Fluoride powder, fluoride coated cathode active material powder, and manufacturing method thereof

Abstract

To provide a powder comprising a lithium ion conductive substance that functions as a protective material of a cathode active material, has excellent withstanding voltage characteristic, and has excellent deposition uniformity to cathode active material particles.SOLUTION: A fluoride powder comprises a powder that contains a lithium-containing metal fluoride containing Li (lithium), a metal element M, and F (fluorine) as the main component, wherein a BET diameter d by the following formula (1) is 300 nm or under: d=6×103 / (ρ×S) (1), here, d is BET diameter (nm), ρ is real density of the powder (g / cm3), and S is BET specific surface area of the powder (m2 / g).SELECTED DRAWING: Figure 7

Description

[Technical Field]

[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. [Background technology]

[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 separator increases, which tends to reduce battery performance, such as battery capacity. This increase in interfacial resistance is mainly caused by the reaction between the positive electrode active material and the solid electrolyte, resulting in the formation of high-resistance regions on the surface of the positive electrode active material. Therefore, attempts have been made to prevent the positive electrode active material from reacting with the solid electrolyte separator by coating the surface of the positive electrode active material with a protective solid electrolyte made of a lithium-ion conductive oxide (a well-known example is lithium niobate, LiNbO3) 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, when lithium niobate is used as a coating material for the positive electrode active 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 a technology using Li3AlF6 as a negative electrode active material, but does not disclose the use of Li3AlF6 as a coating material for a positive electrode active material. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2018-67474 [Patent Document 2] International Publication No. 2020 / 174868 [Patent Document 3] Japanese Patent Publication No. 2022-114320 Summary of the Invention [Problem to be solved by the invention]

[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 variations 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 in Patent Document 1 claims that by using 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 fine powder with a particle diameter on the submicron order for 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 between primary particles, making it difficult to obtain fine coating material powder on the submicron order. 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. [Means for solving the problem]

[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 [1] above, having an average primary particle diameter of 400 nm or less as measured from a scanning electron microscope image. [3] The fluoride powder according to [1] or [2] above, 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], 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 fluoride powder according to any one of the above [1] to [4], wherein the lithium-containing metal fluoride as a main component contains Al as the metal element M and has a β-Li3AlF6 type crystal structure, and in an X-ray diffraction pattern using Cu-Kα radiation with the horizontal axis being 2θ (°), the half-width of the diffraction peak of the (222) plane of the β-Li3AlF6 type crystal structure is 0.2° or more and 1.0° or less.

[0013] [6] 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 under conditions of a pH of 4.0 to 9.5, a liquid temperature of 5°C or higher, and satisfying the following formula (2): a solid-liquid separation step of performing solid-liquid separation and recovering the solid material; A method for producing a fluoride powder having the above formula. T≦-2.5×pH+50 …(2) Here, 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 [6] above, 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 method for producing a fluoride powder according to the above [6] or [7], wherein the lithium-containing metal fluoride contains Al as the metal element M and has a β-Li3AlF6 type crystal structure, and in an X-ray diffraction pattern using Cu-Kα radiation with the horizontal axis being 2θ (°), the half-width of the diffraction peak of the (222) plane of the β-Li3AlF6 type crystal structure is 0.2° or more and 1.0° or less. [9] 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; The method for producing a fluoride powder according to any one of the above [6] to [8], wherein the method comprises:

[0014]

[10] A powder consisting of positive electrode active material particles having a coating layer on the surface thereof, 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, and whose Li / M molar ratio is 2.0 or more and 6.0 or less and whose F / M molar ratio is 5.0 or more and 9.0 or less, 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 A : The total detected amount (in moles) of elements other than Li that make up the lithium-containing metal fluoride, the main component of the coating layer, excluding "elements that make up the positive electrode active material" I B : The total detected amount (molar equivalent) of elements other than O that make up the positive electrode active material, excluding "elements that make up 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

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

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

[10] or

[11] above, wherein the lithium-containing metal fluoride as a main component contains Al as the metal element M and has a β-Li3AlF6 type crystal structure.

[0015]

[13] 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; 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; A method for producing a fluoride-coated positive electrode active material powder having the above-mentioned formula. [Effects of the Invention]

[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. [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 2 is a diagram illustrating the X-ray diffraction patterns of the fluoride powders (coating materials) obtained in Examples 1 to 7 and Comparative Examples 1 to 3. [Figure 2] FIG. 1 is a cross-sectional view schematically showing the laminated structure of an electrochemical cell fabricated for evaluating voltage resistance characteristics. [Figure 3] 1 is a graph illustrating a potential-current curve measured using an electrochemical cell. [Figure 4] 1 is an example of an SEM photograph of the fluoride powder obtained in Example 1. [Figure 5] 1 is an example of an SEM photograph of the fluoride powder obtained in Example 2. [Figure 6] 1 is an example of an SEM photograph of the fluoride powder obtained in Example 3. [Figure 7] 1 is an example of an SEM photograph of the fluoride powder obtained in Example 4. [Figure 8] 1 is an example of an SEM photograph of the fluoride powder obtained in Example 5. [Figure 9] 1 is an example of an SEM photograph of the fluoride powder obtained in Example 6. [Figure 10] 1 is an example of an SEM photograph of the fluoride powder obtained in Example 7. [Figure 11] 1 is an example of an SEM photograph of the fluoride powder obtained in Comparative Example 1. [Figure 12] 1 is an example of an SEM photograph of the fluoride powder obtained in Comparative Example 2. [Figure 13] 1 is an example of an SEM photograph of the fluoride powder obtained in Comparative Example 3. [Figure 14]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. DETAILED DESCRIPTION OF THE INVENTION

[0018] [Fluoride powder] According to the research of the inventors, it has been found that a solid substance whose main component is 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 film 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 metal fluoride having the composition formula Li x MF 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 -11 It is possible to construct a lithium-containing metal fluoride exhibiting a conductivity of 0.5 S / cm or more, and also to increase the band gap, 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, be 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 the preferred composition formula Li of the "lithium-containing metal fluoride". x MF 3+x Similar to the range (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] "Powder primarily composed of lithium-containing metal fluoride" means that the "lithium-containing metal fluoride" is the most abundant compound among the components constituting the powder. In the X-ray diffraction pattern of the powder, the peak height of the highest diffraction peak attributed to the "lithium-containing metal fluoride" is defined as h0, and the peak height of the highest diffraction peak attributed to a different phase (a crystalline phase other than the lithium-containing metal fluoride) is defined as h1. If the relationship h0 > h1 holds, the powder is considered to be "powder primarily composed of lithium-containing metal fluoride." If no different phase is detected, h1 = 0, and the above relationship h0 > h1 is satisfied. For peaks where diffraction peaks from multiple crystal planes overlap, the diffraction intensity corresponding to the peak position of each crystal plane in the overlapped peak shape is used as the diffraction peak height of each crystal plane.

[0022] A preferred specific example of the lithium-containing metal fluoride is one that contains Al as the metal element M and has a β-Li3AlF6 type crystal structure.

[0023] The fluoride powder of the present invention is composed of fine particles, and is specifically specified as a powder having a BET diameter d according to the following formula (1) of 300 nm or less. 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 single-point method. A fluoride powder with a small BET diameter d as described above is extremely useful for obtaining a positive electrode active material powder having a highly uniform coating layer when it is mixed with a positive electrode active material powder to form a slurry and then a coating layer (film) derived from the compound that constituted 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 preferably 250 nm or less, more preferably 150 nm or less, and even more preferably 100 nm or less. There is no particular lower limit for the BET diameter d, but it is usually 10 nm or more. The true density ρ of the powder can be determined by the calculated true density ρ of Li3MF6 as follows.

[0024] (Determining the true density ρ of Li3MF6) 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, the true density ρ value (g / cm) of each M fluoride is as follows: 3 ) is taken as the true density ρ of Li3MF6. Metal element MM fluoride true density ρ Al Li3AlF62.84g / cm 3 Ga Li3GaF63.43g / cm 3 Fe Li3FeF63.16g / cm 3 Cr Li3CrF63.13g / cm 3 Y Li3YF62.60g / cm 3 When there are two metal elements M, the true density ρ value (g / cm 3 ) is taken as the true density ρ of Li3MF6. True density ρ=(ρ1×W1+ρ2×W2) / (W1+W2) …(4) where ρ1 and ρ2 are the true density ρ values ​​(g / cm) of the M fluoride for the first and second M elements, respectively. 3) where W1 and W2 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, ρ1 is 2.84 g / cm 3 , ρ2 is 3.16 g / cm 3 where W1 is the content (mass%) of Li3AlF6, and W2 is the content (mass%) of Li3FeF6. When there are three or more metal elements M, the number of elements is n (where n is an integer of 3 or more and 5 or less), and the true density ρ value (g / cm 3 ) is taken as the true density ρ of Li3MF6. True density ρ=(ρ1×W1+···+ρ n ×W n ) / (W1+···+W n ) …(5) where ρ i (i is an integer between 1 and n) is the true density ρ value (g / cm) of the above M fluoride for the i-th M element. 3 ) is. 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, as the average value of the 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 uniformity. Specifically, the coefficient of variation of the primary particle size 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 of distribution of primary particle size (nm) σ / Average primary particle size (nm)

[0027] When the lithium-containing metal fluoride, the main component of the powder, is crystalline, a smaller crystallite size is advantageous for increasing the uniformity of adhesion to the positive electrode active material, while a larger crystallite size improves electrochemical stability and is advantageous for improving 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 vice versa.

[0028] Taking a powder whose main component is a compound having a β-Li3AlF6 type crystal structure as an example, the half-width of the (222) plane of the β-Li3AlF6 type crystal structure can be used in the X-ray diffraction pattern using Cu-Kα radiation. As shown in ICDD card 01-088-0860, β-Li3AlF6 crystals belong to the monoclinic space group C2 / c, and have a crystal structure in which a diffraction peak from the (222) plane is observed near 2θ = 31.2° in the X-ray diffraction pattern using Cu-Kα radiation. This diffraction peak from the (222) plane does not overlap with diffraction peaks from other crystal planes or diffraction peaks caused by different phases that are likely to be mixed in during manufacturing (e.g., LiF3, AlF3, Al2(OH)3F3), making it suitable for evaluating the half-width. In the case of powders whose main component is a compound having a β-Li3AlF6 type crystal structure, in order to balance the uniformity of deposition and the withstand voltage characteristics, it is preferable that the half-width of the diffraction peak of the (222) plane of the β-Li3AlF6 type crystal structure is 0.2° or more and 1.0° or less, and more preferably 0.3° or more and 0.8° or less.

[0029] [Fluoride powder manufacturing method] The fluoride powder, which is composed of fine particles mainly composed of the lithium-containing metal fluoride mentioned above, can be synthesized by a coprecipitation reaction in an aqueous solution. During the reaction, it is important to appropriately control the pH and temperature of the solution. Specifically, the following process can be adopted.

[0030] (Mixing process) 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, aqueous solution A and 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 aqueous solution A and the entire amount of aqueous solution B to prevent uneven pH in the mixed solution.

[0032] To produce a lithium-containing metal fluoride consisting of fine particles, it is particularly effective to first adjust the pH of the F-containing aqueous solution A to a high value and then mix the aqueous solutions A and B to obtain a mixed solution with a lower pH than the pH of the aqueous solution A before mixing. When using this method, 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 (NH3) water can be used as a pH adjuster. The preferred mixing procedure is to add the entire amount of the Li- and M-containing aqueous solution B all at once in a short period of 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 rather an addition mode in which the entire amount of the solution to be added is added as quickly as possible without interruption within the required addition time range that allows safe mixing depending on the scale of the equipment.

[0033] The pH referred to in this specification is measured using a pH meter calibrated with an appropriate buffer solution according to the pH range to be measured, based on JIS Z8802: 2011. The pH values ​​described in this specification are values ​​measured by a pH meter compensated by a temperature compensation electrode and read directly 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. At this time, stirring is carried out 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 having a lower pH than that of aqueous solution A before mixing is obtained in the mixing step, it is effective to stir under conditions of a pH lower than that of aqueous solution A before mixing, a pH of 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) Here, T is the liquid temperature (°C) and pH is the pH value of the liquid. If the pH is not within the range of 4.0 to 9.5 and the following formula (2) is satisfied, the degree of supersaturation of the target lithium-containing metal fluoride will decrease, which may result in difficulty in synthesizing fine particles. Furthermore, if the liquid temperature is below 10°C, the reaction rate will be slow, 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 be controlled at 10°C to 30°C. The stirring time (reaction time) for allowing the reaction to proceed is preferably set within the 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 an 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 process) 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] (Firing process) In order to further improve the voltage resistance characteristics of crystalline materials primarily composed of lithium-containing metal fluorides, it is effective to perform calcination on 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 below, or even 300°C or below. Even if this firing step is omitted, the firing performed after coating the positive electrode active material can be expected to improve the voltage resistance characteristics.

[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 a 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] Known materials can be used as the positive electrode active material powder, and new positive electrode active materials may also be developed and applied. Representative known positive electrode active materials include LiCoO (LCO type), LiNiO (LNO type), LiMnO (LMO type), LiNiCoAlO (NCA type), LiNiCoMnO (NCM type), LiMnO-LiNiCoMnO (solid solution type), LiNiMnO (spinel type), LiMnFePO (phosphate type), and LiFeSiO (silicate type). The positive electrode active material powder should 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 the volume-based particle size distribution measured by laser diffraction and scattering.

[0040] (Mixed slurry formation) The above-mentioned 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 by 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 shear force of the collision 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. The average thickness of this coating layer is usually 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. It should be noted that a fluoride-coated positive electrode active material powder having a coating layer with high coating uniformity can also be obtained by subjecting the particles constituting the above mixed slurry to a compressive shear treatment using another method instead of spray drying.

[0042] (How to determine the average coating layer thickness) The average thickness of the fluoride coating layer is determined by the true density ρ (g / cm) of the fluoride Li3MF6 determined by the above-mentioned method. 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 × [molecular weight of Li3MF6] / ([atomic weight of F] × 6) The BET specific surface area of ​​the positive electrode active material is defined as Sp(m 2 / g), and the true density of Li3MF6 is ρ (g / cm 3 ), the average thickness t (nm) of the coating layer is expressed by the following equation: t=10×B / (Sp×ρ) Here, the IC analysis for determining the F content A (mass %) can be carried out by the following procedure. A 0.02 g 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 decomposition solution is left to cool, and then diluted sulfuric acid is added to make it acidic and decompose any insoluble residue. Another 10 mL of a 1 mol / L NaOH aqueous solution is added, and the solution is heated to confirm that no insoluble residue remains. The decomposition solution is diluted 20 times in a 100 mL measuring flask. The F concentration in the resulting 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 calcined as necessary to densify the coating layer. The calcination temperature may be set within the same temperature range as that used for the calcination of 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 coating layer with high uniformity 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 composed of positive electrode active material particles whose surfaces are coated with a substance having a composition in which a lithium-containing metal fluoride containing Li (lithium), one or more metal elements M, and F (fluorine) as a main component, the Li / M molar ratio being 2.0 or more and 6.0 or less, and an F / M molar ratio being 5.0 or more and 9.0 or less, and in which the fluoride-coated positive electrode active material powder has a coverage rate of 70% or more and 100% or less according to the following formula (3), as determined by X-ray photoelectron spectroscopy (XPS) by irradiating the outermost surface of the powder particles with X-rays: Coverage (%) = 100 × I A / (I A +I B ) …(3) where: I A : The total detected amount (in moles) of elements that make up the lithium-containing metal fluoride, the main component of the coating layer, excluding "elements contained in the positive electrode active material and Li" IB : The total detected amount (in moles) of elements that make up the positive electrode active material, excluding "Li and O, the elements that make up 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. XPS detects elements present in the surface layer from the outermost surface of the sample to a depth of several nm. The more areas where the coating layer is very thin, less than several nm (hereinafter referred to as "coverage-deficient areas"), the more the amount of elements that constitute only the positive electrode active material is detected (i.e., the above formula I). B becomes 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 insufficient, a positive electrode active material powder with many areas where the coverage is insufficient is considered to have a high risk of causing an increase in interface resistance in an all-solid-state battery. Therefore, a higher coverage rate according to formula (3) can be evaluated as being advantageous in terms of excellent coating uniformity and ensuring stable protective function. When using a fluoride powder (coating material) containing a lithium-containing metal fluoride as a main component as specified 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. [Example]

[0046] [Example 1] (Preparation of fluoride powder) 23.3 g of ammonium fluoride NH4F (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) and 0.07 g of 28 mass % ammonia (NH3) water (manufactured by Nacalai Tesque, Inc.) were mixed with stirring in 297.5 g of pure water until completely dissolved, thereby obtaining aqueous solution A with a pH of 6.1. 21.7 g of lithium nitrate LiNO3 (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) and 39.4 g of aluminum nitrate nonahydrate Al(NO3)3·9H2O (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) were mixed with stirring in 552.5 g of pure water until completely dissolved, to obtain 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, and the solid material was recovered. 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 by the following method. Weigh out 0.05 g of powdered test material, add 5 mL of 36% by mass hydrochloric acid, and decompose by heating. Dilute the decomposition solution 10-fold in a 100 mL measuring flask. Measure the Li and Al concentrations in the diluted solution using an inductively coupled plasma optical emission spectrometer (Agilent Technologies, ICP-720). F was analyzed in the following manner. Weigh out 0.02 g of powder sample of the test material, add 10 mL of 1 mol / L NaOH solution, and heat decompose. After leaving the decomposition solution to cool, add dilute sulfuric acid to make it acidic and decompose any undissolved residue. Add 10 mL of 1 mol / L NaOH solution again and heat to confirm that there is no undissolved residue. Dilute the decomposition solution 20 times in a 100 mL measuring flask. Measure the F concentration in the diluted solution using ion chromatography (Tosoh Corporation, IC-2010). 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 by the BET single-point method using a BET specific surface area measuring device (4Sorb US, manufactured by Yuasa Ionics Co., Ltd.) by flowing nitrogen gas at 105°C for 20 minutes to degas the sample, and then flowing a mixed gas of nitrogen and helium (N: 30 vol%; He: 70 vol%). 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 true density of Li3AlF6 is 2.84 g / cm 3 was applied. The BET diameter of the fluoride powder obtained in this example was 212 nm.

[0052] (Average primary particle size and coefficient of variation) The powder sample used 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 the examples described below, and 10,000 times in the comparative examples described below. 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. The image processing software ImageJ was used to determine the maximum diameter in a specific direction. The average primary particle size of the fluoride powder obtained in this example was 170 nm, and the coefficient of variation was 0.32.

[0053] (X-ray diffraction) The X-ray diffraction pattern of the powder sample was measured using an X-ray diffractometer (RIGAKU, Ultima IV) under the following conditions: Cu-Kα radiation, tube voltage: 40 kV, tube current: 40 mA, divergence slit: 1 / 2°, scattering slit: 8 mm, receiving slit: open, step width: 0.02° / step, scan speed: 0.666667. As a result, it was confirmed that the sample material in this example was mainly composed of a lithium-containing metal fluoride with a β-Li3AlF6 structure (the same applies to each example except for Comparative Example 1). FIG. 1 shows an example of the X-ray diffraction pattern of the obtained fluoride powder (the same applies to Examples 2 to 7 and Comparative Examples 1 to 3).

[0054] Furthermore, for the diffraction peak of the (222) crystal plane of the β-Li3AlF6 type structure observed around 2θ = 31.2°, the half-width of the (222) diffraction peak was calculated using X-ray diffraction pattern analysis software (RIGAKU, PDXL2) based on the measurement data using Kα1 radiation from which Kα2 radiation had been removed, 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°. When the peak height of the highest diffraction peak among the diffraction peaks attributable to the lithium-containing metal fluoride is defined as h0 and the peak height of the highest diffraction peak among the diffraction peaks attributable to a different phase (a crystalline phase other than the lithium-containing metal fluoride) is defined as h1, the peak intensity ratio h1 / h0 was 0.2.

[0055] (Voltage resistance characteristic evaluation) An all-solid-state electrochemical cell was fabricated in which a solid electrolyte (first solid electrolyte) made of 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 conducted as follows.

[0056] An electrochemical cell having the stacked structure shown in FIG. 2 was fabricated as follows. Argyrodite-type sulfide (Li6PS5Cl) was prepared as the second solid electrolyte. 57 mg of this sulfide was placed in an insulating outer cylinder (inner diameter 9.5 mm) made of alumina and pressed at a pressure of 80 MPa to form a layer of the second solid electrolyte. Next, 20 mg of the powder sample was placed on the second solid electrolyte layer and pressed at a pressure of 80 MPa to form a first solid electrolyte layer. Next, 20 mg of a composite material obtained by mixing the powder of the test material and the powder of stainless steel (SUS316) in a mortar in a volume ratio of 50:50 was poured onto the first solid electrolyte layer and pressed at a pressure of 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 the above order under the second solid electrolyte layer (opposite the first solid electrolyte), and then press-molded at a pressure of 80 MPa to form a counter electrode layer consisting of 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, respectively, 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 (high potential electrode) and the counter electrode layer side was used as the counter electrode (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 current fluctuations were measured. Unless otherwise specified, the potential value (V) below represents the potential relative to Li (V vs Li). The potential relative to Li (V vs Li) is the value obtained by adding 0.6 V to the potential relative to In-Li (V vs 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 electrochemical cell is swept toward a higher potential, a small current initially flows, which is thought to be due to non-Faraday reactions that do not involve the transfer of electrons, or to side reactions of absorbed water or impurities. Then, as the first solid electrolyte undergoes an oxidative decomposition reaction and its properties change, a current associated with the decomposition reaction begins to be 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 due to the decomposition reaction. Therefore, in this test, the voltage resistance characteristics were evaluated based on the increase in current value from an applied voltage of 4.0 V to 5.0 V. The greater this increase in current value, the more electrochemically unstable the material is and the poorer its voltage resistance characteristics are. In this example, the increase in current value from 4.0 V to 5.0 V was 0.7 μA. FIG. 3 shows an example of a potential-current curve measured using this electrochemical cell (the same applies to 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 O2 (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 liquid. 30 g of the above positive electrode active material powder was added to the dispersion liquid to obtain a mixed slurry. The pH of this slurry was 8 to 9. The resulting slurry was spray-dried using a spray dryer (JD-1, manufactured by Kawata Corporation) to obtain a coated powder consisting of particles of the positive electrode active material 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, 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 deposition uniformity) The obtained fluoride-coated positive electrode active material powder samples were subjected to semi-quantitative analysis using an X-ray photoelectron spectroscopy (XPS) device (JPS-9200S, manufactured by JEOL Ltd.) by irradiating the outermost surface of the powder particles with X-rays. 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. 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 equation (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 Al, F, Ni, Mn, and Co detected.

[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 of this fluoride-coated positive electrode active material powder was determined according to the above-mentioned "Method for determining the average thickness of a coating layer." 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 preparation conditions and results are shown in Tables 1, 2 and 3 (the same applies to the following examples).

[0063] [Example 2] A fluoride powder was produced under the same conditions as in Example 1, except that the amount of 28 mass% ammonia (NH3) water added to Aqueous Solution A 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 formula (2) above was 33.0. This fluoride powder was examined in the same manner as in Example 1. The results were a BET diameter of 95 nm, an average primary particle diameter of 140 nm, a coefficient of variation of 0.19, a half-width of the (222) diffraction peak of 0.40°, a peak intensity ratio h1 / h0 of 0.8, and a current increase of 1.4 μA from 4.0 V to 5.0 V. 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 in this fluoride-coated positive electrode active material powder was measured in the same manner as in Example 1, and the average film thickness was 20 nm.

[0064] [Example 3] A fluoride powder was produced under the same conditions as in Example 1, except that the amount of 28 mass% ammonia (NH3) water added to Aqueous Solution A 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 formula (2) above was 27.5. This fluoride powder was examined in the same manner as in Example 1. The results were a BET diameter of 135 nm, an average primary particle diameter of 160 nm, a coefficient of variation of 0.26, a half-width of the (222) diffraction peak of 0.34°, a peak intensity ratio h1 / h0 of 0.3, and a current increase from 4.0 V to 5.0 V of 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 in this fluoride-coated positive electrode active material powder was measured in the same manner as in Example 1, and the average film thickness was 20 nm.

[0065] [Example 4] A fluoride powder was produced under the same conditions as in Example 1, except that the pH of aqueous solution A was adjusted to 9 by adding 1.7 g of 28% by mass ammonia (NH3) water, the liquid temperature during the reaction was set to 15°C, and the calcination temperature was set to 150°C. The pH during the reaction was 6.8. The value of the right-hand side of equation (2) above is 33.0. This fluoride powder was examined in the same manner as in Example 1. The results were a BET diameter of 43 nm, an average primary particle diameter of 110 nm, a coefficient of variation of 0.10, a half-width of the (222) diffraction peak of 0.72°, a peak intensity ratio h1 / h0 of 0.6, and a current increase of 3.7 μA from 4.0 V to 5.0 V. 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 in this fluoride-coated positive electrode active material powder was measured in the same manner as in Example 1, and the average film thickness was 20 nm.

[0066] [Example 5] A fluoride powder was produced under the same conditions as in Example 4, except that the firing 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 subjected to the same investigation as in Example 1. The results were as follows: 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°, the peak intensity ratio h1 / h0 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 in this fluoride-coated positive electrode active material powder was measured in the same manner as in Example 1, and the average film thickness was 20 nm.

[0067] [Example 6] A fluoride powder was produced under the same conditions as in Example 4, except that the firing 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 subjected to the same investigation as in Example 1. The results were as follows: 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°, the peak intensity ratio h1 / h0 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 in this fluoride-coated positive electrode active material powder was measured in the same manner as in Example 1, and the average film thickness was 20 nm.

[0068] [Example 7] A fluoride powder was produced under the same conditions as in Example 4, except that the firing temperature was set to 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 subjected to the same investigation as in Example 1. The results were as follows: 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°, the peak intensity ratio h1 / h0 was 0.5, 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 in this fluoride-coated positive electrode active material powder was measured in the same manner as in Example 1, and the average film thickness was 20 nm.

[0069] [Comparative Example 1] In this example, the fluoride powder was synthesized by a dry method as follows. Lithium fluoride LiF and aluminum fluoride AlF3 were weighed out so that the molar ratio of Li:Al:F was 3:1:6, and a total of about 10 g of raw material powder was prepared. The raw material powders were mixed in a ball mill using 10 mm diameter zirconia balls in an Ar atmosphere with a dew point of −50° C., at a rotation speed of 380 rpm, for a mixing time of 36 hours. The resulting mixed powder was fired in an air atmosphere at 900°C for 3 hours. The resulting fired powder was pulverized in a ball mill using 10 mm diameter zirconia balls in an air atmosphere at a rotation speed of 380 rpm for 5 hours.

[0070] The fluoride powder thus obtained was examined in the same manner as in Example 1. As a result, this fluoride powder was amorphous (FIG. 1), with a BET diameter of 537 nm, an average primary particle diameter of 580 nm, a coefficient of variation of 0.54, and an increase in current value 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 according to formula (3) was 53%.

[0071] Comparative Example 2 A fluoride powder was produced under the same conditions as in Example 1, except that the amount of 28 mass% ammonia (NH3) water added to Aqueous Solution A 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 formula (2) above was 25.0. This fluoride powder was examined in the same manner as in Example 1. The results were a BET diameter of 320 nm, an average primary particle diameter of 450 nm, a coefficient of variation of 0.39, a half-width of the (222) diffraction peak of 0.27°, a peak intensity ratio h1 / h0 of 0.6, and a current increase of 0.3 μA from 4.0 V to 5.0 V. 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 according to formula (3) was 62%.

[0072] Comparative Example 3 A fluoride powder was prepared under the same conditions as in Example 1, except that the pH of aqueous solution A was adjusted to 9 by adding 1.7 g of 28% by mass ammonia (NH3) water 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 equation (2) was 33.0, which is outside the definition of equation (3). This fluoride powder was examined in the same manner as in Example 1. The results were a BET diameter of 372 nm, an average primary particle diameter of 500 nm, a coefficient of variation of 0.64, a half-width of the (222) diffraction peak of 0.24°, a peak intensity ratio h1 / h0 of 0.2, and a current increase from 4.0 V to 5.0 V of -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 according to formula (3) was 60%.

[0073] [Table 1]

[0074] [Table 2]

[0075] [Table 3]

[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, demonstrating 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 battery resistance.

[0077] For reference, Fig. 14 shows 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 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 according to the following formula (1) of 300 nm or less: 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. 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. 2. 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. 2. 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 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 under conditions of a pH of 4.0 to 9.5, a liquid temperature of 5°C or higher, and satisfying the following formula (2): a solid-liquid separation step of performing solid-liquid separation and recovering the solid material; A method for producing a fluoride powder having the above formula. T≦−2.5×pH+50 … (2) Here, T is the liquid temperature (°C), and pH is the pH value of the liquid.

7. 7. The method for producing a fluoride powder according to claim 6, wherein the aqueous solution has a Li / M molar ratio of 2.0 or more and 6.0 or less, and a F / M molar ratio of 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. 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; The method for producing a fluoride powder according to claim 6, comprising the steps of:

10. The fluoride-coated positive electrode active material powder is a powder consisting of positive electrode active material particles having a coating layer on the surface thereof, 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, and whose Li / M molar ratio is 2.0 or more and 6.0 or less and whose F / M molar ratio is 5.0 or more and 9.0 or less, and wherein the fluoride-coated positive electrode active material powder has a coverage of 70% or more according to the following formula (3) in terms of atomic ratios detected by irradiating the outermost surfaces of the powder particles with X-rays using X-ray photoelectron spectroscopy (XPS): Coverage (%) = 100 x I Ａ / (I Ａ +I Ｂ ) … (3) where: I Ａ : The total detected amount (in moles) of elements other than Li that constitute the lithium-containing metal fluoride, which is the main component of the coating layer, excluding the "elements that constitute the positive electrode active material" I 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 step of obtaining a slurry by mixing the fluoride powder according to any one of claims 1 to 5 and a positive electrode active material powder in a liquid medium; 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; A method for producing a fluoride-coated positive electrode active material powder having the above-mentioned formula.

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