Method for producing pyrochlore-type oxide

Abstract

A method for producing a pyrochlore-type oxide containing a plurality of cations including alkali metal cations in the components, the method comprising: a mixing step (S10, S20) in which a plurality of raw materials each containing a plurality of cations are mixed; and a heating step (S11, S21) in which a mixture containing a plurality of raw materials is heated by a liquid phase method at a predetermined temperature, thereby producing a composite oxide having a corundum structure, the composition of which contains at least an alkali metal cation.

Description

[0001] Cross Reference to Related Applications

[0002] This application is based on Japanese Patent Application No. 2023-183092 filed October 25, 2023, the content of which is hereby incorporated by reference. TECHNICAL FIELD

[0003] The present disclosure relates to a manufacturing method of a pyrochlore-type oxide. BACKGROUND

[0004] In Non-Patent Literature 1, as a manufacturing method of a pyrochlore-type oxide, a precursor composed of a Li-containing composite oxide is mixed with a raw material composed of LiF and LaF3, and when the precursor contains Ta, calcination is performed at 1200°C.

[0005] Prior Art Documents

[0006] Non-Patent Literature

[0007] Non-Patent Literature 1: Cyrille Galven et al, New Oxyfluoride Pyrochlores Li 2- x La (1+x) / 3 □ (2x-1) / 3 B2O6F (B=Nb, Ta): Average and Local Structure Characterization by XRD, TEM and 19 F Solid-State NMR Spectroscopy, European Journal of Inorganic Chemistry, Germany, October 11, 2010, 33, 5272-5283 (Cyrille Galven et al., New Oxyfluoride Pyrochlores Li 2-x La (1+x) / 3 □ (2x-1) / 3 B2O6F (B=Nb, Ta): Average and Local Structure Characterization by XRD, TEM and 19 F Solid-State NMR Spectroscopy, European Journal of Inorganic Chemistry, Germany, October 11, 2010, 33, 5272-5283 (Cyrille Galven et al., New Oxyfluoride Pyrochlores Li SUMMARY

[0008] Technical Solution to the Problem

[0009] However, in the production method using the conventional technique described above, since the sintering temperature is high, the generated pyrochlore-type oxide particles are coarse due to grain growth. For example, if the pyrochlore-type oxide is used as a solid electrolyte of a secondary battery, in order to reduce the resistance, it is desirable to make the thickness of the solid electrolyte as thin as possible, and it is desirable to make the pyrochlore-type oxide particles fine.

[0010] In view of the above problems, an object of the present disclosure is to provide a production method of a pyrochlore-type oxide capable of making the pyrochlore-type oxide particles fine.

[0011] To achieve the above object, one embodiment of the present disclosure relates to a production method of a pyrochlore-type oxide containing a plurality of cations including alkali metal cations in the composition, the method including a mixing step and a heating step. In the mixing step, a plurality of raw materials each containing a plurality of cations are mixed. In the heating step, the mixture containing the plurality of raw materials is heated at a predetermined temperature by a liquid phase method, thereby generating a corundum-structured composite oxide containing at least an alkali metal cation in the composition.

[0012] Thus, in the heating step, the corundum-structured composite oxide generated by the liquid phase method can further react to generate the pyrochlore-type oxide. The liquid phase method can produce the pyrochlore-type oxide at a lower temperature than the solid phase reaction, and by making the heating temperature low, the pyrochlore-type oxide particles can be made fine. BRIEF DESCRIPTION OF DRAWINGS

[0013] [ Figure 1 ] is a cross-sectional view showing the structure of a secondary battery relating to the first embodiment.

[0014] [ Figure 2 ] is a view showing the crystal structure of a pyrochlore-type oxide.

[0015] [ Figure 3 ] is a view showing the production steps of a pyrochlore-type oxide of the first embodiment.

[0016] [ Figure 4 ] is an SEM image of a pyrochlore-type oxide of the first embodiment.

[0017] [ Figure 5 ] is a chart showing the particle diameter of a pyrochlore-type oxide of the first embodiment using examples and comparative examples.

[0018] [ Figure 6 ] is a view showing the production steps of a pyrochlore-type oxide of the second embodiment.

[0019] [ Figure 7 ] is an SEM image of a pyrochlore-type oxide of the second embodiment.

[0020] [ Figure 8 [This is a graph showing the particle size of the pyrochlore-type oxide of the second embodiment using examples and comparative examples.] Detailed Implementation

[0021] The following description, with reference to the accompanying drawings, outlines various methods for implementing this disclosure. In each method, parts corresponding to those described in the preceding methods are sometimes given the same reference numerals, and repeated descriptions are omitted. In each embodiment, if only a portion of the structure is described, other methods previously described may be applied to the remaining parts. The combination of combinable parts not explicitly shown in each embodiment is not limited to that which will not significantly impede the combination; embodiments may be partially combined together even if not explicitly shown.

[0022] (First Implementation)

[0023] Hereinafter, a first embodiment of the present disclosure, which applies pyrochlore-type oxides to a solid electrolyte for a secondary battery, will be described using the accompanying drawings. The secondary battery 10 of this first embodiment is a lithium-ion battery that is charged and discharged by the movement of lithium ions between the negative electrode 12 and the positive electrode 14.

[0024] like Figure 1 As shown, the secondary battery 10 includes a negative current collector 11, a negative electrode 12, a positive current collector 13, a positive electrode 14, and a solid electrolyte 15. The solid electrolyte 15 is equivalent to a solid electrolyte for a secondary battery.

[0025] A solid electrolyte 15 is sandwiched between the positive electrode 14 and the negative electrode 12. The negative electrode 12 is in contact with the solid electrolyte 15. The positive electrode 14 is in contact with the solid electrolyte 15. The negative electrode 12 and the positive electrode 14 are connected through the solid electrolyte 15. The secondary battery 10 in this first embodiment is a lithium-ion battery that is charged and discharged by lithium ions moving between the negative electrode 12 and the positive electrode 14 via the solid electrolyte 15.

[0026] A laminate comprising the negative electrode 12, the positive electrode 14, and the solid electrolyte 15 is disposed between the negative electrode current collector 11 and the positive electrode current collector 13. The negative electrode current collector 11 is in contact with the negative electrode 12. The positive electrode current collector 13 is in contact with the positive electrode 14. The negative electrode current collector 11 and the positive electrode current collector 13 are connected via the laminate.

[0027] The negative electrode current collector 11 and the positive electrode current collector 13 can be made of any material suitable for use as a current collector in a lithium-ion battery. In this first embodiment, Cu is used as the negative electrode current collector 11, and Al is used as the positive electrode current collector 13.

[0028] The negative electrode material constituting the negative electrode 12 can be any material that can be used as an active material for a lithium-ion battery negative electrode, such as carbon-based negative electrode materials, oxide-based negative electrode materials, metal-based negative electrode materials, etc. In this first embodiment, a lithium-based negative electrode material or a silicon-based negative electrode material is used.

[0029] The cathode material constituting the cathode 14 can be any material that can be used as a cathode active material for lithium-ion batteries. For example, cobalt-based cathode materials (LiCoO2), nickel-based cathode materials (LiNiO2), manganese-based cathode materials (LiMn2O4), iron phosphate-based cathode materials (LiFePO4), and ternary cathode materials (NMC) with nickel / manganese / cobalt as the main components can be used as cathode 14.

[0030] The solid electrolyte 15 has ionic conductivity, enabling lithium ions to move between the negative electrode 12 and the positive electrode 14. To reduce the resistance of the secondary battery 10, it is desirable to form the solid electrolyte 15 as thin as possible.

[0031] Solid electrolyte 15 is an oxide-based solid electrolyte, which has the chemical formula "Aa 2-α Ab (1+α) / 3 B2O 7-β X γ The pyrochlore-type oxide with a pyrochlore structure is indicated by the symbol "". In order to reduce the thickness of the solid electrolyte 15 as much as possible, it is desirable that the particle size of the pyrochlore-type oxide constituting the solid electrolyte 15 be as small as possible. The primary particle size of the pyrochlore-type oxide in this first embodiment is in the nanometer to micrometer range, specifically in the range of 20 nm to 10 μm.

[0032] The particle size of pyrochlore-type oxides refers to the length of the largest portion of the particle's diameter, also known as the maximum diameter or major axis. In this first embodiment, the mode (peak value) of the particle size distribution is used as the particle size. The particle size of pyrochlore-type oxides can be obtained in the following manner.

[0033] The geometry of the particles was observed using scanning electron microscopy (SEM, TEM) and atomic force microscopy (AFM), and the maximum diameter of the particles was measured. The number of samples N was set to, for example, 30 or more. It was assumed that the maximum diameter distribution of each measured particle followed a log-normal distribution, and the mode value estimated based on this was used as the particle size.

[0034] In the above formula, O represents an oxygen atom, and Aa, Ab, B, and X represent arbitrary elements or groups. Aa, Ab, and B are different kinds of cations, and O and X are different kinds of anions. Aa is an alkali metal cation. The pyrochlore-type oxide contains a plurality of cations including the alkali metal cation Aa and a plurality of cations Ab and B other than the alkali metal cation Aa. That is, the pyrochlore-type oxide contains a plurality of cations including the alkali metal cation Aa in the composition thereof.

[0035] As shown in FIG. 1, the solid electrolyte 15 of the pyrochlore structure has a crystal structure formed of a three-dimensional network of octahedra composed of BO6. BO6 has cation B at the center, and O is disposed at the vertices and shares the vertices with adjacent BO6. In the three-dimensional network composed of BO6, a hexagonal tunnel structure in which cations A and anions X are disposed is formed. Figure 2

[0036] In the above formula, 0.6 < α < 2.0, 0 < β ≤ 1, and 0 < γ ≤ 1. The composition ratio of Aa and Ab changes as α changes, and the composition ratio of O and X changes as β changes.

[0037] The cation Aa is an alkali metal cation. As the alkali metal represented by Aa, any one of Li, Na, K, Rb, and Cs can be used. As the cation Aa, Mg or H other than the alkali metal can also be used. That is, the cation Aa includes at least one or more selected from the group consisting of Li, Na, K, Rb, Cs, Mg, and H. In the present first embodiment, Li is used as Aa. The composition ratio of Aa (2 - α) is in the range of 0 < (2 - α) < 1.4.

[0038] The cation Ab includes at least a lanthanoid element. As the lanthanoid element represented by Ab, at least one of La, Ce, Nd, and Sm can be used. In the present first embodiment, La is used as Ab. The composition ratio of Ab ((1 + α) / 3) is in the range of 0.53 < (1 + α) / 3 < 1.

[0039] The basic structure of the cation Ab is composed of a lanthanoid element, and a part of the lanthanoid element constituting Ab can be substituted with an alkaline earth metal (Ca, Mg, Sr, etc.). It is considered that in the solid electrolyte 15 of the present first embodiment, in the above formula, the ion conductivity is improved by containing a lanthanoid element in the pyrochlore structure of 0.6 < α < 2.0 and 0 < β ≤ 1, which generates a defect in the crystal structure. In the present first embodiment, La is used as Ab.

[0040] ​In the solid electrolyte 15 in this first embodiment, the cation A in the chemical formula of the general pyrochlore structure of "A2B2O7" is a complex cation using lithium metal and a lanthanoid element. This is believed to contribute to an improvement in the ion conductivity of the solid electrolyte 15.

[0041] The cation B is a metal cation different from Aa and Ab, and can be a metal selected from a transition metal or a Group 13 to 15 element. B forms an octahedron surrounded by six O atoms in the crystal. As the transition metal represented by B, a Group 4 transition metal or a Group 5 transition metal can be used, and more specifically, at least one of Nb, Ta, Ti, Zr, Hf, and V can be used. As the Group 13 element represented by B, Al, Ga, and In can be used; as the Group 14 element, Ge and Sn can be used; and as the Group 15 element, Sb and Bi can be used. In this first embodiment, B is Ta.

[0042] The anion X is an anion capable of substituting for the O atom constituting the pyrochlore structure. X differs from the O atom in terms of electronegativity and polarizability. As the anion represented by X, at least one of O, F, Cl, Br, I, S, OH, and P can be used. The composition ratio γ of X is in the range of 0 < γ ≤ 1, and at least a portion of the O atoms constituting the pyrochlore structure is substituted by X. In this first embodiment, F is used as X.

[0043] The solid electrolyte 15 of this first embodiment has a defect structure in which a lattice defect is included in the crystal due to a portion of the O atoms constituting the pyrochlore structure being substituted by an anion different from the O atom in terms of electronegativity and polarizability. It is believed that the solid electrolyte 15 of this first embodiment has an improved ion conductivity due to the defect structure being included in the pyrochlore structure.

[0044] In the solid electrolyte 15 of this first embodiment, as the defect structure, a state in which a portion of Aa and Ab is missing. The chemical formula of the general pyrochlore structure is "A2B2O7", and the composition ratio of the cation A is 2. In this first embodiment, the composition ratios of Aa and Ab are "2 - α" and "(1 + α) / 3", respectively, and 0.6 < α < 2.0, and the sum of the composition ratios of Aa and Ab is less than 2. That is, in the crystal structure of the solid electrolyte 15 of this first embodiment, a portion of at least one of Aa and Ab is missing. In addition, the composition ratio corresponding to the missing portion of Aa and Ab is (2α - 1) / 3.

[0045] In addition, in addition to the deviation in the composition ratio, a defect structure can be formed by making the sum of the valence numbers of the cation composed of Aa, Ab, and B and the anion composed of O and X negative in the above chemical formula.

[0046] In addition, the solid electrolyte 15 of the first embodiment is a complex anion compound including a plurality of anions such as O and X in a pyrochlore structure, and the anion represented by X exists in a BO6coordination octahedron structure, so that the alkali metal of Aa can exist in a position at the center of the space of the BO6coordination octahedron without approaching the BO6coordination octahedron. Therefore, it is considered that the solid electrolyte 15 of the first embodiment has high ionic conductivity when used in application of an electric field such as a battery.

[0047] In addition, since α, β, and γ in the above chemical formula affect the crystal lattice defects and the ionic conductivity, it is desirable to use them within appropriate ranges. When the values of α, β, and γ are large, the defect concentration in the crystal lattice increases, but beyond a certain amount, the concentration of the alkali metal represented by Aa decreases, and the ionic conductivity decreases. Therefore, it is desirable to control α within a range of 0.6 < α < 2.0, β within a range of 0 < β ≤ 1, and γ within a range of 0 < γ ≤ 1.

[0048] In the first embodiment, as the solid electrolyte 15, a pyrochlore oxide represented by "Li 1.25 La 0.58 Ta2O6F" or "Li 1.25 La 0.58 Nb2O6F" is used. That is, Li is used as the cation Aa, La is used as the cation Ab, Ta or Nb is used as the cation B, and F is used as the anion X, and α = 0.75, β = 1, and γ = 1 are set. Hereinafter, "Li 1.25 La 0.58 Ta2O6F" is also referred to as "LLTOF", and "Li 1.25 La 0.58 Nb2O6F" is also referred to as "LLNOF".

[0049] Next, a method for manufacturing the solid electrolyte 15 of the first embodiment will be described. Figure 3 A method for manufacturing the solid electrolyte 15 of the first embodiment will be described. Figure 3 A method for manufacturing the solid electrolyte 15 of the first embodiment will be described.

[0050] (First mixing step)

[0051] In the first mixing step S10, a plurality of raw materials each containing a plurality of cations included in the target compound LLTOF are mixed to obtain a mixture. The plurality of raw materials mixed in the first mixing step S10 include a lithium source, a lanthanum source, and a tantalum source. The lithium source is a raw material of the cation Aa, and is an alkali metal compound. The lanthanum source is a raw material of the cation Ab, and the tantalum source is a raw material of the cation B. As the lithium source, the lanthanum source, and the tantalum source, at least one selected from the group consisting of a fluoride, an acetate, a chloride, a hydroxide, a carbonate, and an oxide can be used. In the present first embodiment, LiF is used as the lithium source, La(OH)3is used as the lanthanum source, and Ta205is used as the tantalum source. In the present first embodiment, a fluoride is used as the alkali metal compound, and LiF is also a fluorine source. In addition, in the case of manufacturing LLNOF, a niobium source Nb205is used instead of the tantalum source Ta205.

[0052] The alkali metal compound used in the present first embodiment has water solubility, and LiF is used in a state of an aqueous solution (dissolution solution). The dissolved LiF is ionized in the aqueous solution. It is not necessary to dissolve all of the LiF in the LiF aqueous solution, and it is sufficient that at least a part of the LiF is dissolved. In the first mixing step S10, particles of La(OH)3and Ta205are mixed in a predetermined ratio into the LiF aqueous solution. In the first mixing step S10, the mixture is obtained in a state of a mixed solution. The mixed solution is adjusted to be alkaline by dissolution of LiF. By adjusting the mixed solution to be alkaline, it is possible to improve the reactivity of hydrothermal synthesis in the first heating step described later, and to improve the yield of the precursor.

[0053] In the present first embodiment, the amount of Li supplied as LiF is set to an amount that is an excess amount with respect to the stoichiometric amount of the target compound LLTOF. That is, the mixture contains an excess amount of Li with respect to the target compound. The excess amount of Li can be, for example, 50 to 100 mol%. By the excess amount of Li, it is possible to increase the pH of the mixed solution, and to improve the reactivity.

[0054] (First heating step)

[0055] Next, the first heating step S11 is performed, that is, the mixed solution obtained by mixing La(OH)3and Ta205into the LiF aqueous solution is heated. In the first heating step S11, the mixed solution is heated at a predetermined temperature in an atmospheric environment or an inert environment by a liquid phase method to generate a precursor.

[0056] The liquid phase method is a synthesis method that generates a crystal using a liquid. The liquid used in the liquid phase method can be a solution obtained by dissolving a raw material in a solvent, or a molten liquid phase raw material. As the liquid phase method, a hydrothermal synthesis method, a solid-liquid synthesis method, a flux method, a sol-gel method, a coprecipitation method, or the like can be used.

[0057] According to the liquid phase method, a low-melting-point alkali metal compound (Li compound in the first embodiment) that can be melted to become a liquid phase can dissolve a high-melting-point and stable transition metal compound (Ta compound in the first embodiment), thereby generating a precursor of a corundum structure. Therefore, compared with a solid phase method in which a synthesis reaction is performed in a solid state, the liquid phase method can perform a synthesis reaction at a lower temperature, thereby reducing the particle diameter of a product.

[0058] In the first embodiment, a hydrothermal synthesis method is employed as the liquid phase method. In the hydrothermal synthesis method, a compound is synthesized by a hydrothermal reaction in which water used as a solvent participates in a reaction at a temperature higher than the boiling point of water and at a pressure higher than atmospheric pressure.

[0059] A hydrothermal synthesis apparatus for hydrothermal synthesis can use an autoclave or a flow-through (continuous) hydrothermal synthesis apparatus. In the first embodiment, an autoclave that is a closed container having heat resistance and pressure resistance is used. In the first embodiment, a mixed solution is put into the autoclave and sealed, and then the autoclave is heated by a heating furnace to perform hydrothermal synthesis. In a closed space inside the autoclave, the mixed solution is heated to a temperature higher than the boiling point of water, so that the pressure of the mixed solution is higher than atmospheric pressure.

[0060] The heating time in the first heating step S11 is more desirably several seconds to several tens of hours. When hydrothermal synthesis is performed in the first heating step S11, the heating temperature is desirably in the range of 150°C to 1000°C, and desirably 500°C or lower from the viewpoint of heat resistance of the hydrothermal synthesis apparatus. The heating temperature in the hydrothermal synthesis is more desirably in the range of 200°C to 400°C. Furthermore, the lower the heating temperature in the hydrothermal synthesis, the smaller the particle diameter of the precursor and the target product.

[0061] The hydrothermal synthesis can be performed in a state in which the temperature and the pressure of the mixed solution are in subcritical water that is lower than the critical point of water (374°C, 22.1 MPa), or in a state in which the temperature and the pressure of the mixed solution are in supercritical water that is higher than the critical point.

[0062] In the first heating step S11, LiTaO3 having a corundum structure and LaF3 having a tysonite structure are generated as precursors by heating the mixed solution. The precursor LiTaO3 is a composite oxide containing a plurality of cations, and at least contains an alkali metal cation in its composition.

[0063] In the first heating step S11, the generation reaction of the precursors is performed by dissolving La(OH)3 and Ta2O5 in the solution by heating the mixed solution. The dissolved La(OH)3 and Ta2O5 are ionized in the solution. La(OH)3 and Ta2O5 do not necessarily have to be completely dissolved in the solution, and at least a part of them can be dissolved.

[0064] In the first heating step S11, a precursor generation reaction is carried out via hydrothermal synthesis. Further, the precursor undergoes a reaction to generate the pyrochlore-type oxide, which is the target compound. Specifically, in the first heating step S11, the precursors LiTaO3 and LaF3 generated via hydrothermal synthesis react to generate the target compound LLTOF. Therefore, the hydrothermal synthesis product contains the precursors LiTaO3 and LaF3, and the target compound LLTOF. By performing hydrothermal synthesis in the first heating step S11, the reaction can be carried out at a lower temperature than the solid-state reaction, thereby enabling the particle size of LLTOF to be refined.

[0065] The hydrothermal synthesis product containing the precursor and target compound is washed with water or an organic solvent (e.g., alcohol, acetone, etc.) as needed and then dried. This yields particulate precursor and target compound.

[0066] In this first embodiment, since the target compound LLTOF can be obtained in the first heating step S11, the subsequent second mixing step S12 and second heating step S13 can be performed as needed.

[0067] (Second mixing process)

[0068] Next, a second mixing step S12 is performed, in which LiF is mixed into the hydrothermal synthesis product obtained in the first heating step to obtain a mixture. The mixing of LiF in the precursor can be carried out as needed.

[0069] (Second heating process)

[0070] Next, a second heating step S13 is performed, in which the mixture of the precursor and LiF is heated and calcined. In the second heating step S13, for example, the mixture is heated at a predetermined temperature under an atmospheric atmosphere or an inert atmosphere to calcine the target compound LLTOF. The target compound LLTOF can be generated by the second heating step S13 using any method, such as a solid-phase reaction, a liquid-phase reaction, or a solid-liquid reaction.

[0071] In the second heating step S13, heating is performed at a temperature higher than the hydrothermal synthesis temperature of the first heating step S11. In the second heating step S13 of this first embodiment, the heating temperature is set in the range of 500°C to 1000°C. The heating temperature of the second heating step S13 is preferably 700°C or lower. Through the second heating step, the target compound LLTOF can be generated from the precursors LiTaO3 and LaF3. Furthermore, by heating at a temperature higher than that of the first heating step S11 in the second heating step S13, the particle size of the target compound LLTOF can be increased. The higher the heating temperature in the second heating step S13, the larger the particle size of the target product LLTOF.

[0072] Through the above process, a substance with the chemical formula "Li" can be obtained. 1.25 La 0.58 Ta2O6F” represents pyrochlore-type oxide crystals.

[0073] Furthermore, by changing the mixing ratio of LiF, La(OH)3, and Ta2O5 in the above manufacturing process, it is possible to obtain a product with the chemical formula "Li". 2-α La (1+α) / 3 Ta2O 7-β F γ The figure indicates a pyrochlore crystal structure. The α, β, and γ components in the chemical formula can be adjusted by changing the mixing ratio of LiF, La(OH)3, and Ta2O5. Furthermore, a portion of the material sublimates during heating. Therefore, α, β, and γ can also be adjusted by changing the heating conditions of the first and second heating processes, the furnace atmosphere, and the furnace size.

[0074] Figure 4 SEM images of the pyrochlore-type oxides of this first embodiment and comparative example are shown. In this first embodiment, the pyrochlore-type oxide is generated by heating a precursor produced by a hydrothermal reaction; while in the comparative example, the pyrochlore-type oxide is generated by heating a precursor produced by a solid-state reaction. The pyrochlore-type oxides in both the first embodiment and the comparative example are LLTOF. The scale bar of the SEM images of the first embodiment and the comparative example is 10 μm.

[0075] In the comparative example, the precursor Li was obtained by calcining a mixture of La₂O₃, Li₂CO₃, and Ta₂O₅. 0.5 La 0.5 In Ta2O6, LiF and LaF3 are mixed and calcined at 1200℃ to generate LLTOF through a solid-state reaction.

[0076] like Figure 4As shown, in the comparative example, the particle size of the pyrochlore-type oxide was much larger than 10 μm, while in this first embodiment, pyrochlore-type oxide with a particle size of several μm or less was obtained. Thus, in this first embodiment, the particles of the pyrochlore-type oxide can be miniaturized. Using the pyrochlore-type oxide of this first embodiment as the solid electrolyte 15 of the secondary battery 10 can reduce the thickness of the solid electrolyte 15 and lower the resistance of the secondary battery 10.

[0077] Next, we will use Figure 5 The examples and comparative examples shown illustrate the particle size of the pyrochlore-type oxide in this first embodiment. Examples 1-6 were used to generate the target compound LLTOF by calcining a precursor generated through hydrothermal synthesis. Example 7 was used to generate the target compound LLNOF by calcining a precursor generated through hydrothermal synthesis. Comparative Example 1 was used to generate the pyrochlore-type oxide by calcining a precursor generated through hydrothermal synthesis.

[0078] The Li compound in Examples 1-5, 7 and Comparative Example 1 was LiF, and the Li compound in Example 6 was LiF and LiOH. The amount of excess Li relative to the target compound was 50 mol% in Examples 1 and 7, and 100 mol% in Examples 2-6 and Comparative Example 1.

[0079] The hydrothermal synthesis temperature of Examples 1, 2, and 7 was 200°C, the hydrothermal synthesis temperature of Example 3 was 240°C, the hydrothermal synthesis temperature of Example 4 was 300°C, the hydrothermal synthesis temperature of Examples 5 and 6 was 400°C, and the hydrothermal synthesis temperature of Comparative Example 1 was 130°C.

[0080] The generated phases of the compounds obtained in Examples 1-7 and Comparative Example 1 were evaluated by X-ray diffraction (XRD) to confirm whether the pyrochlore phase, which is the target compound, was generated. As a result, in Examples 1-6, at least a portion of the pyrochlore phase, which is the target compound, was generated. On the other hand, in Comparative Example 1, the generation of the pyrochlore phase, which is the target compound, was not confirmed at all. That is, pyrochlore-type oxides were not obtained at a hydrothermal synthesis temperature of 130°C.

[0081] The primary particle sizes of the obtained pyrochlore-type oxides were 0.3 μm in Example 1, 0.5 μm in Example 2, 0.6 μm in Example 3, 0.9 μm in Example 4, 2.2 μm in Example 5, 3.0 μm in Example 6, and 2.0 μm in Example 7. Furthermore, to distinguish between the precursor and the target compound, the primary particle sizes of Examples 1 to 7 were determined using SEM-EDX.

[0082] In Examples 1-7, pyrochlore-type oxides with a particle size of less than 3.0 μm were obtained. In Examples 1-7, the lower the hydrothermal synthesis temperature, the smaller the particle size of the pyrochlore-type oxides. Furthermore, in Examples 1 and 2, which had the same hydrothermal synthesis temperature, compared to Example 1, Example 2 had a larger excess of Li, resulting in a larger particle size of the pyrochlore-type oxides. This can be attributed to the fact that a larger excess of Li leads to increased reactivity and a larger particle size.

[0083] In the first embodiment described above, in the first heating step S11, a precursor for a composite oxide with a corundum structure is generated through hydrothermal synthesis. During hydrothermal synthesis, the generated precursor reacts to form a pyrochlore-type oxide. In the hydrothermal synthesis reaction, the pyrochlore-type oxide can be generated at a temperature lower than that of the solid-state reaction, and by lowering the heating temperature, the particles of the pyrochlore-type oxide can be miniaturized. Using the miniaturized pyrochlore-type oxide as the solid electrolyte 15 of the secondary battery 10 allows for a reduction in the thickness of the solid electrolyte 15, thereby lowering the resistance of the secondary battery 10.

[0084] Furthermore, in this first embodiment, the composite oxide with a corundum structure generated as a precursor in the first heating step S11 is calcined in the second heating step S13 to generate a pyrochlore-type oxide. Thus, a pyrochlore-type oxide can be obtained from a precursor obtained through hydrothermal synthesis. Moreover, in the second heating step S13, the heating temperature is set to be higher than the hydrothermal synthesis temperature in the first heating step, thereby allowing for an increase in the particle size of the pyrochlore-type oxide as needed.

[0085] Furthermore, in this first embodiment, an alkaline mixed solution is used for hydrothermal synthesis. This improves the reactivity of the hydrothermal synthesis and increases the yield of the precursor.

[0086] (Second Implementation)

[0087] Next, a second embodiment of this disclosure will be described. In this second embodiment, the parts that are the same as those in the first embodiment described above will be omitted, and only the different parts will be described.

[0088] Figure 6 A method for manufacturing the solid electrolyte 15 according to this second embodiment is shown. Figure 6 A method for manufacturing LLTOF is shown. In the manufacturing method of the solid electrolyte 15 of this second embodiment, the same as in the first embodiment described above, a first mixing step S20, a first heating step S21, a second mixing step S22, and a second heating step S23 are performed sequentially. The first mixing step S20 and the first heating step S21 correspond to the mixing step and the heating step, respectively.

[0089] In the first heating step S21 of this second embodiment, a solid-liquid synthesis method is used as the liquid phase method. In the solid-liquid synthesis method, a low-melting-point alkali metal compound (Li compound in this second embodiment) is melted to form a molten liquid, and the liquid phase Li compound reacts with the solid phase transition metal compound (Ta compound in this second embodiment) to generate a precursor with a corundum structure.

[0090] (First mixing process)

[0091] In the first mixing step S20, LiF, La(OH)3, Ta2O5, and a Li compound, which are used as starting materials, are mixed to obtain a mixture. For example, LiF, LiOH, and Li2CO3 can be used as the Li compound. The amount of Li supplied as the Li compound is set to be an excess amount higher than the stoichiometry of the target compound LLTOF. Furthermore, oxides or fluorides of La, or raw materials that become these substances, may be added to the starting materials, or they may be used instead of hydroxides.

[0092] (First heating process)

[0093] Next, a first heating step S21 is performed, in which the mixture prepared in the first mixing step S20 is heated. In the first heating step S21, the mixture is heated at a predetermined temperature in an atmospheric or inert atmosphere by a solid-liquid synthesis method to generate a precursor. In the solid-liquid reaction of the first heating step S21, the Li compound containing LiF melts and becomes a liquid phase.

[0094] In the first heating step S21, the heating temperature for solid-liquid synthesis is above the melting point of the Li compound, ideally within the range of 500°C to 1000°C. More ideally, the heating temperature for solid-liquid synthesis is within the range of 600°C to 900°C. The Li compound does not need to be completely melted; at least a portion needs to be melted.

[0095] In the first heating step S21, LiTaO3 with a corundum structure and LaF3 with a cerium lanthanum fluoride structure are generated as precursors.

[0096] In the first heating step S21, a precursor generation reaction is carried out through solid-liquid synthesis. Further, a reaction is performed to react the precursors to generate the pyrochlore-type oxide, which is the target compound. Specifically, in the first heating step S21, the precursors LiTaO3 and LaF3 generated through solid-liquid synthesis react to generate the target compound LLTOF. Therefore, the solid-liquid synthesis product contains the precursors LiTaO3 and LaF3, and the target compound LLTOF. Sometimes, the solid-liquid synthesis product may contain residual Li compounds from the starting materials. In the first heating step S11, solid-liquid synthesis allows the reaction to proceed at a lower temperature than the solid-phase reaction, thereby enabling the finer particle size of LLTOF.

[0097] In the solid-liquid mixing process of the first heating step S21, heating can be performed only once or more than twice. When heating is performed twice in the first heating step S21, the heating temperature of the first heating step can be lower than the heating temperature of the second heating step.

[0098] When performing a double heating treatment, the product generated during the first heating can be pulverized before the second heating. The product generated during the first heating includes the precursors LiTaO3 and LaF3, as well as the Li compound used as a starting material. If necessary, the Li compound can be added during the second heating.

[0099] During the second heating, the Li compound melts into a liquid phase. By pulverizing the precursor generated during the first heating, the precursor can be homogenized, and the second heating promotes the formation of the target compound LLTOF.

[0100] In this second embodiment, since the target compound LLTOF can be obtained in the first heating step S21, the second mixing step S22 and the second heating step S23 can be performed as needed. In the second mixing step S22, LiF and a Li compound are mixed into the solid-liquid synthesis product obtained in the first heating step S21 to obtain a mixture. In the second mixing step S22, only LiF may be mixed, or only Li compounds other than LiF may be mixed. Furthermore, since the second heating step S23 is the same as in the first embodiment described above, its description is omitted.

[0101] Figure 7 SEM images of the pyrochlore-type oxide in this second embodiment are shown. In this second embodiment, the pyrochlore-type oxide is generated by heating a precursor produced by a solid-liquid reaction. The pyrochlore-type oxide in this second embodiment is LLTOF. The scale bar of the SEM images in this second embodiment is 10 μm.

[0102] like Figure 7As shown, the manufacturing method of this second embodiment yields pyrochlore-type oxides with a particle size of several μm or less. Thus, in this second embodiment, the particles of the pyrochlore-type oxides can be miniaturized.

[0103] Here, use Figure 8 Examples 8-11 and Comparative Example 2 illustrate the particle size of the pyrochlore-type oxide in this second embodiment. Example 2 involves calcining a precursor generated through solid-liquid synthesis to produce the target compound LLTOF. No Li compound was added in Comparative Example 2.

[0104] The Li compound in Examples 10, 11 and Comparative Example 2 was LiF; the Li compound in Example 8 was LiF and LiOH; and the Li compound in Example 9 was LiF and Li₂CO₃. The amount of excess Li relative to the target compound was 100 mol% in Examples 8–11 and 0 mol% in Comparative Example 2.

[0105] The solid-liquid synthesis temperature for Example 8 and Comparative Example 2 was 600°C, the solid-liquid synthesis temperature for Example 9 was 700°C, and the solid-liquid synthesis temperature for Example 10 was 900°C. Example 11 involved two heating processes during solid-liquid synthesis: the first at 400°C and the second at 700°C.

[0106] The generated phases of the compounds obtained in Examples 8-11 and Comparative Example 2 were evaluated by X-ray diffraction (XRD) to confirm whether the pyrochlore phase, which is the target compound, was generated. In Examples 8-11, at least a portion of the pyrochlore phase, which is the target compound, was generated. On the other hand, in Comparative Example 2, the generation of the pyrochlore phase, which is the target compound, was not confirmed at all. That is, when the excess Li amount was 0 mol%, pyrochlore-type oxides could not be obtained.

[0107] The primary particle sizes of the obtained pyrochlore-type oxides were 0.9 μm in Example 8, 2.5 μm in Example 9, 6.1 μm in Example 10, and 3.1 μm in Example 11. Furthermore, to distinguish the precursor from the target compound, the primary particle sizes of Examples 8 to 11 were determined using SEM-EDX.

[0108] In Examples 8 to 11, pyrochlore-type oxides with a particle size of 6.1 μm or less were obtained. In particular, in Examples 8, 9, and 11, where the solid-liquid synthesis temperature was below 700°C, pyrochlore-type oxides with a particle size of 3.1 μm or less were obtained. That is, the lower the solid-liquid synthesis temperature, the smaller the particle size of the pyrochlore-type oxides.

[0109] In the second embodiment described above, a precursor for a composite oxide with a corundum structure is generated through solid-liquid synthesis in the first heating step S21. During solid-liquid synthesis, the generated precursor reacts to generate a pyrochlore-type oxide. In the solid-liquid synthesis reaction, the pyrochlore-type oxide can be generated at a lower temperature than in a solid-phase reaction, and the particle size of the pyrochlore-type oxide can be miniaturized by lowering the heating temperature.

[0110] This disclosure is not limited to the above embodiments, and various modifications can be made without departing from the spirit of this disclosure. Furthermore, the means disclosed in the above embodiments can be appropriately combined within the scope of implementation.

[0111] For example, in the above embodiments, the pyrochlore-type oxide of this disclosure is applied to the solid electrolyte of a lithium-ion battery, but the pyrochlore-type oxide of this disclosure can also be applied to other secondary batteries. Specifically, when K is used as the alkali metal represented by Aa in the chemical formula of the pyrochlore-type oxide, it can be used as a solid electrolyte for potassium-ion batteries; when Na is used as the alkali metal represented by Aa in the chemical formula, it can be used as a solid electrolyte for sodium-ion batteries.

[0112] Furthermore, in the above embodiments, alkali metal fluorides (specifically LiF) were used in the hydrothermal synthesis of the first heating step S11. However, alkali metal compounds other than fluorides (such as hydroxides) can also be used for hydrothermal synthesis. In this case, since the precursor generated in the hydrothermal synthesis does not contain F, a pyrochlore-type oxide containing F as an anion X can be obtained by mixing a compound containing F in the second mixing step S12 and calcining it in the second heating step S13.

[0113] The characteristics of the method for manufacturing pyrochlore-type oxides disclosed in this specification are as follows.

[0114] (Project 1)

[0115] A method for manufacturing pyrochlore-type oxides, comprising a method for manufacturing pyrochlore-type oxides containing multiple cations in their components, wherein the multiple cations include alkali metal cations.

[0116] The method includes:

[0117] The mixing process (S10, S20) involves mixing multiple raw materials, each containing one of the aforementioned cations;

[0118] The heating process (S11, S21) involves heating a mixture containing the aforementioned raw materials at a predetermined temperature using a liquid-phase method to generate a composite oxide with a corundum structure that contains at least the aforementioned alkali metal cations in its composition.

[0119] (Project 2)

[0120] According to the method for manufacturing pyrochlore-type oxides as described in Project 1 or 2, in the liquid phase method, at least a portion of at least one of the plurality of raw materials is dissolved or melted.

[0121] (Project 3)

[0122] The method for manufacturing pyrochlore-type oxides according to Project 1 or 2, wherein the liquid phase method is a hydrothermal synthesis method.

[0123] (Project 4)

[0124] The method for manufacturing pyrochlore-type oxides according to Project 1 or 2, wherein the predetermined temperature is in the range of 150°C to 1000°C.

[0125] (Project 5)

[0126] According to the manufacturing method of pyrochlore-type oxides described in Projects 1 to 4, the liquid used in the liquid phase method is a solution or molten liquid of an alkali metal compound containing the alkali metal cation.

[0127] (Project 6)

[0128] The method for manufacturing pyrochlore-type oxides according to any one of items 1 to 5, wherein the liquid used in the liquid-phase method is alkaline.

[0129] (Project 7)

[0130] The method for manufacturing pyrochlore-type oxide according to any one of items 1 to 6, wherein the pyrochlore-type oxide generated from the composite oxide consists of particles with a particle size ranging from nanometer to micrometer.

[0131] (Project 8)

[0132] The method for manufacturing pyrochlore-type oxides according to any one of items 1 to 7, when the heating step is used as the first heating step, further includes:

[0133] The second heating process (S13, S23) heats the composite oxide generated in the first heating process to generate the pyrochlore-type oxide.

[0134] (Project 9)

[0135] According to the method for manufacturing pyrochlore-type oxides described in Project 8, the heating temperature of the second heating step is higher than the heating temperature of the first heating step.

[0136] (Project 10)

[0137] The method for manufacturing pyrochlore-type oxides according to any one of items 1 to 9, wherein the plurality of raw materials are selected from any one of the group consisting of fluorides, acetates, chlorides, hydroxides, carbonates and oxides.

[0138] (Project 11)

[0139] The method for manufacturing pyrochlore-type oxide according to any one of items 1 to 10, wherein the pyrochlore-type oxide is an electrolyte (15) for secondary batteries.

[0140] While this disclosure is described based on embodiments, it should not be construed as limiting it to those embodiments or structures. This disclosure also includes various modifications and equivalent variations. Furthermore, although various combinations and forms are shown in this disclosure, including only one element, or other combinations and methods above or below it, are also within the scope and spirit of this disclosure.

Claims

1. A method for producing a pyrochlore-type oxide, which is a method for producing a pyrochlore-type oxide containing a plurality of cations in a composition, the plurality of cations including an alkali metal cation, the method for producing including: mixing steps S10, S20, in which a plurality of raw materials each containing the plurality of cations are mixed; heating steps S11, S21, in which the mixture containing the plurality of raw materials is heated at a predetermined temperature by a liquid phase method, thereby generating a corundum-structured composite oxide containing at least the alkali metal cation in the composition.

2. The method for producing a pyrochlore-type oxide according to claim 1, wherein In the liquid phase method, at least a portion of at least one of the plurality of raw materials is dissolved or melted.

3. The method of producing a pyrochlore-type oxide according to claim 1, wherein, The liquid phase method is a hydrothermal synthesis method.

4. The method for producing a pyrochlore-type oxide according to claim 1, wherein, The predetermined temperature is in a range of 150°C to 1000°C.

5. The method of producing a pyrochlore-type oxide according to claim 1, wherein, The liquid used in the liquid phase method is a solution or a molten liquid of an alkali metal compound containing the alkali metal cation.

6. The method of producing a pyrochlore-type oxide according to claim 1, wherein The liquid used in the liquid phase method is alkaline.

7. The method of producing a pyrochlore-type oxide according to claim 1, wherein The pyrochlore-type oxide generated from the composite oxide is a particle having a particle size in a range of nanometers to micrometers.

8. The method for producing a pyrochlore-type oxide according to claim 1, when the heating step is a first heating step, the method for producing further comprising: a second heating step S13, in which the composite oxide generated in the first heating step is heated to generate the pyrochlore-type oxide.

9. The method of producing a pyrochlore-type oxide according to claim 7, wherein The heating temperature of the second heating step is higher than the heating temperature of the first heating step.

10. The method of producing a pyrochlore-type oxide according to claim 1, wherein, The plurality of raw materials are each any one selected from the group consisting of a fluoride, an acetate, a chloride, a hydroxide, a carbonate, and an oxide.

11. The method of producing a pyrochlore-type oxide according to any one of claims 1 to 10, wherein The pyrochlore-type oxide is an electrolyte 15 for a secondary battery.

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