Fluoride powder, fluoride-coated positive electrode active material powder, and method for producing same

Abstract

[Problem] To provide a powder that comprises a lithium-ion conductive substance functioning as a protective material for a positive electrode active material, that has excellent withstand voltage characteristics, and that exhibits excellent uniformity of adhesion to positive electrode active material particles. [Solution] This fluoride powder contains, as the main ingredient, a lithium-containing metal fluoride including lithium (Li), a metal element M, and fluorine (F), and has a BET diameter d of not more than 300 nm as calculated using formula (1).　(1): d=6×103 / (ρ×S) Here, d represents the BET diameter (nm), ρ represents the true density (g / cm3) of the powder, and S represents the BET specific surface area (m2 / g) of the powder.

Description

Fluoride powder, fluoride-coated positive electrode active material powder, and methods for producing the same

[0001] The present invention relates to a fluoride powder useful as a coating material for a positive electrode active material for an all-solid-state lithium ion secondary battery, a fluoride-coated positive electrode active material powder using the fluoride powder for forming a coating layer, and methods for producing the same.

[0002] All-solid-state lithium-ion secondary batteries (hereinafter sometimes referred to as "all-solid-state batteries") have a problem in that the interfacial resistance between the positive electrode active material and the solid electrolyte, which is the separator, increases, and the battery's performance, such as capacity, tends to decrease. This increase in interfacial resistance is mainly caused by the reaction between the positive electrode active material and the solid electrolyte, which forms high-resistance regions on the surface of the positive electrode active material. Therefore, the surface of the positive electrode active material is covered with a lithium-ion conductive oxide (typically lithium niobate, LiNbO ３ Attempts have been made to prevent the positive electrode active material from reacting with the separator solid electrolyte by covering it with a protective solid electrolyte made of a compound such as ethylenediamine (ethylenediamine), propylene glycol (propylene glycol), or a halide.

[0003] Patent Document 1 discloses a method of mixing an aqueous lithium salt solution, fine particles of niobium oxide or niobium hydroxide, and positive electrode active material particles to form a slurry, and then subjecting the slurry to an evaporation-to-dryness method or a spray-drying method to obtain a positive electrode active material coated with lithium niobate.

[0004] However, in the case of a positive electrode active material using lithium niobate as a coating material, the charging voltage of the secondary battery is usually limited to about 4.45 V. There is a demand for the application of coating materials that can achieve a higher charging voltage.

[0005] Patent Document 2 describes the use of a halide containing Cl as a solid electrolyte protective material that covers at least a portion of the surface of a positive electrode active material. This type of halide has been shown to have a higher decomposition voltage than conventional lithium niobate. However, no examples of using fluoride as the halide have been shown.

[0006] On the other hand, Patent Document 3 discloses that Li is used as a negative electrode active material. ３ AlF ６However, a technique using Li ３ AlF ６ There is no description of using it as a coating material for a positive electrode active material.

[0007] JP 2018-67474 A International Publication No. 2020 / 174868 JP 2022-114320 A

[0008] To improve the charging voltage of all-solid-state batteries, it is important to use a material with excellent voltage resistance (i.e., the ability to resist rapid oxidative decomposition due to voltage increases) as a coating material to protect the positive electrode active material. It is also important to achieve a state in which the active material is exposed minimally on the surface of the positive electrode active material, and the entire surface of the active material particles is covered as uniformly as possible with the coating material (this state is referred to as "high coating uniformity" in this specification). Poor coating uniformity leads to greater variation in ionic conductivity at the interface between the positive electrode active material and the solid electrolyte separator, which in turn increases battery resistance and reduces battery performance, such as capacity.

[0009] The technology of Patent Document 1 claims that by using a fine raw material powder (niobium oxide or niobium hydroxide) with a particle diameter of 200 nm or less, it is possible to completely cover the surface of the positive electrode active material with lithium niobate (paragraph 0032). However, there is no known method for producing a fine powder with a particle diameter on the submicron order using a coating material with better voltage resistance characteristics than lithium niobate.

[0010] The halide disclosed in Patent Document 2 has better voltage resistance characteristics than lithium niobate. However, Patent Document 2 employs a method for synthesizing the halide, in which raw materials are pulverized using a planetary ball mill. This dry pulverization method causes adhesion and aggregation of primary particles, making it difficult to obtain fine coating material powder on the submicron order. Therefore, there is still room for improvement in order to realize a positive electrode active material with high coating uniformity.

[0011] The present invention aims to provide a powder of a lithium ion conductive material that functions as a protective material for a positive electrode active material, which has excellent voltage resistance characteristics and is useful for coating positive electrode active material particles with high uniformity. Another object of the present invention is to provide a positive electrode active material powder that is coated with a protective material with high uniformity.

[0012] In order to achieve the above object, the present specification discloses the following invention: [1] A powder containing, as a main component, a lithium-containing metal fluoride containing Li, one or more metal elements M selected from the group consisting of Al, Ga, Fe, Cr, and Y, and F, wherein the fluoride powder has a BET diameter d of 300 nm or less according to the following formula (1): d = 6 × 10 ３ / (ρ×S) (1) where d is the BET diameter (nm), ρ is the true density of the powder (g / cm ３ ), S is the BET specific surface area of ​​the powder (m ２ / g). [2] The fluoride powder according to the above [1], wherein the average primary particle size measured from a scanning electron microscope image is 400 nm or less. [3] The fluoride powder according to the above [1] or [2], wherein the coefficient of variation of the primary particle size measured from a scanning electron microscope image is 0.40 or less. [4] The fluoride powder according to any one of the above [1] to [3], wherein the fluoride powder has a composition in which the Li / M molar ratio is 2.0 or more and 6.0 or less, and the F / M molar ratio is 5.0 or more and 9.0 or less. [5] The lithium-containing metal fluoride as a main component contains Al as the metal element M and β-Li ３ AlF ６ It has a β-Li type crystal structure, and in the X-ray diffraction pattern using Cu-Kα radiation with the horizontal axis being 2θ (°), ３ AlF ６ The fluoride powder according to any one of [1] to [4] above, wherein the half width of the diffraction peak of the (222) plane of the crystalline structure is 0.2° or more and 1.0° or less.

[0013] [6] A method for producing a fluoride powder, comprising: a reaction step of generating a solid substance mainly composed of a lithium-containing metal fluoride by stirring an aqueous solution containing Li, one or more metal elements M selected from the group consisting of Al, Ga, Fe, Cr, and Y, and F, at a pH of 4.0 to 9.5, a liquid temperature of 5°C or higher, and under conditions satisfying the following formula (2): T≦−2.5×pH+50 (2) where T is the liquid temperature (°C), and pH is the pH value of the liquid. [7] A method for producing a fluoride powder according to [6] above, wherein the aqueous solution has a Li / M molar ratio of 2.0 to 6.0 and a F / M molar ratio of 5.0 to 9.0. [8] The lithium-containing metal fluoride contains Al as the metal element M and is β-Li ３ AlF ６ It has a β-Li type crystal structure, and in the X-ray diffraction pattern using Cu-Kα radiation with the horizontal axis being 2θ (°), ３ AlF ６ The method for producing a fluoride powder according to the above [6] or [7], wherein the half-width of the diffraction peak of the (222) plane of the crystalline structure is 0.2° or more and 1.0° or less. [9] The method for producing a fluoride powder according to any one of the above [6] to [8], further comprising a firing step of firing the dried powder of the solid substance recovered in the solid-liquid separation step at a temperature of 130°C or more and 500°C or less.

[0014]

[10] A powder consisting of positive electrode active material particles having a coating layer on the surface, wherein the coating layer is composed of a substance whose main component is a lithium-containing metal fluoride containing Li, one or more metal elements M selected from the group consisting of Al, Ga, Fe, Cr, and Y, and F, with a Li / M molar ratio of 2.0 to 6.0 and an F / M molar ratio of 5.0 to 9.0, and wherein the fluoride-coated positive electrode active material powder has a coverage of 70% or more according to the following formula (3) in atomic ratios detected by irradiating the outermost surface of the powder particles with X-rays using X-ray photoelectron spectroscopy (XPS): Coverage (%) = 100 × I Ａ / (I Ａ +I Ｂ ) ... (3) where I ＡI: Total detected amount (molar equivalent) of elements other than Li constituting the lithium-containing metal fluoride, which is the main component of the coating layer, excluding "elements constituting the positive electrode active material" Ｂ

[11] The fluoride-coated positive electrode active material powder according to

[10] above, wherein the average film thickness of the coating material is 10 nm or more and 200 nm or less.

[12] The lithium-containing metal fluoride, which is the main component, contains Al as the metal element M and β-Li ３ AlF ６ The fluoride-coated positive electrode active material powder according to the above

[10] or

[11] , which has a fluoride-coated positive electrode active material powder having ...

[0015]

[13] A method for producing a fluoride-coated positive electrode active material powder, comprising: a step of mixing the fluoride powder according to any one of the above [1] to [5] and a positive electrode active material powder in a liquid medium to obtain a slurry; and a step of spray-drying the slurry to obtain a coated powder consisting of particles of the positive electrode active material coated with a constituent substance of the fluoride powder.

[0016] According to the present invention, a new powder of a coating material (coating material powder) has been realized that has excellent voltage resistance characteristics and excellent uniformity of adhesion to positive electrode active material particles. The positive electrode active material, which is coated with a protective material with high uniformity and obtained by using this powder to form a coating layer, is advantageous for improving the performance of all-solid-state batteries, such as improving the charging voltage and improving the resistance to deterioration of battery capacity.

[0017] FIG. 1 is a diagram illustrating X-ray diffraction patterns of the fluoride powders (coating materials) obtained in Examples 1 to 7 and Comparative Examples 1 to 3. FIG. 2 is a cross-sectional view schematically showing the layered structure of an electrochemical cell prepared to evaluate voltage resistance characteristics. FIG. 3 is a graph illustrating potential-current curves measured using an electrochemical cell. FIG. 4 is an example of an SEM photograph of the fluoride powder obtained in Example 1. FIG. 5 is an example of an SEM photograph of the fluoride powder obtained in Example 2. FIG. 6 is an example of an SEM photograph of the fluoride powder obtained in Example 3. FIG. 7 is an example of an SEM photograph of the fluoride powder obtained in Example 4. FIG. 8 is an example of an SEM photograph of the fluoride powder obtained in Example 5. FIG. 9 is an example of an SEM photograph of the fluoride powder obtained in Example 6. FIG. 10 is an example of an SEM photograph of the fluoride powder obtained in Example 7. FIG. 11 is an example of an SEM photograph of the fluoride powder obtained in Comparative Example 1. FIG. 12 is an example of an SEM photograph of the fluoride powder obtained in Comparative Example 2. FIG. 13 is an example of an SEM photograph of the fluoride powder obtained in Comparative Example 3. 10 is an example of an SEM photograph of a fluoride-coated positive electrode active material powder in which a coating layer is formed using the fluoride powder of Example 4.

[0018] [Fluoride Powder] Research by the inventors has revealed that a solid substance containing as its main component a "lithium-containing metal fluoride," which is a lithium ion conductive compound containing Li, one or more metal elements M selected from the group consisting of Al, Ga, Fe, Cr, and Y, and F, exhibits superior voltage resistance characteristics compared to lithium niobate. Furthermore, it has been found that a fine powder made of this substance (the fluoride powder of the present invention) is extremely useful for forming a protective coating with high deposition uniformity when used as a coating material for a positive electrode active material.

[0019] The lithium-containing metal fluoride may be, for example, a compound having the composition formula Li ｘ Midfielder ３＋ｘ (where x is 2.0 or more and 6.0 or less) are suitable. Metal elements M such as Al, Ga, Fe, Cr, and Y have lithium ion conductivity (for example, lithium ion conductivity of 1×10 －１１S / cm or more) can be constructed, and the band gap can be increased, which is effective in constructing a coating material in which electrons are less likely to move (i.e., excellent voltage resistance characteristics). Two or more metal elements may be used as M in combination. Among these metal elements M, Al, Y, and Ga are preferably used because the raw materials from which they are supplied have relatively low solubility in water and are less susceptible to hydrolysis. Furthermore, among Al, Y, and Ga, Al is particularly preferred because it has the greatest effect of increasing the band gap.

[0020] The fluoride powder of the present invention may contain phases other than the lithium-containing metal fluoride (referred to as heterogeneous phases) as long as they do not impede the achievement of the object of the present invention, and elements other than Li and the metal elements M and F may be detected. However, it is desirable that the heterogeneous phases, which are impurities, are as small as possible. The composition of the fluoride powder of the present invention, which contains the above-mentioned "lithium-containing metal fluoride" as a main component, is represented by the preferred composition formula Li ｘ Midfielder ３＋ｘ Similar to the range of (x is 2.0 or more and 6.0 or less), it is preferable that the Li / M molar ratio is 2.0 or more and 6.0 or less, and the F / M molar ratio is 5.0 or more and 9.0 or less. The Li / M molar ratio of the fluoride powder of the present invention may be 2.2 or more, or 5.5 or less, or 5.0 or less. The F / M molar ratio of the fluoride powder of the present invention may be 8.5 or less, or 8.0 or less. Furthermore, elements contained other than Li, the metal element M, and F include, for example, N, H, and O. On the other hand, using B as a contained element in the fluoride powder of the present invention may affect stability at high voltage. The B content in the fluoride powder of the present invention is preferably 0.1 mass% or less. The total content of Li, the metal element M, and F in the fluoride powder of the present invention is preferably 88 mass% or more, more preferably 89 mass% or more, and even more preferably 90 mass% or more.

[0021] The term "powder containing a lithium-containing metal fluoride as a main component" means that the component that is most abundant among the compound components that make up the powder is the lithium-containing metal fluoride. In the X-ray diffraction pattern of the powder, the peak height of the diffraction peak with the highest peak height among the diffraction peaks attributable to the lithium-containing metal fluoride is defined as h. ０ The peak height of the highest diffraction peak among the diffraction peaks due to the different phases (crystal phases other than the lithium-containing metal fluoride) is defined as h １ When this is done, h ０ >h １ If the relationship between the above holds, the powder is deemed to be a "powder containing lithium-containing metal fluoride as the main component." If no other phase is detected, １ = 0, and the above h ０ >h １ In addition, for a peak in which diffraction peaks from multiple crystal planes are superimposed, the diffraction intensity corresponding to the peak position of each crystal plane in the peak shape after superposition is used as the diffraction peak height of each crystal plane.

[0022] A preferred example of the lithium-containing metal fluoride is a lithium-containing metal fluoride containing Al as the metal element M and β-Li ３ AlF ６ Examples of such crystal structures include those having a crystalline structure.

[0023] The fluoride powder of the present invention is composed of fine particles. Specifically, it is specified as a powder having a BET diameter d of 300 nm or less according to the following formula (1): d = 6 × 10 ３ / (ρ×S) (1) where d is the BET diameter (nm), ρ is the true density of the powder (g / cm ３ ), S is the BET specific surface area of ​​the powder (m ２ / g). The BET specific surface area is determined by the BET one-point method. A fluoride powder having a small BET diameter d as described above is extremely useful in obtaining a positive electrode active material powder having a coating layer with high uniformity when a slurry is formed together with the positive electrode active material powder and then a coating layer (film) derived from the compound constituting the fluoride powder is formed on the surface of the positive electrode active material particles by, for example, a spray drying method. The BET diameter d is more preferably 250 nm or less, even more preferably 150 nm or less, and even more preferably 100 nm or less. The lower limit of the BET diameter d is not particularly limited, but is usually 10 nm or more. The true density ρ of the powder is determined by the Li ３ Midfielder ６ The calculated true density ρ value can be applied.

[0024] (Li ３ Midfielder ６ As described above, the metal element M is one or more elements selected from the five elements Al, Ga, Fe, Cr, and Y. When the metal element M is only one of the above elements, the true density ρ value (g / cm) of the M fluoride is determined as follows: ３ ) to Li ３ Midfielder ６ The true density ρ of the metal element M is used as the true density ρ of the metal element M fluoride. ３ AlF ６ 2.84 g / cm ３ Ga Li ３ GaF ６ 3.43 g / cm ３ Fe Li ３ FeF ６ 3.16 g / cm ３ Cr Li ３ CrF ６ 3.13 g / cm ３ Y Li ３ YF ６ 2.60 g / cm ３ When there are two kinds of metal elements M, the true density ρ value (g / cm ３ ) to Li ３ Midfielder ６The true density ρ is adopted as the true density ρ. １ ×W １ +ρ ２ ×W ２ ) / (W １ +W ２ ) ... (4) where ρ １ and ρ ２ are the true density ρ values ​​(g / cm) of the M fluorides for the first and second M elements, respectively. ３ ) W １ and W ２ are the contents (mass%) of the first and second M fluorides, respectively, determined by ICP-OES (inductively coupled plasma optical emission spectroscopy) of the powder. For example, when the first M element is Al and the second M element is Fe, ρ １ is 2.84 g / cm ３ , ρ ２ is 3.16 g / cm ３ and W １ Li ３ AlF ６ Content (mass%) of W ２ Li ３ FeF ６ When the number of metal elements M is three or more, the true density ρ (g / cm 3 ) is calculated by the following formula (5), where n is the number of metal elements M (n is an integer of 3 or more and 5 or less). ３ ) to Li ３ Midfielder ６ The true density ρ is adopted as the true density ρ. １ ×W １ +...+ρ ｎ ×W ｎ ) / (W １ +...+W ｎ ) ... (5) where ρ ｉ (i is an integer of 1 or more and n or less) is the true density ρ value (g / cm) of the M fluoride for the i-th M element. ３ ) W ｉ is the content (mass%) of the i-th M fluoride of the powder determined based on ICP-OES.

[0025] The fact that the fluoride powder of the present invention is composed of fine particles can also be confirmed by the average primary particle diameter measured from a scanning electron microscope (SEM) image of the fluoride powder. Specifically, from the viewpoint of obtaining a positive electrode active material powder having a coating layer with high uniformity (e.g., a coverage rate of 70% or more, as described below), the average primary particle diameter is preferably 400 nm or less, more preferably 300 nm or less, and even more preferably 200 nm or less, in terms of the average maximum diameter in a specific direction (e.g., the horizontal direction of the SEM image). There is no particular lower limit to the average primary particle diameter, but considering the cost and ease of handling during production, the average primary particle diameter is preferably 10 nm or more, more preferably 30 nm or more.

[0026] Furthermore, the smaller the variation in the primary particle size of the fluoride powder, the more advantageous it is for obtaining a positive electrode active material powder having a coating layer with high deposition uniformity. Specifically, the coefficient of variation of the primary particle size based on the maximum diameter in a specific direction (e.g., the horizontal direction of the SEM image) measured from a scanning electron microscope (SEM) image is preferably 0.40 or less, more preferably 0.35 or less, and even more preferably 0.30 or less. There is no particular lower limit to the coefficient of variation, but considering the manufacturing cost, it is preferably 0.10 or more. The coefficient of variation of the primary particle size is expressed by the following formula: Coefficient of variation = Standard deviation σ of the distribution of primary particle size (nm) / Average primary particle size (nm)

[0027] When the lithium-containing metal fluoride, which is the main component of the powder, is crystalline, a smaller crystallite size is advantageous for improving the uniformity of adhesion to the positive electrode active material, while a larger crystallite size is advantageous for improving electrochemical stability and voltage resistance characteristics. The crystallite size can be evaluated by the half-width of the diffraction peak in the X-ray diffraction pattern. In general, the smaller the crystallite size, the larger the half-width, and the larger the crystallite size, the smaller the half-width.

[0028] β-Li ３ AlF ６ For example, in the case of a powder mainly composed of a compound having a crystalline structure, β-Li ３ AlF６ The half-width of the (222) plane of the β-Li crystal structure can be used. ３ AlF ６ As shown in ICDD card 01-088-0860, the crystal belongs to the monoclinic space group C2 / c, and has a crystal structure in which a diffraction peak from the (222) plane is observed around 2θ=31.2° in the X-ray diffraction pattern using Cu-Kα radiation. This diffraction peak from the (222) plane is not accompanied by diffraction peaks from other crystal planes or heterophases (e.g., LiF ３ , AlF ３ , Al ２ (OH) ３ F ３ ) and therefore is suitable for evaluating the half-width. ３ AlF ６ In the case of powders whose main component is a compound having a β-Li type crystal structure, the balance between coating uniformity and voltage resistance characteristics is important. ３ AlF ６ The half-width of the diffraction peak of the (222) plane of the crystalline structure is preferably 0.2° or more and 1.0° or less, and more preferably 0.3° or more and 0.8° or less.

[0029] [Method for producing fluoride powder] The fluoride powder composed of fine particles mainly composed of the above-mentioned lithium-containing metal fluoride can be synthesized by a coprecipitation reaction in an aqueous solution. In this case, it is important to appropriately control the pH and the solution temperature during the reaction. Specifically, the following steps can be adopted.

[0030] (Mixing Step) An aqueous solution A containing F (fluorine) and an aqueous solution B containing Li (lithium) and one or more of the metal elements M are prepared. The compositions of these aqueous solutions are preferably such that, in the total amounts of aqueous solutions A and B used for mixing, the Li / M molar ratio is 2.0 to 6.0 and the F / M molar ratio is 5.0 to 9.0. The Li / M molar ratio is more preferably 2.2 to 5.5 or 5.0, and the F / M molar ratio is more preferably 8.5 or 8.0. Examples of raw materials that can be used as the F source include ammonium fluoride. Examples of raw materials that can be used as the Li source include lithium nitrate. Examples of raw materials that can be used as the metal element M source include water-soluble metal salts, metal oxides, and metal hydroxides. When the metal element M is Al, aluminum nitrate nonahydrate is a suitable raw material.

[0031] Next, the aqueous solution A and the aqueous solution B are mixed to obtain an aqueous solution containing dissolved Li, one or more metal elements M selected from the group consisting of Al, Ga, Fe, Cr, and Y, and F. At this time, it is preferable to quickly mix the entire amount of the aqueous solution A and the entire amount of the aqueous solution B to prevent uneven pH in the mixed solution.

[0032] In order to produce a lithium-containing metal fluoride consisting of fine particles, it is particularly effective to adjust the pH of an aqueous solution A containing F to a high value in advance, and then mix the aqueous solutions A and B to obtain a mixed solution having a lower pH than the pH of the aqueous solution A before mixing. When this method is adopted, the pH of the aqueous solution A before mixing is preferably adjusted to 5.5 or higher, more preferably 8.0 or higher. In this case, ammonia (NH ３ ) water can be used. As a mixing procedure, it is preferable to add the entire amount of the Li- and M-containing aqueous solution B all at once in a short time to the F-containing aqueous solution A adjusted to a predetermined pH value. The all at once addition is not dropwise or intermittent addition, but an addition mode in which the entire amount of the liquid to be added is added as quickly as possible without interruption within the addition time range that allows safe mixing depending on the scale of the apparatus.

[0033] The pH referred to in this specification is measured using a pH meter calibrated with an appropriate buffer solution depending on the pH range to be measured, based on JIS Z8802: 2011. The pH values ​​described in this specification are values ​​obtained by directly reading the measurement value indicated by the pH meter compensated by a temperature compensation electrode under the temperature conditions of the liquid.

[0034] (Reaction Step) The aqueous solution obtained in the mixing step is stirred to produce a solid substance. Stirring is performed under conditions of pH 4.0 to 9.5, a liquid temperature of 5°C or higher, and satisfying the following formula (2). If a mixed solution exhibiting a lower pH than that of aqueous solution A before mixing is obtained in the mixing step, it is effective to stir the solution under conditions of a pH lower than that of aqueous solution A before mixing, pH 4.0 to 9.5, a liquid temperature of 5°C or higher, and satisfying the following formula (2): T≦−2.5×pH+50 (2), where T is the liquid temperature (°C), and pH is the pH value of the solution. Deviation from the condition of "pH 4.0 to 9.5 and satisfying the following formula (2)" may result in a decrease in the degree of supersaturation of the target lithium-containing metal fluoride, which may result in difficulty in synthesizing fine particles. Furthermore, a liquid temperature below 10°C slows the reaction rate, which is disadvantageous in terms of productivity. It is more preferable to maintain the liquid temperature at 10°C to 35°C, and it may also be controlled at 10°C to 30°C. The stirring time (reaction time) for allowing the reaction to proceed is preferably set within a range of, for example, 25 minutes to 60 minutes. The pH and temperature in the reaction step are measured 5 minutes after starting stirring of the aqueous solution in which Li, one or more metal elements M selected from the group consisting of Al, Ga, Fe, Cr, and Y, and F are dissolved.

[0035] In this way, a solid substance consisting of fine particles containing lithium-containing metal fluoride as a main component can be obtained. According to the method of the present invention, the particles can be made fine without the need for a dispersant to prevent aggregation of the particles, which has the secondary effect of preventing the inclusion of impurities derived from the dispersant.

[0036] (Solid-liquid separation step) Next, solid-liquid separation is carried out to recover the solid material obtained in the reaction step. As the solid-liquid separation method, known techniques such as pressure filtration and vacuum filtration can be used.

[0037] (Caustic Treatment) To further improve the voltage resistance characteristics of a crystalline material primarily composed of a lithium-containing metal fluoride, it is effective to calcinate the dried powder obtained by removing the water solvent from the solid material recovered in the solid-liquid separation process. Calcination can be performed by heat treatment at a temperature range of 130°C to 500°C. The holding time in this temperature range can be, for example, 0.5 hours to 6 hours. The calcination atmosphere can be air. Since higher calcination temperatures tend to increase the crystallite size, the optimal calcination temperature is set based on the balance between uniformity of deposition on the positive electrode active material and voltage resistance characteristics (electrochemical stability). For example, when uniformity of deposition is important, it is effective to maintain the temperature at a relatively low temperature, such as 350°C or less or 300°C or less. Even if this calcination process is omitted, improved voltage resistance characteristics can be expected by calcining after coating the positive electrode active material.

[0038] [Method for producing fluoride-coated positive electrode active material powder] The fluoride powder of the present invention obtained by the above method is extremely useful as a protective material for coating the surface of positive electrode active material particles with high uniformity. A suitable method for producing fluoride-coated positive electrode active material powder using the fluoride powder of the present invention is a process that combines mixed slurry formation and spray drying. This process is described below.

[0039] As the positive electrode active material powder, known materials can be used, and if a new positive electrode active material is developed, it can also be used. A representative known positive electrode active material is LiCoO ２ (LCO type), LiNiO ２ (LNO type), LiMn ２ O ４ (LMO type), LiNiCoAlO ２ (NCA type), LiNiCoMnO ２ (NCM type), Li ２ MnO ３ -LiNiCoMnO ２ (solid solution type), LiNiMnO ４ (spinel type), LiMnFePO ４(phosphate type), Li ２ FeSiO ４ The positive electrode active material powder may have a cumulative 50% particle diameter D50 of 5.0 μm or more and 20.0 μm or less, more preferably 5.0 μm or more and 10.0 μm or less, in a volume-based particle size distribution measured by a laser diffraction / scattering method.

[0040] (Formation of Mixed Slurry) The above-described fluoride powder and positive electrode active material powder are mixed in a liquid medium to form a mixed slurry. Water can be used as the liquid medium. The particles constituting this slurry are considered to be composite particles in which fine fluoride particles are attached to the surfaces of active material particles through hetero-coagulation. Since the fluoride powder according to the present invention is composed of extremely fine particles, it is presumed that the positive electrode active material particles are uniformly covered with a thin particle layer of fluoride particles. The mixing ratio of the fluoride powder and the positive electrode active material powder is set to a quantity ratio sufficient to cover the entire surface of the positive electrode active material particles with fluoride particles. The amount of water, which is the liquid medium, may be set, for example, in the range of 0.3 to 1.0 times the total mass of the fluoride powder and the positive electrode active material powder.

[0041] (Spray Drying) The above slurry is spray-dried. During spray drying, particles constituting the slurry collide with each other in a high-speed airflow, and the collision shear force deforms the fluoride particles attached to the surface of the active material particles, forming a coating layer made of the constituent materials of the fluoride powder on the surface of the active material particles. Typically, the average thickness of this coating layer is thinner than the BET diameter d of the fluoride particles before spray drying. A thin average coating layer can be advantageous in ensuring electrical conduction (electron transfer) between the positive electrode active material and the conductive additive, so the average thickness is preferably, for example, 10 nm to 200 nm, and more preferably, a thin coating layer of 10 nm to 50 nm is formed. Note that, instead of spray drying, a fluoride-coated positive electrode active material powder having a highly uniform coating layer can also be obtained by subjecting the particles constituting the above mixed slurry to a compression shear treatment using another method.

[0042] (Method of determining the average film thickness of the coating layer) The average film thickness of the fluoride coating layer is determined by the above-mentioned method. ３ Midfielder ６ True density ρ (g / cm ３ ) and the F (fluorine) content (mass%) based on IC (ion chromatography) analysis of the fluoride-coated positive electrode active material powder, as follows: When the measured F content in the fluoride-coated positive electrode active material powder is A (mass%), the mass proportion B (mass%) of the coating layer is expressed by the following formula: B = A × [Li ３ Midfielder ６ [molecular weight of F] / ([atomic weight of F]×6) The BET specific surface area of ​​the positive electrode active material is defined as Sp (m ２ / g), Li ３ Midfielder ６ The true density of ρ (g / cm ３ ), the average film thickness t (nm) of the coating layer is expressed by the following formula: t = 10 × B / (Sp × ρ) Here, IC analysis to determine the F content A (mass%) can be performed by the following procedure. 0.02 g of a sample of fluoride-coated positive electrode active material powder is weighed out, and 10 mL of a 1 mol / L NaOH aqueous solution is added and thermally decomposed. The decomposed solution is allowed to cool, and then dilute sulfuric acid is added to make it acidic and decompose the insoluble residue. 10 mL of a 1 mol / L NaOH aqueous solution is added again, and the solution is heated to confirm that no insoluble residue remains. The decomposed solution is diluted 20 times in a 100 mL measuring flask. The F concentration in the obtained diluted solution is measured by IC analysis, and the F content (mass%) in the fluoride-coated positive electrode active material powder is calculated based on the measured F concentration value.

[0043] (Firing) The fluoride-coated positive electrode active material powder obtained as described above can be fired as necessary to densify the coating layer. The firing temperature may be set within the same temperature range as that used for firing the fluoride powder described above, and within a temperature range in which the performance of the positive electrode active material does not deteriorate.

[0044] [Fluoride-Coated Positive Electrode Active Material Powder] In this manner, a fluoride-coated positive electrode active material powder having a highly uniform coating layer can be obtained. This fluoride-coated positive electrode active material powder is specified, for example, as follows: The fluoride-coated positive electrode active material powder is a powder consisting of positive electrode active material particles whose surfaces are coated with a substance having a composition containing a lithium-containing metal fluoride containing Li (lithium), one or more metal elements M, and F (fluorine) as a main component, with a Li / M molar ratio of 2.0 to 6.0 and a F / M molar ratio of 5.0 to 9.0, and the fluoride-coated positive electrode active material powder has a coverage of 70% to 100% according to the following formula (3) in atomic ratios detected by irradiating the outermost surface of the powder particles with X-rays using X-ray photoelectron spectroscopy (XPS): Coverage (%) = 100 × I Ａ / (I Ａ +I Ｂ ) ... (3) where I Ａ I: Total detected amount (molar equivalent) of elements constituting the lithium-containing metal fluoride, which is the main component of the coating layer, excluding "elements contained in the positive electrode active material and Li" Ｂ : The total detected amount (in moles) of elements constituting the positive electrode active material, excluding "Li and O, the elements constituting the lithium-containing metal fluoride that is the main component of the coating layer"

[0045] The coverage rate calculated by the above formula (3) is an index for evaluating the "uniformity of coverage" of the coating layer formed on the fluoride-coated positive electrode active material powder. In XPS, elements present in the surface layer from the outermost surface of the sample to a depth of several nanometers are detected. The more areas where the coating layer is very thin, less than several nanometers (hereinafter referred to as "coverage-deficient areas"), the more the amount of elements detected that constitute only the positive electrode active material increases (i.e., the above I Ｂbecomes larger), so the coverage rate according to formula (3) decreases. Since the protective function of the positive electrode active material decreases in areas where the coverage is deficient, a positive electrode active material powder with many areas where the coverage is deficient is considered to have a high risk of increasing the interface resistance in an all-solid-state battery. Therefore, the higher the coverage rate according to formula (3), the better the coating uniformity and the more advantageous it is in ensuring stable protective function. When using a fluoride powder (coating material) containing a lithium-containing metal fluoride as a main component as defined in the present invention, the coverage rate according to formula (3) is preferably 70% or more and 100% or less, and more preferably 80% or more and 100% or less.

[0046] [Example 1] (Preparation of fluoride powder) Ammonium fluoride NH ４ F (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) 23.3 g, 28 mass % ammonia (NH ３ 0.07 g of water (manufactured by Nacalai Tesque, Inc.) was mixed with 297.5 g of pure water under stirring to completely dissolve the water, thereby obtaining an aqueous solution A having a pH of 6.1. ３ (Fujifilm Wako Pure Chemical Industries, Ltd.) 21.7 g, aluminum nitrate nonahydrate Al(NO ３ ) ３ ・9H ２ 39.4 g of PEG-400 (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was mixed with stirring in 552.5 g of purified water until it was completely dissolved, thereby obtaining an aqueous solution B.

[0047] The entire amount of aqueous solution B was added all at once in about 5 seconds to a container containing the entire amount of aqueous solution A, to obtain a mixed solution. During mixing, both aqueous solutions A and B were adjusted to the same temperature as the liquid temperature during the reaction (the same applies to the following Examples and Comparative Examples 2 and 3).

[0048] The above mixture was stirred for 30 minutes while maintaining the liquid temperature at 35°C, to obtain a slurry. The pH during the reaction was 4.4. The right-hand side of the above equation (2) is -2.5 x 4.4 + 50 = 39.0. The pH and liquid temperature conditions in this example satisfy equation (2) (the same applies to each example described below).

[0049] The obtained slurry was subjected to solid-liquid separation by pressure filtration to recover the solid material. The cake of this solid material was vacuum dried at 120°C for 12 hours to obtain a dry powder, which was then calcined in air at 200°C for 2 hours to obtain a fluoride powder. The obtained fluoride powder was used as a test material for the following investigations.

[0050] (Composition Analysis) Li and Al were analyzed using the following method. 0.05 g of a powder sample of the test material was weighed out, and 5 mL of 36% by mass hydrochloric acid was added and thermally decomposed. The decomposed solution was diluted 10-fold in a 100 mL measuring flask. The Li and Al concentrations in the diluted solution were measured using an inductively coupled plasma optical emission spectrometer (ICP-720, manufactured by Agilent Technologies). F was analyzed using the following method. 0.02 g of a powder sample of the test material was weighed out, and 10 mL of 1 mol / L NaOH solution was added and thermally decomposed. The decomposed solution was allowed to cool, and then diluted sulfuric acid was added to make it acidic and decompose the insoluble residue. 10 mL of 1 mol / L NaOH solution was added again, and the solution was heated to confirm that no insoluble residue remained. The decomposed solution was diluted 20-fold in a 100 mL measuring flask. The F concentration in the diluted solution was measured using ion chromatography (IC-2010, manufactured by Tosoh Corporation). The composition of the fluoride powder based on the analytical results is shown in Table 2 (same for each example).

[0051] (BET diameter) The BET specific surface area of ​​the powder sample used as the test material was measured using a BET specific surface area measuring device (Yuasa Ionics Co., Ltd., 4Sorb US). The powder was degassed by flowing nitrogen gas at 105°C for 20 minutes, and then purged with a mixed gas of nitrogen and helium (N ２ The BET diameter d was calculated from the measured BET specific surface area using the following formula (1): d = 6 × 10 ３ / (ρ×S) (1) where d is the BET diameter (nm), ρ is the true density of the powder (g / cm ３ ), S is the BET specific surface area of ​​the powder (m ２ / g). Here, the ρ value is Li ３ AlF ６ True density of 2.84 g / cm ３The BET diameter of the fluoride powder obtained in this example was 212 nm.

[0052] (Average Primary Particle Diameter and Coefficient of Variation) The powder sample as the test material was observed using a scanning electron microscope (JSM-7200F, manufactured by JEOL Ltd.) at an acceleration voltage of 3 kV to obtain an SEM image. The magnification was 30,000 times in this example and each example described later, and 10,000 times in each comparative example described later. Forty primary particles were randomly selected from the obtained SEM image. The maximum diameter in a specific direction (horizontal) was determined for these particles, and the data for the 10 smallest particle diameters were removed from the obtained data, and the average value of the remaining 30 data was used as the average primary particle diameter of the powder sample. The coefficient of variation of the primary particle diameter was also determined from the remaining 30 data. ImageJ, an image processing software, was used to determine the maximum diameter in a specific direction. The average primary particle diameter of the fluoride powder obtained in this example was 170 nm, and the coefficient of variation was 0.32.

[0053] (X-ray diffraction) Using an X-ray diffractometer (RIGAKU Corporation, Ultima IV), the X-ray diffraction pattern of the powder sample was measured under the following conditions: Cu-Kα radiation, tube voltage: 40 kV, tube current: 40 mA, divergence slit: 1 / 2°, scattering slit: 8 mm, light receiving slit: open, step width: 0.02° / step, scan speed: 0.666667. As a result, the sample material in this example was found to be β-Li ３ AlF ６ It was confirmed that the main component was a lithium-containing metal fluoride having a type fluoride structure (this was the case for each example except for Comparative Example 1). Figure 1 shows an example of the X-ray diffraction pattern of the obtained fluoride powder (this was the case for Examples 2 to 7 and Comparative Examples 1 to 3).

[0054] In addition, β-Li observed around 2θ = 31.2° ３ AlF ６ For the diffraction peak of the (222) crystal plane of the type structure, Kα ２ Kα with lines removed １Based on the measured data by X-ray, the half width of the (222) diffraction peak was determined using X-ray diffraction pattern analysis software (PDXL2, manufactured by RIGAKU Corporation) under the condition that the σ cut value, which is the threshold value between the background and the peak, was set to 3.0. As a result, the half width of the (222) diffraction peak of the sample material in this example was 0.30°. The peak height of the diffraction peak with the highest peak height among the diffraction peaks attributed to the lithium-containing metal fluoride was determined as h ０ The peak height of the highest diffraction peak among the diffraction peaks due to the different phase (crystal phase other than the lithium-containing metal fluoride) is defined as h １ When this is the case, the peak intensity ratio h １ / h ０ was 0.2.

[0055] (Evaluation of voltage resistance characteristics) An all-solid-state electrochemical cell was fabricated in which a solid electrolyte (first solid electrolyte) made of a test material powder and a sulfide-based solid electrolyte (second solid electrolyte) were adjacent to each other, and the current generated due to the oxidation reaction of the test material was investigated. Specifically, the experiment was carried out as follows.

[0056] An electrochemical cell having the stacked structure shown in FIG. 2 was fabricated as follows. An argyrodite-type sulfide (Li ６ P.S. ５Cl) was prepared. 57 mg of this sulfide was placed in an insulating outer cylinder (inner diameter 9.5 mm) made of alumina and pressed at 80 MPa to form a second solid electrolyte layer. Next, 20 mg of the test material powder was placed on top of the second solid electrolyte layer and pressed at 80 MPa to form a first solid electrolyte layer. Next, 20 mg of a composite material obtained by mixing the test material powder and stainless steel (SUS316) powder in a mortar at a volume ratio of 50:50 was placed on top of the first solid electrolyte layer and pressed at 360 MPa to form a composite layer. Next, a 200 μm thick sheet of metallic In (indium), a 300 μm thick sheet of metallic Li, and a 200 μm thick sheet of metallic In were stacked in this order under the second solid electrolyte layer (the side opposite to the first solid electrolyte), and the stacked layers were press-molded at a pressure of 80 MPa to form a counter electrode layer having an In / Li / In three-phase structure. Next, current collectors made of stainless steel (SUS316) plates were placed on the composite layer and under the counter electrode layer, and current collecting leads were attached to each current collector.

[0057] The electrochemical cell prepared as described above was placed in a thermostatic chamber at 25 ° C., and the composite layer side was used as the working electrode, which was the high-potential electrode, and the counter electrode member layer side was used as the counter electrode, which was the low-potential electrode. The voltage was swept from the open circuit voltage of the electrochemical cell to 5.0 V vs. Li (synonymous with 4.4 V vs. In-Li) at a sweep rate of 1 mV / s, and the fluctuation in current was measured. Hereinafter, unless otherwise specified, the potential value (V) represents the potential (V vs. Li) relative to Li. The potential (V vs. Li) relative to Li is the value obtained by adding 0.6 V to the potential (V vs. In-Li) relative to In-Li.

[0058] Solid electrolytes are inherently insulating materials with respect to electrical conduction based on the movement of electrons. However, when the applied voltage to the working electrode in the above-described electrochemical cell is swept toward a higher potential, a small current initially flows, which is thought to be due to non-Faraday reactions not involving the transfer of electrons, or side reactions of adsorbed water or impurities. Then, when the first solid electrolyte undergoes an oxidative decomposition reaction and is altered, a current associated with the decomposition reaction is observed. When a voltage exceeding 4.0 V is applied, a point appears where the current value increases. This increase in current value is considered to be associated with the decomposition reaction. Therefore, in this test, the withstand voltage characteristics were evaluated based on the increase in current value from an applied voltage of 4.0 V to 5.0 V. The greater the increase in current value, the more electrochemically unstable the sample is and the poorer its withstand voltage characteristics can be evaluated. In this example, the increase in current value from 4.0 V to 5.0 V was 0.7 μA. Figure 3 shows an example of a potential-current curve measured using this electrochemical cell (same for Examples 2 to 6 and Comparative Examples 1 to 3).

[0059] An SEM (scanning electron microscope) photograph of the fluoride powder obtained in this example is shown in Figure 4. The length of the white scale bar at the bottom of the photograph corresponds to 100 nm (the same applies to Figures 5 to 12).

[0060] [Preparation of fluoride-coated positive electrode active material powder] As the positive electrode active material powder, LiNi ０．３３ Mn ０．３３ Co ０．３３ O ２A sample (D50 = 9.5 μm) was prepared. 0.96 g of the above fluoride powder as a test material was dispersed in 14 g of water to prepare a dispersion. 30 g of the above positive electrode active material powder was added to the dispersion to obtain a mixed slurry. The pH of this slurry was 8 to 9. The obtained slurry was spray-dried using a spray dryer (JD-1, manufactured by Kawata Corporation) to obtain a coated powder consisting of positive electrode active material particles coated with the constituent substances of the fluoride powder. The spray-drying conditions were as follows: slurry supply rate: 3 mL / min, dispersion air pressure: 500 kPa, dispersion air temperature: 200°C, drying air temperature: 135°C, drying blower air flow rate: 900 L / min, and classification blower air flow rate: 890 L / min. The spray-dried coated powder was calcined for 5 hours at the same temperature (200°C in this example) as that used in the calcination of the fluoride powder, to obtain a sample of fluoride-coated positive electrode active material powder. The following investigations were carried out on this sample.

[0061] (Evaluation of Coating Uniformity) A sample of the obtained fluoride-coated positive electrode active material powder was subjected to semi-quantitative analysis by irradiating the outermost surface of the powder particles with X-rays using an X-ray photoelectron spectroscopy (XPS) device (JPS-9200S, manufactured by JEOL Ltd.). The analysis conditions were: X-ray source: Al tube, output: 200 W, analysis angle: 45°, background processing: Shirley method. From the obtained analysis data, the coverage (%) defined by the above formula (3) was calculated. In the fluoride-coated positive electrode active material powder of this example, it can be assumed that Ni, Mn, and Co are present only in the positive electrode active material, and Al and F are present only in the coating layer. Therefore, in the above formula (3), I Ａ = Al + F, I Ｂ = Ni + Mn + Co, and the above formula (3) is expressed by the following formula (3)' (the same applies to each of the following examples): Coverage (%) = 100 × (Al + F) / (Al + F + Ni + Mn + Co) (3)' In formula (3)', (Al + F) / (Al + F + Ni + Mn + Co) corresponds to the ratio (molar ratio) of the total number of moles of Al and F to the total number of moles of detected Al, F, Ni, Mn, and Co.

[0062] The coverage of the fluoride-coated positive electrode active material powder in this example was 72%. The average thickness of the fluoride coating layer on this fluoride-coated positive electrode active material powder was determined according to the "Method for Determining the Average Thickness of a Coating Layer" described above. As a result, the average thickness of the fluoride coating layer was 20 nm. The IC device used was an IC-2010 manufactured by Tosoh Corporation. The above production conditions and results are shown in Tables 1, 2, and 3 (the same applies to each of the following examples).

[0063] [Example 2] Aqueous solution A was diluted with 28% ammonia (NH ３ ) A fluoride powder was produced under the same conditions as in Example 1, except that the amount of water added was 1.7 g, the pH was adjusted to 9, and the liquid temperature during the reaction was 25°C. The pH during the reaction was 6.8. The value of the right-hand side of the above equation (2) is 33.0. This fluoride powder was examined in the same manner as in Example 1. As a result, the BET diameter was 95 nm, the average primary particle diameter was 140 nm, the coefficient of variation was 0.19, the half-width of the (222) diffraction peak was 0.40°, and the peak intensity ratio h １ / h ０ The voltage drop was 0.8, and the increase in current value from 4.0 V to 5.0 V was 1.4 μA. FIG. 5 shows an example of an SEM photograph of the fluoride powder obtained in this example. Furthermore, using the fluoride powder obtained in this example, a fluoride-coated positive electrode active material powder was produced in the same manner as in Example 1. The coverage rate according to formula (3) was 80%. The average film thickness of the fluoride coating layer on this fluoride-coated positive electrode active material powder was measured in the same manner as in Example 1, and was found to be 20 nm.

[0064] [Example 3] Aqueous solution A was diluted with 28% by mass of ammonia (NH ３ ) A fluoride powder was produced under the same conditions as in Example 1, except that the amount of water added was 1.9 g, the pH was adjusted to 10, and the liquid temperature during the reaction was 15°C. The pH during the reaction was 9. The value of the right-hand side of the above equation (2) was 27.5. This fluoride powder was examined in the same manner as in Example 1. The results showed that the BET diameter was 135 nm, the average primary particle diameter was 160 nm, the coefficient of variation was 0.26, the half-width of the (222) diffraction peak was 0.34°, and the peak intensity ratio h １ / h ０The increase in current value from 4.0 V to 5.0 V was 0.3 μA, and the increase in current value from 4.0 V to 5.0 V was 0.5 μA. FIG. 6 shows an example of an SEM photograph of the fluoride powder obtained in this example. Furthermore, using the fluoride powder obtained in this example, a fluoride-coated positive electrode active material powder was produced in the same manner as in Example 1. The coverage rate according to formula (3) was 76%. The average film thickness of the fluoride coating layer on this fluoride-coated positive electrode active material powder was measured in the same manner as in Example 1, and was found to be 20 nm.

[0065] [Example 4] Aqueous solution A was diluted with 28% by mass of ammonia (NH ３ ) A fluoride powder was produced under the same conditions as in Example 1, except that the amount of water added was 1.7 g, the pH was adjusted to 9, the liquid temperature during the reaction was 15°C, and the baking temperature was 150°C. The pH during the reaction was 6.8. The value of the right-hand side of the above equation (2) is 33.0. This fluoride powder was examined in the same manner as in Example 1. The results showed that the BET diameter was 43 nm, the average primary particle diameter was 110 nm, the coefficient of variation was 0.10, the half-width of the (222) diffraction peak was 0.72°, and the peak intensity ratio h １ / h ０ The voltage drop was 0.6, and the increase in current value from 4.0 V to 5.0 V was 3.7 μA. FIG. 7 shows an example of an SEM photograph of the fluoride powder obtained in this example. Furthermore, using the fluoride powder obtained in this example, a fluoride-coated positive electrode active material powder was produced in the same manner as in Example 1. The coverage rate according to formula (3) was 94%. The average film thickness of the fluoride coating layer on this fluoride-coated positive electrode active material powder was measured in the same manner as in Example 1, and was found to be 20 nm.

[0066] [Example 5] A fluoride powder was produced under the same conditions as in Example 4, except that the calcination temperature was set to 200°C. The pH during the reaction was 6.8. The value of the right-hand side of the formula (2) was 33.0. This fluoride powder was examined in the same manner as in Example 1. As a result, the BET diameter was 44 nm, the average primary particle diameter was 120 nm, the coefficient of variation was 0.15, the half-width of the (222) diffraction peak was 0.51°, and the peak intensity ratio h １ / h ０The increase in current value from 4.0 V to 5.0 V was 0.4, and the increase in current value from 4.0 V to 5.0 V was 3.2 μA. FIG. 8 shows an example of an SEM photograph of the fluoride powder obtained in this example. Furthermore, using the fluoride powder obtained in this example, a fluoride-coated positive electrode active material powder was produced in the same manner as in Example 1. The coverage rate according to formula (3) was 90%. The average film thickness of the fluoride coating layer on this fluoride-coated positive electrode active material powder was measured in the same manner as in Example 1, and was found to be 20 nm.

[0067] [Example 6] A fluoride powder was produced under the same conditions as in Example 4, except that the calcination temperature was set to 250°C. The pH during the reaction was 6.8. The value of the right-hand side of the formula (2) was 33.0. This fluoride powder was examined in the same manner as in Example 1. As a result, the BET diameter was 45 nm, the average primary particle diameter was 120 nm, the coefficient of variation was 0.18, the half-width of the (222) diffraction peak was 0.41°, and the peak intensity ratio h １ / h ０ The average current value was 0.4, and the increase in current value from 4.0 V to 5.0 V was 1.5 μA. FIG. 9 shows an example of an SEM photograph of the fluoride powder obtained in this example. Furthermore, using the fluoride powder obtained in this example, a fluoride-coated positive electrode active material powder was produced in the same manner as in Example 1. The coverage rate according to formula (3) was 85%. The average film thickness of the fluoride coating layer on this fluoride-coated positive electrode active material powder was measured in the same manner as in Example 1, and was found to be 20 nm.

[0068] [Example 7] A fluoride powder was produced under the same conditions as in Example 4, except that the calcination temperature was 300°C. The pH during the reaction was 6.8. The value of the right-hand side of the formula (2) was 33.0. This fluoride powder was examined in the same manner as in Example 1. As a result, the BET diameter was 45 nm, the average primary particle diameter was 120 nm, the coefficient of variation was 0.21, the half-width of the (222) diffraction peak was 0.35°, and the peak intensity ratio h １ / h ０The increase in current value from 4.0 V to 5.0 V was 0.5 μA, and the increase in current value from 4.0 V to 5.0 V was 0.3 μA. FIG. 10 shows an example of an SEM photograph of the fluoride powder obtained in this example. Furthermore, using the fluoride powder obtained in this example, a fluoride-coated positive electrode active material powder was produced in the same manner as in Example 1. The coverage rate according to formula (3) was 82%. The average film thickness of the fluoride coating layer on this fluoride-coated positive electrode active material powder was measured in the same manner as in Example 1, and was found to be 20 nm.

[0069] Comparative Example 1 In this example, a fluoride powder was synthesized by a dry method as follows: lithium fluoride LiF and aluminum fluoride AlF ３ The above raw material powders were weighed out so that the molar ratio of Li:Al:F was 3:1:6, and a total of approximately 10 g of raw material powder was prepared. The raw material powders were mixed using a ball mill. The mixing conditions were as follows: 10 mm diameter zirconia balls were used, an Ar atmosphere with a dew point of -50°C, a rotation speed of 380 rpm, and a mixing time of 36 hours. The obtained mixed powder was fired by holding it in an air atmosphere at 900°C for 3 hours. The obtained fired powder was pulverized using a ball mill. The pulverization conditions were as follows: 10 mm diameter zirconia balls were used, an air atmosphere, a rotation speed of 380 rpm, and a pulverization time of 5 hours.

[0070] The fluoride powder thus obtained was examined in the same manner as in Example 1. The results showed that this fluoride powder was amorphous ( FIG. 1 ), had a BET diameter of 537 nm, an average primary particle diameter of 580 nm, a coefficient of variation of 0.54, and a current increase from 4.0 V to 5.0 V of 6.1 μA. FIG. 11 shows an example of an SEM photograph of the fluoride powder obtained in this example. Furthermore, using the fluoride powder obtained in this example, a fluoride-coated positive electrode active material powder was produced in the same manner as in Example 1. The coverage rate according to formula (3) was 53%.

[0071] Comparative Example 2: Aqueous solution A was diluted with 28% ammonia (NH ３) A fluoride powder was produced under the same conditions as in Example 1, except that the amount of water added was 18.9 g, the pH was adjusted to 10.8, and the liquid temperature during the reaction was 15°C. The pH during the reaction was 10. The value of the right-hand side of the above equation (2) was 25.0. This fluoride powder was examined in the same manner as in Example 1. The results showed that the BET diameter was 320 nm, the average primary particle diameter was 450 nm, the coefficient of variation was 0.39, the half-width of the (222) diffraction peak was 0.27°, and the peak intensity ratio h １ / h ０ The voltage drop was 0.6, and the increase in current value from 4.0 V to 5.0 V was 0.3 μA. FIG. 12 shows an example of an SEM photograph of the fluoride powder obtained in this example. Furthermore, using the fluoride powder obtained in this example, a fluoride-coated positive electrode active material powder was produced in the same manner as in Example 1. The coverage rate according to formula (3) was 62%.

[0072] Comparative Example 3: Aqueous solution A was diluted with 28% ammonia (NH ３ ) A fluoride powder was produced under the same conditions as in Example 1, except that the amount of water added was 1.7 g, the pH was adjusted to 9, and the liquid temperature during the reaction was 40°C. The pH during the reaction was 6.8. The value of the right-hand side of the formula (2) was 33.0, which is outside the definition of formula (3). This fluoride powder was examined in the same manner as in Example 1. The results showed that the BET diameter was 372 nm, the average primary particle diameter was 500 nm, the coefficient of variation was 0.64, the half-width of the (222) diffraction peak was 0.24°, and the peak intensity ratio h １ / h ０ The increase in current value from 4.0 V to 5.0 V was 0.2 μA, and the increase in current value from 4.0 V to 5.0 V was -0.3 μA. FIG. 13 shows an example of an SEM photograph of the fluoride powder obtained in this example. Furthermore, using the fluoride powder obtained in this example, a fluoride-coated positive electrode active material powder was produced in the same manner as in Example 1. The coverage rate according to formula (3) was 60%.

[0073]

[0074]

[0075]

[0076] The fluoride powders (coating materials) obtained in each example exhibited good voltage resistance characteristics and, when used as coating materials for cathode active material powders, were able to form coating layers with high coverage rates, confirming their usefulness as coating materials for obtaining cathode active material powders with high coating uniformity. Therefore, the cathode active material powders coated with the fluoride powders obtained in each example as coating materials can contribute to improving the performance of all-solid-state batteries, such as improving the charging voltage and improving the resistance to deterioration of battery capacity. Furthermore, the high uniformity of the coating is thought to reduce variations in ionic conductivity at the interface between the cathode and solid electrolyte, thereby lowering the resistance of the battery.

[0077] For reference, Fig. 14 shows an example of an SEM photograph of a fluoride-coated positive electrode active material powder in which a coating layer was formed using the fluoride powder (coating material) of Example 4. The length of the white scale bar at the bottom of the photograph corresponds to 1 µm.

Claims

1. A powder containing as its main component a lithium-containing metal fluoride containing Li, one or more metal elements M selected from the group consisting of Al, Ga, Fe, Cr, and Y, and F, wherein the fluoride powder has a BET diameter d of 300 nm or less according to the following formula (1): d = 6 × 10 ３ / (ρ×S) (1) where d is the BET diameter (nm), ρ is the true density of the powder (g / cm ３ ), S is the BET specific surface area of ​​the powder (m ２ / g).

2. The fluoride powder according to claim 1, wherein the average primary particle size measured from a scanning electron microscope image is 400 nm or less.

3. The fluoride powder according to claim 1, wherein the coefficient of variation of the primary particle size measured from a scanning electron microscope image is 0.40 or less.

4. The fluoride powder according to claim 1, having a composition in which the Li / M molar ratio is 2.0 or more and 6.0 or less, and the F / M molar ratio is 5.0 or more and 9.0 or less.

5. The lithium-containing metal fluoride, which is the main component, contains Al as the metal element M and β-Li ３ AlF ６ It has a β-Li type crystal structure, and in the X-ray diffraction pattern using Cu-Kα radiation with the horizontal axis being 2θ (°), ３ AlF ６ 2. The fluoride powder according to claim 1, wherein the half-width of the diffraction peak of the (222) plane of the crystalline structure is 0.2° or more and 1.0° or less.

6. A method for producing a fluoride powder, comprising: a reaction step of generating a solid substance primarily composed of lithium-containing metal fluoride by stirring an aqueous solution containing Li, one or more metal elements M selected from the group consisting of Al, Ga, Fe, Cr, and Y, and F, at a pH of 4.0 to 9.5, a liquid temperature of 5°C or higher, and under conditions that satisfy the following formula (2): T≦-2.5×pH+50 ... (2) where T is the liquid temperature (°C), and pH is the pH value of the liquid.

7. The method for producing a fluoride powder according to claim 6, wherein the Li / M molar ratio in the aqueous solution is 2.0 or more and 6.0 or less, and the F / M molar ratio is 5.0 or more and 9.0 or less.

8. The lithium-containing metal fluoride contains Al as the metal element M and β-Li ３ AlF ６ It has a β-Li type crystal structure, and in the X-ray diffraction pattern using Cu-Kα radiation with the horizontal axis being 2θ (°), ３ AlF ６ 7. The method for producing a fluoride powder according to claim 6, wherein the half width of the diffraction peak of the (222) plane of the crystalline structure is 0.2° or more and 1.0° or less.

9. The method for producing a fluoride powder according to claim 6, further comprising a calcination step of calcining the dried powder of the solid substance recovered in the solid-liquid separation step at a temperature of 130°C or higher and 500°C or lower.

10. A fluoride-coated positive electrode active material powder comprising particles of positive electrode active material having a coating layer on the surface thereof, the coating layer being composed of a substance whose main component is a lithium-containing metal fluoride containing Li, one or more metal elements M selected from the group consisting of Al, Ga, Fe, Cr, and Y, and F, with a Li / M molar ratio of 2.0 to 6.0 and an F / M molar ratio of 5.0 to 9.0, and wherein the atomic ratio detected by X-ray photoelectron spectroscopy (XPS) by irradiating the outermost surface of the powder particles with X-rays is a coating rate of 70% or more according to the following formula (3): Coating rate (%) = 100 x I Ａ / (I Ａ +I Ｂ ) ... (3) where I Ａ I: Total detected amount (molar equivalent) of elements other than Li constituting the lithium-containing metal fluoride, which is the main component of the coating layer, excluding "elements constituting the positive electrode active material" Ｂ : The total detected amount (in moles) of elements other than O that constitute the positive electrode active material, excluding "elements that constitute the lithium-containing metal fluoride that is the main component of the coating layer" 11. The fluoride-coated positive electrode active material powder according to claim 10, wherein the average film thickness of the coating material is 10 nm or more and 200 nm or less.

12. The lithium-containing metal fluoride, which is the main component, contains Al as the metal element M and β-Li ３ AlF ６ The fluoride-coated positive electrode active material powder according to claim 10, which has a fluoride-coated crystalline structure.

13. A method for producing a fluoride-coated positive electrode active material powder, comprising: a step of obtaining a slurry by mixing the fluoride powder according to any one of claims 1 to 5 with a positive electrode active material powder in a liquid medium; and a step of spray-drying the slurry to obtain a coated powder consisting of particles of positive electrode active material coated with a constituent substance of the fluoride powder.

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