Coating liquid for positive electrode active material and method for producing positive electrode active material with coating film

Abstract

To provide a coating liquid for a positive electrode active material capable of uniformly forming a coating film on a surface of the positive electrode active material, suppressing a reaction between the positive electrode active material of a positive electrode member and a solid electrolyte layer, and constituting an all-solid-state lithium ion secondary battery having stable battery performance, and to provide a method of manufacturing the positive electrode active material with the coating film.SOLUTION: A coating liquid for a positive electrode active material, which is used for forming a coating layer made of a composite compound of lithium and tungsten on a positive electrode active material containing lithium, includes an aqueous solution containing a lithium compound, a tungsten compound, and ammonia, wherein an ammonia concentration is in a range of 0. 7mol / L or more and 3. 9mol / L or less. A method for producing a positive electrode active material with a coating film having a coating film composed of a composite compound of lithium and tungsten on a surface of a positive electrode active material containing lithium includes attaching the above-described coating liquid for a positive electrode active material to the surface of the positive electrode active material, followed by drying.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a coating solution for a positive electrode active material used when forming a coating film on the surface of a lithium-containing positive electrode active material, and a method for producing a positive electrode active material with a coating film. [Background technology]

[0002] Lithium-ion secondary batteries are widely used as power sources in a wide range of devices, from vehicles such as EVs (electric vehicles) and HEVs (hybrid electric vehicles) to electronic devices such as mobile phones and laptops. In recent years, sulfide-based solid electrolytes have attracted attention as electrolytes for lithium-ion secondary batteries because they have high ionic conductivity and are safer than liquid electrolytes, and all-solid-state lithium-ion secondary batteries using these solid electrolytes have been proposed.

[0003] As shown in Patent Document 1, for example, an all-solid-state lithium ion secondary battery has a structure in which a solid electrolyte is disposed between a negative electrode member containing a negative electrode active material and a positive electrode member containing a positive electrode active material. In the above-mentioned all-solid-state lithium ion secondary battery, the positive electrode active material of the positive electrode member reacts with the solid electrolyte layer, resulting in the formation of a high-resistance film and a significant deterioration in battery characteristics.

[0004] Therefore, in order to suppress the reaction between the positive electrode active material of the positive electrode member and the solid electrolyte layer, a technique has been proposed in which a coating film is formed on the surface of the positive electrode active material. Here, as the coating film formed on the surface of the positive electrode active material, for example, as shown in Patent Document 2, one made of lithium niobate (LiNbO3) is used.

[0005] When forming this coating film made of lithium niobate (LiNbO3), a coating liquid containing lithium niobate is used. For example, a coating liquid containing lithium niobate is applied to the surface of the positive electrode active material, and then the coating liquid is heated, whereby the lithium niobate is crystallized to form the coating film described above. However, lithium niobate is relatively expensive, which increases costs. Also, because lithium niobate is poorly soluble in water, alcohol is generally used as a solvent, which also increases costs.

[0006] As a coating film to replace lithium niobate (LiNbO3), for example, Patent Documents 3 and 4 propose a technique for forming a coating film made of lithium tungstate (Li2WO4). Lithium tungstate is relatively inexpensive and dissolves in water, so water can be used as a solvent, which makes it possible to reduce the production cost of the coating film. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-257878 [Patent Document 2] Japanese Patent Application Laid-Open No. 2010-170715 [Patent Document 3] Patent No. 5772626 [Patent Document 4] Patent No. 6978182 Summary of the Invention [Problem to be solved by the invention]

[0008] However, when a coating film is formed using a coating solution of lithium tungstate using water as a solvent, water has a lower wettability with the positive electrode active material than alcohol, and there is a risk that a uniform coating film cannot be formed on the surface of the positive electrode active material.

[0009] The present invention has been made in view of the above-mentioned circumstances, and aims to provide a coating solution for a positive electrode active material, which can form a uniform coating film on the surface of a positive electrode active material, can suppress the reaction between the positive electrode active material of the positive electrode member and the solid electrolyte layer, and can form an all-solid-state lithium ion secondary battery with stable battery performance, and a method for producing a positive electrode active material with a coating film. [Means for solving the problem]

[0010] In order to solve the above problems, a coating liquid for a positive electrode active material according to a first aspect of the present invention is a coating liquid for a positive electrode active material used when forming a coating layer made of a composite compound of lithium and tungsten on the surface of a lithium-containing positive electrode active material, and is characterized in that the coating liquid is made of an aqueous solution containing a lithium compound, a tungsten compound, and ammonia, and the ammonia concentration is within the range of 0.7 mol / L or more and 3.9 mol / L or less.

[0011] The coating solution for a positive electrode active material according to the first aspect of the present invention is an aqueous solution containing a lithium compound and a tungsten compound. By attaching the coating solution to the surface of a positive electrode active material and drying it, it is possible to form a coating film made of a composite compound of lithium and tungsten (for example, lithium tungstate). The coating solution for a positive electrode active material according to the first aspect of the present invention contains ammonia, and the ammonia concentration is set to be within the range of 0.7 mol / L or more and 3.9 mol / L or less, so that it is possible to form a uniform coating film on the surface of the positive electrode active material. Furthermore, a composite compound of lithium and tungsten (lithium tungstate) can be crystallized at a relatively low temperature, and a coating film can be formed on the surface of the positive electrode active material without performing a heat treatment at a high temperature. This allows for efficient formation of the coating film and also makes it possible to suppress thermal deterioration of the positive electrode active material.

[0012] The method for producing a coated cathode active material according to the second aspect of the present invention is a method for producing a coated cathode active material having a coating film made of a composite compound of lithium and tungsten on the surface of a lithium-containing cathode active material, and is characterized by applying the coating liquid for a cathode active material according to the first aspect of the present invention to the surface of the cathode active material and drying it.

[0013] According to the method for producing a coated cathode active material of aspect 2 of the present invention, the coating liquid for a cathode active material of aspect 1 of the present invention is used, so that a uniform coating film can be formed on the surface of the cathode active material, the reaction between the cathode active material of the cathode member and the solid electrolyte layer can be suppressed, and an all-solid-state lithium ion secondary battery with stable battery performance can be constructed. Furthermore, a composite compound of lithium and tungsten (lithium tungstate) can be crystallized at a relatively low temperature, and a coating film can be formed on the surface of the positive electrode active material without performing a high-temperature heat treatment or the like. This allows for the efficient formation of the coating film and also suppresses the deterioration of the positive electrode active material due to heat, making it possible to construct an all-solid-state lithium-ion secondary battery with stable battery performance. [Effects of the Invention]

[0014] According to the present invention, it is possible to provide a coating solution for a positive electrode active material, which can form a uniform coating film on the surface of a positive electrode active material, can suppress the reaction between the positive electrode active material of the positive electrode member and the solid electrolyte layer, and can form an all-solid-state lithium ion secondary battery with stable battery performance, and a method for producing a positive electrode active material with a coating film. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1 is a schematic diagram of an all-solid-state lithium ion secondary battery. [Figure 2] FIG. 2 is an explanatory diagram of a positive electrode active material used in the all-solid-state lithium ion secondary battery shown in FIG. [Figure 3] 1 is a flow diagram showing a method for producing a coating film-attached positive electrode active material using a coating liquid for a positive electrode active material according to the present embodiment. [Figure 4]1 shows SEM photographs of coated positive electrode active materials in Examples, where (a) is Example 1, (b) is Example 2, (c) is Comparative Example 1, and (d) is Comparative Example 2. [Figure 5] 2 is a graph showing the results of an X-ray diffraction test of a coating film in an example. DETAILED DESCRIPTION OF THE INVENTION

[0016] An example of an embodiment of the present invention will be described below.

[0017] The coating liquid for a positive electrode active material of this embodiment is used, for example, when forming a coating film on the surface of a positive electrode active material contained in a positive electrode member of an all-solid-state lithium ion secondary battery shown in FIG. As shown in FIG. 1, the all-solid-state lithium-ion secondary battery 1 includes a positive electrode member 10 containing a positive electrode active material 16, a negative electrode member 20 containing a negative electrode active material 25, and a solid electrolyte layer 30 interposed between the positive electrode member 10 and the negative electrode member 20.

[0018] The negative electrode member 20 is constructed by applying a negative electrode active material 25 mixed with a binder 22 to a negative electrode current collector 21 made of, for example, copper foil. The negative electrode active material 25 may be, for example, a lithium-indium alloy, lithium metal, silicon, lithium titanate (Li2TiO3), or a carbon material capable of absorbing and desorbing lithium ions.

[0019] The solid electrolyte layer 30 is made of a sulfide solid electrolyte containing lithium sulfide, such as a Li2S-P2S5-based, Li2S-P2S3-based, Li2S-SiS2-based, Li2S-Ga2S2-based, or Li2S-GeS2-based electrolyte.

[0020] The positive electrode member 10 is constructed by applying a positive electrode active material 16 (coated positive electrode active material 15) mixed with a binder 12 to a positive electrode current collector 11 made of, for example, aluminum foil. If necessary, a conductive additive 13 such as a carbon material may be mixed together with the binder 12 and the positive electrode active material 16 (coated positive electrode active material 15). The positive electrode active material 16 may be, for example, lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), or lithium manganese oxide (LiMn2O4).

[0021] In the all-solid-state lithium-ion secondary battery 1, the positive electrode active material 16 contained in the positive electrode member 10 reacts with the solid electrolyte layer 30 to form a highly resistive film, which may significantly degrade the battery characteristics. Therefore, in order to suppress the reaction between the positive electrode active material 16 and the solid electrolyte layer 30, a coating film 17 is formed on the surface of the positive electrode active material 16 as shown in FIG. In this embodiment, the coating film 17 formed on the positive electrode active material 16 is made of a composite compound of lithium and tungsten (lithium tungstate in this embodiment).

[0022] The coating liquid for a positive electrode active material according to this embodiment is used when forming a coating film 17 made of a composite compound of lithium and tungsten (lithium tungstate) on the surface of the positive electrode active material 16.

[0023] In the coating liquid for a positive electrode active material of this embodiment, a lithium compound and a tungsten compound that constitute a composite compound of lithium and tungsten (lithium tungstate) are contained in a solvent. In this embodiment, water is used as the solvent, and the coating liquid is an aqueous solution containing the lithium compound and the tungsten compound. Lithium and tungsten composite compounds (lithium tungstate) include Li2WO4, Li4WO5, Li6WO6, and Li2W4O 13 , Li2W2O7, Li6W2O9, Li2W2O7, Li2W5O 16 , Li9W19 O 55 , Li3W 10 O 30 , Li 18 W5O 15 , Li2WO4·0.5H2O, etc.

[0024] Examples of lithium compounds that can be used as the lithium source include lithium hydroxide, lithium hydroxide hydrate, lithium carbonate, lithium nitrate, lithium acetate, and lithium oxide. Examples of tungsten compounds that can be used as the tungsten source include tungsten trioxide, ammonium tungstate, sodium tungstate, and lithium tungstate. The coating liquid for a positive electrode active material of this embodiment contains ammonia, and the ammonia concentration is set within the range of 0.7 mol / L or more and 3.9 mol / L or less.

[0025] In the coating liquid for a positive electrode active material according to this embodiment, the lithium concentration is preferably in the range of more than 3.0 mol / L and not more than 5.3 mol / L. In addition, in the coating liquid for a positive electrode active material of this embodiment, the lithium concentration is preferably equal to or higher than the saturation concentration. The above-mentioned saturated concentration is the lithium concentration in the filtrate obtained by dissolving a large amount of lithium compound and then filtering out the remaining lithium compound.

[0026] The reason why the coating liquid for a positive electrode active material according to this embodiment is configured as described above will be explained below.

[0027] (ammonia concentration) In the coating solution for a positive electrode active material according to this embodiment, the ammonia content is in the range of 0.7 mol / L to 3.9 mol / L, which improves wettability with the positive electrode active material and enables the formation of a uniform coating film on the surface of the positive electrode active material. Furthermore, the addition of ammonia allows the lithium-tungsten composite compound (lithium tungstate) to be crystallized at a relatively low temperature, enabling the formation of a crystalline coating film 17 without the need for a high-temperature heat treatment or the like. In this embodiment, the lower limit of the ammonia concentration in the coating solution for a positive electrode active material is preferably 3.0 mol / L or more, and more preferably 3.5 mol / L or more.

[0028] (Lithium concentration) In the coating liquid for a positive electrode active material of this embodiment, water is used as a solvent. Therefore, when the coating liquid for a positive electrode active material is applied to the surface of the positive electrode active material, lithium contained in the positive electrode active material may dissolve into the coating liquid for a positive electrode active material, causing a deviation in the composition of the positive electrode active material and resulting in a deterioration in its characteristics. Here, by setting the lithium concentration in the coating liquid for a positive electrode active material to more than 3.0 mol / L, lithium is sufficiently dissolved in the coating liquid for a positive electrode active material, and it is possible to prevent the lithium contained in the positive electrode active material from dissolving into the coating liquid for a positive electrode active material. On the other hand, by setting the lithium concentration in the coating liquid for a positive electrode active material to 5.3 mol / L or less, it is possible to prevent the use of excessive lithium.

[0029] The lower limit of the lithium concentration in the coating solution for a positive electrode active material is preferably 4.2 mol / L or more, and more preferably 5.0 mol / L or more. Furthermore, in order to reliably prevent the lithium contained in the positive electrode active material from dissolving into the coating liquid for the positive electrode active material, it is preferable to set the lithium concentration in the coating liquid for the positive electrode active material to a saturation concentration or higher.

[0030] Next, a method for producing the coating film-attached positive electrode active material 15 using the coating liquid for a positive electrode active material according to the present embodiment will be described with reference to the flow chart of FIG. As shown in FIG. 3, the method for producing the coated cathode active material 15 according to this embodiment includes a coating solution applying step S01 for a cathode active material and a drying step S02.

[0031] (Positive electrode active material coating liquid application step S01) First, the coating liquid for a positive electrode active material according to the present embodiment is applied to the surface of the positive electrode active material. There are no particular limitations on the means for application. The coating liquid for a positive electrode active material may be sprayed, applied, or immersed in the coating liquid for a positive electrode active material. In the coating liquid for a positive electrode active material of this embodiment, water is used as the solvent, so that the coating liquid for a positive electrode active material of this embodiment can be applied to the surface of the positive electrode active material by a relatively simple means.

[0032] (Drying process S02) Next, the coating liquid for the positive electrode active material attached to the surface of the positive electrode active material is dried to remove the water solvent, thereby forming a coating film 17 made of a composite compound of lithium and tungsten (lithium tungstate). In addition, if the composite compound of lithium and tungsten (lithium tungstate) is crystallized by drying, there is no need to perform a heat treatment at a temperature higher than that for removing the solvent (for example, 120°C or higher).

[0033] Through the above steps, a coating film made of lithium tungstate is formed on the surface of the positive electrode active material, and positive electrode active material 15 with a coating film is produced.

[0034] The coating solution for a positive electrode active material according to the present embodiment configured as described above is an aqueous solution containing a lithium compound and a tungsten compound. By attaching the coating solution to the surface of the positive electrode active material 16 and drying it, it is possible to form a coating film 17 made of a composite compound of lithium and tungsten (lithium tungstate). The coating solution for a positive electrode active material of this embodiment contains ammonia, and the ammonia concentration is set to a range of 0.10 mol / L or more and 3.00 mol / L or less, so that it is possible to form a uniform coating film 17 on the surface of the positive electrode active material 16. Furthermore, the composite compound of lithium and tungsten (lithium tungstate) can be crystallized at a relatively low temperature, and the coating film 17 can be efficiently formed on the surface of the positive electrode active material 16 without performing a heat treatment at a high temperature, and deterioration of the positive electrode active material 16 due to heat, etc. can be suppressed.

[0035] According to the method for producing the coated cathode active material 15 of the present embodiment, the coating liquid for the cathode active material of the present embodiment is used, so that the coating film 17 made of a composite compound of lithium and tungsten (lithium tungstate) can be uniformly formed, and an all-solid-state lithium ion secondary battery with stable battery performance can be constructed.

[0036] Although the embodiment of the present invention has been described above, the present invention is not limited to this and can be modified as appropriate within the scope of the technical idea of ​​the invention. [Example]

[0037] The results of confirmation experiments conducted to confirm the effectiveness of the present invention will be described below.

[0038] <Coating liquid for positive electrode active material> Example 1 53 mL of 28 vol% aqueous ammonia (14.8 mol / L) was added to 200 mL of water and mixed with stirring. Next, 32.1 g of lithium hydroxide was added and stirred, and then 155.4 g of tungsten trioxide was further added and stirred, and the mixture was heated to 70° C. to completely dissolve the solid content. This produced a coating solution for a positive electrode active material of Example 1. The concentrations of lithium and tungsten were analyzed using ion chromatography, and it was confirmed that the lithium concentration was 5.30 mol / L and the tungsten concentration was 2.65 mol / L. Furthermore, the ammonia concentration was measured using neutralization titration using 0.1 mol / L hydrochloric acid, and it was confirmed that the ammonia concentration was 3.9 mol / L.

[0039] Example 2 9.5 mL of 28 vol% aqueous ammonia (14.8 mol / L) was added to 200 mL of water and mixed with stirring. Next, 15.0 g of lithium hydroxide was added and stirred, and then 72.8 g of tungsten trioxide was added and stirred, and the mixture was heated to 70° C. to completely dissolve the solid content. This produced a coating liquid for a positive electrode active material of Example 2. The concentrations of lithium and tungsten were analyzed using ion chromatography, and it was confirmed that the lithium concentration was 3.00 mol / L and the tungsten concentration was 1.50 mol / L.

[0040] (Comparative Example 1) 20.6 g of lithium hydroxide was added to 200 mL of water and stirred, after which 99.7 g of tungsten trioxide was added and stirred, and the mixture was heated to 70° C. to completely dissolve the solid matter. Note that no ammonia water was added. This produced a coating solution for a positive electrode active material of Comparative Example 1. The concentrations of lithium and tungsten were analyzed using ion chromatography, and it was confirmed that the lithium concentration was 4.30 mol / L and the tungsten concentration was 2.15 mol / L. Furthermore, the ammonia concentration was measured using neutralization titration with 0.1 mol / L hydrochloric acid, and it was confirmed that the ammonia concentration was 0.0 mol / L.

[0041] (Comparative Example 2) 13.5 mL of 28 vol% aqueous ammonia (14.8 mol / L) was added to 200 mL of water and mixed with stirring. Next, 6.6 g of lithium hydroxide was added and stirred, and then 32.0 g of tungsten trioxide was added and stirred, and the mixture was heated to 70° C. to completely dissolve the solid content. This produced a coating solution for a positive electrode active material of Comparative Example 2. The concentrations of lithium and tungsten were analyzed using ion chromatography, and it was confirmed that the lithium concentration was 0.45 mol / L and the tungsten concentration was 0.23 mol / L. Furthermore, the ammonia concentration was measured using neutralization titration using 0.1 mol / L hydrochloric acid, and it was confirmed that the ammonia concentration was 0.3 mol / L.

[0042] <Coating liquid for positive electrode active material> Cathode active material NMC532 (LiNi 0.5 Mn 0.3 Co 0.2 The coating solutions for the positive electrode active material of Examples 1 and 2 and Comparative Examples 1 and 2 prepared as described above were prepared in the amounts shown in Table 1, and coated onto 1200 g of the positive electrode active material using a tumbling fluidized bed apparatus. The coating was carried out at 85°C. As a result, a coating film made of lithium tungstate was formed on the surface of the positive electrode active material, and a positive electrode active material with a coating film was obtained.

[0043] (Coated state) The obtained coating film-coated positive electrode active material coating film was observed under an SEM. A case in which a coating film was formed on the entire surface of the positive electrode active material was evaluated as "Good", and a case in which the surface of the positive electrode active material was exposed without a coating film was evaluated as "Poor". The evaluation results are shown in Table 1. The SEM observation results are also shown in Figure 4.

[0044] (Crystalline nature of coating film) The coating film of the obtained positive electrode active material with a coating film was measured using an X-ray diffractometer (Rigaku SmartLab SE). The measurement results are shown in FIG. In Examples 1 and 2 and Comparative Examples 1 and 2, the peak of lithium tungstate was observed, confirming that the material was crystallized.

[0045] [Table 1]

[0046] In Comparative Example 1, the coating solution for the positive electrode active material did not contain ammonia, and a uniform portion of the coating film could not be formed. In Comparative Example 2, the ammonia concentration in the coating solution for the positive electrode active material was as low as 0.3 mol / L, and a uniform coating film could not be formed.

[0047] In Example 1, the ammonia concentration in the coating solution for the positive electrode active material was set to 3.9 mol / L, and a uniform coating film could be formed. In Example 2, the ammonia concentration in the coating solution for the positive electrode active material was set to 0.7 mol / L, and a uniform coating film could be formed.

[0048] As a result of the above confirmatory experiments, it was confirmed that the present invention can provide a coating solution for a positive electrode active material, which can form a uniform coating film on the surface of a positive electrode active material, can suppress the reaction between the positive electrode active material of the positive electrode member and the solid electrolyte layer, and can form an all-solid-state lithium ion secondary battery with stable battery performance, and a method for producing a positive electrode active material with a coating film.

Claims

1. A coating solution for a positive electrode active material used in forming a coating layer made of a composite compound of lithium and tungsten on a surface of a lithium-containing positive electrode active material, A coating solution for a positive electrode active material, comprising an aqueous solution containing a lithium compound, a tungsten compound, and ammonia, wherein the ammonia concentration is in the range of 0.7 mol / L or more and 3.9 mol / L or less.

2. A method for producing a coated positive electrode active material having a coating film made of a composite compound of lithium and tungsten on a surface of a lithium-containing positive electrode active material, comprising: A method for producing a positive electrode active material with a coating film, comprising: applying the coating solution for a positive electrode active material according to claim 1 to a surface of the positive electrode active material; and drying the coating solution.

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