JP2010225309A - Method for manufacturing cathode active material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a cathode active material for effectively restraining generation of high-resistance layers. <P>SOLUTION: The method for manufacturing the cathode active material including an oxide cathode active material and a coating part consisting of lithium niobate system compound formed on the surface of the oxide cathode active material, includes a coating process coating a coat solution containing a precursor of the lithium niobate system compound on the surface of the oxide cathode active material to form a precursor coating part, and a heat treatment process putting the oxide cathode active material having the precursor coating part under heat treatment in an atmosphere with oxygen concentration of 25 volume% or less. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a method for producing a positive electrode active material capable of effectively suppressing generation of a high resistance layer.

  With the rapid spread of information-related equipment and communication equipment such as personal computers, video cameras, and mobile phones in recent years, the development of batteries (eg, lithium batteries) that are excellent as power sources has been regarded as important. In fields other than information-related equipment and communication-related equipment, for example, in the automobile industry, development of lithium batteries and the like used for electric cars and hybrid cars is being promoted.

  Here, since a commercially available lithium battery uses an organic electrolyte solution that uses a flammable organic solvent, it has a structure to prevent the installation of a safety device and to prevent a short circuit. Improvements in materials are necessary. In contrast, an all-solid-state battery in which the liquid electrolyte is changed to a solid electrolyte does not use a flammable organic solvent in the battery, so the safety device can be simplified, and the manufacturing cost and productivity are considered excellent. Yes.

In the field of such all-solid-state batteries, there have been attempts to improve the performance of all-solid-state batteries by paying attention to the interface between the oxide positive electrode active material and the solid electrolyte material. For example, Non-Patent Document 1 discloses a material in which the surface of LiCoO ₂ (oxide positive electrode active material) is coated with LiNbO ₃ (lithium niobate). This technique is to coat the surface of LiCoO ₂ in LiNbO _3, reduce the interfacial resistance of LiCoO ₂ and a solid electrolyte material, those which attained higher output of the battery. Patent Document 1 discloses an electrode material for an all-solid-state secondary battery that has been surface-treated with sulfur and / or phosphorus. This is intended to improve the ion conduction path by surface treatment. Patent Document 2 discloses a sulfide-based solid battery in which lithium chloride is supported on the surface of an oxide positive electrode active material. This is intended to reduce the interfacial resistance by supporting lithium chloride.

JP 2008-027581 A JP 2001-052733 A

Narumi Ohta et al., “LiNbO3-coated LiCoO2 as cathode material for all solid-state lithium secondary batteries”, Electrochemistry Communications 9 (2007) 1486-1490

As described in Non-Patent Document 1, when the surface of LiCoO ₂ is coated with LiNbO ₃ , the interface resistance between LiCoO ₂ and the solid electrolyte material can be reduced. This is considered to be because it is possible to suppress the formation of a high resistance layer by using LiNbO ₃ to react LiCoO ₂ and the solid electrolyte material. Further, Non-Patent Document 1, on the surface of LiCoO _2, a coating solution containing the precursor of the LiNbO ₃ was applied, followed by heat treatment in pure oxygen atmosphere, coating the surface of LiCoO ₂ in LiNbO ₃ A method is disclosed. However, the material (positive electrode active material) obtained by this method has a problem that the formation of the high resistance layer cannot be effectively suppressed. This invention is made | formed in view of the said problem, and it aims at providing the manufacturing method of the positive electrode active material material which can suppress the production | generation of a high resistance layer effectively.

As a result of intensive studies by the present inventors to solve the above-mentioned problems, the reason why the coating portion made of LiNbO ₃ cannot effectively suppress the generation of the high resistance layer is that by heat treatment in an atmosphere having a high oxygen concentration, It was found that the components of the coat portion penetrated into the oxide positive electrode active material and the thickness of the coat portion formed on the surface of the oxide positive electrode active material was reduced. Therefore, when heat treatment was performed in an atmosphere having a low oxygen concentration, it was found that the decrease in the thickness of the coat portion due to the penetration of the coat portion component was suppressed. The present invention has been made based on such knowledge.

  That is, in the present invention, a method for producing a positive electrode active material having an oxide positive electrode active material and a coat portion made of a lithium niobate compound formed on the surface of the oxide positive electrode active material, A coating solution containing a precursor of the lithium niobate-based compound is applied on the surface of the oxide positive electrode active material to form a precursor coating portion, and an oxide positive electrode having the precursor coating portion And a heat treatment step of heat treating the active material in an atmosphere having an oxygen concentration of 25% by volume or less.

  According to the present invention, by performing the heat treatment in the above oxygen concentration atmosphere, it is possible to suppress the components of the coat portion from penetrating into the oxide positive electrode active material. Thereby, it can prevent that the thickness of a coating part becomes thin too much, and can suppress the production | generation of a high resistance layer effectively.

In the above invention, the lithium niobate compound is preferably lithium niobate (LiNbO ₃ ). This is because the generation of the high resistance layer can be effectively suppressed.

  In the said invention, it is preferable that the average particle diameter of the said oxide positive electrode active material is 7.8 micrometers or less. It is because it can suppress more effectively that the component of a coat part penetrates into the inside of an oxide cathode active material.

  In the said invention, it is preferable that the average particle diameter of the said oxide positive electrode active material is 2.6 micrometers or less. This is because the reaction of desorbing oxygen from the lithium niobate compound can be suppressed.

  In the present invention, a positive electrode active material layer forming step of forming a positive electrode active material layer using a composition containing a positive electrode active material obtained by the above-described method for producing a positive electrode active material, and a sulfide solid And a solid electrolyte layer forming step of forming a solid electrolyte layer using a composition containing an electrolyte material. A method for producing an all-solid battery is provided.

  According to the present invention, by using the positive electrode active material obtained by the above-described manufacturing method, it is possible to effectively suppress the generation of the high resistance layer and obtain an all-solid battery excellent in use durability. Can do.

It is a schematic sectional drawing which shows an example of the manufacturing method of the positive electrode active material of this invention. It is a schematic sectional drawing explaining the difference between the positive electrode active material obtained by the conventional manufacturing method, and the positive electrode active material obtained by the manufacturing method of this invention. It is a schematic sectional drawing which shows an example of the manufacturing method of the all-solid-state battery of this invention.

  Hereinafter, the manufacturing method of the positive electrode active material of the present invention and the manufacturing method of the all solid state battery will be described in detail.

A. First, a method for producing a positive electrode active material according to the present invention will be described. The method for producing a positive electrode active material according to the present invention is a production of a positive electrode active material having an oxide positive electrode active material and a coating portion made of a lithium niobate compound formed on the surface of the oxide positive electrode active material. A coating step of coating a coating solution containing the precursor of the lithium niobate compound on the surface of the oxide positive electrode active material to form a precursor coating portion; and the precursor coating portion. And a heat treatment step of heat-treating the oxide positive electrode active material having an oxygen concentration in an atmosphere having an oxygen concentration of 25% by volume or less.

  According to the present invention, by performing the heat treatment in the above oxygen concentration atmosphere, it is possible to suppress the components of the coat portion from penetrating into the oxide positive electrode active material. Thereby, it can prevent that the thickness of a coating part becomes thin too much, and can suppress the production | generation of a high resistance layer effectively. Moreover, since the coating solution is usually expensive, the amount of the coating solution used can be reduced by suppressing the penetration of the coating component. Thereby, the manufacturing cost of the positive electrode active material can be reduced. Moreover, when the component of the coat portion penetrates into the oxide positive electrode active material, the internal resistance of the oxide positive electrode active material increases. Therefore, the increase in the internal resistance of the oxide positive electrode active material can be suppressed by suppressing the penetration of the coating part component. In addition, the lithium niobate compound constituting the coat portion has a property that a deoxygenation reaction easily occurs. Therefore, conventionally, in order to prevent oxygen from being desorbed from the lithium niobate-based compound, heat treatment is performed under conditions rich in oxygen such as a pure oxygen atmosphere. On the other hand, in the present invention, although there is a possibility that the influence of the deoxygenation reaction is slightly caused, the oxygen concentration is lowered from the viewpoint of suppressing the penetration of the coating part component. The following merit can be obtained.

The lithium niobate compound in the present invention is usually a compound represented by Li _x Nb _y O _z . Here, x is normally 0.9 ≦ x ≦ 1.1, and preferably 0.95 ≦ x ≦ 1.05. y is usually 0.9 ≦ y ≦ 1.1, and preferably 0.95 ≦ y ≦ 1.05. z is usually 2.8 ≦ z ≦ 3.2, and preferably 2.9 ≦ z ≦ 3.1. In particular, in the present invention, the lithium niobate compound is preferably lithium niobate (LiNbO ₃ ). This is because the generation of the high resistance layer can be effectively suppressed.

  FIG. 1 is a schematic cross-sectional view showing an example of a method for producing a positive electrode active material of the present invention. In the manufacturing method shown in FIG. 1, first, an oxide positive electrode active material 1 is prepared (FIG. 1 (a)), and then a coating solution containing a precursor of a lithium niobate compound is used. A precursor coat portion 2 is formed on the surface of the oxide positive electrode active material 1 (FIG. 1B). Thereafter, the oxide positive electrode active material 1 having the precursor coat portion 2 is heat-treated in the above-described oxygen concentration atmosphere to form the coat portion 3 (FIG. 1C). Thereby, the positive electrode active material 1 which has the oxide positive electrode active material 1 and the coating part 3 which consists of a lithium niobate type compound formed on the surface of the oxide positive electrode active material 1 can be obtained.

FIG. 2 is a schematic cross-sectional view for explaining the difference between the positive electrode active material obtained by the conventional production method and the positive electrode active material obtained by the production method of the present invention. As shown in FIG. 2 (a), the positive electrode active material 10 obtained by the conventional manufacturing method has an oxide positive electrode active material because the components of the coat part 3 penetrate into the oxide positive electrode active material 1. The thickness of the coat part 3 formed on the surface of 1 becomes small. On the other hand, as shown in FIG. 2B, the positive electrode active material 10 obtained by the production method of the present invention does not penetrate into the oxide positive electrode active material 1, and therefore the oxide positive electrode active material 1 The coat part 3 having a sufficient thickness can be formed on the surface. As a result, the generation of the high resistance layer can be effectively suppressed. In addition, the reason why the components of the coat portion penetrate is not yet clear, but the exposed portion of the oxide cathode active material present on the surface of the cathode active material material reacts with oxygen, so that the component of the coat portion penetrates. It is presumed that this is because the state is easily changed.
Hereafter, the manufacturing method of the positive electrode active material of this invention is demonstrated for every process.

1. Application Process First, the application process in the present invention will be described. This step is a step of applying a coating solution containing the precursor of the lithium niobate compound on the surface of the oxide positive electrode active material to form a precursor coating portion.

(1) Coating solution The coating solution in this invention contains the precursor of a lithium niobate type compound. The precursor of the lithium niobate-based compound is not particularly limited as long as the lithium niobate-based compound can be obtained by a heat treatment step to be described later. For example, a material in which a Li source and an Nb source are mixed is used. be able to. Examples of the Li source include Li alkoxide. Examples of the Li alkoxide include LiOC ₂ H ₅ . Examples of the Nb source include Nb alkoxide. Examples of the Nb alkoxide include Nb (OC ₂ H ₅ ) _{5 and the} like. Moreover, it is preferable to select suitably the ratio of the Li source and Nb source which are contained in a coating solution according to the composition of the target lithium niobate compound. Usually, paying attention to the molar ratio of Li contained in the Li source and Nb contained in the Nb source, the ratio of the Li source and the Nb source is determined. Furthermore, the molar ratio of Li and Nb is preferably in the same range as the numerical range of Li and Nb in the lithium niobate-based compound described above.

  The coating solution in the present invention usually contains a solvent. The solvent is not particularly limited as long as it can dissolve and disperse the precursor of the lithium niobate compound, and examples thereof include ethanol. Further, the ratio of the Li source contained in the coating solution is, for example, preferably in the range of 0.05 mol / kg to 1.0 mol / kg, and more preferably in the range of 0.1 mol / kg to 0.6 mol / kg. . The same applies to the proportion of the Nb source contained in the coating solution.

  In the present invention, the Li source and Nb source may be dissolved in a solvent, and then stirred and refluxed to obtain a solution containing a composite alkoxide. Furthermore, after forming the composite alkoxide, the composite alkoxide may be hydrolyzed by preparing a solution containing water, adding this aqueous solution to the solution containing the composite alkoxide, and mixing them. In addition, stirring and refluxing can be performed for the hydrolysis reaction. In the present invention, the solution thus obtained can be used as a coating solution.

(2) Oxide positive electrode active material Next, the oxide positive electrode active material in this invention is demonstrated. In the present invention, by using an oxide positive electrode active material, for example, an all-solid battery having a high energy density can be obtained. Examples of the oxide positive electrode active material include a general formula Li _x M _y O _z (M is a transition metal element, x = 0.02 to 2.2, y = 1 to 2, z = 1.4 to 4). ) Can be used as the positive electrode active material. In the above general formula, M is preferably at least one selected from the group consisting of Co, Mn, Ni, V, Fe and Si, and is at least one selected from the group consisting of Co, Ni and Mn. It is more preferable. As such an oxide positive electrode active material, specifically, LiCoO ₂ , LiMnO ₂ , LiNiO ₂ , LiVO ₂ , LiNi _1/3 Co _1/3 Mn _1/3 O ₂ , LiMn ₂ O ₄ , Li ( _{Ni 0.5 Mn 1.5) O 4,} Li 2 FeSiO 4, Li 2 MnSiO 4 , and the like. Examples of the positive electrode active material other than the above general formula Li _x M _y O _z include olivine-type positive electrode active materials such as LiFePO ₄ and LiMnPO ₄ .

Examples of the shape of the oxide positive electrode active material include a particle shape, and among them, a true spherical shape or an elliptical spherical shape is preferable. In addition, when the oxide positive electrode active material has a particle shape, the average particle size is preferably 7.8 μm or less. It is because it can suppress more effectively that the component of a coat part penetrates into the inside of an oxide cathode active material. Furthermore, the average particle size is more preferably 2.6 μm or less. This is because the reaction of desorbing oxygen from the lithium niobate compound can be suppressed. The reason why the desorption can be suppressed is not yet clear, but since the average particle size is small, the voids between the particles are small, and the surface area of the coat part in contact with the gas is small, so that the desorption of oxygen occurs. This is thought to be due to difficulty. On the other hand, the average particle diameter of the oxide positive electrode active material is preferably 0.1 μm or more, for example. This is because it is difficult to produce an oxide positive electrode active material having an average particle size smaller than the above range. Incidentally, the average particle diameter of the oxide positive electrode active material, it is possible to employ a median diameter (D _50).

(3) Coating method In this invention, a coating solution is apply | coated on the surface of an oxide positive electrode active material. The method for applying the coating solution is not particularly limited as long as it is a method capable of applying the coating solution on the surface of the oxide positive electrode active material. For example, a method using a coating apparatus having a rolling fluidized bed, etc. Can be mentioned. Moreover, it is preferable to select the coating amount of the coating solution as appropriate according to the thickness of the target coating portion. In the present invention, it is preferable to perform drying to remove the solvent of the coating solution after coating the coating solution on the surface of the oxide positive electrode active material.

2. Next, the heat treatment process in the present invention will be described. This step is a step of heat-treating the oxide positive electrode active material having the precursor coat portion in an atmosphere having a predetermined oxygen concentration.

  The oxygen concentration during the heat treatment is usually 25% by volume or less, preferably 24% by volume or less, and more preferably 23% by volume or less. This is because it is possible to further suppress the components of the coat portion from penetrating into the oxide positive electrode active material. On the other hand, the oxygen concentration is preferably 5% by volume or more, and more preferably 10% by volume or more. This is because if the oxygen concentration is too low, oxygen may be insufficient, and a desired lithium niobate compound may not be obtained.

  Further, as the gas component other than oxygen, a gas that does not adversely affect the synthesis of the lithium niobate compound is preferable, and specifically, an inert gas is preferable. Examples of the inert gas include nitrogen and argon. Among them, nitrogen is preferable. In the present invention, the heat treatment may be performed in an air atmosphere. This is because the oxygen concentration in the atmosphere is about 20% by volume, and the remaining gas component is almost nitrogen, which can be regarded as a mixed gas of oxygen and nitrogen. Further, in the present invention, it is preferable to perform the heat treatment under the condition of less moisture. This is because moisture may cause deterioration of the lithium niobate compound.

  The temperature and time of the heat treatment are not particularly limited as long as a desired lithium niobate compound can be obtained. As temperature of heat processing, it exists in the range of 300 to 500 degreeC, for example. Moreover, as heat processing time, it exists in the range of 0.5 hour-10 hours, for example.

3. Next, the positive electrode active material obtained by the present invention will be described. The positive electrode active material obtained by the present invention has an oxide positive electrode active material and a coat portion made of a lithium niobate-based compound formed on the surface of the oxide positive electrode active material. The average thickness of the coat part is, for example, preferably in the range of 0.1 nm to 30 nm, and more preferably in the range of 1 nm to 15 nm. This is because if the average thickness is too small, the generation of the high resistance layer may not be effectively suppressed, and if the average thickness is too large, the ionic conductivity may be reduced. Moreover, the coverage of the coating part in the surface of an oxide positive electrode active material is 50% or more, for example, and it is preferable to exist in the range of 75%-95%. It is because the production | generation of a high resistance layer can be suppressed more effectively if it is in the said range.

B. Next, a method for producing an all solid state battery of the present invention will be described. The all-solid-state battery production method of the present invention includes a positive electrode active material layer forming step of forming a positive electrode active material layer using a composition containing a positive electrode active material obtained by the above-described method for producing a positive electrode active material. And a solid electrolyte layer forming step of forming a solid electrolyte layer using a composition containing a sulfide solid electrolyte material.

  According to the present invention, by using the positive electrode active material obtained by the above-described manufacturing method, it is possible to effectively suppress the generation of the high resistance layer and obtain an all-solid battery excellent in use durability. Can do.

FIG. 3 is a schematic cross-sectional view showing an example of the method for producing an all solid state battery of the present invention. In the manufacturing method in FIG. 3, first, a composition containing the positive electrode active material obtained by the above-described manufacturing method and Li ₇ P ₃ S ₁₁ (sulfide solid electrolyte material) is used. The positive electrode active material layer 11 is formed by pressing (FIG. 3A). Next, Li ₇ P ₃ S ₁₁ is added to the surface of the positive electrode active material layer 11 and pressed to form the solid electrolyte layer 12 (FIG. 3B). Next, an In foil (negative electrode active material) is disposed on the surface of the solid electrolyte layer 12 and pressed to form the negative electrode active material layer 13 (FIG. 3C). Thereby, the electric power generation element 20 which has the positive electrode active material layer 11, the solid electrolyte layer 12, and the negative electrode active material layer 13 can be obtained. Finally, the power generation element 20 is accommodated in the battery case, and the battery case is caulked to obtain an all-solid battery. In addition, the order which forms each layer of an electric power generation element is not specifically limited, Arbitrary orders can be employ | adopted. Moreover, you may form the several layer which comprises an electric power generation element simultaneously.

1. Positive electrode active material layer forming step The positive electrode active material layer forming step in the present invention uses a composition containing a positive electrode active material obtained by the method described in "A. Method for producing positive electrode active material" above. This is a step of forming a positive electrode active material layer. As a method for forming the positive electrode active material layer, for example, a pressing method can be given. Moreover, the said composition contains positive electrode active material material, and may contain at least one of the solid electrolyte material and the electrically conductive material as needed.

  Although the solid electrolyte material contained in the said composition will not be specifically limited if Li ion conductivity can be improved, It is preferable that it is a sulfide solid electrolyte material. This is because a high-resistance layer is easily generated and the effects of the present invention can be sufficiently exhibited. The sulfide solid electrolyte material will be described later in “2. Solid electrolyte layer forming step”. The content of the solid electrolyte material in the positive electrode active material layer is, for example, preferably in the range of 1% by weight to 90% by weight, and more preferably in the range of 10% by weight to 80% by weight. On the other hand, the conductive material contained in the composition is not particularly limited as long as it can improve the conductivity of the positive electrode active material layer. For example, acetylene black, ketjen black, carbon fiber, etc. Can be mentioned. The content of the conductive material in the positive electrode active material layer is, for example, in the range of 0.1% by weight to 20% by weight. Moreover, the thickness of the positive electrode active material layer is, for example, in the range of 1 μm to 100 μm.

2. Solid electrolyte layer forming step The solid electrolyte layer forming step in the present invention is a step of forming a solid electrolyte layer using a composition containing a sulfide solid electrolyte material. As a method for forming the solid electrolyte layer, for example, a pressing method can be cited. Moreover, although the said composition should just contain a sulfide solid electrolyte material at least, it is preferable to contain only a sulfide solid electrolyte material especially.

The sulfide solid electrolyte material in the present invention may have crosslinked sulfur, or may not have crosslinked sulfur. Especially, what has bridge | crosslinking sulfur is preferable for a sulfide solid electrolyte material. This is because a high-resistance layer is easily generated and the effects of the present invention can be sufficiently exhibited. The generation of the high resistance layer can be confirmed by a transmission electron microscope (TEM) or energy dispersive X-ray spectroscopy (EDX). Examples of the sulfide solid electrolyte material having bridging sulfur include Li ₇ P ₃ S ₁₁ , 0.6Li ₂ S-0.4SiS ₂ , 0.6Li ₂ S-0.4GeS _{2 and the} like. Here, the above _Li 7 _P 3 _{S 11 includes} a PS ₃ -S-PS 3 structure, which has a PS _{4 structure,} PS ₃ -S-PS 3 structure has a bridging sulfur. Thus, in the present invention, the sulfide solid electrolyte material preferably has a PS ₃ —S—PS ₃ structure. In addition, the sulfide solid electrolyte material in the present invention may be amorphous or crystalline. The crystalline sulfide solid electrolyte material can be obtained, for example, by subjecting an amorphous sulfide solid electrolyte material to a heat treatment. Examples of the shape of the sulfide solid electrolyte material include a particle shape, and among them, a spherical shape or an oval shape is preferable. When the sulfide solid electrolyte material has a particle shape, the average particle diameter is preferably in the range of, for example, 0.1 μm to 50 μm. The thickness of the solid electrolyte layer is preferably in the range of, for example, 0.1 μm to 1000 μm, and more preferably in the range of 0.1 μm to 300 μm.

3. Negative electrode active material layer forming step In the present invention, in addition to the steps described above, a negative electrode active material layer forming step of forming a negative electrode active material layer using a composition containing a negative electrode active material is usually performed. Examples of a method for forming the negative electrode active material layer include a pressing method. The composition contains a negative electrode active material, and may further contain at least one of a solid electrolyte material and a conductive material as necessary. Examples of the negative electrode active material include a metal active material and a carbon active material. Examples of the metal active material include In, Al, Si, and Sn. On the other hand, examples of the carbon active material include mesocarbon microbeads (MCMB), highly oriented graphite (HOPG), hard carbon, and soft carbon. In addition, about the solid electrolyte material and electroconductive material used for a negative electrode active material layer, it is the same as that of the case in the positive electrode active material layer mentioned above. The thickness of the negative electrode active material layer is, for example, in the range of 0.1 μm to 1000 μm.

4). Other Steps In the present invention, in addition to the steps described above, a step of disposing a positive electrode current collector on the surface of the positive electrode active material layer, a step of disposing a negative electrode current collector on the surface of the negative electrode active material layer, power generation You may have the process of accommodating an element in a battery case. Examples of the material for the positive electrode current collector include SUS, aluminum, nickel, iron, titanium, and carbon. On the other hand, examples of the material for the negative electrode current collector include SUS, copper, nickel, and carbon. The thickness, shape, and the like of the positive electrode current collector and the negative electrode current collector are preferably appropriately selected according to the use of the all solid state battery. Moreover, the battery case in this invention can use the battery case of a general all-solid-state battery, for example, the battery case made from SUS etc. can be mentioned. In the present invention, the power generation element may be formed inside the insulating ring. Further, the all solid state battery obtained by the present invention may be a primary battery or a secondary battery, and among these, a secondary battery is preferable. This is because it can be repeatedly charged and discharged and is useful, for example, as an in-vehicle battery. Furthermore, examples of the shape of the all solid state battery include a coin type, a laminate type, a cylindrical type, and a square type.

  The present invention is not limited to the above embodiment. The above-described embodiment is an exemplification, and the present invention has substantially the same configuration as the technical idea described in the claims of the present invention, and any device that exhibits the same function and effect is the present invention. It is included in the technical scope of the invention.

  Hereinafter, the present invention will be described in more detail with reference to examples.

[Example 1]
First, 30 g of lithium ethoxide and 182 g of niobium pentaethoxide were dissolved in 1000 g of ethanol to obtain a coating solution. Note that lithium ethoxide and niobium pentaethoxide have a molar ratio of 1: 1. Next, the obtained coating solution at coating apparatus using a tumbling fluidized bed, was applied on the surface of the oxide positive electrode active material (LiNiO _2, the average particle diameter _D 50 = _2.5μm). At this time, the coating solution was applied so that the average thickness of the coating portion was 3 nm. The coating amount of the coating solution was determined in consideration of the product of the surface area of the oxide positive electrode active material and the average thickness of the coating portion. Thereafter, the coated oxide positive electrode active material was dried with warm air. Next, the obtained powder is heat-treated at 400 ° C. for 30 minutes under the atmosphere (oxygen concentration: about 20% by volume), and a positive electrode active material having a coating portion made of LiNbO ₃ on the surface of LiNiO ₂ Got.

[Example 2]
D ₅₀ = 2.5 [mu] m to LiNiO _{2 of,} except for changing the LiNiO ₂ of D 50 = 7.6 [mu] m, in the same manner as in Example 1 to obtain a positive electrode active material.

[Comparative Example 1]
A positive electrode active material was obtained in the same manner as in Example 1 except that the air atmosphere was changed to a pure oxygen atmosphere.

[Evaluation]
(Coating thickness evaluation)
Using the positive electrode active material obtained in Example 1 and Comparative Example 1, the average thickness of the coat portion was measured. The average thickness of the coat part was evaluated using X-ray photoelectron spectroscopy (XPS). Moreover, the average thickness of the coat part was computed by performing argon etching and measuring element distribution by XPS. The obtained results are shown in Table 1.

  As shown in Table 1, in Comparative Example 1, the average thickness of the coat portion was smaller than that in Example 1. This is considered to be because when the heat treatment is performed in a pure oxygen atmosphere, the components in the coat portion penetrate into the oxide positive electrode active material. In addition, the reason why the components of the coat portion penetrate is not yet clear, but the exposed portion of the oxide cathode active material present on the surface of the cathode active material material reacts with oxygen, so that the component of the coat portion penetrates. It is presumed that this is because the state is easily changed.

(Evaluation of interface resistance)
Using the positive electrode active material obtained in Examples 1 and 2 and Comparative Example 1, all solid lithium secondary batteries were produced. First, the electric power generation element 20 as shown in FIG. 3 mentioned above was produced with the press machine. Here, the positive electrode active material layer 11 includes a positive electrode active material obtained by the above-described method and a solid electrolyte material (Li ₇ P ₃ S ₁₁ ) produced by the method described in JP-A-2005-228570. Were mixed at a weight ratio of 7: 3. Further, Li ₇ P ₃ S ₁₁ was used for the solid electrolyte layer 12, and graphite was used for the negative electrode active material layer 13. Next, an all solid lithium secondary battery was obtained using the power generation element 20.

  Thereafter, the obtained all solid lithium secondary battery was charged. The charging conditions were such that constant current charging was performed at 0.1 C to 3.96 V, followed by constant voltage charging at 3.96 V for 2 hours. After charging, the interface resistance values of the positive electrode active material layer and the solid electrolyte layer were determined by impedance measurement using an alternating current impedance method. The impedance measurement conditions were a voltage amplitude of 10 mV, a measurement frequency of 1 MHz to 0.1 Hz, and 25 ° C. The results are shown in Table 2.

  As shown in Table 2, in Example 1, the interface resistance value was reduced by about 47% compared to Comparative Example 1. This is presumably because, in Comparative Example 1, the components in the coat portion penetrated into the oxide positive electrode active material, and the internal resistance of the oxide positive electrode active material increased. In addition, it was confirmed that the interface resistance value of Example 1 was significantly smaller than that of Example 2. This is considered to be because in Example 1, the average particle size is small, so that the desorption of oxygen is difficult to occur.

DESCRIPTION OF SYMBOLS 1 ... Oxide positive electrode active material 2 ... Precursor coat part 3 ... Coat part 10 ... Positive electrode active material material 11 ... Positive electrode active material layer 12 ... Solid electrolyte layer 13 ... Negative electrode active material layer 20 ... Power generation element

Claims (5)

A method for producing a positive electrode active material comprising: an oxide positive electrode active material; and a coat portion made of a lithium niobate compound formed on the surface of the oxide positive electrode active material,
A coating solution containing a precursor of the lithium niobate-based compound is coated on the surface of the oxide positive electrode active material to form a precursor coating portion;
A heat treatment step of heat-treating the oxide positive electrode active material having the precursor coat portion in an atmosphere having an oxygen concentration of 25% by volume or less;
A method for producing a positive electrode active material characterized by comprising: The method for producing a positive electrode active material according to claim 1, wherein the lithium niobate-based compound is lithium niobate (LiNbO ₃ ).   3. The method for producing a positive electrode active material according to claim 1, wherein an average particle diameter of the oxide positive electrode active material is 7.8 μm or less.   3. The method for producing a positive electrode active material according to claim 1, wherein an average particle size of the oxide positive electrode active material is 2.6 μm or less. The positive electrode active material which forms a positive electrode active material layer using the composition containing the positive electrode active material obtained by the manufacturing method of the positive electrode active material of any one of Claim 1 to 4 A layer forming step;
And a solid electrolyte layer forming step of forming a solid electrolyte layer using a composition containing a sulfide solid electrolyte material.

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