Positive electrode active material for all-solid-state lithium-ion batteries, positive electrode for all-solid-state lithium-ion batteries, all-solid-state lithium-ion battery, and method for manufacturing the positive electrode active material for all-solid-state lithium-ion batteries.

Abstract

To provide: a positive electrode active material for an all-solid-state lithium-ion battery, which leads to excellent battery characteristics when used in an all-solid-state lithium-ion battery; and a method for manufacturing the positive electrode active material.SOLUTION: The positive electrode active material for an all-solid-state lithium-ion battery includes: positive electrode active material particles; and coating layers provided respectively on surfaces of the positive electrode active material particles. The positive electrode active material particles are expressed by a composition indicated in the following formula (1): LiaNibCocMndMeOf (in the formula (1), relationships 1.0≤a≤1.05, 0.8≤b≤0.9, b+c+d+e=1, 0.002≤e / (b+c+d)≤0.016 and 1.8≤f≤2.2 hold true, and M is at least one element selected from Zr, Ta and W). The coating layer contains Li, Nb and Ti. In ICP emission spectral analysis, a content of Ti contained in the coating layer with respect to a total amount of the positive electrode active material is 10 to 30 mass ppm and a mass ratio Ti / Nb of Ti and Nb in the positive electrode active material is 0.0015 to 0.0040.SELECTED DRAWING: Figure 2

Description

[Technical Field] 【0001】 The present invention relates to a positive electrode active material for all-solid-state lithium-ion batteries, a positive electrode for all-solid-state lithium-ion batteries, an all-solid-state lithium-ion battery, and a method for producing a positive electrode active material for all-solid-state lithium-ion batteries. [Background technology] 【0002】 Currently used lithium-ion rechargeable batteries utilize organic electrolytes. However, these electrolytes are flammable, and the risk of fire during charging is difficult to completely eliminate despite all efforts. For these reasons, the development of all-solid-state lithium-ion rechargeable batteries, which use a solid electrolyte, is actively underway. 【0003】 Conventional solid electrolytes have poor lithium-ion conductivity, making battery design difficult. However, in recent years, solid electrolytes with good conductivity have been discovered, and many inventions applying these to all-solid-state lithium-ion secondary batteries have emerged. 【0004】 However, all-solid-state lithium-ion secondary batteries have a problem in that a high-resistance layer is formed due to interfacial reactions between the positive electrode and the solid electrolyte, resulting in a decrease in output. One known method to suppress the formation of this high-resistance layer is to coat the surface of the positive electrode active material with a Li composite oxide. A typical Li composite oxide for coating the surface of the positive electrode active material is LiNbO3, which exhibits high ionic conductivity (Patent Document 1). [Prior art documents] [Patent Documents] 【0005】 [Patent Document 1] Patent No. 6293338 [Overview of the Initiative] [Problems that the invention aims to solve] 【0006】 LiNbO3 is 10 in an amorphous state. -6It becomes a Li composite oxide having a high ionic conductivity of about S / cm, but the ionic conductivity of the sulfide-based solid electrolyte material is 10 -4 ~10 -3 S / cm, which is about 1 / 100 to 1 / 1000 compared to that, and the Li ion diffusion in the coating layer becomes the rate-determining process for the entire battery. Therefore, there is still room for development for the positive electrode active material for all-solid-state lithium ion batteries that exhibits good battery characteristics when used in all-solid-state lithium ion batteries. 【Means for Solving the Problems】 【0007】 The present invention has been made to solve the above problems, and an object thereof is to provide a positive electrode active material for all-solid-state lithium ion batteries, a positive electrode for all-solid-state lithium ion batteries, an all-solid-state lithium ion battery, and a method for manufacturing a positive electrode active material for all-solid-state lithium ion batteries that exhibit good battery characteristics when used in all-solid-state lithium ion batteries. 【0008】 The present invention completed based on the above findings is defined as follows. 1. A positive electrode active material for all-solid-state lithium ion batteries, comprising positive electrode active material particles and a coating layer provided on the surface of the positive electrode active material particles, The positive electrode active material particles are represented by the composition shown in the following formula (1), Li a Ni b Co c Mn d M e O f (1) (In the formula (1), 1.0 ≦ a ≦ 1.05, 0.8 ≦ b ≦ 0.9, b + c + d + e = 1, 0.002 ≦ e / (b + c + d) ≦ 0.016, 1.8 ≦ f ≦ 2.2, and M is at least one selected from Zr, Ta, and W.) The coating layer contains Li, Nb, and Ti, A positive electrode active material for an all-solid-state lithium-ion battery, wherein, in ICP emission spectroscopy analysis, the Ti content in the coating layer relative to the total amount of positive electrode active material for the all-solid-state lithium-ion battery is 10 to 30 ppm by mass, and the Ti to Nb mass ratio (Ti / Nb) in the positive electrode active material for the all-solid-state lithium-ion battery is 0.0015 to 0.0040. 2. The positive electrode active material for an all-solid-state lithium-ion battery according to claim 1, wherein when ion sputtering is performed with a sputtering rate set to 0.25 nm / second in TOF-SIMS analysis of the surface of the positive electrode active material for an all-solid-state lithium-ion battery, Ti is detected for up to 50 seconds from the start of the analysis. 3. A positive electrode active material for an all-solid-state lithium-ion battery according to claim 1 or 2, wherein the 3.50% cumulative volume particle size D50 is 4 to 7 μm. 4. A positive electrode for an all-solid-state lithium-ion battery, comprising the positive electrode active material for all-solid-state lithium-ion batteries described in any of items 1 to 3 above. 5. An all-solid-state lithium-ion battery comprising the positive electrode and negative electrode for an all-solid-state lithium-ion battery described in item 4 above. 6. A step of preparing a precursor for a positive electrode active material for an all-solid-state lithium-ion battery, represented by the composition shown in formula (2) below, Ni b Co c Mn d (OH)2(2) (In equation (2) above, 0.8 ≤ b ≤ 0.9, 0.07 ≤ c ≤ 0.15, and b + c + d = 1.) A step of obtaining a mixture by wet mixing at least one selected from Zr oxide, Ta oxide, and W oxide, having a 50% cumulative volume particle size D50 of 1 μm or less, with the precursor of the positive electrode active material for the all-solid-state lithium-ion battery, The process involves dry-mixing the aforementioned mixture with a lithium source and firing it at 700°C or higher for 4 hours or more to obtain positive electrode active material particles. A step of forming a coating layer containing Li, Nb, and Ti on the surface of the positive electrode active material particles by coating the surface of the positive electrode active material particles with a coating solution containing Li, Nb, and Ti, and performing heat treatment at 300°C or below, A method for producing a positive electrode active material for all-solid-state lithium-ion batteries, including the material itself. 7. The method for producing a positive electrode active material for an all-solid-state lithium-ion battery according to 6, wherein the coating solution containing Li, Nb, and Ti is an aqueous solution obtained by mixing an aqueous solution of a peroxo complex of Li and Nb having a Li content and an aqueous solution of a peroxo complex of Ti having a Ti content of 0.002 to 0.01 mol / L with the aqueous solution of a peroxo complex of Ti having a Ti content of 0.002 to 0.01 mol / L, such that the mass ratio of the aqueous solution of the peroxo complex of Li and Nb to the aqueous solution of the peroxo complex of Ti is 30:1 to 30:5. [Effects of the Invention] 【0009】 According to the present invention, it is possible to provide a positive electrode active material for all-solid-state lithium-ion batteries, a positive electrode for all-solid-state lithium-ion batteries, an all-solid-state lithium-ion battery, and a method for producing the positive electrode active material for all-solid-state lithium-ion batteries, which result in good battery characteristics when used in all-solid-state lithium-ion batteries. [Brief explanation of the drawing] 【0010】 [Figure 1] This is a schematic diagram of an all-solid-state lithium-ion battery according to an embodiment of the present invention. [Figure 2] This is a graph of the spectral distribution obtained from the TOF-SIMS analysis in Example 1. [Figure 3] This is a graph of the spectral distribution obtained from the TOF-SIMS analysis in Example 2. [Figure 4] This is a graph of the spectral distribution obtained from the TOF-SIMS analysis in Example 3. [Figure 5] This is a graph of the spectral distribution obtained from the TOF-SIMS analysis of Comparative Example 1. [Figure 6] This is a graph of the spectral distribution obtained from the TOF-SIMS analysis of Comparative Example 2. [Figure 7] This is a graph of the spectral distribution obtained from the TOF-SIMS analysis of Comparative Example 3. [Modes for carrying out the invention] 【0011】 Next, embodiments for carrying out the present invention will be described in detail with reference to the drawings. The present invention is not limited to the following embodiments, and it should be understood that appropriate design changes, improvements, etc., can be made based on the ordinary knowledge of those skilled in the art, without departing from the spirit of the invention. 【0012】 (Positive electrode active material for all-solid-state lithium-ion batteries) The positive electrode active material for an all-solid-state lithium-ion battery according to an embodiment of the present invention comprises positive electrode active material particles and a coating layer provided on the surface of the positive electrode active material particles. The positive electrode active material particles are represented by the composition shown in the following formula (1). Li a Ni b Co c Mn d M e O f (1) (In equation (1) above, 1.0 ≤ a ≤ 1.05, 0.8 ≤ b ≤ 0.9, b + c + d + e = 1, 0.002 ≤ e / (b + c + d) ≤ 0.016, 1.8 ≤ f ≤ 2.2, and M is at least one selected from Zr, Ta, and W.) 【0013】 In the positive electrode active material for all-solid-state lithium-ion batteries, the positive electrode active material particles are controlled such that a, which represents the lithium composition in formula (1) above, is 1.0 ≤ a ≤ 1.05. Since a, which represents the lithium composition, is 1.0 or greater, the reduction of nickel due to lithium deficiency can be suppressed. Furthermore, since a, which represents the lithium composition, is 1.05 or less, residual alkaline components such as lithium carbonate and lithium hydroxide present on the surface of the positive electrode active material particles, which can become a resistive component when used in a battery, can be suppressed. 【0014】 In the positive electrode active material for all-solid-state lithium-ion batteries, the positive electrode active material particles have a nickel composition (b) controlled to 0.8 ≤ b ≤ 0.9 in formula (1) above, which is a so-called high-nickel composition. Since b, which represents the nickel composition, is 0.8 or higher, a good battery capacity can be obtained for the all-solid-state lithium-ion battery. 【0015】 In the positive electrode active material for all-solid-state lithium-ion batteries, the sum of b (representing nickel composition), c (representing cobalt composition), d (representing manganese composition), and e (representing the composition of additive element M) in formula (1) above is controlled to b+c+d+e=1, i.e., 0.1≦c+d+e≦0.2. This improves cycle characteristics and reduces the expansion and contraction behavior of the crystal lattice due to lithium insertion and deinsertion during charging and discharging. If c+d+e falls below 0.1, it becomes difficult to obtain the above-mentioned effects of improved cycle characteristics and reduced expansion and contraction behavior. If c+d+e exceeds 0.2, the amount of cobalt and manganese added may be too high, leading to a significant decrease in initial discharge capacity or becoming costly. 【0016】 In the positive electrode active material for all-solid-state lithium-ion batteries, the positive electrode active material particles satisfy the following condition in formula (1) above: 0.002 ≤ e / (b+c+d) ≤ 0.016, where M is at least one element selected from Zr, Ta, and W. That is, the positive electrode active material particles of the positive electrode active material for all-solid-state lithium-ion batteries contain at least one element selected from Zr, Ta, and W. This element, by solid-solubilating into the positive electrode active material, has the effect of reducing the expansion and contraction behavior of the crystal lattice due to the insertion and removal of lithium during charging and discharging. For this reason, if the composition ratio of this element, e / (b+c+d), is 0.002 or higher, the cycle characteristics are improved. On the other hand, this element does not contribute to charge compensation during charging and discharging. For this reason, if the composition ratio of this element, e / (b+c+d), is 0.016 or lower, it has the effect of suppressing the decrease in discharge capacity. Furthermore, preferably 0.002 ≤ e / (b+c+d) ≤ 0.012, and more preferably 0.002 ≤ e / (b+c+d) ≤ 0.008. 【0017】 The coating layer provided on the surface of the positive electrode active material particles of the positive electrode active material for an all-solid-state lithium-ion battery according to an embodiment of the present invention contains Li, Nb, and Ti. Preferably, the coating layer is in a form in which lithium niobate (LiNbO3) contains a small amount of Ti. The effect of including a small amount of Ti in the lithium niobate (LiNbO3) of the coating layer will be explained. Because the coating layer contains Ti, which exists as a tetravalent ion with a lower valence than Nb, which exists as a pentavalent ion, holes are formed within the coating layer. During charging and discharging, in conventional lithium niobate coating layers, lithium ions move only by interstitial diffusion, but when a small amount of Ti is included, Li ions can also move by vacancy diffusion through the formed holes. Therefore, the movement of Li ions becomes easier, which can improve rate characteristics and reduce diffusion resistance within the coating layer. 【0018】 In ICP (Inductively Coupled Plasma) emission spectroscopy, the Ti content in the coating layer relative to the total amount of positive electrode active material for all-solid-state lithium-ion batteries is controlled to 10 to 30 ppm by mass. When the Ti content is 10 ppm by mass or more, holes are formed in the coating layer containing lithium niobate, improving the rate characteristics of the all-solid-state lithium-ion battery using this positive electrode active material and reducing diffusion transfer resistance. When the Ti content is greater than 30 ppm by mass, the number of holes formed in the coating layer containing lithium niobate increases, which may worsen the rate characteristics and diffusion transfer resistance. Preferably, the Ti content in the coating layer relative to the total amount of positive electrode active material for all-solid-state lithium-ion batteries is 10 to 20 ppm by mass, and more preferably 10 to 15 ppm by mass. 【0019】 In ICP emission spectroscopy, the mass ratio Ti / Nb of Ti to Nb in the positive electrode active material for all-solid-state lithium-ion batteries is controlled to 0.0015 to 0.0040. When the mass ratio Ti / Nb is 0.0015 or higher, holes are formed in the coating layer containing lithium niobate, improving the rate characteristics of the all-solid-state lithium-ion battery using this positive electrode active material and reducing diffusion transfer resistance. When the mass ratio Ti / Nb is greater than 0.0040, more holes are formed in the lithium niobate coating layer, worsening the rate characteristics and diffusion transfer resistance. In ICP emission spectroscopy, the mass ratio Ti / Nb of Ti to Nb in the positive electrode active material for all-solid-state lithium-ion batteries is preferably 0.0015 to 0.0035, and more preferably 0.0015 to 0.0025. 【0020】 The above-mentioned ICP emission spectroscopy analysis can be performed, for example, by weighing 0.2 g of positive electrode active material (powder), decomposing it by alkaline fusion, and then performing a compositional analysis using Hitachi High-Tech Corporation's ICP (inductively coupled plasma) emission spectrometer (ICP-OES) "PS7800". 【0021】 Furthermore, the Ti content obtained by analyzing the positive electrode active material by ICP emission spectroscopy can be analyzed, for example, by analyzing the Ti data detected by time-of-flight secondary ion mass spectrometry (TOF-SIMS), to determine that it represents the Ti content contained in the coating layer on the surface of the positive electrode active material particles. Specifically, in TOF-SIMS analysis of the surface of positive electrode active material for all-solid-state lithium-ion batteries, if ion sputtering is performed with a sputtering rate set to 0.25 nm / second and Ti is detected within 50 seconds from the start of the analysis, it can be determined that Ti is contained in the coating layer. For example, TOF-SIMS analysis of the surface of positive electrode active material was performed using a TOF-SIMS analyzer (TOF-SIMS 4S manufactured by ION-TOF). Specifically, Bi was used as the primary ion. 3+ , Cs +TOF-SIMS analysis will be performed in high mass resolution mode (negative) with a sputtering rate of 0.25 nm / second (SiO2 equivalent). The sputtering area will be 300 μm × 300 μm, and the measurement area will be 150 μm × 150 μm. The coating layer has a thickness of 6-10 nm, and in TOF-SIMS analysis, if Ti is detected within approximately 50 seconds from the start of the analysis, when the thickness of the coating layer is sputtered, it will be determined that Ti is present in the coating layer. Here, if the positive electrode active material particles contain Ti, the graph will show an increasing trend for the first 50 seconds after sputtering, as shown in Figures 2-4 below for Ni in TOF-SIMS. If the coating layer contains Ti, the graph will show a peak at the start of sputtering and a decreasing trend thereafter, as shown in Figures 2-4 below for Ti and Nb in TOF-SIMS. If the coating layer does not contain Ti, the ICP analysis results will be below the limit of quantification. 【0022】 The thickness of the coating layer is preferably 10 nm or less, and more preferably 6 nm or less. A coating layer thickness of 6 nm or less allows for better avoidance of adverse effects such as inhibition of Li ion migration. The lower limit of the coating layer thickness is not particularly limited, but is typically 4 nm or more, and preferably 5 nm or more. The thickness of the coating layer can be measured by elemental mapping analysis and line analysis using a scanning transmission electron microscope (STEM). 【0023】 The positive electrode active material for all-solid-state lithium-ion batteries mostly has the form of secondary particles formed by the aggregation of multiple primary particles, and may also contain primary particles that do not aggregate as secondary particles. The shape of the primary particles constituting the secondary particles, and the shape of the primary particles existing individually, are not particularly limited and may be various shapes such as approximately spherical, approximately elliptical, approximately plate-shaped, or approximately needle-shaped. Furthermore, the form in which multiple primary particles aggregate is not particularly limited and may be various forms such as aggregation in random directions, or aggregation radially from the center almost uniformly to form approximately spherical or approximately elliptical secondary particles. 【0024】 It is preferable that the 50% cumulative volume particle size D50 of the positive electrode active material for all-solid-state lithium-ion batteries is 4 to 7 μm. Here, the 50% cumulative volume particle size D50 is the volume particle size at 50% accumulation in the volume-based cumulative particle size distribution curve. If the 50% cumulative volume particle size D50 of the positive electrode active material for all-solid-state lithium-ion batteries is 4 μm or more, the specific surface area can be suppressed and the amount of coating of Li, Nb, and Ti can be reduced. If the 50% cumulative volume particle size D50 of the positive electrode active material for all-solid-state lithium-ion batteries is 7 μm or less, it is possible to suppress the specific surface area from becoming excessively small. It is more preferable that the 50% cumulative volume particle size D50 of the positive electrode active material for all-solid-state lithium-ion batteries is 5 to 6 μm. 【0025】 The 50% cumulative volume particle size (D50) of the positive electrode active material for all-solid-state lithium-ion batteries can be measured, for example, as follows. First, 100 mg of the positive electrode active material powder is dispersed by irradiating it with 40 W of ultrasound for 60 seconds at a flow rate of 50% using a Microtrac MT3300EXII laser diffraction particle size distribution analyzer, and then the particle size distribution is measured to obtain a volume-based cumulative particle size distribution curve. Next, the volume particle size at 50% accumulation in the obtained cumulative particle size distribution curve is defined as the 50% cumulative volume particle size (D50) of the positive electrode active material powder. The water-soluble solvent used for measurement is filtered, with a solvent refractive index of 1.333, particle permeability conditions set to permeable, particle refractive index of 1.81, shape set to non-spherical, measurement range set to 0.021 to 2000 μm, and measurement time set to 30 seconds. 【0026】 (Method for manufacturing positive electrode active material for all-solid-state lithium-ion batteries) Next, a method for producing a positive electrode active material for an all-solid-state lithium-ion battery according to an embodiment of the present invention will be described in detail. The method for producing a positive electrode active material for an all-solid-state lithium-ion battery according to an embodiment of the present invention first involves preparing a precursor of a positive electrode active material for an all-solid-state lithium-ion battery represented by the composition shown in formula (2) below. Ni b Co c Mn d (OH)2(2) (In equation (2) above, 0.8 ≤ b ≤ 0.9, 0.07 ≤ c ≤ 0.15, and b + c + d = 1.) 【0027】 As a method for producing a precursor of positive electrode active material for all-solid-state lithium-ion batteries, first, an aqueous solution containing (a) a nickel salt, (b) a cobalt salt, (c) a manganese salt, and (d) a basic aqueous solution containing ammonia and a basic aqueous solution of an alkali metal is prepared. (a) Nickel salts include nickel sulfate, nickel nitrate, or nickel hydrochloride. (b) Cobalt salts include cobalt sulfate, cobalt nitrate, or cobalt hydrochloride. (c) Manganese salts include manganese sulfate, manganese nitrate, or manganese hydrochloride. (d) Basic aqueous solutions containing ammonia include aqueous solutions of ammonia, ammonium sulfate, ammonium carbonate, ammonium hydrochloride, etc. The basic aqueous solution of the alkali metal may be an aqueous solution of sodium hydroxide, potassium hydroxide, carbonate, etc. Furthermore, as the aqueous solution of the carbonate, for example, an aqueous solution using a salt of the carbonate group, such as an aqueous solution of sodium carbonate, an aqueous solution of potassium carbonate, an aqueous solution of sodium bicarbonate, or an aqueous solution of potassium bicarbonate, may be used. 【0028】 Furthermore, the composition of the aqueous solution can be appropriately adjusted depending on the composition of the precursor to be produced, but it is preferable that it is (a) an aqueous solution containing 45 to 110 g / L of nickel ions, (b) an aqueous solution containing 4 to 20 g / L of cobalt ions, (c) an aqueous solution containing 1 to 4 g / L of manganese ions, (d) an aqueous ammonia solution with a concentration of 10 to 28% by mass, and a basic aqueous solution with an alkali metal concentration of 10 to 30% by mass. 【0029】 Next, an aqueous solution containing (a) nickel salt, (b) cobalt salt, (c) manganese salt, and (d) a basic aqueous solution containing ammonia and a basic aqueous solution of an alkali metal is used as the reaction solution, and a coprecipitation reaction is carried out while controlling the pH of the reaction solution to 10.8-11.4, the ammonium ion concentration to 10-22 g / L, and the liquid temperature to 55-65°C. At this time, the chemical solutions may be supplied to the reaction vessel from three tanks: a tank containing a mixed aqueous solution of nickel salt, cobalt salt, and manganese salt, a tank containing a basic aqueous solution containing ammonia, and a tank containing a basic aqueous solution of an alkali metal. In this way, a precursor of the positive electrode active material represented by formula (2) above can be produced. 【0030】 Next, a mixture is obtained by wet mixing the precursor of the positive electrode active material for all-solid-state lithium-ion batteries with at least one selected from Zr oxide, Ta oxide, and W oxide, having a 50% cumulative volume particle size D50 of 1 μm or less. The total amount of at least one selected from Zr oxide, Ta oxide, and W oxide to be mixed can be appropriately adjusted depending on the target composition of the positive electrode active material for all-solid-state lithium-ion batteries. ZrO2 can be used as the Zr oxide, Ta2O5 as the Ta oxide, and WO2 or WO3 as the W oxide. The wet mixing is not particularly limited, but one method is to add the precursor of the positive electrode active material for all-solid-state lithium-ion batteries and at least one selected from Zr oxide, Ta oxide, and W oxide to an aqueous solvent, mix them by mechanical means to prepare a slurry, and then dry the slurry while it is standing or dry the slurry by spray drying to obtain the mixture. 【0031】 As described above, by wet-mixing Zr oxide, Ta oxide, and W oxide with the precursor of the positive electrode active material for all-solid-state lithium-ion batteries before mixing with the lithium source, the adhesion rate of dissimilar elemental oxides (Zr, Ta, W) to the surface of the positive electrode active material precursor for all-solid-state lithium-ion batteries is improved. Furthermore, by controlling the D50 of the Zr oxide, Ta oxide, and W oxide particles to be mixed to 1 μm or less, the adhesion rate of dissimilar elemental oxides (Zr, Ta, W) to the surface of the positive electrode active material precursor for all-solid-state lithium-ion batteries is improved. When the adhesion rate of dissimilar elemental oxides (Zr, Ta, W) to the surface of the positive electrode active material precursor for all-solid-state lithium-ion batteries is improved, it is possible to suppress the existence of dissimilar elemental oxides (Zr, Ta, W) as independent particles without adhering to the surface of the positive electrode active material particles for all-solid-state lithium-ion batteries. The D50 of the Zr oxide, Ta oxide, and W oxide particles to be mixed is preferably 0.3 to 1.0 μm, and more preferably 0.3 to 0.5 μm. 【0032】 Next, a lithium mixture is formed by dry mixing the precursor of the positive electrode active material for all-solid-state lithium-ion batteries obtained as described above with at least one of Zr oxide, Ta oxide, and W oxide, and a lithium source. The amount of lithium source to be mixed can be appropriately adjusted depending on the target composition of the positive electrode active material for all-solid-state lithium-ion batteries. Lithium hydroxide is an example of a lithium source. The mixing method involves adjusting the mixing ratio of each raw material and dry mixing using a Henschel mixer, automatic mortar and pestle, or V-type mixer. 【0033】 Next, the lithium mixture obtained as described above is calcined at 700°C or higher for 4 hours or more. By calcining the lithium mixture at a temperature of 700°C or higher and for a long period of 4 hours or more in one go, the solid solution rate of dissimilar elements (Zr, Ta, W) into the positive electrode active material for all-solid-state lithium-ion batteries is improved. Furthermore, this suppresses the existence of oxides of dissimilar elements (Zr, Ta, W) as independent particles instead of adhering to the surface of the positive electrode active material particles for all-solid-state lithium-ion batteries. The calcination temperature is preferably 700 to 800°C, and the calcination time is preferably 4 to 12 hours. The calcination atmosphere is preferably an oxygen atmosphere. 【0034】 Subsequently, if necessary, the calcined body can be crushed using, for example, a pulverizer to obtain positive electrode active material particles. 【0035】 Next, the surface of the positive electrode active material particles described above is coated with an aqueous solution (coating solution) containing Li, Nb, and Ti. In this case, it is preferable that the coating solution is an aqueous solution prepared by mixing an aqueous solution of Li and Nb peroxo complexes having a Li content of 0.1 to 0.2 mol / L and an aqueous solution of Ti peroxo complexes having a Ti content of 0.002 to 0.01 mol / L, in a mass ratio of aqueous solution of Li and Nb peroxo complexes to aqueous solution of Ti peroxo complexes = 30:1 to 30:5. 【0036】 Furthermore, the coating method is not particularly limited as long as it is a method that can adhere the coating liquid to the surface of the positive electrode active material particles. For example, a method using a coating apparatus having a rolling fluidized bed or a method by spray drying may be used. 【0037】 After coating the surface of the positive electrode active material particles with an aqueous solution (coating solution) containing Li, Nb, and Ti, a heat treatment is performed at 300°C or below. This heat treatment is preferably carried out at 200-300°C for 1-5 hours. In this way, a positive electrode active material for an all-solid-state lithium-ion battery according to the present invention can be obtained, having a coating layer containing Li, Nb, and Ti formed on the surface of the positive electrode active material particles. 【0038】 (Positive electrode for all-solid-state lithium-ion batteries and all-solid-state lithium-ion batteries) A positive electrode can be formed using a positive electrode active material for an all-solid-state lithium-ion battery according to an embodiment of the present invention, and this positive electrode can be used as the positive electrode layer. An all-solid-state lithium-ion battery can be manufactured comprising this positive electrode layer, a solid electrolyte layer, and a negative electrode layer. The solid electrolyte layer and negative electrode layer constituting the all-solid-state lithium-ion battery according to an embodiment of the present invention are not particularly limited and can be formed from known materials, and can have known configurations as shown in Figure 1. 【0039】 The positive electrode layer of the all-solid-state lithium-ion battery can be made by forming a positive electrode composite material in layers, which is obtained by mixing a positive electrode active material for all-solid-state lithium-ion batteries according to the present invention with a solid electrolyte. The content of the positive electrode active material in the positive electrode layer is preferably 50% by mass or more and 99% by mass or less, and more preferably 60% by mass or more and 90% by mass or less. 【0040】 The positive electrode composite may further contain a conductive additive. A carbon material can be used as the conductive additive. Examples of carbon materials include carbon black such as Ketjenblack, acetylene black, Denka black, thermal black, and channel black, as well as graphite, carbon fiber, activated carbon, and the like. 【0041】 The average thickness of the positive electrode layer of an all-solid-state lithium-ion battery is not particularly limited and can be designed appropriately depending on the purpose. The average thickness of the positive electrode layer of an all-solid-state lithium-ion battery may be, for example, 1 μm to 100 μm, or 1 μm to 10 μm. 【0042】 The method for forming the positive electrode layer of an all-solid-state lithium-ion battery is not particularly limited and can be appropriately selected depending on the purpose. Examples of methods for forming the positive electrode layer of an all-solid-state lithium-ion battery include a method of compression molding the positive electrode active material for the all-solid-state lithium-ion battery. 【0043】 The negative electrode layer of an all-solid-state lithium-ion battery may be formed by creating a known metal foil for all-solid-state lithium-ion batteries or by forming a negative electrode active material in layers. Alternatively, the negative electrode layer may be formed by creating a negative electrode composite material by mixing a known negative electrode active material for all-solid-state lithium-ion batteries with a solid electrolyte in layers. The content of the negative electrode active material in the negative electrode layer is preferably, for example, 10% by mass or more and 99% by mass or less, and more preferably 20% by mass or more and 90% by mass or less. 【0044】 The negative electrode layer can be made of, for example, metallic lithium, metallic indium, silicon, or other metals, alloys combined with other elements or compounds, or graphite, silicon oxide, or composite materials thereof. 【0045】 The average thickness of the negative electrode layer of an all-solid-state lithium-ion battery is not particularly limited and can be appropriately selected depending on the purpose. The average thickness of the negative electrode layer of an all-solid-state lithium-ion battery may be, for example, 1 μm to 100 μm, or 1 μm to 10 μm. 【0046】 The method for forming the negative electrode layer of an all-solid-state lithium-ion battery is not particularly limited and can be appropriately selected depending on the purpose. Examples of methods for forming the negative electrode layer of an all-solid-state lithium-ion battery include inserting metal foil and compression molding of negative electrode active material particles. 【0047】 A known solid electrolyte for all-solid-state lithium-ion batteries can be used as the solid electrolyte. Sulfide-based solid electrolytes, etc., can be used. 【0048】 Examples of sulfide-based solid electrolytes include LiI-Li2S-P2S5, LiI-Li2S-B2S3, Li3PO4-Li2S-Si2S, Li3PO4-Li2S-SiS2, LiPO4-Li2S-SiS, LiI-Li2S-P2O5, LiI-Li3PO4-P2S5, Li3PS4, and Li2S-P2S5. 【0049】 The average thickness of the solid electrolyte layer in an all-solid-state lithium-ion battery is not particularly limited and can be designed appropriately depending on the purpose. The average thickness of the solid electrolyte layer in an all-solid-state lithium-ion battery may be, for example, 50 μm to 500 μm, or 50 μm to 100 μm. 【0050】 Methods for forming the solid electrolyte layer of an all-solid-state lithium-ion battery include methods such as compression molding of the solid electrolyte. 【0051】 Other components constituting the all-solid-state lithium-ion battery are not particularly limited and can be appropriately selected depending on the purpose, and examples include a positive electrode current collector, a negative electrode current collector, and a battery case. 【0052】 The size and structure of the positive electrode current collector are not particularly limited and can be appropriately selected according to the purpose. Examples of materials for the positive electrode current collector include die steel, stainless steel, aluminum, and aluminum alloys. Possible shapes for the positive electrode current collector include foil-like or plate-like forms. The average thickness of the positive electrode current collector may be, for example, 10 μm to 500 μm, or 50 μm to 100 μm. 【0053】 The size and structure of the negative electrode current collector are not particularly limited and can be appropriately selected according to the purpose. Examples of materials for the negative electrode current collector include die steel, indium, copper, and stainless steel. Examples of negative electrode current collector shapes include foil-like and plate-like forms. The average thickness of the negative electrode current collector may be, for example, 10 μm to 500 μm, or 50 μm to 100 μm. 【0054】 The battery case is not particularly limited and can be selected as appropriate depending on the purpose. Examples include known laminate films that can be used with conventional solid-state batteries. Examples of laminate films include resin laminate films and films in which metal has been vapor-deposited onto a resin laminate film. The shape of the battery is not particularly limited and can be selected as appropriate depending on the purpose. Examples include cylindrical, rectangular, button-shaped, coin-shaped, and flat-shaped batteries. [Examples] 【0055】 The following examples are provided to better understand the present invention and its advantages, but the present invention is not limited to these examples. 【0056】 (Example 1) First, the chemical formula is: Ni 0.82 Co 0.15 Mn 0.03 A precursor material for the positive electrode active material of an all-solid-state lithium-ion battery, represented as (OH)2, was prepared. Next, a precursor of a positive electrode active material for all-solid-state lithium-ion batteries having a D50 of 5.4 μm and ZrO2 having a D50 of 0.35 μm were added to an aqueous solvent in a charge amount of 0.3 mol%, and these were mixed by mechanical means (wet mixing) to prepare a slurry. Then, the slurry was allowed to stand and dried to obtain a mixture. Next, lithium carbonate (lithium source) was added to the resulting mixture and mixed in a Henschel mixer (dry mixing) to form a lithium mixture. Next, the lithium mixture obtained as described above was calcined in an oxygen atmosphere at 740°C for 12 hours to produce positive electrode active material particles. Next, as a coating solution, an aqueous solution (Li-Nb-Ti peroxo complex aqueous solution) was prepared by mixing an aqueous solution of Li and Nb peroxo complexes, each containing 0.18 mol / L of Li and Nb, with an aqueous solution of Ti peroxo complex, each containing 0.005 mol / L of Ti, in a mass ratio of Li and Nb peroxo complex aqueous solution:Ti peroxo complex aqueous solution = 30:4. Next, using this coating solution, the surface of the prepared positive electrode active material particles was coated with an oxide precursor containing Li, Nb, and Ti using a rolling fluidized bed coating apparatus, and heat treatment was performed at 230°C in an oxygen atmosphere to produce a positive electrode active material with a coating layer on its surface. 【0057】 (Example 2) First, the chemical formula is: Ni 0.82 Co 0.15 Mn 0.03 A precursor material for the positive electrode active material of an all-solid-state lithium-ion battery, represented as (OH)2, was prepared. Next, a precursor of a positive electrode active material for all-solid-state lithium-ion batteries having a D50 of 5.4 μm and WO2 having a D50 of 0.56 μm were added to an aqueous solvent in a charge amount of 0.5 mol%, and these were mixed by mechanical means (wet mixing) to prepare a slurry. Then, the slurry was allowed to stand and dried to obtain a mixture. Next, lithium carbonate (lithium source) was added to the resulting mixture and mixed in a Henschel mixer (dry mixing) to form a lithium mixture. Next, the lithium mixture obtained as described above was calcined in an oxygen atmosphere at 720°C for 12 hours to produce cathode active material particles. Next, a positive electrode active material was prepared by providing a coating layer on the surface of positive electrode active material particles using the same method as in Example 1. 【0058】 (Example 3) First, the chemical formula is: Ni 0.82 Co 0.15 Mn 0.03 A precursor material for the positive electrode active material of an all-solid-state lithium-ion battery, represented as (OH)2, was prepared. Next, a precursor of a positive electrode active material for all-solid-state lithium-ion batteries having a D50 of 5.4 μm and Ta2O5 having a D50 of 0.31 μm were added to an aqueous solvent in a charge amount of 0.5 mol%, and these were mixed by mechanical means (wet mixing) to prepare a slurry. Then, the slurry was allowed to stand and dried to obtain a mixture. Next, lithium carbonate (lithium source) was added to the resulting mixture and mixed in a Henschel mixer (dry mixing) to form a lithium mixture. Next, the lithium mixture obtained as described above was calcined in an oxygen atmosphere at 720°C for 12 hours to produce cathode active material particles. Next, a positive electrode active material was prepared by providing a coating layer on the surface of positive electrode active material particles using the same method as in Example 1. 【0059】 (Comparative Example 1) First, the chemical formula is: Ni 0.82 Co 0.15 Mn 0.03 A precursor material for the positive electrode active material of an all-solid-state lithium-ion battery, represented as (OH)2, was prepared. Next, a precursor of a positive electrode active material for all-solid-state lithium-ion batteries having a D50 of 5.5 μm and ZrO2 having a D50 of 0.35 μm were added to an aqueous solvent in a charge amount of 0.3 mol%, and these were mixed by mechanical means (wet mixing) to prepare a slurry. Then, the slurry was allowed to stand and dried to obtain a mixture. Next, lithium carbonate (lithium source) was added to the resulting mixture and mixed in a Henschel mixer (dry mixing) to form a lithium mixture. Next, the lithium mixture obtained as described above was calcined in an oxygen atmosphere at 720°C for 12 hours to produce cathode active material particles. Next, an aqueous solution of Li and Nb peroxo complex (Li-Nb peroxo complex aqueous solution) with Li and Nb content of 0.18 mol / L each was prepared as a coating solution. Then, using this coating solution, the surface of the prepared positive electrode active material particles was coated with an oxide precursor containing Li and Nb using a rolling fluidized bed coating apparatus, and heat treatment was performed at 250°C in an oxygen atmosphere to produce a positive electrode active material with a coating layer on its surface. 【0060】 (Comparative Example 2) First, the chemical formula is: Ni 0.82 Co 0.15 Mn 0.03 A precursor material for the positive electrode active material of an all-solid-state lithium-ion battery, represented as (OH)2, was prepared. Next, a precursor of a positive electrode active material for all-solid-state lithium-ion batteries having a D50 of 5.5 μm and WO2 having a D50 of 0.56 μm were added to an aqueous solvent in a charge amount of 0.5 mol%, and these were mixed by mechanical means (wet mixing) to prepare a slurry. Then, the slurry was allowed to stand and dried to obtain a mixture. Next, lithium carbonate (lithium source) was added to the resulting mixture and mixed in a Henschel mixer (dry mixing) to form a lithium mixture. Next, the lithium mixture obtained as described above was calcined in an oxygen atmosphere at 720°C for 12 hours to produce cathode active material particles. Next, a positive electrode active material was prepared by providing a coating layer on the surface of positive electrode active material particles using the same method as in Comparative Example 1. 【0061】 (Comparative Example 3) First, the chemical formula is: Ni 0.82 Co 0.15 Mn 0.03 A precursor material for the positive electrode active material of an all-solid-state lithium-ion battery, represented as (OH)2, was prepared. Next, a precursor of a positive electrode active material for all-solid-state lithium-ion batteries having a D50 of 5.5 μm and Ta2O5 having a D50 of 0.31 μm were added to an aqueous solvent in a charge amount of 0.5 mol%, and these were mixed by mechanical means (wet mixing) to prepare a slurry. Then, the slurry was allowed to stand and dried to obtain a mixture. Next, lithium carbonate (lithium source) was added to the resulting mixture and mixed in a Henschel mixer (dry mixing) to form a lithium mixture. Next, the lithium mixture obtained as described above was calcined in an oxygen atmosphere at 720°C for 12 hours to produce cathode active material particles. Next, a positive electrode active material was prepared by providing a coating layer on the surface of positive electrode active material particles using the same method as in Comparative Example 1. 【0062】 (Composition of positive electrode active material) 0.2 g of each positive electrode active material sample (powder) was weighed out, decomposed by alkaline fusion, and then its composition was analyzed using a Hitachi High-Tech ICP (Inductively Coupled Plasma) Atomic Emission Spectrometer (ICP-OES) "PS7800". The oxygen content is determined by subtracting the impurity concentration and residual alkali content, in addition to the analytical values ​​of Li and metal components, from the total amount of the analytical sample, and this is expressed in formula (1) as "O f The value of f was calculated. 【0063】 (50% cumulative volume particle size D50) 100 mg of each positive electrode active material sample (powder) was dispersed by irradiating it with 40 W ultrasonic waves for 60 seconds at a flow rate of 50% using a Microtrac MT3300EXII laser diffraction particle size distribution analyzer. The particle size distribution was then measured to obtain a volume-based cumulative particle size distribution curve. Next, the volume particle size at 50% accumulation in the obtained cumulative particle size distribution curve was defined as the 50% cumulative volume particle size D50 of the positive electrode active material powder. The water-soluble solvent used for measurement was filtered, with a solvent refractive index of 1.333, particle permeability conditions set to permeable, particle refractive index of 1.81, and shape set to non-spherical. The measurement range was 0.021 to 2000 μm, and the measurement time was 30 seconds. 【0064】 (TOF-SIMS analysis) TOF-SIMS analysis of the surface of the positive electrode active material was performed using a TOF-SIMS analyzer (ION-TOF TOF 4S). Specifically, Bi was used as the primary ion. 3+ , Cs + TOF-SIMS analysis was performed in high mass resolution mode (negative) with a sputtering rate of 0.25 nm / second (SiO2 equivalent). The sputtering area was 300 μm × 300 μm, and the measurement area was 150 μm × 150 μm. The coating layer had a thickness of 6-10 nm, and in TOF-SIMS analysis, if Ti was detected within approximately 50 seconds from the start of the analysis, during which the thickness of the coating layer was sputtered, it was determined that Ti was present in the coating layer. Figures 2-7 show graphs of the spectral distributions obtained by TOF-SIMS analysis for Examples 1-3 and Comparative Examples 1-3. In the graphs in Figures 2-7, the vertical axis represents ionic intensity, and the horizontal axis represents the elapsed time from the start of analysis (sputtering time [seconds]). According to the graphs of the spectral distribution obtained by TOF-SIMS analysis in Examples 1-3 shown in Figures 2-4, Ti(Ti) was observed in approximately 50 seconds from the start of the analysis until the thickness of the coating layer was sputtered. + ) was detected, and it was determined that Ti is present in the coating layer. According to the spectral distribution graphs obtained from the TOF-SIMS analysis of Comparative Examples 1 to 3 in Figures 5 to 7, the amount of Ti detected during the approximately 50 seconds from the start of the analysis until the thickness of the coating layer is sputtered is recognized as noise, and therefore it can be determined that it was not significantly detected, and as a result it was determined that there is no Ti in the coating layer. Furthermore, in the graphs of the spectral distribution obtained from TOF-SIMS analysis in Figures 2-7, Ti(Ti) is observed in the approximately 50 seconds from the start of the analysis until the thickness of the coating layer is sputtered. + In addition to ), Li( LiCs + ), Nb(Nb + O(OCs) have also been detected, and it is determined that Nb is also present in the coating layer. 2+ ), Li( LiCs + ), Ni( NiCs +Regarding ), a constant amount is distributed from approximately 50 seconds after the start of the analysis, indicating that it is uniformly present in the depth direction within the positive electrode active material particles. 【0065】 (Battery characteristics) <Method for manufacturing all-solid-state lithium-ion batteries> The positive electrode active materials obtained in Examples 1-3 and Comparative Examples 1-3 were mixed with a sulfide-based solid electrolyte (75Li2S-25P2S5), acetylene black, and a binder in the order of 60:35:5:1.5 by mass. Anisole was added as a solvent so that the solid content of the slurry was 65% by mass, and the mixture was mixed in a Mazelstar for 400 seconds to obtain a positive electrode mixture slurry. This slurry was then coated onto the surface of a 0.03 mm thick aluminum foil, which served as the positive electrode current collector. At this time, the positive electrode mixture slurry was coated onto the surface of the positive electrode current collector by using an applicator with a gap of 400 μm and moving the applicator at a travel speed of 15 mm / s. Next, a positive electrode composite slurry was coated onto the surface of a positive electrode current collector, and the solvent was removed by drying the collector on a hot plate at 100°C for 30 minutes to form a positive electrode composite layer on the surface of the current collector. Next, the aforementioned positive electrode composite layer was placed on top of a sulfide-based solid electrolyte having the same composition as the sulfide-based solid electrolyte used in the preparation of the positive electrode composite layer, and pressed at 333 MPa to produce a laminate of solid electrolyte layer / positive electrode composite layer / positive electrode current collector. Next, a metallic Li-In alloy was pressed onto the negative electrode side of the solid electrolyte layer at 37 MPa to form the negative electrode layer. The laminate thus fabricated was placed in a SUS304 battery test cell and confined under pressure to create an all-solid-state secondary battery. Furthermore, this all-solid-state secondary battery, with confined under pressure, was placed in a sealed container to block out air. 【0066】 <Evaluation of discharge capacity> The discharge capacity of the all-solid-state lithium-ion battery was evaluated by measuring the impedance and resistance after the initial charge at 0.1C at 55°C, and then discharging at 0.1C. 【0067】 <Evaluation of Rate Characteristics> The rate characteristics (%) of the all-solid-state lithium-ion battery were evaluated by first measuring the initial capacity obtained at a discharge rate of 0.1C (55°C, upper charge voltage limit: 3.7V, lower discharge voltage limit: 2.5V vs Li-In), and then measuring the high-rate capacity obtained at a discharge rate of 0.5C (55°C, upper charge voltage limit: 3.7V, lower discharge voltage limit: 2.5V vs Li-In), and the ratio of (high-rate capacity) / (initial capacity) was expressed as a percentage. 【0068】 <Evaluation of resistance> The resistance of the all-solid-state lithium-ion battery was evaluated as the initial resistance of the all-solid-state cell by measuring the AC impedance from 0.1 Hz to 1 MHz and analyzing the resulting Cole-Cole plot. The above manufacturing conditions and test results are shown in Tables 1 and 2. 【0069】 [Table 1] 【0070】 [Table 2] 【0071】 (Evaluation results) The positive electrode active materials of Examples 1 to 3 all had the composition of formula (1) below. The "Li / Me ratio" in Table 2 indicates the composition ratio of Li to the total of Ni, Co, Mn, and M in the positive electrode active material. Li a Ni b Co c Mn d M e O f (1) (In equation (1) above, 1.0 ≤ a ≤ 1.05, 0.8 ≤ b ≤ 0.9, b + c + d + e = 1, 0.002 ≤ e / (b + c + d) ≤ 0.016, 1.8 ≤ f ≤ 2.2, and M is at least one selected from Zr, Ta, and W.) Furthermore, in all of the positive electrode active materials of Examples 1 to 3, the coating layer contained Li, Nb, and Ti. ICP emission spectrometry analysis showed that the Ti content in the coating layer relative to the total amount of positive electrode active material was 10 to 30 ppm by mass, and the Ti / Nb mass ratio in the positive electrode active material for all-solid-state lithium-ion batteries was 0.0015 to 0.0040. Therefore, in all three examples (1-3), the initial discharge capacity, rate characteristics, and initial resistance of the all-solid-state cell were all excellent. 【0072】 The positive electrode active materials in Comparative Examples 1-3 all lacked Ti in their coating layer, and all exhibited high initial resistance in their all-solid-state cells. Furthermore, Comparative Example 1 also had poor rate characteristics.

Claims

[Claim 1] A positive electrode active material for an all-solid-state lithium-ion battery comprising positive electrode active material particles and a coating layer provided on the surface of the positive electrode active material particles, The positive electrode active material particles are represented by the composition shown in the following formula (1): Li a Ni b Co c Mn d M e O f (1) (In equation (1) above, 1.0 ≤ a ≤ 1.05, 0.8 ≤ b ≤ 0.9, b + c + d + e = 1, 0.002 ≤ e / (b + c + d) ≤ 0.016, 1.8 ≤ f ≤ 2.2, and M is at least one selected from Zr, Ta, and W.) The coating layer comprises Li, Nb, and Ti. A positive electrode active material for an all-solid-state lithium-ion battery, wherein, in ICP emission spectroscopy analysis, the Ti content in the coating layer relative to the total amount of positive electrode active material for the all-solid-state lithium-ion battery is 10 to 30 ppm by mass, and the mass ratio Ti / Nb of Ti in the positive electrode active material for the all-solid-state lithium-ion battery is 0.0015 to 0.0040. [Claim 2] The positive electrode active material for an all-solid-state lithium-ion battery according to claim 1, wherein when ion sputtering is performed on the surface of the positive electrode active material for an all-solid-state lithium-ion battery with the sputtering rate set to 0.25 nm / second, Ti is detected within 50 seconds from the start of the analysis. [Claim 3] The positive electrode active material for an all-solid-state lithium-ion battery according to claim 1, wherein the 50% cumulative volume particle size D50 is 4 to 7 μm. [Claim 4] A positive electrode for an all-solid-state lithium-ion battery, comprising the positive electrode active material for an all-solid-state lithium-ion battery described in any one of claims 1 to 3. [Claim 5] An all-solid-state lithium-ion battery comprising a positive electrode and a negative electrode for an all-solid-state lithium-ion battery as described in claim 4. [Claim 6] A step of preparing a precursor for a positive electrode active material for an all-solid-state lithium-ion battery, represented by the composition shown in formula (2) below, Ni b Co c Mn d (OH) 2 (2) (In equation (2) above, 0.8 ≤ b ≤ 0.9, 0.07 ≤ c ≤ 0.15, and b + c + d = 1.) A step of obtaining a mixture by wet mixing at least one selected from Zr oxide, Ta oxide, and W oxide, having a 50% cumulative volume particle size D50 of 1 μm or less, with the precursor of the positive electrode active material for the all-solid-state lithium-ion battery, The process involves dry-mixing the aforementioned mixture with a lithium source and firing it at 700°C or higher for 4 hours or more to obtain positive electrode active material particles. A step of forming a coating layer containing Li, Nb, and Ti on the surface of the positive electrode active material particles by coating the surface of the positive electrode active material particles with a coating solution containing Li, Nb, and Ti, and performing heat treatment at 300°C or below, A method for producing a positive electrode active material for all-solid-state lithium-ion batteries, including the material itself. [Claim 7] The method for producing a positive electrode active material for an all-solid-state lithium-ion battery according to claim 6, wherein the coating solution containing Li, Nb, and Ti is an aqueous solution obtained by mixing an aqueous solution of a peroxo complex of Li and Nb having a Li content of 0.1 to 0.2 mol / L and an aqueous solution of a peroxo complex of Ti having a Ti content of 0.002 to 0.01 mol / L, such that the mass ratio of the aqueous solution of the peroxo complex of Li and Nb to the aqueous solution of the peroxo complex of Ti is 30:1 to 30:5.

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