Lithium-containing fluoride coating solution, method for manufacturing lithium-containing fluoride coating solution, lithium-containing fluoride coated active material, and method for manufacturing lithium-containing fluoride coated active material

Abstract

To provide a lithium-containing fluoride coating positive electrode active material that, when the surface of a positive electrode active material of an all-solid-state lithium battery is coated with lithium-containing fluoride, has a high coverage even when the coating film is thin, and a method for manufacturing the same.SOLUTION: A lithium-containing fluoride coating liquid contains a lithium-containing fluoride powder having a composition Li2+xMF6+y (M is one or more metal elements that can take a tetravalent oxidation number), -0.5≤x≤1, -0.5≤y≤1), characterized in that the lithium-containing fluoride coating liquid has a pH of 2.5 or more and 7.0 or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a lithium-containing fluoride coating solution, a method for producing a lithium-containing fluoride coating solution, a lithium-containing fluoride coated active material, and a method for producing a lithium-containing fluoride coated active material. [Background technology]

[0002] In recent years, in order to increase the energy density of batteries, there has been a demand for coating materials for cathode materials that are stable even under high voltages. For example, lithium-containing fluorides are expected to be coating materials that are stable even under high voltages because fluorine has a high electronegativity. According to Non-Patent Document 1, the results of first-principles calculations suggest that the lithium-containing fluorides are stable even under high voltages.

[0003] On the other hand, for example, Non-Patent Document 2 describes an example of a positive electrode active material used in a lithium ion secondary battery (liquid LIB) using an electrolyte solution, in which a first mixed slurry is produced by mixing LiMn2O4 powder, Li2CO3 powder, and water, which are positive electrode active materials, and a second mixed slurry is produced by mixing an H2TiF6 aqueous solution containing 60 mass % H2TiF6 with the first mixed slurry so that the obtained first mixed slurry has a composition of Li2TiF6, and the obtained second mixed slurry is dried at 60°C for 24 hours, thereby producing a positive electrode active material in which LiMn2O4 particles are coated with Li2TiF6, which is a lithium-containing fluoride. [Prior art documents] [Patent documents]

[0004] [Non-Patent Document 1] Bo Liu,et al High-Throughput Computational Screening of Li-Containing Fluorides for Battery Cathode Coatings. ACS Sustainable Chem. Eng. 2020, 8, 948-957 [Non-patent document 2] Min-Kun Kim et al A Facile Process for Surface Modification with Lithium Ion Conducting Material of Li2TiF6 for LiMn2O4 in Lithium Ion Batteries J. Electrochem. Sci. Technol., 2019, 10(2), 223-230 Summary of the Invention [Problem to be solved by the invention]

[0005] On the other hand, in all-solid-state lithium batteries, high-resistance regions are formed at the interface between the positive electrode active material and the solid electrolyte, increasing the interfacial resistance and sometimes preventing good battery characteristics. Therefore, in order to reduce the interfacial resistance between the positive electrode active material and the solid electrolyte, attempts have been made to coat the surface of the positive electrode active material with a coating film having lithium conductivity.

[0006] A high coverage rate is desirable for the coating film in order to minimize the exposed area of ​​the positive electrode active material surface. Therefore, increasing the thickness of the coating film is considered to increase the coverage rate. However, increasing the thickness of the coating film reduces the Li conductivity of the positive electrode active material, making it impossible to obtain the desired battery characteristics. Therefore, a thin coating film (e.g., 40 nm or less) is desirable.

[0007] In recent years, studies have been conducted to coat the surface of a positive electrode active material of an all-solid-state lithium battery with a coating film made of a lithium-containing fluoride, as described in, for example, Non-Patent Document 1. However, no technology has been established for coating the surface of a positive electrode active material with a thin coating film made of a lithium-containing fluoride at a high coverage rate.

[0008] For example, according to the investigations of the present inventors, when a coating film of lithium-containing fluoride is formed on the surface of a positive electrode active material of an all-solid-state lithium battery using the method described in Non-Patent Document 2, there is a problem in that the coverage rate of the coating film is low.

[0009] The present invention has been made under the above circumstances, and the problem to be solved by the present invention is to provide a lithium-containing fluoride-coated cathode active material that, when coated with lithium-containing fluoride on the surface of a cathode active material of an all-solid-state lithium battery, has a high coverage rate even when the coating film has a thickness as thin as, for example, 40 nm or less; a method for producing the same; and a lithium-containing fluoride coating solution that is suitable for coating the surface of a cathode active material with lithium-containing fluoride in a thin film thickness and at a high coverage rate; and a method for producing the same. [Means for solving the problem]

[0010] That is, the first invention to solve the above-mentioned problems is: A lithium-containing fluoride coating solution containing Li, F, and M (M is one or more metal elements that can have a tetravalent oxidation number), the molar ratio of Li, F, and M contained in the lithium-containing fluoride coating solution, Li:F:M, is 2+x:1:6+y (-0.5≦x≦1, -0.5≦y≦1); The lithium-containing fluoride coating solution is characterized in that the pH of the lithium-containing fluoride coating solution is 2.5 or more and 7.0 or less. The second invention is: Li is 1.0 mass% or less, F is 7.5 mass% or less, The lithium-containing fluoride coating solution according to the first aspect of the present invention is characterized in that it contains 3.5 mass % or less of M. The third invention is The lithium-containing fluoride coating solution according to the first or second invention is characterized in that the pH is 4.0 or more and 7.0 or less. The fourth invention is The lithium-containing fluoride coating solution according to any one of the first to third aspects of the present invention is characterized in that the pH is 4.5 or more and 7.0 or less. The fifth invention is The lithium-containing fluoride coating solution according to any one of the first to fourth aspects of the present invention is characterized in that M is one or more metal elements selected from Ti, Si, and Zr. The sixth invention is Li 2+x MF 6+y (M is one or more metal elements that can have a tetravalent oxidation number, -0.5≦x≦1, -0.5≦y≦1) is dissolved in water to form a solution, and a lithium-containing fluoride coating solution is obtained, The method for producing the lithium-containing fluoride coating liquid is characterized in that the pH of the lithium-containing fluoride coating liquid is set to 2.5 or more and 7.0 or less. The seventh invention is A sixth aspect of the present invention provides a method for producing a lithium-containing fluoride coating solution, characterized in that Li fluoride and a fluoride of M are dry-mixed to produce the lithium-containing fluoride powder. The eighth invention is a step of dissolving a water-soluble Li salt and a water-soluble salt containing M and F in water to obtain an aqueous solution containing Li, F, and M and having a pH of 2.5 or more and 7.0 or less; drying the aqueous solution containing Li, F, and M to obtain a dry powder; and a step of calcining the dried powder at 150°C or higher and 500°C or lower, thereby producing the lithium-containing fluoride powder. The ninth invention is A lithium-containing fluoride-coated active material in which a positive electrode active material is coated with a lithium-containing fluoride, The lithium-containing fluoride contains Li, F, and M (M is one or more metal elements that can have a tetravalent oxidation number), the thickness of the lithium-containing fluoride coating is 5 nm or more and 40 nm or less, The lithium-containing fluoride-coated active material is characterized in that the coverage of the lithium-containing fluoride with respect to the positive electrode active material is 80% or more. The tenth invention is The lithium-containing fluoride is Li 2+x MF 6+y The ninth aspect of the present invention provides a lithium-containing fluoride-coated active material, characterized in that (M is one or more metal elements that can take a tetravalent oxidation number, -0.5≦x≦1, -0.5≦y≦1). The eleventh invention is a lithium-containing fluoride coating solution containing Li, F, and M (M is one or more metal elements capable of taking a tetravalent oxidation number), wherein the molar ratio of Li, F, and M is Li:F:M is 2+x:1:6+y (-0.5≦x≦1, -0.5≦y≦1), and a positive electrode active material is brought into contact with the lithium-containing fluoride coating solution to obtain a lithium-containing fluoride-coated active material in which the positive electrode active material is coated with the lithium-containing fluoride, The method for producing a lithium-containing fluoride coated active material is characterized in that the pH of the lithium-containing fluoride coating solution is 2.5 or more and 7.0 or less. The twelfth invention is The lithium-containing fluoride coating solution is Li is 1.0 mass% or less, F is 7.5 mass% or less, The eleventh aspect of the present invention provides a method for producing a lithium-containing fluoride-coated active material, characterized in that M is contained in an amount of 3.5 mass % or less. The thirteenth invention is The method for producing a lithium-containing fluoride coated active material according to the eleventh or twelfth invention is characterized in that the pH of the lithium-containing fluoride coating solution is 4.0 or more and 7.0 or less. The fourteenth invention is The method for producing a lithium-containing fluoride coated active material according to the eleventh or twelfth invention is characterized in that the lithium-containing fluoride coating solution has a pH of 4.5 or more and 7.0 or less. The fifteenth invention is The method for producing a lithium-containing fluoride-coated active material according to any one of the eleventh to fourteenth aspects of the present invention is characterized in that M is one or more metal elements selected from Ti, Si, and Zr. [Effects of the Invention]

[0011] By using the lithium-containing fluoride coating solution according to the present invention, a lithium-containing fluoride coating film can be provided on the surface of the positive electrode active material of an all-solid-state lithium battery, with a thin film thickness and a high coverage rate. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is an XRD pattern of Li2TiF6 powder according to Example 1 of the present invention. [Figure 2] 1 is an XRD pattern of a dried powder of a coating liquid according to Example 1 of the present invention. [Figure 3] 1 is an XRD pattern of Li2ZrF6 powder according to Example 2 of the present invention. [Figure 4] 1 is an XRD pattern of a dried powder of a coating liquid according to Example 2 of the present invention. [Figure 5] 1 is an XRD pattern of Li2SiF6 powder according to Example 3 of the present invention. [Figure 6] 10 is an XRD pattern of a dried powder of a coating liquid according to Example 3 of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0013] In order to solve the above problems, the present inventors have conducted extensive research. In the technology described in Non-Patent Document 2, a second mixed slurry is produced by adding an H2TiF6 aqueous solution to a first mixed slurry, which is a mixture of a positive electrode active material, lithium carbonate, and water. This first mixed slurry contains lithium carbonate, which has a pH of approximately 12 when dissolved in water. Therefore, the first mixed slurry is considered to have a high pH, ​​in the alkaline range. Furthermore, to obtain the second mixed slurry, a strong acid H2TiF6 aqueous solution is added to the first mixed slurry. Meanwhile, H2TiF6 hydrolyzes to TiO2 in the alkaline range. Therefore, when H2TiF6 is added to the first mixed slurry, it hydrolyzes through the high pH range, resulting in the precipitation of TiO2 in the second mixed slurry. Therefore, Non-Patent Document 2 speculates that this may ultimately result in a decrease in the coverage of the coating film on the positive electrode active material.

[0014] The present inventors have conducted research based on the above considerations. They have conceived a method for producing a lithium-containing fluoride coating solution without using H2MF6, such as H2TiF6, a strong acid, as a raw material for the lithium-containing fluoride coating solution, and a method for producing a lithium-containing fluoride coating solution using H2MF6, such as H2TiF6, as a raw material for the lithium-containing fluoride coating solution, but without mixing the H2MF6 with a high pH solution. They have also conceived that these production methods can suppress the precipitation of MO2, a hydrolysis product of H2MF6, in the lithium-containing fluoride coating solution, and can coat the surface of a positive electrode active material with lithium-containing fluoride at a high coverage even when the coating film is thin. In the present invention, "M" refers to one or more metal elements capable of assuming a tetravalent oxidation number. In the lithium-containing fluoride coating solution of the present invention, the metal element M is a concept that includes metalloid elements. The metal element M capable of assuming a tetravalent oxidation number is preferably one or more of Ti, Si, Zr, Ge, Sn, and Hf, and more preferably one or more metal elements selected from Ti, Si, and Zr.

[0015] Furthermore, the present inventors have conceived that by adjusting the pH of the lithium-containing metal fluoride coating solution within a predetermined range, it is possible to provide a coating film made of a lithium-containing fluoride on the surface of a positive electrode active material of an all-solid-state lithium battery with a thin film thickness but a high coverage rate, and have completed the present invention.

[0016] Hereinafter, embodiments for carrying out the present invention will be described in the order of 1. lithium-containing fluoride, 2. lithium-containing fluoride coating solution, and 3. lithium-containing fluoride coated active material.

[0017] 1. Lithium-containing fluorides (1) Composition of lithium-containing fluoride The lithium-containing fluoride of the present invention is Li 2+x MF 6+y (M is one or more metal elements that can take on a tetravalent oxidation number, -0.5≦x≦1, -0.5≦y≦1).

[0018] By using one or more metal elements capable of assuming a tetravalent oxidation number as the metal element M, the solubility of the lithium-containing fluoride in water can be improved, and therefore a lithium-containing fluoride coating solution with a high solids concentration (amount of lithium-containing fluoride dissolved in water) can be realized. The metal element M capable of assuming a tetravalent oxidation number is preferably one or more of Ti, Si, Zr, Ge, Sn, and Hf. Furthermore, by using one or more metal elements selected from Ti, Si, and Zr as the metal element M, the solubility of the lithium-containing fluoride in water can be further improved, and therefore it is possible to realize a lithium-containing fluoride coating liquid having a higher solid concentration (amount of lithium-containing fluoride dissolved in water), which is preferable. The lithium-containing fluoride coating solution will be described later in the section "2. Lithium-containing fluoride coating solution."

[0019] In contrast, when a lithium-containing fluoride in which the metal element M does not have a tetravalent oxidation number is used (for example, a fluoride with low solubility in water such as Li3AlF6), the coating liquid has a low solids concentration. When an active material is coated using a coating liquid with a low solids concentration, the amount of coating liquid becomes large relative to the active material, and when dried using a spray dryer or the like, the number of droplets without active material increases, resulting in a lower coverage rate (droplets containing only the coating liquid are generated, and a higher proportion of these droplets dry).

[0020] In addition, the composition of lithium-containing fluoride is Li 2+x MF 6+y In the above formula, when x is less than -0.5, an oxide of the metal element M is generated in the lithium-containing fluoride coating solution as described above, resulting in a decrease in the coverage of the lithium-containing fluoride on the surface of the positive electrode active material. Similarly, when y is less than -0.5, a precipitate of MO2 is generated in the lithium-containing fluoride coating solution as described above, resulting in a decrease in the coverage of the lithium-containing fluoride on the surface of the positive electrode active material.

[0021] On the other hand, if x exceeds 1, when a mixed slurry obtained by mixing a lithium-containing fluoride coating liquid with a positive electrode active material is dried using a spray dryer or the like, the excess lithium becomes lithium hydroxide, resulting in a decrease in the coverage of the lithium-containing fluoride on the surface of the positive electrode active material.If y exceeds 1, when a mixed slurry obtained by mixing a lithium-containing fluoride coating liquid with a positive electrode active material is dried using a spray dryer or the like, the excess fluorine volatilizes, which may cause a problem of corroding the device.

[0022] (2) Manufacturing method of lithium-containing fluoride There is no particular limitation on the method for producing the lithium-containing fluoride according to the present invention, but convenient methods include a dry method in which the starting materials are pulverized and mixed using a ball mill or the like, and a wet method in which solutions of the starting materials are mixed and dried.

[0023] (I) Dry method This is a method for producing lithium-containing fluoride powder without using H2MF6, such as H2TiF6, which is a strong acid. As the starting materials, it is preferable to use LiF powder and MF4 powder from the viewpoint of reducing impurities in the lithium-containing fluoride powder to be produced. The starting materials are weighed according to the composition of the lithium-containing fluoride having the predetermined composition described above, and are charged into a ball mill. Note that this reaction is a mechanochemical reaction, and it is preferable that the mechanical energy, such as shear stress, shear stress, and friction, is large. Therefore, it is preferable to use a planetary ball mill as the ball mill.

[0024] The starting materials are pulverized and mixed using a ball mill. At this time, the dew point temperature of the pulverization atmosphere is preferably 10°C dp or lower. If the dew point temperature of the pulverization atmosphere is 10°C dp or lower, hydrolysis of MF4 can be prevented, and the oxide of the metal element M that is insoluble in HO can be prevented from being produced in the lithium-containing fluoride powder produced. The amount of the oxide of the metal element M in the lithium-containing fluoride powder produced is preferably 10% by mass or lower, more preferably 1% by mass or lower, and most preferably, no oxide is contained. From these viewpoints, the pulverization atmosphere is preferably an inert atmosphere such as Ar or N2, a vacuum atmosphere, or a dry air atmosphere.

[0025] (II) Wet method Of the starting materials, it is preferable to use a water-soluble Li salt as the Li source. Examples of water-soluble Li salts that can be used include Li acetate, sulfate, chloride, and nitrate. For example, lithium acetate, lithium sulfate, lithium chloride, and lithium nitrate are preferably used. The acetate, sulfate, chlorine, and nitric acid components contained in the Li source can be removed by heating.

[0026] On the other hand, as the M source and the F source, a hydrofluoride salt of the metal element M, an ammonium hydrogen fluoride salt, or the like, which is a water-soluble salt containing the metal element M and F, can be preferably used. Furthermore, from the viewpoint of suppressing the deposition of LiF and the like and increasing the coverage of the lithium-containing fluoride on the surface of the positive electrode active material, a hydrofluoride salt of the metal element M is more preferred.

[0027] These starting materials, a water-soluble Li salt and a water-soluble salt containing metal elements M and F, are dissolved in water to obtain an aqueous solution containing Li, F, and M and having a pH of 2.5 to 7.0. This aqueous solution is mixed according to the composition of the lithium-containing fluoride having the predetermined composition described above, and then heated to volatilize the solvent and impurities such as acetate, sulfate, chlorine, and nitrate, thereby producing a dry powder of lithium-containing fluoride. When the water-soluble Li salt starting material is dissolved in water, the pH becomes approximately 2.5 to 7.0. In other words, even when a lithium-containing fluoride coating solution is produced using H2MF6 such as H2TiF6, this production method produces a lithium-containing fluoride powder without the H2MF6 passing through a high pH range.

[0028] The aqueous solution containing Li, F, and the metal element M obtained by dissolving the produced water-soluble Li salt and the water-soluble salt containing the metal elements M and F in water preferably has a pH of 2.5 or more and 7.0 or less, more preferably a pH of 4.0 or more and 7.0 or less, and even more preferably a pH of 4.5 or more and 7.0 or less, from the viewpoint of suppressing the precipitation of the hydrolysis product MO2. The pH of the aqueous solution containing Li, F, and M may be 7.0 or less, but may also be, for example, pH 6.0 or less or pH 5.0 or less.

[0029] In order to prevent deterioration of the characteristics of the final all-solid-state lithium battery, the heating temperature is preferably 150°C or higher in order to volatilize impurities such as acetate, sulfate, chlorine, and nitrate. On the other hand, in order to avoid decomposition of fluoride, the heating temperature is preferably 500°C or lower. There are no particular restrictions on the firing atmosphere, but air is preferred from the viewpoint of productivity.

[0030] 2. Lithium-containing fluoride coating solution (1) Composition and properties of lithium-containing fluoride coating solution By mixing the lithium-containing fluoride produced by the above-mentioned dry method or wet method with pure water to form an aqueous solution, it is possible to obtain the lithium-containing fluoride coating liquid according to the present invention, which contains Li, F, and M (M is one or more metal elements that can take a tetravalent oxidation number), and in which the molar ratio Li:F:M of Li, F, and M contained in the lithium-containing fluoride coating liquid is 2+x:1:6+y (-0.5≦x≦1, -0.5≦y≦1).

[0031] The total concentration of the elements Li, M, and F in the lithium-containing fluoride coating solution is preferably 0.5% by mass or more and 12% by mass or less. This is because, if the total concentration of the elements Li, M, and F is 0.5% by mass or more, it is easy to adjust the thickness of the lithium-containing fluoride coating film when it is formed on the surface of the positive electrode active material. On the other hand, if the total concentration of the elements Li, M, and F is 12% by mass or less, hydrolysis of M ions does not occur, and the generation and precipitation of an oxide of the metal element M can be avoided. From the same perspective, the total concentration of the elements Li, M, and F in the lithium-containing fluoride coating solution is more preferably 1% by mass or more, and even more preferably 2% by mass or more. The total concentration of the elements Li, M, and F in the lithium-containing fluoride coating solution is more preferably 10% by mass or less, even more preferably 8% by mass or less, and even more preferably 5% by mass or less.

[0032] Furthermore, the preferred concentrations of each element in the lithium-containing fluoride coating liquid are 0.1% by mass or more and 1.0% by mass or less for Li, 0.3% by mass or more and 7.5% by mass or less for F, and 0.2% by mass or more and 3.5% by mass or less for the metal element M, more preferably 0.15% by mass or more and 0.8% by mass or less for Li, 0.5% by mass or more and 6% by mass or less for F, and 0.3% by mass or more and 2.8% by mass or less for the metal element M, and even more preferably 0.2% by mass or more and 0.4% by mass or less for Li, 0.6% by mass or more and 3% by mass or less for F, and 0.4% by mass or more and 1.5% by mass or less for the metal element M.

[0033] The lithium-containing fluoride coating solution is a coating solution that is dried by spray drying the coating solution using, for example, a spray dryer, and the resulting powder is measured using an XRD device to obtain an XRD profile that shows the presence of Li 2+x MF 6+y , (-0.5≦x≦1, -0.5≦y≦1) as the main phase. When the metal element M is one kind, the "main phase" refers to the crystalline phase to which the most intense diffraction peak belongs in the XRD spectrum obtained by XRD measurement. When the metal element M is two kinds or more, the main phase can be determined in the same way as when there is one kind of M. For example, when the metal element M is two kinds, Ti and Si, the most intense diffraction peak belongs to Li in the XRD spectrum obtained by XRD measurement. 2+x SiF 6+y or Li 2+x TiF 6+y If it can be attributed to either Li 2+x MF 6+y can be determined to be the main phase.

[0034] Whether or not the diffraction peaks can be assigned can be determined by comparing the XRD spectrum of the dried powder with the diffraction peaks of lithium-containing fluorides registered in a database. For example, the XRD spectrum of the dried powder of the coating liquid can be compared with the diffraction peaks of lithium-containing fluorides registered in a database, such as Li2TiF6 (ICDD: 01-086-8694), Li2ZrF6 (ICDD: 01-071-2775), Li2SiF6 (ICDD: 01-084-4762), Li2GeF6 (ICDD: 01-080-1786), Li2SnF6 (ICDD: 00-056-0531), and Li2HfF6 (ICDD: 01-084-3281), which are PDFs (Powder Diffraction Files) of the ICDD (International Centre for Diffraction Data). 2+x MF 6+y , (-0.5≦x≦1, -0.5≦y≦1) to determine whether the value is correct.

[0035] (2) Method for producing lithium-containing fluoride coating solution The lithium-containing fluoride produced by the dry method or the wet method described above is mixed with pure water to produce a lithium-containing fluoride coating solution.

[0036] The pH of the produced lithium-containing fluoride coating solution is 2.5 or more and 7.0 or less, but from the viewpoint of ensuring the coverage rate of the lithium-containing fluoride on the surface of the positive electrode active material, the pH of the lithium-containing fluoride coating solution is preferably 4.0 or more and 7.0 or less, and more preferably 4.5 or more and 7.0 or less.

[0037] By adjusting the pH of the lithium-containing fluoride coating solution to 4.0 or more and 7.0 or less, the coverage rate of the lithium-containing fluoride on the surface of the positive electrode active material can be 85% or more, and by adjusting the pH to 4.5 or more and 7.0 or less, the coverage rate of the lithium-containing fluoride on the surface of the positive electrode active material can be 90% or more. The pH of the lithium-containing fluoride coating solution may be 7.0 or less, but may also be, for example, pH 6.0 or less or pH 5.0 or less.

[0038] Therefore, it is preferable to add a base to the produced lithium-containing fluoride coating solution to adjust the pH. The base can be an inorganic base such as LiOH, NaOH, KOH, or NH4OH, or an organic base such as ethylamine, ethylenediamine, diethylamine, or triethylamine.

[0039] As a result, it is thought that the total of Li, M, F, and water in the produced lithium-containing fluoride coating solution is 70 mass % or more, with the remainder being ammonia and organic salts added as a base.

[0040] Here, from the viewpoint of increasing the ionic conductivity of the finally obtained lithium-containing fluoride coating film, the total content of Li, M, F, and water in the lithium-containing fluoride coating solution is preferably 90 mass % or more, more preferably 95 mass % or more, and most preferably 99 mass % or more. From the viewpoint of reducing the amount of impurities other than Li, M, and F in the lithium-containing fluoride coating, the base to be added is preferably LiOH or NH4OH. When LiOH or NH4OH is used as the added base, the generated HO or NH3 volatilizes in the drying step after the lithium-containing fluoride coating film is provided on the surface of the positive electrode active material, and therefore, it is also preferable from the viewpoint of preventing impurities from remaining in the lithium-containing fluoride coating film.

[0041] 3. Lithium-containing fluoride-coated active material (1) Positive electrode active material used for lithium-containing fluoride-coated active material There are no particular limitations on the positive electrode active material to which the lithium-containing fluoride coating solution is applied to form a coating film, and positive electrode active materials of general compositions can be used. 1 / 3 Co 1 / 3 Mn 1 / 3 In addition to O2 (NCM111), lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMnO4), and cathode active materials in which some of the transition metals are replaced with Al, Cr, Fe, Y, W, Ta, or Nb (LiNi 0.95 Al 0.05 O2, etc.), and further, a positive electrode active material obtained by combining these positive electrode active materials can also be used.

[0042] The thickness of the lithium-containing fluoride coating provided on the surface of the positive electrode active material is 5 nm or more and 40 nm or less, preferably 8 nm or more and 30 nm or less, and more preferably 10 nm or more and 20 nm or less.

[0043] The coverage of the lithium-containing fluoride coating on the surface of the positive electrode active material is 80% or more, preferably 85% or more, and more preferably 90% or more.

[0044] (2) Method for producing lithium-containing fluoride-coated active material The method for contacting the lithium-containing fluoride coating liquid of the present invention with a positive electrode active material to form a coating on the surface of the positive electrode active material is not particularly limited. For example, a mixed slurry obtained by mixing the lithium-containing fluoride coating liquid with a positive electrode active material may be dried by a spray dryer method to form a coating film on the surface of the positive electrode active material. Alternatively, for example, a positive electrode active material that has not undergone the mixed slurry process may be sprayed with the lithium-containing fluoride coating liquid to contact it, and simultaneously dried using a so-called tumbling flow method, in which the lithium-containing fluoride coating liquid is dried to form a coating film on the surface of the positive electrode active material.

[0045] However, from the viewpoint of productivity, a method of forming a coating film on the surface of the positive electrode active material by drying a mixed slurry obtained by mixing a lithium-containing fluoride coating solution with a positive electrode active material is preferable from the viewpoint of enabling continuous processing. In addition, as a method of forming a coating film on the surface of the positive electrode active material by drying the mixed slurry, for example, a spray dryer method is preferable from the viewpoint of reducing unevenness in the thickness of the coating film.

[0046] The drying temperature may be appropriately set so that no moisture remains in the resulting dry powder. Specifically, the inlet temperature of the spray dryer, which is a spray dryer, is preferably 150 to 250°C and the hot air outlet temperature is preferably 60 to 120°C. This is because an inlet temperature of 150°C or higher can achieve a sufficient drying rate and prevent the dried powder from remaining in the drying tower of the spray dryer, while an inlet temperature of 250°C or lower can prevent components in the slurry from thermally decomposing and failing to produce the desired dry powder. Furthermore, a hot air outlet temperature of 60°C or higher, preferably 80°C or higher, can sufficiently reduce the moisture content in the dry powder, while a hot air outlet temperature of 120°C or lower, preferably 110°C or lower, can prevent segregation of germanium dioxide.

[0047] If necessary, the lithium-containing fluoride coating solution may be further diluted with pure water and then mixed with the positive electrode active material. For example, when a spray dryer method is used to dry the obtained mixed slurry, the total concentration of each element of Li, M, and F in the lithium-containing fluoride coating liquid is preferably 0.5 mass% or more and 5 mass% or less in order to facilitate spraying.

[0048] Furthermore, the preferred concentrations of each element in the mixed slurry are 0.1 to 0.4 mass % for Li, 0.3 to 3 mass % for F, and 0.2 to 1.5 mass % for the metal element M.

[0049] In the present invention, when drying the mixed slurry, for example, in accordance with the use of a spray dryer method, the lithium-containing fluoride coating liquid may be diluted with pure water to produce and use a liquid having an adjusted lithium-containing fluoride concentration. In the present invention, this lithium-containing fluoride coating liquid having an adjusted lithium-containing fluoride concentration may be referred to as a "coating liquid". Note that the lithium-containing fluoride coating liquid according to the present invention is a concept that includes both the above-mentioned lithium-containing fluoride coating liquid and the "coating liquid" within its scope of rights. [Example]

[0050] The present invention will be described in more detail below with reference to examples, although the present invention is not limited to these examples.

[0051] Example 1 1. Preparation of Li2TiF6 powder 0.74 g of LiF (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), 1.76 g of TiF4 (manufactured by Sigma-Aldrich Co., Ltd.), and 17 zirconia balls with a diameter of 10 mm were placed in a grinding pot (made of zirconia, capacity 45 ml). The milling pot was set in a planetary ball mill (Fritsch, Pulverisette-7) and milling was carried out for 12 hours at a rotation speed of 380 rpm, a dew point of -52°C, and an Ar atmosphere. After the milling process, the zirconia balls were separated from the contents of the milling pot to obtain a powder. The blending of the raw materials for obtaining the powder is shown in Table 1. The same applies to Examples 2 to 7 and Comparative Examples 1 and 3 below. The powder obtained in the above process was subjected to XRD measurement, and the XRD pattern confirmed that it was Li2TiF6 powder. The XRD pattern of the obtained Li2TiF6 powder according to Example 1 is shown in Figure 1. Details of the XRD measurement will be described later.

[0052] 2. Preparation of lithium-containing fluoride coating solution 0.80 g of the obtained Li2TiF6 powder was added to 19.20 g of pure water and stirred in the air at 25°C for 60 minutes using a stirrer to obtain a lithium-containing fluoride coating solution according to Example 1. The solution after stirring was transparent and had a pH of 3.2. Table 2 shows the composition of the obtained lithium-containing fluoride coating solution, the solid concentration of lithium-containing fluoride dissolved in the lithium-containing fluoride coating solution, and the pH. The pH of the lithium-containing fluoride coating solution was measured using a glass electrode with a pH meter equipped with a temperature-compensated electrode, after calibrating it with an appropriate buffer solution according to the pH range to be measured, in accordance with JIS Z8802. Table 3 also shows the concentrations of each element contained in the obtained lithium-containing fluoride coating solution, expressed as mass %, molar concentration, and molar ratio (the molar ratio of the metal element M is defined as 1). The same applies to Examples 2 to 7 and Comparative Examples 1 and 3.

[0053] The concentrations (mass %) of Li and M (M is one or more metal elements that can take a tetravalent oxidation number) contained in the lithium-containing fluoride coating solution were analyzed by the following method. 0.05 g of the resulting lithium-containing fluoride coating solution was weighed out, and 5 mL of hydrochloric acid (36 mass% aqueous solution) was added and heated to decompose. The decomposed solution was diluted 10 times in a 100 mL measuring flask. The Li concentration and the metal element M concentration in the resulting diluted solution were measured using an inductively coupled plasma atomic emission spectrometer (ICP-AES, ICP-720, manufactured by Agilent Technologies), and these were defined as the concentrations (mass%) of Li and metal element M contained in the lithium-containing fluoride coating solution.

[0054] The F contained in the lithium-containing fluoride coating solution was analyzed by the following method. 0.05 g of the obtained lithium-containing fluoride coating solution was weighed out and added to 20 mL of 1 mol / L NaOH solution for thermal decomposition, and the decomposed solution was diluted 20 times in a 100 mL measuring flask. The F concentration in the obtained diluted solution was measured using ion chromatography (Tosoh Corporation, IC-2010) and was defined as the F concentration (mass%) contained in the lithium-containing fluoride coating solution. Furthermore, based on the measurement results, the molar concentrations and molar ratios of the elements Li, F, and M contained in the lithium-containing fluoride coating solution were calculated.

[0055] The obtained lithium-containing fluoride coating liquid was then spray-dried using a spray dryer (Tokyo Rika SD-1000) to obtain a dried coating liquid powder. The spray drying conditions were a spray dryer inlet temperature of 200°C, a hot air outlet temperature of 90°C, and a slurry addition rate to the spray dryer of 10 g / min. The obtained dried coating liquid powder was subjected to the XRD measurement described below, and the XRD pattern confirmed that it was Li2TiF6 powder. The XRD pattern of the obtained dried coating liquid powder according to Example 1 is shown in Figure 2. Details of the XRD measurement will be described later.

[0056] 3. Preparation of lithium-containing fluoride-coated active material 12 g of pure water was added to 18 g of the obtained lithium-containing fluoride coating solution according to Example 1 to obtain a coating solution according to Example 1. The obtained coating solution according to Example 1 was mixed with a positive electrode active material (NCM111:LiNi 1 / 3 Mn 1 / 3 Co 1 / 3 02) was added to obtain a slurry. The composition of the obtained coating liquid according to Example 1, the concentration of each element contained in the slurry expressed as mass %, molar concentration, and molar ratio (the molar ratio of metal element M is defined as 1), the pH of the slurry, and the amount of positive electrode active material in the slurry are shown in Table 3. The same applies to Examples 2 to 7 and Comparative Examples 1 and 3.

[0057] The obtained slurry was dried using a spray dryer (SD-1000 manufactured by Tokyo Rika Co., Ltd.) to obtain a lithium-containing fluoride-coated active material having a surface coated with lithium-containing fluoride according to Example 1. The spray drying conditions were a spray dryer inlet temperature of 200°C, a hot air outlet temperature of 90°C, and a slurry addition rate to the spray dryer of 10 g / min.

[0058] The thickness of the lithium-containing fluoride coating film on the obtained lithium-containing fluoride-coated active material was determined to be 14.4 nm by ICP and IC (ion chromatography) analysis. The method for measuring and calculating the thickness of the lithium-containing fluoride coating film will be described in detail later. The coverage of the lithium-containing fluoride in the obtained lithium-containing fluoride-coated active material was measured by XPS and found to be 82%. The method for measuring and calculating the coverage of the lithium-containing fluoride coating film will be described in detail below. The film thickness (target value, measured value) and coverage of the lithium-containing fluoride coating film in the obtained lithium-containing fluoride coated active material are shown in Table 4. The same applies to the following Examples 2 to 7 and Comparative Examples 1 to 3.

[0059] Example 2 1. Preparation of Li2ZrF6 powder 0.59 g of LiF (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), 1.91 g of ZrF4 (manufactured by Sigma-Aldrich Co., Ltd.), and 17 zirconia balls with a diameter of 10 mm were placed in a grinding pot (made of zirconia, capacity 45 cc). The milling pot was set in a planetary ball mill (Fritsch, Pulverisette-7) and milling was carried out for 12 hours at a rotation speed of 380 rpm, a dew point of -52°C, and an Ar atmosphere. After the milling process, the zirconia balls were separated from the contents of the milling pot to obtain a powder. The powder obtained in the above step was subjected to XRD measurement, and the XRD pattern confirmed that it was Li2ZrF6 powder. The XRD pattern of the obtained Li2ZrF6 powder according to Example 2 is shown in Figure 3.

[0060] 2. Preparation of lithium-containing fluoride coating solution 0.80 g of the obtained Li2ZrF6 powder was added to 19.20 g of pure water and stirred in the air at 25°C for 60 minutes using a stirrer to obtain a lithium-containing fluoride coating solution according to Example 2. The solution after stirring was transparent and had a pH of 3.5. The obtained lithium-containing fluoride coating solution was spray-dried in the same manner as in Example 1 to obtain a dried coating solution powder. The obtained dried coating solution powder was subjected to the XRD measurement described below, and the XRD pattern confirmed that it was Li2ZrF6 powder. The XRD pattern of the obtained dried coating solution powder according to Example 2 is shown in Figure 4. Details of the XRD measurement will be described later.

[0061] 3. Preparation of lithium-containing fluoride-coated active material 9 g of pure water was added to 21 g of the obtained lithium-containing fluoride coating solution according to Example 2 to obtain a coating solution according to Example 2. 1 / 3 Mn 1 / 3 Co 1 / 3 30 g of O2) was added, and the same operation as in Example 1 was carried out to obtain a lithium-containing fluoride-coated active material according to Example 2, the surface of which was coated with lithium-containing fluoride. Then, in the same manner as in Example 1, the thickness and coverage of the coating film of the lithium-containing fluoride coated active material according to Example 2 were measured.

[0062] Example 3 1. Preparation of Li2SiF6 powder A solution was obtained by dissolving 1.20 g of lithium acetate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) in 10.0 g of pure water. 3.73 g of 60% H2SiF6 solution (manufactured by Sigma-Aldrich) was added to the solution to obtain a transparent aqueous solution containing Li, F, and Si as the metal element M. The pH of the resulting aqueous solution containing Li, F, and Si was 2.6. The resulting aqueous solution was heated on a hot plate at 80°C until the solvent was completely removed, and then dried at 110°C in a vacuum for 5 hours to obtain a powder. XRD measurement was performed on the powder obtained in the above process, and the XRD pattern confirmed that it was Li2SiF6 powder. The XRD pattern of the resulting Li2SiF6 powder of Example 3 is shown in Figure 5.

[0063] 2. Preparation of lithium-containing fluoride coating solution 0.80 g of the obtained Li2SiF6 powder was added to 19.20 g of pure water and stirred for 60 minutes at 25°C in the air using a stirrer to obtain a lithium-containing fluoride coating solution according to Example 3. The solution after stirring was transparent and had a pH of 4.0. The obtained lithium-containing fluoride coating solution was spray-dried in the same manner as in Example 1 to obtain a dried coating solution powder. The obtained dried coating solution powder was subjected to the XRD measurement described below, and the XRD pattern confirmed that it was Li2SiF6 powder. The XRD pattern of the obtained dried coating solution powder according to Example 2 is shown in Figure 6. Details of the XRD measurement will be described later.

[0064] 3. Preparation of lithium-containing fluoride-coated active material 13 g of pure water was added to 17 g of the obtained lithium-containing fluoride coating solution according to Example 3 to obtain a coating solution according to Example 3. 1 / 3 Mn 1 / 3 Co 1 / 3 30 g of O2) was added, and the same operation as in Example 1 was carried out to obtain a lithium-containing fluoride-coated active material according to Example 3. Then, in the same manner as in Example 1, the film thickness and coverage of the lithium-containing fluoride coating film in the lithium-containing fluoride coated active material according to Example 3 were measured.

[0065] Example 4 1. Preparation of Li2TiF6 powder The same procedure as in Example 1 was carried out to obtain Li2TiF6 powder.

[0066] 2. Preparation of lithium-containing fluoride coating solution 0.80 g of the obtained Li2TiF6 powder was added to 18.83 g of pure water and stirred in the air at 25°C using a stirrer for 60 minutes. After stirring, the solution appeared transparent and had a pH of 3.2. 0.37 g of 28% aqueous ammonia (manufactured by Nacalai Tesque) was added to the Li2TiF6 solution as an additional base to obtain a lithium-containing fluoride coating solution according to Example 4. The lithium-containing fluoride coating solution according to Example 4 was transparent and had a pH of 4.8. The obtained lithium-containing fluoride coating solution was spray-dried in the same manner as in Example 1 to obtain a dried coating solution powder. The obtained dried coating solution powder was subjected to the XRD measurement described below, and the XRD pattern confirmed that it was Li2TiF6 powder.

[0067] 3. Preparation of lithium-containing fluoride-coated active material 12 g of pure water was added to 18 g of the obtained lithium-containing fluoride coating solution according to Example 4 to obtain a coating solution according to Example 4. The coating solution according to Example 4 thus obtained was added with a positive electrode active material (NCM111:LiNi 1 / 3 Mn 1 / 3 Co 1 / 3 30 g of O2) was added, and the same operation as in Example 1 was carried out to obtain a lithium-containing fluoride coated active material according to Example 4. The pH of the slurry after mixing the lithium-containing fluoride coating solution according to Example 4 and the positive electrode active material was 4.9. Then, in the same manner as in Example 1, the film thickness and coverage of the lithium-containing fluoride coating film in the lithium-containing fluoride coated active material according to Example 4 were measured.

[0068] Example 5 1. Preparation of Li2TiF6 powder The same procedure as in Example 1 was carried out to obtain Li2TiF6 powder.

[0069] 2. Preparation of lithium-containing fluoride coating solution 0.80 g of the obtained Li2TiF6 powder was added to 18.83 g of pure water and stirred in the air at 25°C for 60 minutes using a stirrer to obtain a Li2TiF6 solution. After stirring, the solution was transparent and had a pH of 3.2. 0.37 g of an aqueous LiOH solution with a Li concentration of 2.6 mass% was added to the Li2TiF6 solution as an additional base to obtain a lithium-containing fluoride coating solution according to Example 5. The lithium-containing fluoride coating solution according to Example 5 was transparent and had a pH of 4.9. The obtained lithium-containing fluoride coating solution was spray-dried in the same manner as in Example 1 to obtain a dried coating solution powder. The obtained dried coating solution powder was subjected to the XRD measurement described below, and the XRD pattern confirmed that it was Li2TiF6 powder.

[0070] 3. Preparation of lithium-containing fluoride-coated active material 12 g of pure water was added to 18 g of the obtained lithium-containing fluoride coating solution according to Example 5 to obtain a coating solution according to Example 5. The obtained coating solution according to Example 5 was mixed with a positive electrode active material (NCM111:LiNi 1 / 3 Mn 1 / 3 Co 1 / 3 30 g of O2) was added, and the same operation as in Example 1 was carried out to obtain a lithium-containing fluoride coated active material according to Example 5. The pH of the slurry after mixing the lithium-containing fluoride coating solution according to Example 5 and the positive electrode active material was 4.9. Then, in the same manner as in Example 1, the film thickness and coverage of the lithium-containing fluoride coating film in the lithium-containing fluoride coated active material according to Example 5 were measured.

[0071] Example 6 1. Preparation of Li2TiF6 powder A solution was obtained by dissolving 1.12 g of lithium acetate (Fujifilm Wako Pure Chemical Industries, Ltd.) in 10.0 g of pure water. 2.31 g of 60% H2TiF6 solution (Sigma-Aldrich) was added to the solution to obtain a transparent aqueous solution containing Li, F, and Ti as the metal element M. The pH of the resulting aqueous solution containing Li, F, and Ti was 2.7. The resulting aqueous solution was heated on a hot plate at 80°C until the solvent was completely removed, and then dried at 110°C in a vacuum for 5 hours to obtain a powder. XRD measurement was performed on the powder obtained in the above process, and the XRD pattern confirmed that it was Li2TiF6 powder.

[0072] 2. Preparation of lithium-containing fluoride coating solution 0.80 g of the obtained Li2TiF6 powder was added to 19.20 g of pure water and stirred for 60 minutes using a stirrer at 25°C in the air to obtain a lithium-containing fluoride coating solution according to Example 6. The solution after stirring was transparent and had a pH of 3.3. The obtained lithium-containing fluoride coating solution was spray-dried in the same manner as in Example 1 to obtain a dried coating solution powder. The obtained dried coating solution powder was subjected to the XRD measurement described below, and the XRD pattern confirmed that it was Li2TiF6 powder.

[0073] 3. Preparation of lithium-containing fluoride-coated active material The same operation as in Example 1 was carried out to obtain a lithium-containing fluoride coating solution according to Example 6. 12 g of pure water was added to 18 g of the obtained lithium-containing fluoride coating solution according to Example 6 to obtain a coating solution according to Example 6. The positive electrode active material (NCM111:LiNi 1 / 3 Mn 1 / 3 Co 1 / 3 30 g of O2) was added, and the same operation as in Example 1 was carried out to obtain a lithium-containing fluoride-coated active material according to Example 6. Then, in the same manner as in Example 1, the film thickness and coverage of the lithium-containing fluoride coating film in the lithium-containing fluoride coated active material according to Example 6 were measured.

[0074] Example 7 1. Production of Li2TiF6 powder, Li2ZrF6 powder, and Li2SiF6 powder The same procedure as in Example 1 was carried out to obtain Li2TiF6 powder, the same procedure as in Example 2 was carried out to obtain Li2ZrF6 powder, and the same procedure as in Example 3 was carried out to obtain Li2SiF6 powder.

[0075] 2. Preparation of lithium-containing fluoride coating solution 0.267 g of the obtained Li2TiF6 powder, 0.267 g of Li2ZrF6 powder, and 0.267 g of Li2SiF6 powder were added to 18.83 g of pure water and stirred for 60 minutes at 25°C using a stirrer in the air to obtain a Li2TiF6-Li2ZrF6-Li2SiF6 solution. After stirring, the solution was transparent and had a pH of 3.8. 0.37 g of an aqueous LiOH solution with a Li concentration of 2.6 mass% was added to the Li2TiF6-Li2ZrF6-Li2SiF6 solution as an additional base to obtain a lithium-containing fluoride coating solution according to Example 7. The lithium-containing fluoride coating solution according to Example 7 was transparent and had a pH of 4.9. The obtained lithium-containing fluoride coating solution was spray-dried in the same manner as in Example 1 to obtain a dried coating solution powder. The dried powder obtained from the coating liquid was subjected to the XRD measurement described below. The XRD pattern showed that the most intense diffraction peak at 2θ=21.5° was attributable to the diffraction peak of Li2TiF6, confirming that the main phase was Li2TiF6. Peaks of Li2ZrF6 and Li2SiF6 were also confirmed.

[0076] 3. Preparation of lithium-containing fluoride-coated active material 12 g of pure water was added to 18 g of the obtained lithium-containing fluoride coating solution according to Example 7 to obtain a coating solution according to Example 7. The obtained coating solution according to Example 7 was mixed with a positive electrode active material (NCM111:LiNi 1 / 3 Mn 1 / 3 Co 1 / 3 30 g of O2) was added, and the same operation as in Example 1 was carried out to obtain a lithium-containing fluoride-coated active material according to Example 7, the surface of which was coated with lithium-containing fluoride. Then, in the same manner as in Example 1, the film thickness and coverage of the lithium-containing fluoride coating film in the lithium-containing fluoride coated active material according to Example 7 were measured.

[0077] Example 8 1. Preparation of Li2TiF6 powder A solution was obtained by dissolving 0.72 g of lithium chloride (Fujifilm Wako Pure Chemical Industries, Ltd.) in 10.0 g of pure water. 1.67 g of (NH4)2TiF6 (Sigma-Aldrich) was added to the solution to obtain a clear aqueous solution. The pH of the resulting aqueous solution containing Li, F, and Ti was 3.3. The resulting aqueous solution was heated on a hot plate at 80°C until the solvent evaporated, and then baked in an argon atmosphere at 300°C for 5 hours to obtain a powder. XRD measurement was performed on the powder obtained in the above process, and the XRD pattern confirmed that it was Li2TiF6 powder.

[0078] 2. Preparation of lithium-containing fluoride coating solution 0.80 g of the obtained Li2TiF6 powder was added to 19.20 g of pure water and stirred for 60 minutes using a stirrer at 25°C in the air to obtain a lithium-containing fluoride coating solution according to Example 8. The solution after stirring was transparent and had a pH of 3.3. The obtained lithium-containing fluoride coating solution was spray-dried in the same manner as in Example 1 to obtain a dried coating solution powder. The obtained dried coating solution powder was subjected to the XRD measurement described below, and the XRD pattern confirmed that it was Li2TiF6 powder.

[0079] 3. Preparation of lithium-containing fluoride-coated active material The same operation as in Example 1 was carried out to obtain a lithium-containing fluoride coating solution according to Example 8. 12 g of pure water was added to 18 g of the obtained lithium-containing fluoride coating solution according to Example 8 to obtain a coating solution according to Example 8. The positive electrode active material (NCM111:LiNi 1 / 3 Mn 1 / 3 Co 1 / 3 30 g of O2) was added, and the same operation as in Example 1 was carried out to obtain a lithium-containing fluoride-coated active material according to Example 8. Then, in the same manner as in Example 1, the film thickness and coverage of the lithium-containing fluoride coating film of the lithium-containing fluoride coated active material according to Example 8 were measured.

[0080] (Comparative Example 1) 1. Preparation of Li2TiF6 powder The same procedure as in Example 1 was carried out to obtain Li2TiF6 powder.

[0081] 2. Preparation of Li2TiF6 solution 0.8 g of the obtained Li2TiF6 powder was added to 18.83 g of pure water and stirred in the air at 25°C for 60 minutes using a stirrer to obtain a Li2TiF6 solution. After stirring, the solution was transparent and had a pH of 3.2. 0.37 g of an aqueous acetic acid solution with a concentration of 99.7 mass% was added to the Li2TiF6 solution as an additional acid to obtain a lithium-containing fluoride coating solution according to Comparative Example 1. The lithium-containing fluoride coating solution according to Comparative Example 1 was transparent and had a pH of 2.2. The obtained lithium-containing fluoride coating solution was spray-dried in the same manner as in Example 1 to obtain a dried coating solution powder. The obtained dried coating solution powder was subjected to the XRD measurement described below, and the XRD pattern confirmed that it was Li2TiF6 powder.

[0082] 3. Preparation of lithium-containing fluoride-coated active material 12 g of pure water was added to 18 g of the obtained lithium-containing fluoride coating solution according to Comparative Example 1 to obtain a coating solution according to Comparative Example 1. The obtained coating solution according to Comparative Example 1 was mixed with a positive electrode active material (NCM111:LiNi 1 / 3 Mn 1 / 3 Co 1 / 3 30 g of O2) was added, and the same operation as in Example 1 was carried out to obtain a lithium-containing fluoride-coated active material according to Comparative Example 1. Then, in the same manner as in Example 1, the film thickness and coverage of the lithium-containing fluoride coating film in the lithium-containing fluoride coated active material according to Comparative Example 1 were measured.

[0083] (Comparative Example 2) 1. Production of a positive electrode active material whose surface is coated with Li2TiF6 (This production method conforms to the production method described in Non-Patent Document 2) 28.57 g of pure water, 0.31 g of Li carbonate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), and 30 g of positive electrode active material (NCM111) were weighed and mixed to obtain a mixed slurry. The pH of the obtained mixed slurry was 11.8. 1.12 g of 60% H2TiF6 solution (manufactured by Sigma-Aldrich) was added to the mixed slurry. The pH of the slurry after the addition was 2.1. The slurry after the addition of the H2TiF6 solution was dried using a spray dryer (Tokyo Rika SD-1000) to obtain a positive electrode active material whose surface was coated with Li2TiF6. Then, in the same manner as in Example 1, the thickness and coverage of the coating film of the positive electrode active material whose surface was coated with Li2TiF6 were measured.

[0084] (Comparative Example 3) 1. Preparation of Li2TiF6 powder The same procedure as in Example 1 was carried out to obtain Li2TiF6 powder.

[0085] 2. Preparation of lithium-containing fluoride coating solution according to Comparative Example 3 0.8 g of the obtained Li2TiF6 powder was added to 16.20 g of pure water and stirred in the air at 25°C for 60 minutes using a stirrer to obtain a Li2TiF6 solution. After stirring, the solution was transparent and had a pH of 3.2. 3.0 g of 28% aqueous ammonia (manufactured by Nacalai Tesque) was added to the Li2TiF6 solution as an additional base to obtain a lithium-containing fluoride coating solution according to Comparative Example 3. The lithium-containing fluoride coating solution according to Comparative Example 3 became cloudy and had a pH of 8.6. The obtained lithium-containing fluoride coating solution was spray-dried in the same manner as in Example 1 to obtain a dried coating solution powder. The obtained dried coating solution powder was subjected to the XRD measurement described below, and the XRD pattern confirmed that it was Li2TiF6 powder.

[0086] 3. Preparation of lithium-containing fluoride-coated active material 12 g of pure water was added to 18 g of the obtained lithium-containing fluoride coating solution according to Comparative Example 3 to obtain a coating solution according to Comparative Example 3. The obtained coating solution according to Comparative Example 3 was mixed with a positive electrode active material (NCM111:LiNi 1 / 3 Mn 1 / 3 Co 1 / 330 g of O2 was introduced, and the same operations as in Example 1 were carried out to obtain a lithium-containing fluoride-coated active material according to Comparative Example 3. Then, in the same manner as in Example 1, the film thickness and coating rate of the lithium-containing fluoride coating film in the lithium-containing fluoride-coated active material according to Comparative Example 3 were measured.

[0087]

Table 1

Table 2

Table 3

Table 4

[0088] (XRD Measurement of Lithium-Containing Fluoride, Measurement and Calculation Method of Film Thickness and Coating Rate of Coating Film) 1. XRD Measurement of Lithium-Containing Fluoride (1) Measurement Conditions For the Li2TiF6 powder, Li2ZrF6 powder, and Li2SiF6 powder according to Examples 1-7 and Comparative Examples 1 and 3, XRD measurements were carried out under the following measurement conditions. And in each example, a plot with 2θ (°) on the horizontal axis and intensity (cps) on the vertical axis was created.

[0089] <XRD Measurement Conditions> Measurement Device: XRD-6100 (manufactured by Shimadzu Corporation) Tube Ball: Measured at the wavelength of Cu (Kα1) Tube Voltage: 40 kv Tube Current: 30 mA Divergence Slit: 1.0° Scattering Slit: 1.0° Receiving Slit: 0.3 mm Step Width: 0.02° / step Measurement Time: 0.60 sec Integration Number: 1 time

[0090] (2) Evaluation of XRD measurements The obtained XRD profile was confirmed to have diffraction peaks derived from Li2TiF6 (attributed to ICDD: 01-086-8694), Li2ZrF6 (attributed to ICDD: 01-071-2775), or Li2SiF6 (attributed to ICDD: 01-084-4762).

[0091] 2.Method for measuring and calculating the thickness of lithium-containing fluoride coating film (1) Quantitative analysis of F 0.05 g of each sample of the positive electrode active material whose surface was coated with a lithium-containing fluoride according to Examples 1-7 and Comparative Examples 1 and 3 was weighed out, and 20 mL of a 1 mol / L NaOH solution was added thereto, followed by thermal decomposition to obtain a decomposition solution. The resulting decomposition solution was diluted 20 times in a 100 mL measuring flask, and the F concentration in the resulting diluted solution was measured using an ion chromatograph (IC-2010, manufactured by Tosoh Corporation) to determine the F concentration (mass%) contained in the lithium-containing fluoride coating solution.

[0092] (2) Calculation method for film thickness Since F is not contained in the composition of general positive electrode active materials, the F content in the positive electrode active material coated with lithium-containing fluoride can be considered to be the F content in the coating film. 2+x MF 6+y In the case of a lithium-containing fluoride having a composition (M is one or more metal elements that can take a tetravalent oxidation number, -0.5≦x≦1, -0.5≦y≦1), the coating thickness can be calculated by the following (Equation 1) and (Equation 2). When the lithium-containing fluoride is Li2MF6, if the F content in the positive electrode active material coated with the lithium-containing fluoride measured by the above method is taken as A (mass %), the mass proportion B (mass %) of the coating film can be calculated by the following (Equation 1) from the atomic weight of F and the molecular weight of Li2MF6. B (mass%) = A (mass%) × molecular weight of Li2MF6 / (atomic weight of F × 6) (Equation 1) Furthermore, the BET specific surface area S (m 2 / g), and the true density d of Li2MF6 (g / cm 3 ), the average film thickness t (nm) of the coating film is expressed by the following (Equation 2): Note that the constant 10 in Equation 2 is a conversion coefficient. t(nm)=10×B(mass%) / (d(g / cm 3 )×S(m 2 / g))...(Formula 2) however, The BET value of the positive electrode active material is 0.538m 2 / g (when the positive electrode active material of Example 1 was used), Also, Li 2+x MF 6+y The true density of a lithium-containing fluoride having a composition of (M is one or more metal elements that can take a tetravalent oxidation number, -0.5≦x≦1, -0.5≦y≦1) is calculated using, for example, the following values. The true density of Li2TiF6 is 3.12 g / cm 3 , The true density of Li2SiF6 is 3.04g / cm 3 , The true density of Li2ZrF6 is 3.73 g / cm 3 , The true density of Li2GeF6 is 3.71 g / cm 3 , The true density of Li2SnF6 is 4.32 g / cm 3 , The true density of Li2HfF6 is 5.11g / cm 3 ,

[0093] As described above, the metal element M is one or more metal elements that can have a tetravalent oxidation number. When there is only one metal element M, the true density ρ value (g / cm3) of each of the above M fluorides is used as the true density ρ of Li2MF6.

[0094] On the other hand, when there are multiple types of metal elements M, the ratio of each element constituting the metal element M can be calculated from the ICP analysis value, and the true density of the metal element M can be calculated. For example, when the metal element M is two elements (M1, M2), the true density can be calculated using the following (Equation 3). True density of metal element M = (ρ1W1 + ρ2W2) / (W1 + W2) ··· (Equation 3) Here, ρ1 is the true density of element M1, W1 is the mass % of element M1 obtained by ICP analysis, ρ2 is the true density of element M2, and W2 is the mass % of element M2 obtained by ICP analysis.

[0095] Also, when the metal element M is composed of three elements (M1, M2, M3), the true density can be calculated by the following (Equation 4). True density of metal element M = (ρ1W1 + ρ2W2 + ρ3W3) / (W1 + W2 + W3) ··· (Equation 4) Here, ρ1 is the true density of element M1, W1 is the mass % of element M1 obtained by ICP analysis, ρ2 is the true density of element M2, W2 is the mass % of element M2 obtained by ICP analysis, ρ3 is the true density of element M3, and W3 is the mass % of element M3 obtained by ICP analysis.

[0096] 3. Measurement and calculation method of the coating rate of the lithium-containing fluoride coating (1) Measurement conditions For the cathode active materials with surfaces coated with lithium-containing fluoride according to Examples 1-7 and Comparative Examples 1 and 3, XPS measurement was carried out under the following measurement conditions. <XPS measurement conditions> Measurement device: Narrow scan analysis was performed using JPS-9200S from JEOL under the following measurement conditions. X-ray source: Al tube target Output of X-ray source: 200 W Analysis angle: 45° Spot diameter Φ: 700 μm Step width: 0.1 eV Scan times: 5 times

[0097] (2) Calculation method of the coating rate Regarding the measurement data, analysis processing was performed using the JEOL analysis software "Product name SpecSurf". The background was processed using the Shirley method, and the peak area of ​​the spectrum for each element was calculated. The molar ratio of each element was calculated from the calculated peak area of ​​the spectrum for each element, and this was then substituted into the following (Equation 5) to calculate the coverage. However, the following peaks were used as the spectral species for each element. Co:2p3 / 2 orbital Mn:2p3 / 2 orbital Ni:2p3 / 2 orbital Ti:2p3 / 2 orbital Si:2p orbital Zr:3d5 / 2 orbit F :1s orbit Coverage (%) = 100 × (M + F) / (M + F + Ni + Mn + Co) (Equation 5)

Claims

1. A lithium-containing fluoride coating solution containing Li, F, and M (M is one or more metal elements capable of taking a tetravalent oxidation number), the molar ratio of Li:F:M of Li, F, and M contained in the lithium-containing fluoride coating solution is 2+x:1:6+y (-0.5≦x≦1, -0.5≦y≦1); The lithium-containing fluoride coating solution has a pH of 2.5 or more and 7.0 or less.

2. Li is 1.0 mass% or less, F is 7.5 mass% or less, 2. The lithium-containing fluoride coating solution according to claim 1, wherein M is contained in an amount of 3.5 mass % or less.

3. 2. The lithium-containing fluoride coating solution according to claim 1, wherein the pH is 4.0 or more and 7.0 or less.

4. 2. The lithium-containing fluoride coating solution according to claim 1, wherein the pH is 4.5 or more and 7.0 or less.

5. 5. The lithium-containing fluoride coating solution according to claim 1, wherein M is at least one metal element selected from the group consisting of Ti, Si, and Zr.

6. Li ２＋ｘ Midfielder ６＋ｙ (M is one or more metal elements capable of taking a tetravalent oxidation number, −0.5≦x≦1, −0.5≦y≦1) in water to form a solution, thereby obtaining a lithium-containing fluoride coating solution, A method for producing a lithium-containing fluoride coating solution, characterized in that the pH of the lithium-containing fluoride coating solution is set to 2.5 or more and 7.0 or less.

7. 7. The method for producing a lithium-containing fluoride coating solution according to claim 6, wherein the lithium-containing fluoride powder is produced by dry-mixing Li fluoride and M fluoride.

8. a step of dissolving a water-soluble Li salt and a water-soluble salt containing M and F in water to obtain an aqueous solution containing Li, F, and M and having a pH of 2.5 or more and 7.0 or less; drying the aqueous solution containing Li, F, and M to obtain a dry powder; 7. The method for producing a lithium-containing fluoride coating solution according to claim 6, wherein the lithium-containing fluoride powder is produced by a step of calcining the dried powder at 150° C. or more and 500° C. or less.

9. A lithium-containing fluoride-coated active material in which a positive electrode active material is coated with a lithium-containing fluoride, The lithium-containing fluoride contains Li, F, and M (M is one or more metal elements that can have a tetravalent oxidation number), the thickness of the lithium-containing fluoride coating is 5 nm or more and 40 nm or less; A lithium-containing fluoride-coated active material, characterized in that the coverage of the lithium-containing fluoride with respect to the positive electrode active material is 80% or more.

10. The lithium-containing fluoride is Li ２＋ｘ Midfielder ６＋ｙ 10. The lithium-containing fluoride coated active material according to claim 9, wherein M is one or more metal elements capable of taking a tetravalent oxidation number, −0.5≦x≦1, −0.5≦y≦1.

11. a lithium-containing fluoride coating solution containing Li, F, and M (M is one or more metal elements capable of taking a tetravalent oxidation number), wherein the molar ratio of Li, F, and M is 2+x:1:6+y (-0.5≦x≦1, -0.5≦y≦1), and a positive electrode active material is brought into contact with the lithium-containing fluoride coating solution to obtain a lithium-containing fluoride-coated active material in which the positive electrode active material is coated with the lithium-containing fluoride; The method for producing a lithium-containing fluoride coated active material, wherein the lithium-containing fluoride coating solution has a pH of 2.5 or more and 7.0 or less.

12. The lithium-containing fluoride coating solution is Li is 1.0 mass% or less, F is 7.5 mass% or less, 12. The method for producing a lithium-containing fluoride-coated active material according to claim 11, wherein M is contained in an amount of 3.5 mass % or less.

13. 12. The method for producing a lithium-containing fluoride coated active material according to claim 11, wherein the lithium-containing fluoride coating solution has a pH of 4.0 or more and 7.0 or less.

14. 12. The method for producing a lithium-containing fluoride coated active material according to claim 11, wherein the lithium-containing fluoride coating solution has a pH of 4.5 or more and 7.0 or less.

15. 15. The method for producing a lithium-containing fluoride-coated active material according to claim 11, wherein M is at least one metal element selected from the group consisting of Ti, Si, and Zr.

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