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 suppressing elution of lithium contained in the positive electrode active material into the coating liquid and constituting an all-solid-state lithium ion secondary battery having stable battery performance, and to provide a method of manufacturing a positive electrode active material with a coating film.SOLUTION: A coating liquid for a positive electrode active material, which is used for forming a coating film 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 and a tungsten compound, and has a lithium concentration in a range of more than 3. 0mol / L and 5. 3mol / 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 is used.

[0005] When forming this coating film made of lithium niobate, 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, for example, Patent Documents 3 and 4 propose a technique for forming a coating film made of lithium tungstate. 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, there is a risk that lithium contained in the positive electrode active material will dissolve into the coating solution, resulting in a decrease in battery performance.

[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 that can suppress the dissolution of lithium contained in the positive electrode active material into the coating solution and that 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 film 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 an aqueous solution containing a lithium compound and a tungsten compound, and has a lithium concentration in the range of more than 3.0 mol / L and not more than 5.3 mol / L.

[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). Furthermore, since the lithium concentration is set to a range of more than 3.0 mol / L and not more than 5.3 mol / L, the coating liquid for the positive electrode active material contains sufficient lithium, and it is possible to prevent the lithium contained in the positive electrode active material from dissolving into the coating liquid for the positive electrode active material. As a result, it is possible to construct an all-solid-state lithium-ion secondary battery with stable battery performance, and to obtain sufficient power storage capacity as a rechargeable battery.

[0012] A coating liquid for a positive electrode active material according to a second aspect of the present invention is characterized in that, in the coating liquid for a positive electrode active material according to the first aspect of the present invention, the lithium concentration is equal to or higher than the saturation concentration. According to the coating liquid for a positive electrode active material of the second aspect of the present invention, the lithium concentration is set to be equal to or higher than the saturated concentration, so that it is possible to reliably prevent the lithium contained in the positive electrode active material from dissolving into the coating liquid for a positive electrode active material.

[0013] A coating liquid for a positive electrode active material according to a third aspect of the present invention is characterized in that the coating liquid for a positive electrode active material according to the first or second aspect of the present invention further contains ammonia, and the ammonia concentration is within the range of 0.7 mol / L or more and 3.9 mol / L or less. According to the coating solution for a positive electrode active material of the third aspect of the present invention, ammonia is contained 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.

[0014] The method for producing a coated cathode active material according to the fourth 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 solution for a cathode active material according to any one of the first to third aspects of the present invention to the surface of the cathode active material and drying it. According to the method for producing a coated cathode active material of the fourth aspect of the present invention, the coating liquid for a cathode active material of any one of the first to third aspects of the present invention is used, so that when forming a coating film made of a composite compound of lithium and tungsten (e.g., lithium tungstate), it is possible to prevent the lithium contained in the cathode active material from dissolving into the coating liquid for a cathode active material, and it is possible to construct an all-solid-state lithium ion secondary battery with stable battery performance. As a result, it is possible to obtain sufficient power storage capacity as a rechargeable battery. [Effects of the Invention]

[0015] According to the present invention, it is possible to provide a coating solution for a positive electrode active material that can suppress the dissolution of lithium contained in the positive electrode active material into the coating solution and that 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]

[0016] [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] 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

[0017] An example of an embodiment of the present invention will be described below.

[0018] 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.

[0019] 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.

[0020] The solid electrolyte layer 30 is made of a sulfide-based solid electrolyte 31 containing lithium sulfide, such as a Li2S-P2S5-based, Li2S-P2S3-based, Li2S-SiS2-based, Li2S-Ga2S2-based, or Li2S-GeS2-based solid electrolyte.

[0021] 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).

[0022] 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 coated positive electrode active material 15 in which a coating film 17 is formed on the surface of the positive electrode active material 16 is used, 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).

[0023] The coating liquid for a positive electrode active material of 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.

[0024] In the coating liquid for a positive electrode active material of this embodiment, a lithium compound and a tungsten compound constituting 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 Li2WO 4, Li4WO 5, Li6WO 6, Li2W4O 13, Li2W2O 7,Li6W2O 9, Li2W2O 7, Li2W5O 16, Li9W 19 O 55, Li3W 10 O 30, Li 18 W5O 15, Examples include Li2WO4·0.5H2O.

[0025] 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. In the coating liquid for a positive electrode active material according to this embodiment, the lithium concentration is set to be in the range of more than 3.0 mol / L and not more than 5.3 mol / L.

[0026] Here, in the coating liquid for a positive electrode active material of this embodiment, it is preferable that the lithium concentration is equal to or higher than the saturation concentration. The saturated concentration mentioned above is the lithium concentration in the filtrate obtained by dissolving a large amount of lithium compound and then filtering out the remaining lithium compound.

[0027] The coating liquid for a positive electrode active material of this embodiment may further contain ammonia, and the ammonia concentration may be set within the range of 0.7 mol / L or more and 3.9 mol / L or less.

[0028] 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.

[0029] (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.

[0030] 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.

[0031] (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. When ammonia is contained, the lower limit of the ammonia concentration in the coating liquid for a positive electrode active material is more preferably 3.0 mol / L or more, and even more preferably 3.5 mol / L or more.

[0032] 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 positive electrode active material 15 according to this embodiment includes a coating solution applying step S01 for a positive electrode active material and a drying step S02.

[0033] (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.

[0034] (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 as the solvent, thereby forming a coating film 17 made of lithium tungstate. In addition, when lithium tungstate is crystallized by drying, there is no need to perform a heat treatment or the like at a temperature higher than that for removing the solvent (for example, 120° C. or higher).

[0035] Through the above steps, a coating film made of a composite compound of lithium and tungsten (lithium tungstate) is formed on the surface of the positive electrode active material, and thus positive electrode active material 15 with a coating film is produced.

[0036] 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). Furthermore, because the lithium concentration is in the range of more than 3.0 mol / L and not more than 5.3 mol / L, the coating liquid for the positive electrode active material contains sufficient lithium, and it is possible to prevent the lithium contained in the positive electrode active material from dissolving into the coating liquid for the positive electrode active material. This prevents the composition of the positive electrode active material from changing and its characteristics from deteriorating, making it possible to construct an all-solid-state lithium-ion secondary battery 1 with stable battery performance. As a result, it is possible to obtain sufficient power storage capacity as a rechargeable battery.

[0037] In the coating liquid for a positive electrode active material of this embodiment, when the lithium concentration is equal to or higher than the saturation concentration, it is possible to reliably prevent the lithium contained in the positive electrode active material 16 from dissolving into the coating liquid for a positive electrode active material, and it is possible to reliably prevent the composition of the positive electrode active material from changing and the characteristics from deteriorating.

[0038] When the coating liquid for a positive electrode active material of this embodiment further contains ammonia and the ammonia concentration is set within a range of 0.7 mol / L or more and 3.9 mol / L or less, it becomes possible to form a uniform coating film 17 on the surface of the positive electrode active material 16. Furthermore, a 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 or the like at a high temperature, and thermal deterioration of the positive electrode active material 16 can be suppressed.

[0039] According to the method for producing the coated cathode active material 15 of this embodiment, the coating liquid for the cathode active material of this embodiment is used, so that when forming the coating film 17 made of a composite compound of lithium and tungsten (lithium tungstate), it is possible to prevent the lithium contained in the cathode active material 16 from dissolving into the coating liquid for the cathode active material, and it is possible to construct an all-solid-state lithium ion secondary battery with stable battery performance. Therefore, it is possible to obtain sufficient power storage capacity as a rechargeable battery.

[0040] 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]

[0041] The results of confirmation experiments conducted to confirm the effectiveness of the present invention will be described below.

[0042] <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.

[0043] 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 solution 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. 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 0.7 mol / L.

[0044] Example 3 5.4 mL of 28 vol% aqueous ammonia (14.8 mol / L) was added to 200 mL of water and mixed with stirring. Next, 19.7 g of lithium hydroxide was added and stirred, and then 95.2 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 Example 3. The concentrations of lithium and tungsten were analyzed using ion chromatography, and it was confirmed that the lithium concentration was 4.00 mol / L and the tungsten concentration was 2.00 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 0.4 mol / L.

[0045] (Comparative Example 1) 5.4 mL of 28 vol% aqueous ammonia (14.8 mol / L) was added to 200 mL of water and mixed with stirring. Next, 8.9 g of lithium hydroxide was added and stirred, and then 42.9 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 Comparative Example 1. The concentrations of lithium and tungsten were analyzed using ion chromatography, and it was confirmed that the lithium concentration was 1.80 mol / L and the tungsten concentration was 0.90 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 0.4 mol / L.

[0046] (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.

[0047] <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.

[0048] (Coated state) The obtained coating film-coated positive electrode active material coating film was observed under an SEM, and a case where a coating film was formed on the entire surface of the positive electrode active material was evaluated as "Good", and a case where 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.

[0049] (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.

[0050] <All-solid-state lithium-ion secondary battery> For the positive electrode, a mixture obtained by mixing a positive electrode active material powder, an LGPS-based solid electrolyte, and a carbon material serving as a conductive additive in a mass ratio of 70:25:5 was used. A lithium-indium alloy was used for the negative electrode. In this experiment, the solid electrolyte layer used was the same as the solid electrolyte mixed with the positive electrode (LGPS-based solid electrolyte). A battery was constructed using the above-mentioned positive electrode, negative electrode, and solid electrolyte layer, and a Hosen cell (KP-SolidCell) as the cell.

[0051] (discharge capacity) The discharge capacity of each of the various all-solid-state lithium-ion secondary batteries obtained as described above was evaluated at 25°C. Charging was performed with CC charging at 0.1C, CV charging at 3.6V, and cutoff at 0.01C. Discharging was performed with CC discharging at 0.5C and cutoff at 1.9V. The results of evaluating the initial discharge capacity are shown in Table 1.

[0052] [Table 1]

[0053] In Comparative Example 1, the lithium concentration in the coating solution for the positive electrode active material was as low as 1.80 mol / L, and a coating film could not be stably formed. Furthermore, the initial discharge capacity was as low as 118 mAh / g. It is presumed that the battery characteristics deteriorated due to the dissolution of lithium contained in the positive electrode active material.

[0054] In Comparative Example 2, the lithium concentration in the coating solution for the positive electrode active material was as low as 0.45 mol / L, and a coating film could not be stably formed. Furthermore, the initial discharge capacity was as low as 95 mAh / g. It is presumed that the battery characteristics deteriorated due to the dissolution of lithium contained in the positive electrode active material.

[0055] In Example 1, the lithium concentration in the coating solution for the positive electrode active material was 5.30 mol / L, which was above the saturated concentration, and a coating film could be stably formed. Furthermore, the initial discharge capacity was high at 152 mAh / g. It is presumed that the dissolution of lithium contained in the positive electrode active material could be suppressed, resulting in stable battery characteristics.

[0056] In Example 2, the lithium concentration in the coating solution for the positive electrode active material was 3.05 mol / L, and a coating film was stably formed. Furthermore, the initial discharge capacity was high at 147 mAh / g. It is presumed that the dissolution of lithium contained in the positive electrode active material was suppressed, resulting in stable battery characteristics.

[0057] In Example 3, the lithium concentration in the coating solution for the positive electrode active material was 4.00 mol / L, and a coating film was stably formed. Furthermore, the initial discharge capacity was high at 148 mAh / g. It is presumed that the dissolution of lithium contained in the positive electrode active material was suppressed, resulting in stable battery characteristics.

[0058] 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 that can suppress the dissolution of lithium contained in the positive electrode active material into the coating solution and that 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 film made of a composite compound of lithium and tungsten on a surface of a positive electrode active material containing lithium, A coating solution for a positive electrode active material, comprising an aqueous solution containing a lithium compound and a tungsten compound, the lithium concentration of which is in the range of more than 3.0 mol / L and not more than 5.3 mol / L.

2. 2. The coating solution for a positive electrode active material according to claim 1, wherein the lithium concentration is a saturated concentration.

3. 2. The coating solution for a positive electrode active material according to claim 1, further comprising ammonia, the ammonia concentration of which is in the range of 0.7 mol / L to 3.9 mol / L.

4. A method for producing a coated positive electrode active material having a coating film made of a composite compound of lithium and tungsten on the 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 liquid for a positive electrode active material according to claim 1 to a surface of the positive electrode active material; and drying the coating liquid.

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