _X M
The value of n _Y O _Z of X / Z is 0.25 in the center layer had the surface layer becomes 0.24.

Next, the sample no. In producing the positive electrode active material of E2, the Mn valence was 3.
52 were prepared, and as the solution, an aqueous solution prepared by preparing lithium nitrate and manganese nitrate so that Li: Mn was 0.512: 1 was prepared. For others, sample N
o. Same as E1.

Thus, the obtained positive electrode active material (sample N
o. In E2), as shown in Table 1, the Mn valence of the central layer was 3.52, and the Mn valence of the surface layer was 3.65. The value of Li _X Mn _Y O _Z of X / Z is 0.25 in the center layer had the surface layer becomes 0.26.

Next, the sample no. In E3 and E4, first, a powder for the center layer of Li _x Mn _Y O _{Z having} a spinel structure was prepared.
Next, the powder for the center layer was added to and mixed with a solution containing Li ions and Mn ions to prepare a mixed raw material, and then the mixed raw material was heat-treated. The heat treatment in this case was performed by simply heating in an oxygen (or air) atmosphere.

Specifically, the sample No. In preparing the cathode active material of E3, a powder having a Mn valence of 3.6 was prepared as the powder for the center layer, and the above solution was prepared by mixing lithium nitrate and manganese nitrate with Li: Mn of 0.56. :
An aqueous solution prepared to be 1 was prepared. The powder for the center layer and the solution were mixed to obtain a mixed raw material. The solvent in this case is also water.

Next, the mixed raw material was subjected to a heat treatment by keeping it in a heating furnace at a temperature of 800 ° C. in an oxygen atmosphere for 2 hours. As a result, the solution around the center layer powder reacted to synthesize a surface layer composed of Li _x Mn _Y O _Z , and a positive electrode active material composed of particles having a two-layer structure was obtained.

The obtained positive electrode active material (Sample No. E3)
As shown in Table 1, the Mn valence of the central layer was 3.6, and the Mn valence of the surface layer was 3.64. Li _X Mn
The X / Z value of _Y O _Z was 0.24 in the center layer and 0.25 in the surface layer.

Next, the sample no. In producing the positive electrode active material of E4, the powder for the center layer had an Mn valence of 3.
62 were prepared, and as a solution, an aqueous solution prepared by preparing lithium nitrate and manganese nitrate such that Li: Mn was 0.572: 1 was prepared. For others, sample N
o. Same as E3.

Thus, the obtained positive electrode active material (sample N
o. In E4), as shown in Table 1, the Mn valence of the central layer was 3.62, and the Mn valence of the surface layer was 3.58. The value of Li _X Mn _Y O _Z of X / Z is 0.25 in the center layer had the surface layer becomes 0.25. The sample No. The powder for the center layer from E1 to E4
In each case, a raw material solution is prepared to have a desired composition,
This was processed by a spray pyrolysis method and used.

Next, three types of positive electrode active materials were prepared for comparison (Sample Nos. C5 to C7). Sample No. As the positive electrode active material of C5, particles were produced from the above solution by a spray pyrolysis method, and this was used as it was as a positive electrode active material. That is, the particles have a single particle structure instead of a two-layer structure.

Thus, the obtained positive electrode active material (sample N
o. As for C5), as shown in Table 1, the Mn valence of the central portion and the surface portion were both 3.54. In addition, Li _X
The value of Mn _Y O _Z of X / Z had also become 0.25 in both the central portion and the surface portion.

Next, the sample No. The C6 positive electrode active material is a single-layer particle produced by a so-called solid phase method. That is,
As raw materials, LiCO ₃ (lithium carbonate) and MnO
₂ (manganese dioxide) and Li: Mn 0.548: 1
Use the mixture so that Then, this raw material is heated in an oxygen atmosphere at a temperature of 800 ° C. for 20 hours.
Thereafter, the pulverized powder was pressed and reacted by reheating, and repeatedly fired so that the structure became uniform, thereby synthesizing a positive electrode active material.

Thus, the obtained positive electrode active material (sample N
o. For C6), as shown in Table 1, the Mn valence of both the central portion and the surface layer portion was 3.58. In addition, Li _X
The value of Mn _Y O _Z of X / Z had also become 0.26 in both the central portion and surface layer portion.

Next, the sample No. In producing the positive electrode active material of C7, the above sample No. The mixing ratio of the C6 raw material was changed so that Li: Mn became 0.584: 1. For other samples, Same as C6. As a result, the obtained positive electrode active material (Sample No. C7)
As shown in Table 1, the Mn valence of the central portion and the surface portion both became 3.64. In addition, _X of Li _X Mn _Y O _Z
The value of / Z was 0.28 in both the central part and the surface part.

Next, using the above seven 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 (polytetrafluoroethylene) as a binder.

For the negative electrode, Li metal was used as a negative electrode active material. As the organic electrolyte, a solution obtained by dissolving 1M LiPF ₆ in a solution obtained by mixing EC (ethylene carbide) and DEC (diethylene carbonate) at a ratio of 1: 1 was used. The obtained seven types of lithium secondary batteries are respectively referred to as the positive electrode active material sample N for convenience of explanation.
o. The same sample No. And

Next, a charge / discharge test was performed using the above seven 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 products of the present invention (Sample Nos. E1 to E4) all had high initial capacities and a discharge capacity of about 110 mAh / g even at the discharge capacity at the 100th charge / discharge cycle. Excellent results were maintained at high values. On the other hand, the comparative products (sample Nos. C5 to C5)
In 7), although the initial capacity was good, the discharge capacity at the 100th cycle was significantly deteriorated.

In this example, L was used as the positive electrode active material.
i _X Mn was shown a case of using a _Y O _Z is, for the case of using the _{Li X (Mn + M) Y} O Z as a positive active material also, it was confirmed similarly that good results of cycle characteristics are obtained .

[0055]

[Table 1]

[0056]

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

Claims (8)

[Claims] 1. A lithium secondary battery comprising a negative electrode, a positive electrode, a separator interposed between the two electrodes, and an organic electrolyte, wherein the negative electrode contains a negative electrode active material capable of inserting and extracting lithium ions. and which, on the other hand, the positive electrode, Li _X Mn _Y O _Z or Li _X (Mn having a spinel structure +
M) _Y O _Z (where M is Co, Ni, Cr, Fe, T
i, V, Mo, B, and C), and the powder has a two-layer structure including a center layer and a surface layer. With
A lithium secondary battery, wherein the Mn valence is different between the center layer and the surface layer. 2. The method according to claim 1, wherein one of the center layer and the surface layer is Li _x Mn _Y O _Z , and the other is Li _x (Mn + M) _Y O _Z (where M is Co, Ni,
And at least one of Cr, Fe, Ti, V, Mo, B, and C). 3. A method for producing a positive electrode active material for a lithium secondary battery comprising a powder having a two-layer structure of a center layer and a surface layer, wherein a center layer comprising Li _x Mn _Y O _Z having a spinel structure is provided. Powder for the center layer, and then the powder for the center layer is Li
A positive electrode for a lithium secondary battery, characterized by adding and mixing a solution containing ions and Mn ions to produce a mixed material, and then subjecting the mixed material to heat treatment to obtain a powder having a two-layer structure. Active material manufacturing method. 4. The method for producing a positive electrode active material for a lithium secondary battery according to claim 3, wherein the heat treatment of the mixed raw material is performed in a state of being sprayed in a droplet form. 5. The method according to claim 4, wherein the heating of the mixed raw material is performed by externally heating a container for spraying the raw material, heating by injecting a heated gas into the spray container,
A method for producing a positive electrode active material for a lithium secondary battery, wherein at least one of heating by combustion heat of a solvent is used. 6. The method according to claim 3, wherein:
The solution containing the above-mentioned Li ion and Mn ion comprises a lithium compound and manganese or a manganese compound,
The method for producing a positive electrode active material for a lithium secondary battery, wherein the lithium compound is at least one compound among oxides, hydroxides, carbonates, nitrates, acetates, and oxalates of Li. . 7. The lithium secondary battery according to claim 6, wherein the manganese compound is one or more of oxides, hydroxides, carbonates, nitrates, acetates, and oxalates of Mn. A method for producing a positive electrode active material for a battery. 8. The method according to claim 3, wherein:
A method for producing a positive electrode active material for a lithium secondary battery, wherein the solvent of the solution is at least one of water, an aqueous acid solution, an aqueous alkaline solution, and an organic solvent.