Method for preparing coating solution, coating solution and coating method

Abstract

To provide a coating solution capable of forming a thin and smooth film of lithium-rich lithium silicate on the surface of coated particles, a method for preparing the same, and a coating method for forming a film on the surface of coated particles using the coating solution.SOLUTION: The LiSi sol stock solution and the chelating agent preparation solution are prepared. The LiSi sol stock solution contains lithium hydroxide, a lithium-containing solute that includes lithium as a constituent element, and silica gel, a silicon-containing solute that includes silicon as a constituent element, dissolved in decarbonated water as the solvent. The chelating agent preparation solution contains lithium hydroxide, a lithium compound, and citric acid, a chelating agent, dissolved in it. By mixing the LiSi sol stock solution with the chelating agent preparation solution, a coating solution consisting of this mixture is obtained.SELECTED DRAWING: Figure 3C

Description

[Technical Field]

[0001] The present invention relates to a coating solution used for forming a film on the surface of particles to be coated, a method for preparing the same, and a coating method for forming a film on the surface of particles to be coated using the coating solution. [Background technology]

[0002] In all-solid-state batteries using a sulfide-based solid electrolyte, it is known that the resistance (interface resistance) generated at the interface between the positive electrode active material and the solid electrolyte can be reduced by forming a coating of lithium metal composite oxide on the surface of the positive electrode active material (see, for example, Patent Documents 1 and 2).

[0003] An example of a lithium metal composite oxide that can be used as the coating material is lithium niobate (LiNbO3). Lithium niobate is a material that can form a smooth thin film with excellent crystallinity. However, because niobium (Nb) is a rare metal, there are resource limitations when it comes to establishing a full-scale mass production system for positive electrode active materials with a lithium niobate coating.

[0004] It has been proposed to use lithium silicate as a coating material instead of lithium niobate. Also, a technique has been proposed in which a lithium transition metal oxide powder is mixed with an aqueous solution of lithium silicate, and the mixture is heat-treated to evaporate water from the aqueous solution, thereby forming a coating made of lithium silicate on the surface of the lithium transition metal oxide powder (see, for example, Patent Document 3). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 4982866 [Patent Document 2] International Publication No. 2012 / 043321 [Patent Document 3] Patent No. 5989087 Summary of the Invention [Problem to be solved by the invention]

[0006] The lithium-richer the composition of a lithium metal composite oxide, the higher the lithium ion conductivity tends to be.

[0007] However, commercially available lithium silicate aqueous solutions have a Li / Si molar ratio in the range of 0.25 to 0.67, meaning they contain little lithium. Using conventional technology, it is difficult to prepare a colorless, transparent lithium silicate aqueous solution with a lithium-rich composition. Making a lithium-rich lithium silicate aqueous solution promotes the polymerization reaction of silicic acid, causing the silicic acid polymer sol to grow to a size that scatters light. In lithium silicate aqueous solutions containing polymer sols of this size, segregation and particle aggregation occur during film formation, making it impossible to form a thin, smooth film.

[0008] An object of the present invention is to provide a coating solution capable of forming a thin and smooth coating of lithium silicate having a lithium-rich composition on the surface of particles to be coated, a method for preparing the same, and a coating method for forming a coating on the surface of particles to be coated using the coating solution. [Means for solving the problem]

[0009] In order to achieve the above-mentioned object, a coating solution preparation method according to one aspect of the present invention is a method for preparing a coating solution used to form a coating on the surface of a particle to be coated, and includes a first preparation step of preparing a LiSi sol stock solution in which a coating solute containing lithium and silicon as constituent elements is dissolved, a second preparation step of preparing a chelating agent preparation solution in which a lithium compound and a chelating agent are dissolved, and a mixing step of mixing the LiSi sol stock solution prepared in the first preparation step with the chelating agent preparation solution prepared in the second preparation step.

[0010] According to this method, a LiSi sol stock solution and a chelating agent preparation solution are prepared. Then, the LiSi sol stock solution and the chelating agent preparation solution are mixed. The LiSi sol stock solution contains a coating solute containing lithium and silicon as constituent elements dissolved in a solvent. The chelating agent preparation solution contains a lithium compound and a chelating agent dissolved in the solvent. Because both the LiSi sol stock solution and the chelating agent preparation solution contain lithium, by mixing the LiSi sol stock solution and the chelating agent preparation solution, the resulting mixed solution becomes a lithium-rich coating solution with a Li / Si molar ratio of 1 or more. Furthermore, because the chelating agent is contained in the chelating agent preparation solution, the dispersion stability of the coating solution can be maintained.

[0011] Therefore, a coating solution having a lithium-rich composition can be obtained that is free from cloudiness and gelation. By using this coating solution to form a coating on the surface of particles to be coated, a thin and smooth coating of lithium silicate having a lithium-rich composition can be formed on the surface of the particles to be coated.

[0012] The coating solution preparation method preferably further includes an adjusting step of measuring and adjusting the hydrogen ion exponent of the solution obtained in the mixing step, i.e., the solution obtained by mixing the LiSi sol stock solution and the chelating agent preparation solution.

[0013] By adjusting the pH of the solution, it is possible to suppress the absorption of carbon dioxide gas by the solution and the gelation of the solution.

[0014] If the pH of the solution is less than 7.5, the solution will gel quickly, and when the solution is mixed with the positive electrode active material to form a slurry, the transition metal elements in the positive electrode active material, such as cobalt and nickel, will ionize and dissolve into the solution, potentially contaminating the coating components and reducing the effectiveness of the coating. On the other hand, if the pH of the solution is more than 10, the solution will be more likely to absorb carbon dioxide, which could result in granular deposits on the surface of the coating formed from the coating solution.

[0015] Therefore, in the preparation step, it is preferable to adjust the hydrogen ion exponent of the solution to a range of 7.5 to 10. This makes it possible to effectively suppress absorption of carbon dioxide gas by the solution and gelation of the solution.

[0016] The coating solute may include a lithium-containing solute containing lithium as a constituent element and a silicon-containing solute containing silicon as a constituent element, and in the first preparation step, the lithium-containing solute and the silicon-containing solute may be dissolved in a solvent.

[0017] The lithium-containing solute may be lithium hydroxide.

[0018] The silicon-containing solute may be silica gel.

[0019] The solvent may be decarbonated water.

[0020] In the first preparation step, the lithium-containing solute may be dissolved in a solvent, the silicon-containing solute may be added to the solution in which the lithium-containing solute has been dissolved, and the solution containing the silicon-containing solute may be heated.

[0021] From the above, the coating solute may include lithium hydroxide and silica gel, and in the first preparation step, lithium hydroxide may be dissolved in deoxygenated water, silica gel may be added to the solution in which the lithium hydroxide has been dissolved, and the solution containing the silica gel may be heated.

[0022] The first preparation step is preferably carried out in an inert atmosphere.

[0023] This makes it possible to prevent the LiSi sol stock solution from absorbing carbon dioxide gas in the first preparation step.

[0024] The lithium compound may be lithium hydroxide.

[0025] The chelating agent may be citric acid.

[0026] In the second preparation step, the lithium compound and the chelating agent may be dissolved in a solvent.

[0027] 12. The method for preparing a coating solution according to claim 11, wherein the solvent is decarbonated water.

[0028] The second preparation step is preferably carried out in an inert atmosphere.

[0029] This makes it possible to prevent the chelating agent preparation solution from absorbing carbon dioxide gas in the second preparation step.

[0030] Not only the first preparation step and the second preparation step, but also the mixing step may be performed in an inert atmosphere, and further, the preparation step may be performed in an inert atmosphere.

[0031] A coating solution according to another aspect of the present invention is a coating solution used to form a coating film of lithium silicate on the surface of particles to be coated, which has a Li / Si molar ratio of 1 or more and is transparent.

[0032] By using this coating solution to form a film on the surface of particles to be coated, a thin and smooth film of lithium silicate with a lithium-rich composition can be formed on the surface of the particles to be coated.

[0033] The coating solution may also contain a chelating agent.

[0034] The inclusion of a chelating agent can maintain the dispersion stability of the coating solution.

[0035] The coating solution preferably has a hydrogen ion exponent in the range of 7.5 to 10. If the hydrogen ion exponent of the coating solution is less than 7.5, not only will the coating solution gel quickly, but when the coating solution is mixed with the positive electrode active material to form a slurry, transition metal elements in the positive electrode active material, such as cobalt and nickel, will ionize and dissolve into the coating solution, potentially contaminating the components of the coating film and reducing the effectiveness of the coating. If the hydrogen ion exponent of the coating solution exceeds 10, the coating solution will be prone to absorbing carbon dioxide gas, potentially causing granular deposits to form on the surface of the film formed from the coating solution.

[0036] A coating method according to yet another aspect of the present invention is a coating method for forming a coating film of lithium silicate on the surface of particles to be coated, and includes a coating solution preparation step for preparing a coating solution; a slurry preparation step for preparing a slurry by mixing a powder of particles to be coated with the coating solution; a coating step for dispersing the slurry prepared in the slurry preparation step onto particles to be coated having a film of the coating solution attached to their surfaces and drying the film of the coating solution attached to the particles to be coated; and a firing step for firing the powder of particles to be coated covered with the dried film of the coating solution after the coating step, wherein the coating solution preparation method described above is used in the coating solution preparation step.

[0037] This coating method makes it possible to form a thin, smooth coating of lithium silicate with a lithium-rich composition on the surface of the particles to be coated.

[0038] A coating method according to yet another aspect of the present invention is a coating method for forming a coating of lithium silicate on the surface of particles to be coated, and includes a slurry preparation step of preparing a slurry by mixing powder of particles to be coated with a coating solution, a coating step of dispersing the slurry prepared in the slurry preparation step onto particles to be coated having a film of the coating solution attached to their surfaces, and drying the film of the coating solution attached to the particles to be coated, and a firing step of firing the powder of particles to be coated covered with the dried film of the coating solution after the coating step, wherein the coating solution described in any one of claims 15 to 17 is used as the coating solution.

[0039] This coating method makes it possible to form a thin, smooth coating of lithium silicate with a lithium-rich composition on the surface of the particles to be coated.

[0040] The particles to be coated may be a positive electrode active material.

[0041] By using a cathode active material having a lithium-rich lithium silicate coating formed on its surface in an all-solid-state battery, the resistance (interface resistance) generated at the interface between the cathode active material and the solid electrolyte is reduced and lithium ion conductivity is increased, thereby achieving high performance in the all-solid-state battery. [Effects of the Invention]

[0042] According to the present invention, a coating solution can be obtained that has a lithium-rich composition but is free from cloudiness and gelation. By using this coating solution to form a coating on the surface of a particle to be coated, a thin and smooth coating of lithium silicate having a lithium-rich composition can be formed on the surface of the particle to be coated. [Brief explanation of the drawings]

[0043] [Figure 1] 1 is a process diagram showing the flow of a manufacturing process of a composite powder using a coating method according to one embodiment of the present invention. [Figure 2] FIG. 2 is a diagram showing the details of a slurry preparation step. [Figure 3A] FIG. 1 is a diagram showing the details of a stock solution preparation step. [Figure 3B] FIG. 1 is a diagram showing the details of a chelating agent preparation solution preparation step. [Figure 3C] FIG. 2 is a diagram showing the contents of a solution mixing step and a solution adjusting step. [Figure 4] FIG. 2 is a cross-sectional view illustrating the configuration of a coating device. DETAILED DESCRIPTION OF THE INVENTION

[0044] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[0045] <Composite powder manufacturing method> FIG. 1 is a process diagram showing the flow of a manufacturing process for composite powder using a coating method according to one embodiment of the present invention.

[0046] In the manufacturing process of the composite powder, a slurry preparation step (step S1), a coating step (step S2), and a firing step (step S3) are carried out in this order.

[0047] <Slurry preparation process> FIG. 2 is a diagram showing the details of the slurry preparation step.

[0048] In the slurry preparation step, a powder of particles to be coated and a LiSi coating solution are mixed in the glove box GB to prepare a slurry to be used in the coating step.

[0049] The glove box GB is controlled to an inert atmosphere at atmospheric pressure with a carbon dioxide concentration of 20 ppm or less. The inert atmosphere is an atmosphere filled with an inert gas, and may be an atmosphere filled with nitrogen gas or an atmosphere filled with a rare gas such as argon gas. The glove box GB may be either a sealed type or a flow type, as long as it can be controlled to an inert atmosphere at atmospheric pressure. The glove box GB used in the coating solution preparation process described below (the inert atmosphere in the glove box GB and the type of glove box GB) is the same as the glove box GB used in the slurry preparation process.

[0050] In preparing the slurry, the powder and the coating solution are each weighed out in appropriate amounts. For example, when the particles to be coated are a positive electrode active material for an all-solid-state battery using a sulfide-based solid electrolyte, the powder of the particles to be coated and the coating solution are each weighed out so that the weight ratio of powder to coating solution is 10:3 to 8. Examples of the positive electrode active material include LiCoO2, LiNiO2, LiMn2O4, and LiNi 1 / 3 Mn 1 / 3 Co 1 / 3 O2, Li4Ti5O 12 , LiFePO4, LiNi 0.8 Co 0.15 Al 0.05 Examples include lithium metal composite oxides such as O2.

[0051] The powder and coating solution, each weighed in the appropriate amounts, are stirred in a mixing vessel to form a slurry. The slurry is then transferred from the mixing vessel to a slurry tank 2 (see Figure 4), which is then sealed to isolate the inside of the slurry tank 2 from the outside air. The slurry tank 2 is then removed from the glove box GB.

[0052] It should be noted that the containers and tools such as the electronic balance used in the glove box GB are placed in the glove box GB before use to remove moisture (dry).

[0053] <Coating process> In the coating process, a coating device is used to disperse the slurry prepared in the slurry preparation process onto particles to be coated, which have a film of coating solution attached to their surfaces, and then dry the film of coating solution attached to the particles to be coated, thereby producing a powder in which the surfaces of the particles to be coated are coated with a precursor of lithium silicate (hereinafter, this powder is referred to as "dry gel powder").

[0054] The coating device may be a spray-drying type coating device that sprays the slurry into hot air to obtain particles to be coated with a film of the coating solution attached to their surfaces, but it is preferable to use a high-speed air current dispersion type coating device that continuously introduces the slurry into a high-speed air current and disperses the slurry onto the particles to be coated with a film of the coating solution attached to their surfaces using the shear force of the high-speed air current. By using a high-speed air current dispersion type coating device, it is possible to improve dispersibility and shorten the time required for the coating process, and also to obtain a uniform thin film coating (improved surface concentration and surface coverage). The configuration of a high-speed air current dispersion type coating device will be described later.

[0055] <Firing process> In the firing step, a firing furnace is used to fire the dry gel powder produced in the coating step at 300°C to 400°C (for example, at 350°C for 4 hours), thereby obtaining a powder in which the particles to be coated are covered with a thin film of lithium silicate. In order to reduce residual carbon in the powder, atmospheric control, such as increasing the oxygen concentration in the firing furnace, may be performed.

[0056] The calcination furnace may be of any type, such as a muffle furnace, a tubular furnace, or a rotary kiln, as long as it can heat the dry gel powder to 300° C. or higher in the atmosphere.

[0057] <Coating solution preparation process> 3A, 3B, and 3C are diagrams showing the details of the coating solution preparation process.

[0058] In the coating solution preparation step, a stock solution preparation step (step S11), a chelating agent preparation solution preparation step (step S12), a solution mixing step (step S13), and a solution adjustment step (step S14) are carried out in this order.

[0059] <Stock solution preparation process> In the stock solution preparation step (step S11), as shown in FIG. 3A, a LiSi sol stock solution is prepared in a glove box GB, in which a lithium-containing solute containing lithium as a constituent element and a silicon-containing solute containing silicon as a constituent element are dissolved.

[0060] To prepare the LiSi sol stock solution, decarbonated water is placed in a sealable container with a lid in the glove box GB. The decarbonated water is water from which carbon dioxide has been removed by a decarbonation treatment, and preferably has a carbon dioxide concentration of 100 ppm or less.

[0061] The lithium-containing solute is, for example, lithium hydroxide. In the glove box GB, a predetermined amount of lithium hydroxide is weighed, and the weighed lithium hydroxide is placed in decarbonated water in a container with a lid, and the lithium hydroxide is completely dissolved in the decarbonated water by stirring. The amount of lithium hydroxide dissolved in the decarbonated water (predetermined amount) is, for example, 2 to 10 wt %.

[0062] The silicon-containing solute is, for example, silica gel. In the glove box GB, an equimolar amount of silica gel to lithium hydroxide is weighed, and the weighed silica gel is placed in the lithium hydroxide aqueous solution in a container with a lid. The silicon-containing solute is not limited to silica gel, and may be, for example, silica sand, as long as it contains silicon dioxide at a high purity. However, silica gel is preferred because it has a larger specific surface area than silica sand and is more easily dissolved in the lithium hydroxide aqueous solution.

[0063] Thereafter, in the glove box GB, the solution in the covered container (lithium hydroxide aqueous solution containing silica gel) is heated while being stirred. Heating is performed, for example, using a hot plate. By heating, the temperature of the solution in the covered container rises to a temperature of 50°C or higher and lower than 80°C. When the silica gel is completely dissolved in the lithium hydroxide aqueous solution, a transparent LiSi sol stock solution in which silicon dioxide is uniformly dissolved is obtained. Once the silica gel is completely dissolved in the lithium hydroxide aqueous solution, heating is stopped, and the LiSi sol stock solution is allowed to cool naturally while being stirred. After being allowed to cool naturally, the covered container containing the LiSi stock solution is sealed.

[0064] <Chelating agent preparation solution preparation process> In the chelating agent preparation solution preparation step (step S12), as shown in FIG. 3B, a chelating agent preparation solution in which a lithium compound and a chelating agent are dissolved is prepared in a glove box GB.

[0065] To prepare the chelating agent preparation solution, decarbonated water is placed in a sealable container with a lid in the glove box GB. The decarbonated water preferably has a carbon dioxide concentration of 100 ppm or less, similar to the process for preparing the stock solution.

[0066] The lithium compound is, for example, lithium hydroxide. In the glove box GB, a predetermined amount of lithium hydroxide is weighed, and the weighed lithium hydroxide is placed in decarbonated water in a container with a lid, and the lithium hydroxide is completely dissolved in the decarbonated water by stirring. The amount of lithium hydroxide dissolved in the decarbonated water (predetermined amount) is, for example, 2 to 10 wt %.

[0067] The chelating agent is, for example, citric acid. In the glove box GB, a predetermined amount of citric acid is weighed and placed in the lithium hydroxide aqueous solution in a container with a lid, and the citric acid is dissolved in the lithium hydroxide aqueous solution. The amount (predetermined amount) of citric acid dissolved in the lithium hydroxide aqueous solution is, for example, 3 to 12 wt %. When the citric acid is completely dissolved in the lithium hydroxide aqueous solution, a transparent chelating agent preparation solution is obtained. Once the chelating agent preparation solution is obtained, the container with a lid containing the chelating agent preparation solution is sealed.

[0068] The chelating agent may be any organic acid that has chelating properties and that decomposes thermally at a relatively low temperature, and is not limited to citric acid. For example, it may be a hydroxy acid such as glycolic acid, malic acid, lactic acid, or tartaric acid, a dicarboxylic acid such as oxalic acid or malonic acid, or a carboxylic acid or carboxylic acid derivative such as acetic acid, gluconic acid, or propionic acid.

[0069] <Solution mixing process> In the solution mixing step (step S13), as shown in FIG. 3C, the LiSi sol stock solution and the chelating agent preparation solution are mixed in the glove box GB.

[0070] Specifically, in the glove box GB, the lidded containers containing the LiSi sol stock solution and the chelating agent preparation solution are opened. Then, the chelating agent preparation solution is added dropwise to the LiSi sol stock solution, and the LiSi sol stock solution to which the chelating agent preparation solution has been added is thoroughly stirred. This mixes the LiSi sol stock solution and the chelating agent preparation solution, producing a coating solution consisting of the mixture.

[0071] Because both the LiSi sol stock solution and the chelating agent preparation solution contain lithium, the coating solution made from their mixture has a lithium-rich composition with a Li / Si molar ratio of 1 or more. In addition, because the chelating agent preparation solution contains a chelating agent, the dispersion stability of the coating solution is maintained. Therefore, in the solution mixing step (step S13), a transparent coating solution can be obtained that is free from cloudiness and gelation despite its lithium-rich composition.

[0072] <Solution preparation process> In the solution adjustment step (step S14), the hydrogen ion concentration (pH) of the coating solution is adjusted in the glove box GB, as shown in FIG. 3C.

[0073] Specifically, the pH of the coating solution is measured in the glove box GB. It is then confirmed whether the pH of the coating solution is within the range of 7.5 to 10. If the pH of the coating solution is less than 7.5, decarbonated water is added to the coating solution to adjust the pH to within the range of 7.5 to 10. If the pH of the coating solution is greater than 10, a chelating agent (citric acid) is added to the coating solution to adjust the pH to within the range of 7.5 to 10.

[0074] <Coating equipment> FIG. 4 is a cross-sectional view illustrating the configuration of the coating apparatus 1. As shown in FIG.

[0075] The coating apparatus 1 is a high-speed air current dispersion type coating apparatus, and is equipped with a slurry tank 2, a dispersion section 3, a drying section 4 and a collection section 5.

[0076] The slurry tank 2 taken out from the glove box GB is introduced into the coating apparatus 1. The coating apparatus 1 is equipped with, for example, a magnetic stirrer 21, and the slurry in the slurry tank 2 introduced into the coating apparatus 1 is stirred by the magnetic stirrer 21. A slurry delivery pipe 22 is connected to the slurry tank 2, and the slurry in the slurry tank 2 is supplied to the dispersion section 3 through the slurry delivery pipe 22 by the action of a delivery pump 23 installed midway through the slurry delivery pipe 22.

[0077] The dispersion section 3 includes a slurry inlet channel 31, a first dry air channel 32, a second dry air channel 33, and a third dry air channel .

[0078] The slurry inlet channel 31 extends linearly in the vertical direction. The first dry air channel 32 also extends linearly in the vertical direction, and its lower end is connected to the middle of the slurry inlet channel 31 and communicates with the slurry inlet channel 31.

[0079] The second dry air passage 33 is provided separately on one side and the other side of the slurry inlet passage 31 in a direction perpendicular to the vertical direction, and each is inclined at a predetermined angle (for example, 45°) with respect to the vertical direction. The second dry air passage 33 is connected to a position in the middle of the slurry inlet passage 31 below the connection position of the first dry air passage 32, and communicates with the slurry inlet passage 31, with its malleable end opening toward the collision position P set in the slurry inlet passage 31.

[0080] The third dry air flow path 34 is provided separately on one side and the other side of the slurry inlet path 31 in a direction perpendicular to the up-down direction, and each extends horizontally. The third dry air flow path 34 is connected to the slurry inlet path 31 at a position below the connection position of the first dry air flow path 32 in the middle of the slurry inlet path 31 and above the collision position P, and is in communication with the slurry inlet path 31.

[0081] The slurry inlet channel 31 is connected to an upper end thereof with a slurry delivery pipe 22. The slurry flowing through the slurry delivery pipe 22 flows from the slurry delivery pipe 22 into the slurry inlet channel 31. Meanwhile, instrument air from which moisture has been removed by a dry air unit is supplied to the first dry air flow path 32, the second dry air flow path 33, and the third dry air flow path 34. The instrument air is air (atmospheric air) pressurized to a predetermined air pressure. That is, high-pressure dry air is supplied to the first dry air flow path 32, the second dry air flow path 33, and the third dry air flow path 34.

[0082] As the dry air, various gases other than air, such as nitrogen, carbon dioxide, and inert gases, can be used.

[0083] The slurry flowing from the slurry delivery pipe 22 into the slurry inlet channel 31 flows downward through the slurry inlet channel 31. Meanwhile, while the dry air flows through the second dry air channel 33, the flow velocity of the dry air increases significantly, reaching, for example, the sonic velocity. A dry air stream is ejected from the open end of the second dry air channel 33 at a flow velocity exceeding the sonic velocity, and the two streams collide at a collision position P. As the slurry flowing through the slurry inlet channel 31 passes through the collision position P, it is subjected to a shear force from the colliding air stream at the collision position P, dispersing the slurry into the particles to be coated, the surfaces of which have a film of the coating solution attached. Furthermore, by discharging dry air from the first dry air channel 32 into the slurry inlet channel 31, the influence of the collision pressure of the dry air from the second dry air channel 33 is suppressed, thereby regulating the flow of the slurry. By discharging dry air from the third dry air channel 34 horizontally into the slurry inlet channel 31, the spreading of the slurry is suppressed and the flow of the slurry is regulated.

[0084] The drying section 4 integrally includes a cylindrical section 41 having a cylindrical peripheral surface and a conical section 42 that is continuous with the cylindrical section 41 and has a generally conical shape that narrows with increasing distance from the cylindrical section 41. The drying section 4 is disposed directly below the dispersion section 3 such that the center line of the cylindrical section 41 extends in the vertical direction. A drying gas inlet 43 is formed on the peripheral surface of the cylindrical section 41. One end of a drying gas flow path 44 is connected to the drying gas inlet 43. The other end of the drying gas flow path 44 is connected to an intake filter 45. The drying gas flow path 44 passes through a heat exchanger 46 and a drying heater 47 in the middle.

[0085] The collecting section 5 includes a collecting cyclone 51 and a collecting filter 52. One end of an intake pipe 53 is connected to the collecting cyclone 51. The other end of the intake pipe 53 is connected to the suction port of a drying blower 54. The collecting cyclone 51 is also provided with a powder introducing section 55. A sanitary pipe 56 extending from the lower end of the conical section 42 of the drying section 4 is connected to the powder introducing section 55. The collecting filter 52 is installed midway through the intake pipe 53.

[0086] When the drying blower 54 is activated, air in the collecting cyclone 51 is sucked into the intake pipe 53, creating a negative pressure inside the collecting cyclone 51. When the negative pressure inside the collecting cyclone 51 is created, a negative pressure is created inside the drying section 4 (cylindrical section 41 and conical section 42) via the sanitary pipe 56. Due to the negative pressure inside the drying section 4, air (atmospheric air) is sucked into the drying gas passage 44 through the intake filter 45, and this air flows as dry gas through the drying gas passage 44 toward the drying section 4. As the air flows through the drying gas passage 44, heat is exchanged between the air and the exhaust gas of the drying blower 54 in the heat exchanger 46, and the air is heated by the drying heater 47. As a result, the air flowing through the drying gas passage 44 becomes heated air, and the heated air flows from the drying gas passage 44 into the drying section 4 through the drying gas inlet 43.

[0087] The drying gas inlet 43 and the drying gas flow path 44 are formed so that heated air is blown out from the drying gas inlet 43 in a tangential direction to the inner circumferential surface of the cylindrical portion 41 of the drying unit 4. Therefore, the heated air introduced into the drying unit 4 from the drying gas inlet 43 becomes a vortex air current that flows along the inner circumferential surface of the cylindrical portion 41.

[0088] Meanwhile, the drying section 4 communicates with the slurry inlet channel 31 of the dispersion section 3 as a drying path. As a result, particles to be coated, with a film of coating liquid adhering to their surfaces, flow from the slurry inlet channel 31 into the drying section 4. The particles to be coated are then transported within the drying section 4 from the cylindrical section 41 toward the conical section 42, riding on the vortex of heated air generated within the drying section 4. During this transport, the coating liquid adhering to the surface of each particle to be coated dries, producing dry gel powder. Furthermore, droplets of the coating liquid that are not adhering to the surface of the particles to be coated dry, producing dried pieces of the coating liquid.

[0089] The dry gel powder and dried pieces of coating liquid that reach the bottom end of the drying section 4 (conical section 42) flow through the sanitary pipe 56 due to the negative pressure inside the collecting cyclone 51 and are sucked into the collecting cyclone 51. Inside the collecting cyclone 51, the airflow containing the dry gel powder and dried pieces of coating liquid swirls, and the resulting centrifugal force and gravity separate the dry gel powder, which has a relatively large particle size and mass, from the dried pieces of coating liquid, which has a relatively small particle size and mass. The dry gel powder is stored in a collection box 57 connected to the bottom end of the collecting cyclone 51.

[0090] The dried pieces of coating liquid separated in the collecting cyclone 51 are carried by air and sucked into the intake pipe 53, and are captured by the collecting filter 52. Therefore, the collecting filter 52 allows only air to pass through.

[0091] As the drying gas, various gases other than air, such as nitrogen, carbon dioxide, or an inert gas, can be used.

[0092] Furthermore, to promote drying of the coating solution in the drying section 4, the drying section 4 may be heated by a heater. For the same reason, the dry air supplied to the first dry air flow path 32, the second dry air flow path 33, and the third dry air flow path 34 of the dispersion section 3 may be heated by a heater.

[0093] <Action and effect> As described above, the LiSi sol stock solution is prepared in the stock solution preparation process, and the chelating agent preparation solution is prepared in the chelating agent preparation solution preparation process, thereby preparing the LiSi sol stock solution and the chelating agent preparation solution. Then, in the solution mixing process, the LiSi sol stock solution and the chelating agent preparation solution are mixed. The LiSi sol stock solution contains lithium hydroxide, a lithium-containing solute containing lithium as a constituent element, and silica gel, a silicon-containing solute containing silicon as a constituent element, dissolved in decarbonated water as a solvent. The chelating agent preparation solution contains lithium hydroxide, a lithium compound, and citric acid, a chelating agent. Because both the LiSi sol stock solution and the chelating agent preparation solution contain lithium, the resulting mixed coating solution has a lithium-rich composition with a Li / Si molar ratio of 1 or greater. Furthermore, the chelating agent preparation solution contains a chelating agent, thereby maintaining the dispersion stability of the coating solution.

[0094] Therefore, a coating solution can be obtained that has a lithium-rich composition but is not cloudy or gelled. When this coating solution is used to form a coating on the surface of a particle to be coated, a thin and smooth coating of lithium silicate with a lithium-rich composition can be formed on the surface of the particle to be coated.

[0095] The coating solution preparation process also includes a solution adjustment process in which the hydrogen ion exponent of the solution obtained in the solution mixing process, i.e., the coating solution obtained by mixing the LiSi sol stock solution and the chelating agent preparation solution, is measured and adjusted. By adjusting the hydrogen ion exponent of the coating solution, it is possible to suppress the absorption of carbon dioxide gas by the coating solution and the gelation of the solution.

[0096] If the pH of the coating solution is less than 7.5, the coating solution will gel quickly. In addition, when the coating solution is mixed with the positive electrode active material to form a slurry, the transition metal elements in the positive electrode active material, such as cobalt and nickel, will ionize and dissolve into the solution, potentially contaminating the coating components and reducing the effectiveness of the coating. On the other hand, if the pH of the coating solution is more than 10, the coating solution will be more likely to absorb carbon dioxide, which may result in granular deposits on the surface of the coating formed from the coating solution.

[0097] Therefore, in the solution preparation step, the hydrogen ion exponent of the coating solution is adjusted to a range of 7.5 to 10. This effectively prevents the coating solution from absorbing carbon dioxide gas and from gelling.

[0098] The coating solution preparation steps (stock solution preparation step, chelating agent preparation solution preparation step, solution mixing step, and solution adjustment step) are performed in an inert atmosphere in a glove box GB, which prevents the LiSi sol stock solution from absorbing carbon dioxide gas in the first preparation step.

[0099] When the coated particles are a positive electrode active material for an all-solid-state battery using a sulfide-based solid electrolyte, a positive electrode active material can be obtained in which a coating made of lithium silicate with a lithium-rich composition is formed on the surface. By using this positive electrode active material in an all-solid-state battery, the resistance (interface resistance) generated at the interface between the positive electrode active material and the solid electrolyte can be reduced, and lithium ion conductivity can be increased, thereby achieving high performance in the all-solid-state battery.

[0100] <Modification> Although the embodiment of the present invention has been described above, the present invention can be embodied in other forms.

[0101] For example, in the stock solution preparation step, the step of dissolving lithium hydroxide in decarbonated water may be omitted, and a commercially available lithium hydroxide aqueous solution (for example, a lithium hydroxide aqueous solution adjusted to 4 mol / L) may be used.

[0102] Alternatively, instead of the LiSi sol stock solution prepared in the stock solution preparation step, a commercially available lithium silicate aqueous solution (such as Lithium Silicate 35 manufactured by Nippon Chemical Industry Co., Ltd. or Lithium Silicate 35 manufactured by Nissan Chemical Industries, Ltd.) may be used. In this case, the lithium silicate aqueous solution does not need to be heated, but since the amount of lithium is small (the Li / Si molar ratio is low at around 0.57), it is necessary to add lithium hydroxide to the chelating agent preparation solution in an amount sufficient to make up for the deficiency.

[0103] In the chelating agent preparation solution preparation step, a water-soluble organic acid lithium salt such as lithium citrate and lithium acetate may be dissolved in decarbonated water instead of lithium hydroxide and citric acid.

[0104] In the step of preparing the stock solution, silica gel is placed in the lithium hydroxide aqueous solution in the container with a lid, and then the solution in the container with a lid is heated in the glove box GB. However, the solution may be transferred from the container with a lid to a bottle, the bottle is sealed, the bottle is removed from the glove box GB, and the solution sealed in the bottle is heated in a constant temperature and humidity bath. In this case, the solution in the bottle can be stirred using, for example, a magnetic stirrer.

[0105] In the coating step, dry gel powder (powder in which the surfaces of particles to be coated are coated with a lithium silicate precursor) is produced by a spray drying method or a high-speed airflow dispersion method. Alternatively, the dry gel powder may be produced by a so-called tumbling fluidized bed method in which the powder of particles to be coated is placed in a container, and the particles are floated and fluidized by a mechanical rotation mechanism or the like while the coating solution prepared in the above-mentioned coating solution preparation step is sprayed in a mist.

[0106] In addition, various design modifications can be made to the above-described configuration within the scope of the claims. [Explanation of symbols]

[0107] S1: Slurry preparation process S2: Coating process S3: Firing process S11: Stock solution preparation process (first preparation process) S12: Chelating agent preparation solution preparation step (second preparation step) S13: Solution mixing process (mixing process) S14: Solution adjustment process (adjustment process)

Claims

1. 1. A method for preparing a coating solution used to form a coating on the surface of a particle to be coated, comprising the steps of: a first preparation step of preparing a LiSi sol stock solution in which a coating solute containing lithium and silicon as constituent elements is dissolved; a second preparation step of preparing a chelating agent preparation solution in which a lithium compound and a chelating agent are dissolved; a mixing step of mixing the LiSi sol stock solution prepared in the first preparation step and the chelating agent preparation solution prepared in the second preparation step; A method for preparing a coating solution, comprising:

2. an adjusting step of measuring and adjusting the hydrogen ion exponent of the solution obtained in the mixing step; The method of preparing a coating solution according to claim 1, further comprising:

3. the coating solute includes a lithium-containing solute containing lithium as a constituent element and a silicon-containing solute containing silicon as a constituent element; The coating solution preparation method according to claim 1 , wherein the first preparation step comprises dissolving the lithium-containing solute and the silicon-containing solute in a solvent.

4. 4. The method for preparing a coating solution according to claim 3, wherein the lithium-containing solute is lithium hydroxide.

5. 4. The method for preparing a coating solution according to claim 3, wherein the silicon-containing solute is silica gel.

6. The method for preparing a coating solution according to claim 3 , wherein the solvent is decarbonated water.

7. 4. The coating solution preparation method according to claim 3, wherein in the first preparation step, the lithium-containing solute is dissolved in the solvent, the silicon-containing solute is added to the solution in which the lithium-containing solute has been dissolved, and the solution containing the silicon-containing solute is heated.

8. 2. The method for preparing a coating solution according to claim 1, wherein the lithium compound is lithium hydroxide.

9. The method for preparing a coating solution according to claim 1 , wherein the chelating agent is a hydroxy acid.

10. The coating solution preparation method according to claim 1 , wherein the second preparation step comprises dissolving the lithium compound and the chelating agent in a solvent.

11. 3. The method for preparing a coating solution according to claim 2, wherein the adjusting step adjusts the hydrogen ion exponent of the solution obtained in the mixing step to a range of 7.5 or more and 10 or less.

12. A coating solution used to form a coating film of lithium silicate on the surface of particles to be coated, A coating solution having a Li / Si molar ratio of 1 or more and being transparent.

13. The coating solution of claim 12 , further comprising a chelating agent.

14. 14. The coating solution according to claim 13, wherein the hydrogen ion exponent is in the range of 7.5 or more and 10 or less.

15. A coating method for forming a coating film of lithium silicate on the surface of particles to be coated, comprising the steps of: a coating solution preparation step of preparing a coating solution; a slurry preparation step of preparing a slurry by mixing the powder of the particles to be coated with the coating solution; a coating step of dispersing the slurry prepared in the slurry preparation step onto the particles to be coated, the particles having a film of the coating solution attached to their surfaces, and drying the film of the coating solution attached to the particles to be coated; a firing step of firing the powder of the coated particles coated with the dried film of the coating solution after the coating step; Including, A coating method, wherein the coating solution preparation step uses the coating solution preparation method according to any one of claims 1 to 11.

16. A coating method for forming a coating film of lithium silicate on the surface of particles to be coated, comprising the steps of: a slurry preparation step of preparing a slurry by mixing the powder of the particles to be coated with a coating solution; a coating step of dispersing the slurry prepared in the slurry preparation step onto the particles to be coated, the particles having a film of the coating solution attached to their surfaces, and drying the film of the coating solution attached to the particles to be coated; a firing step of firing the powder of the coated particles coated with the dried film of the coating solution after the coating step; Including, A coating method, wherein the coating solution according to any one of claims 12 to 14 is used as the coating solution.

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