Method for producing pyrochlore-type oxide

Abstract

A method for producing a pyrochlore-type oxide including, in the composition, a plurality of cations including an alkali metal cation, the method comprising: a mixing step (S10, S20) for mixing a plurality of raw materials each containing a plurality of cations; and a heating step (S11, S21) for heating the mixture containing the plurality of raw materials at a prescribed temperature by using a liquid phase method to generate a composite oxide including, in the composition, at least an alkali metal cation and having a corundum structure.

Description

Method for producing pyrochlore-type oxide CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is based on Japanese Patent Application No. 2023-183092, filed on October 25, 2023, the contents of which are incorporated herein by reference.

[0002] The present disclosure relates to a method for producing a pyrochlore-type oxide.

[0003] Non-Patent Document 1 discloses a method for producing a pyrochlore-type oxide by combining a precursor made of a composite oxide containing Li with LiF and LaF. ３ When the precursor contains Ta, the precursor is fired at 1200°C.

[0004] Cyrille Galven et al, New Oxyfluoride Pyrochlores Li2-xLa(1+x) / 3□(2x-1) / 3B2O6F(B=Nb, Ta): Average and Local Structure Characterization by XRD, TEM and 19F Solid-State NMR Spectroscopy, European Journal of Inorganic Chemistry, Germany, October 11, 2010, 33, 5272-5283

[0005] However, in the above-mentioned conventional manufacturing methods, the high calcination temperature causes the particles of the resulting pyrochlore oxide to grow together, resulting in coarse grains. For example, when using a pyrochlore oxide as a solid electrolyte for a secondary battery, it is desirable to make the thickness of the solid electrolyte as thin as possible to reduce resistance, and it is therefore desirable to make the particles of the pyrochlore oxide finer.

[0006] In view of the above, an object of the present disclosure is to provide a method for producing a pyrochlore-type oxide that can reduce the size of the particles of the pyrochlore-type oxide.

[0007] To achieve the above object, one aspect of the present disclosure provides a method for producing a pyrochlore-type oxide containing multiple cations, including alkali metal cations, in its composition, comprising a mixing step and a heating step. In the mixing step, multiple raw materials, each containing multiple cations, are mixed. In the heating step, the mixture containing the multiple raw materials is heated at a predetermined temperature by a liquid-phase method to produce a composite oxide with a corundum structure containing at least alkali metal cations in its composition.

[0008] As a result, in the heating step, the corundum-structure composite oxide produced by the liquid-phase method reacts further to produce a pyrochlore-type oxide. The liquid-phase method can produce a pyrochlore-type oxide at a lower temperature than the solid-phase reaction, and lowering the heating temperature can make the particles of the pyrochlore-type oxide finer.

[0009] FIG. 1 is a cross-sectional view showing the configuration of a secondary battery according to a first embodiment; FIG. 2 is a diagram showing the crystal structure of a pyrochlore oxide; FIG. 3 is a diagram showing a manufacturing process for the pyrochlore oxide according to the first embodiment; FIG. 4 is an SEM image of the pyrochlore oxide according to the first embodiment; FIG. 5 is a table showing the particle size of the pyrochlore oxide according to the first embodiment using examples and comparative examples; FIG. 6 is a diagram showing the manufacturing process for the pyrochlore oxide according to a second embodiment; FIG. 7 is an SEM image of the pyrochlore oxide according to the second embodiment; and FIG. 8 is a table showing the particle size of the pyrochlore oxide according to the second embodiment using examples and comparative examples.

[0010] Hereinafter, several embodiments for carrying out the present disclosure will be described with reference to the drawings. In each embodiment, parts corresponding to matters described in the preceding embodiment will be assigned the same reference numerals, and duplicated explanations may be omitted. In each embodiment, when only a part of the configuration is described, other previously described embodiments may be applied to the other parts of the configuration. In addition to combinations of parts that are specifically specified as being combinable in each embodiment, it is also possible to partially combine embodiments even if not specified, as long as there is no particular problem with the combination.

[0011] First Embodiment A first embodiment in which the pyrochlore oxide of the present disclosure is applied to a solid electrolyte for a secondary battery will be described below with reference to the 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.

[0012] 1, a secondary battery 10 includes an anode current collector 11, an anode 12, a cathode current collector 13, a cathode 14, and a solid electrolyte 15. The solid electrolyte 15 corresponds to the solid electrolyte for the secondary battery.

[0013] A solid electrolyte 15 is sandwiched between the positive electrode 14 and the negative electrode 12. The negative electrode 12 and the solid electrolyte 15 are in contact with each other. The positive electrode 14 and the solid electrolyte 15 are in contact with each other. The negative electrode 12 and the positive electrode 14 are connected via the solid electrolyte 15. The secondary battery 10 of the 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.

[0014] A laminate including the negative electrode 12, the positive electrode 14, and the solid electrolyte 15 is provided between the negative electrode current collector 11 and the positive electrode current collector 13. The negative electrode current collector 11 and the negative electrode 12 are in contact. The positive electrode current collector 13 and the positive electrode 14 are in contact. The negative electrode current collector 11 and the positive electrode current collector 13 are connected via the laminate.

[0015] Any material that can be used as a current collector for a lithium ion battery can be used for the negative electrode current collector 11 and the positive electrode current collector 13. In the first embodiment, Cu is used as the negative electrode current collector 11, and Al is used as the positive electrode current collector 13.

[0016] The negative electrode material constituting the negative electrode 12 can be any material that can be used as a negative electrode active material for lithium ion batteries, such as a carbon-based negative electrode material, an oxide-based negative electrode material, a metal-based negative electrode material, etc. In the first embodiment, a lithium-based negative electrode material or a Si-based negative electrode material is used.

[0017] The positive electrode material constituting the positive electrode 14 may be any material that can be used as a positive electrode active material for a lithium ion battery. For example, the positive electrode 14 may be a cobalt-based positive electrode material (LiCoO ２ ), nickel-based positive electrode material (LiNiO ２ ), manganese-based positive electrode material (LiMn ２ O ４ ), iron phosphate-based positive electrode material (LiFePO ４ ), a ternary positive electrode material (NMC) mainly composed of nickel, manganese and cobalt, etc. can be used.

[0018] The solid electrolyte 15 has ion conductivity and is capable of transferring lithium ions between the negative electrode 12 and the positive electrode 14. It is desirable that the solid electrolyte 15 be formed as thin as possible in order to reduce the resistance of the secondary battery 10.

[0019] The solid electrolyte 15 is an oxide-based solid electrolyte, and the composition formula is "Aa ２－α Ab （１＋α）／３ B ２ O ７－β X γ In order to make the solid electrolyte 15 as thin as possible, it is desirable that the particle diameter of the pyrochlore oxide constituting the solid electrolyte 15 is as small as possible. The pyrochlore oxide of the first embodiment has a primary particle diameter on the order of nanometers to micrometers, specifically within the range of 20 nm to 10 μm.

[0020] The particle diameter of the pyrochlore oxide is the length of the largest diameter part of the particle, and can also be called the maximum diameter or major diameter. In the first embodiment, the mode (peak value) of the particle diameter distribution is taken as the particle diameter. The particle diameter of the pyrochlore oxide can be obtained as follows.

[0021] The geometric shape of particles is observed using an electron microscope (SEM, TEM) or an atomic force microscope (AFM), and the maximum diameter of the particles to be measured is measured. The number of measurement samples N is, for example, 30 or more. The mode estimated by assuming that the distribution of the maximum diameters of the measured particles follows a log-normal distribution is obtained as the particle diameter.

[0022] In the above composition formula, O represents an oxygen atom, and Aa, Ab, B, and X represent any element or group. Aa, Ab, and B are each different types of cations, and O and X are each different types of anion. Aa is an alkali metal cation. A pyrochlore oxide contains multiple cations in its composition, including the alkali metal cation Aa and multiple cations Ab and B other than the alkali metal cation Aa. In other words, a pyrochlore oxide contains multiple cations in its composition, including the alkali metal cation Aa.

[0023] As shown in FIG. 2, the solid electrolyte 15 having the pyrochlore structure is made of BO ６ It has a crystal structure in which a three-dimensional network of octahedra consisting of BO is formed. ６ is a cation B in the center, with O at the vertex, and adjacent B ６ It shares a vertex with BO. ６ In the three-dimensional network, a hexagonal tunnel structure is formed in which cations A and anions X are arranged.

[0024] In the above composition formula, 0.6<α<2.0, 0<β≦1, and 0<γ≦1. A change in α changes the composition ratio of Aa and Ab, and a change in β changes the composition ratio of O and X.

[0025] The cation Aa is an alkali metal cation. The alkali metal represented by Aa can be any of Li, Na, K, Rb, and Cs. Mg or H, other than alkali metals, may also be used as the cation Aa. That is, the cation Aa contains at least one selected from Li, Na, K, Rb, Cs, Mg, and H. In the first embodiment, Li is used as Aa. The composition ratio (2-α) of Aa is within the range of 0<(2-α)<1.4.

[0026] The cation Ab contains at least a lanthanoid. The lanthanoid represented by Ab can be at least one of La, Ce, Nd, and Sm. In the first embodiment, La is used as Ab. The composition ratio (1+α) / 3 of Ab is within the range of 0.53<(1+α) / 3<1.

[0027] The basic structure of the cation Ab is a lanthanoid, and a portion of the lanthanoid constituting Ab may be substituted with an alkaline earth metal (Ca, Mg, Sr, etc.). The solid electrolyte 15 of the first embodiment has a pyrochlore structure in which α is 0.6<α<2.0 and β is 0<β≦1 in the composition formula, and the inclusion of a lanthanoid in the pyrochlore structure generates defects in the crystal structure, which is thought to improve the ionic conductivity. In the first embodiment, La is used as Ab.

[0028] The solid electrolyte 15 of the first embodiment has a general pyrochlore structure and a composition formula of "A ２ B ２ O ７ " is a composite cation using lithium metal and lanthanoid. This is thought to contribute to the improvement of the ionic conductivity of the solid electrolyte 15.

[0029] The cation B is a metal cation different from Aa and Ab, and is a transition metal or a metal selected from Groups 13 to 15 elements. 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 the first embodiment, Ta is used as B.

[0030] The anion X is an anion that can substitute for the O atoms that constitute the pyrochlore structure. X has electronegativity and polarizability different from those of the O atoms. At least one of O, F, Cl, Br, I, S, OH, and P can be used as the anion represented by X. The composition ratio γ of X is in the range of 0<γ≦1, and at least a portion of the O atoms that constitute the pyrochlore structure are substituted with X. In the first embodiment, F is used as X.

[0031] The solid electrolyte 15 of the first embodiment has a defect structure in which lattice defects are included in the crystal due to some of the O atoms constituting the pyrochlore structure being substituted with anions having electronegativity and polarizability different from those of the O atoms. It is believed that the ionic conductivity of the solid electrolyte 15 of the first embodiment is improved due to the defect structure included in the pyrochlore structure.

[0032] In the solid electrolyte 15 of the first embodiment, a defect structure is formed in which a portion of Aa and Ab is missing. ２ B ２ O ７ ", and the composition ratio of cation A is 2. In contrast, in the solid electrolyte 15 of the first embodiment, the composition ratios of Aa and Ab are "2-α" and "(1+α) / 3", respectively, and 0.6<α<2.0, so the sum of the composition ratios of Aa and Ab is less than 2. In other words, in the crystal structure of the solid electrolyte 15 of the first embodiment, at least one of Aa and Ab is partially deficient. The composition ratio corresponding to the deficient portions of Aa and Ab is (2α-1) / 3.

[0033] In addition to the deviation in the composition ratio, a defect structure can also be formed by making the sum of the valences of the cations consisting of Aa, Ab, and B and the anions consisting of O and X in the above composition formula negative.

[0034] The solid electrolyte 15 of the first embodiment is a composite anion compound in which a plurality of anions such as O and X are contained in a pyrochlore structure, and BO ６ Since there is an anion represented by X in the coordinated octahedral structure, the alkali metal of Aa is BO ６ Without relying on the coordination octahedron, BO ６ It can be positioned in the center of the space between the coordination octahedron, which is thought to be why the solid electrolyte 15 of the first embodiment has high ionic conductivity when used in an electric field such as a battery.

[0035] Furthermore, since α, β, and γ in the composition formula affect lattice defects and ionic conductivity, it is desirable to use them within appropriate ranges. Large values ​​of α, β, and γ increase the defect concentration in the crystal lattice, but if they exceed a certain amount, the concentration of the alkali metal represented by Aa decreases, resulting in a decrease in ionic conductivity. Therefore, it is desirable to control α within the range of 0.6<α<2.0, β within the range of 0<β≦1, and γ within the range of 0<γ≦1.

[0036] In the first embodiment, the solid electrolyte 15 is “Li １．２５ La ０．５８ Ta ２ O ６ F" or "Li １．２５ La ０．５８ Nb ２ O ６ In other words, Li is used as the cation Aa, La as the cation Ab, Ta or Nb as the cation B, and F as the anion X, with α = 0.75, β = 1, and γ = 1. Hereinafter, "Li １．２５ La ０．５８ Ta ２ O ６ F" is also called "LLTOF" and "Li １．２５ La ０．５８ Nb ２ O ６ "LLNOF" is also referred to as "LLNOF."

[0037] Next, a method for producing the solid electrolyte 15 of the first embodiment will be described with reference to Fig. 3. Fig. 3 shows a method for producing an LLTOF. In the method for producing the solid electrolyte 15, a first mixing step S10, a first heating step S11, a second mixing step S12, and a second heating step S13 are performed in this order. The first mixing step S10 and the first heating step S11 correspond to the mixing step and the heating step.

[0038] (First Mixing Step) In the first mixing step S10, a mixture is obtained by mixing multiple raw materials each containing multiple cations contained in the target compound LLTOF. The multiple 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 for cation Aa and is an alkali metal compound. The lanthanum source is a raw material for cation Ab, and the tantalum source is a raw material for cation B. As the lithium source, the lanthanum source, and the tantalum source, at least one selected from the group consisting of fluorides, acetates, chlorides, hydroxides, carbonates, and oxides can be used. In this first embodiment, LiF is used as the lithium source, and La(OH) is used as the lanthanum source. ３ , Ta as a tantalum source ２ O ５ In the first embodiment, a fluoride is used as the alkali metal compound, and LiF is also a fluorine source. ２ O ５ Instead of the niobium source Nb ２ O ５ Just use

[0039] The alkali metal compound used in this first embodiment is water-soluble, and LiF is used in the form of an aqueous solution (dissolution liquid). The dissolved LiF is ionized in the aqueous solution. The entire amount of LiF does not necessarily have to be dissolved in the LiF aqueous solution, as long as at least a portion of LiF is dissolved. In the first mixing step S10, La(OH) ３ and Ta ２ O ５ The particles are mixed in a predetermined ratio. In the first mixing step S10, the mixture is obtained in the form of a mixed solution. The mixed solution is adjusted to be alkaline due to the dissolved LiF. By making the mixed solution alkaline, the reactivity of the hydrothermal synthesis can be improved in the first heating step described below, and the yield of the precursor can be improved.

[0040] In the first embodiment, the amount of Li supplied as LiF is an excess amount exceeding the stoichiometric amount relative to the target compound LLTOF. In other words, the mixture contains an excess amount of Li relative to the target compound. The excess amount of Li can be, for example, 50 to 100 mol %. By providing an excess amount of Li, the pH of the mixed solution can be increased, thereby enhancing reactivity.

[0041] (First heating step) Next, La(OH) ３ and Ta ２ O ５ In the first heating step S11, the mixed solution is heated to a predetermined temperature by a liquid phase method in an air atmosphere or an inert atmosphere to generate a precursor.

[0042] The liquid phase method is a synthesis method for producing crystals using a liquid. The liquid used in the liquid phase method may be a solution in which raw materials are dissolved in a solvent, or may be a molten liquid-phase raw material. Examples of liquid phase methods that can be used include hydrothermal synthesis, solid-liquid synthesis, flux synthesis, sol-gel synthesis, and coprecipitation.

[0043] According to the liquid phase method, a low-melting-point alkali metal compound (Li compound in the first embodiment) that can be melted into a liquid phase dissolves a high-melting-point stable transition metal compound (Ta compound in the first embodiment), thereby producing a precursor of a corundum structure. Therefore, the liquid phase method allows the synthesis reaction to be carried out at a lower temperature than the solid phase method in which the synthesis reaction is carried out in a solid state, and the particle size of the product can be made smaller.

[0044] In the first embodiment, a hydrothermal synthesis method is used as the liquid phase method. In the hydrothermal synthesis method, a compound is synthesized by a hydrothermal reaction involving water used as a solvent at a temperature higher than the boiling point of water and at a pressure higher than atmospheric pressure.

[0045] The hydrothermal synthesis apparatus used for hydrothermal synthesis may be an autoclave or a flow-through (continuous) hydrothermal synthesis apparatus. In this first embodiment, an autoclave, which is a sealed container that is heat-resistant and pressure-resistant, is used. In this first embodiment, the mixed solution is placed in the autoclave, sealed, and then heated in a heating furnace to perform hydrothermal synthesis. By heating the mixed solution in the sealed space inside the autoclave to a temperature higher than the boiling point of water, the pressure of the mixed solution becomes higher than atmospheric pressure.

[0046] The heating time in the first heating step S11 is preferably several seconds to several tens of hours. The heating temperature when performing hydrothermal synthesis in the first heating step S11 is preferably within the range of 150°C to 1000°C, and is preferably 500°C or less from the viewpoint of the heat resistance of the hydrothermal synthesis apparatus. The heating temperature in the hydrothermal synthesis is more preferably within the range of 200°C to 400°C. Furthermore, a lower heating temperature in the hydrothermal synthesis can reduce the particle sizes of the precursor and the target product.

[0047] The hydrothermal synthesis may be carried out in a subcritical water state in which the temperature and pressure of the mixed solution are lower than the critical point of water (374°C, 22.1 MPa), or in a supercritical water state in which the temperature and pressure of the mixed solution are higher than the critical point.

[0048] In the first heating step S11, a corundum structure LiTaO ３ and LaF with a Tysonite structure ３ The precursor LiTaO ３ is a composite oxide containing a plurality of cations, and contains at least an alkali metal cation in its composition.

[0049] In the first heating step S11, the mixed solution is heated to form La(OH) ３ and Ta ２ O5 dissolves in the solution, and the precursor production reaction proceeds. ３ and Ta ２ O ５ is ionized in solution. La(OH) ３ and Ta ２ O ５The entire amount of the compound does not necessarily have to be dissolved in the solution, as long as at least a portion of the compound is dissolved.

[0050] In the first heating step S11, a precursor production reaction proceeds by hydrothermal synthesis, and the precursor further reacts to produce a pyrochlore-type oxide, which is the target compound. That is, in the first heating step S11, the precursor LiTaO produced by hydrothermal synthesis ３ , LaF ３ reacts to produce the target compound LLTOF, and the precursor LiTaO ３ , LaF ３ and the target compound LLTOF. In the first heating step S11, hydrothermal synthesis is performed, which allows the reaction to proceed at a temperature lower than that of a solid-state reaction, and makes it possible to reduce the particle size of LLTOF.

[0051] The hydrothermal synthesis product containing the precursor and the target compound is washed with water or an organic solvent (e.g., alcohol, acetone, etc.) as needed, and then dried, thereby obtaining the precursor and target compound in particulate form.

[0052] In the first embodiment, the target compound LLTOF is obtained in the first heating step S11, and therefore the following second mixing step S12 and second heating step S13 may be performed as needed.

[0053] (Second Mixing Step) Next, the second mixing step S12 is carried out in which LiF is mixed with the hydrothermal synthesis product obtained in the first heating step to obtain a mixture. Mixing of LiF with the precursor may be carried out as needed.

[0054] (Second Heating Step) Next, the mixture of precursor and LiF is heated and fired in the second heating step S13. In the second heating step S13, the mixture is heated at a predetermined temperature in, for example, an air atmosphere or an inert atmosphere to fire the target compound LLTOF. The target compound LLTOF can be produced by any method using, for example, a solid-phase reaction, a liquid-phase reaction, or a solid-liquid reaction.

[0055] In the second heating step S13, heating is performed at a temperature higher than the hydrothermal synthesis temperature in the first heating step S11. In the second heating step S13 of the first embodiment, the heating temperature is set to a range of 500°C to 1000°C. The heating temperature in the second heating step S13 is preferably set to 700°C or less. By the second heating step, the precursor LiTaO ３ , LaF ３ The target compound LLTOF can be produced from the second heating step S13. Furthermore, by heating at a temperature higher than that in 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.

[0056] By the above steps, the composition formula "Li １．２５ La ０．５８ Ta ２ O ６ It is possible to obtain a crystal of a pyrochlore-type oxide represented by "F".

[0057] In the above manufacturing process, LiF, La(OH) ３ , Ta ２ O ５ By changing the mixing ratio of ２－α La （１＋α）／３ Ta ２ O ７－β F γ " can be obtained. LiF, La(OH) ３ , Ta ２ O ５ By changing the mixing ratio of the elements, it is possible to adjust α, β, and γ in the composition formula. Furthermore, when heated, a portion of the material sublimes. Therefore, it is possible to adjust α, β, and γ by changing the heating conditions in the first and second heating steps, the furnace atmosphere, and the furnace size.

[0058] 4 shows SEM images of the pyrochlore oxides of the first embodiment and the comparative example. In the first embodiment, the pyrochlore oxide was produced by heating a precursor produced by a hydrothermal reaction, while in the comparative example, the pyrochlore oxide was produced by heating a precursor produced by a solid-state reaction. The pyrochlore oxide in the first embodiment and the comparative example is LLTOF. The scale of the SEM images of the first embodiment and the comparative example is 10 μm.

[0059] In the comparative example, La ２ O ３ , Li ２ CO ３ , Ta ２ O ５ Precursor Li obtained by calcining the mixture ０．５ La ０．５ Ta ２ O ６ LiF and LaF ３ The mixture was mixed and fired at 1200°C to produce LLTOF through a solid-state reaction.

[0060] 4, in the comparative example, the particle size of the pyrochlore-type oxide is significantly larger than 10 μm, whereas in the first embodiment, a pyrochlore-type oxide having a particle size of several μm or less is obtained. In this way, in the first embodiment, the particles of the pyrochlore-type oxide can be made fine. By using the pyrochlore-type oxide of the first embodiment as the solid electrolyte 15 of the secondary battery 10, the solid electrolyte 15 can be made thinner, and the resistance of the secondary battery 10 can be reduced.

[0061] Here, the particle size of the pyrochlore oxide of this first embodiment will be explained using examples and comparative examples shown in Figure 5. In examples 1 to 6, a precursor produced by hydrothermal synthesis was calcined to produce the target compound LLTOF. In example 7, a precursor produced by hydrothermal synthesis was calcined to produce the target compound LLNOF. In comparative example 1, a precursor produced by hydrothermal synthesis was calcined to produce a pyrochlore oxide.

[0062] The Li compound in Examples 1 to 5, 7, and Comparative Example 1 was LiF, and the Li compounds in Example 6 were 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 to 6 and Comparative Example 1.

[0063] The hydrothermal synthesis temperature for Examples 1, 2, and 7 was 200°C, the hydrothermal synthesis temperature for Example 3 was 240°C, the hydrothermal synthesis temperature for Example 4 was 300°C, the hydrothermal synthesis temperature for Examples 5 and 6 was 400°C, and the hydrothermal synthesis temperature for Comparative Example 1 was 130°C.

[0064] The product phases of the compounds obtained in Examples 1 to 7 and Comparative Example 1 were evaluated by X-ray diffraction (XRD) to confirm whether or not the pyrochlore phase, the target compound, was formed. As a result, it was confirmed that the pyrochlore phase, the target compound, was at least partially formed in Examples 1 to 6. On the other hand, the formation of the pyrochlore phase, the target compound, was not confirmed at all in Comparative Example 1. In other words, a pyrochlore-type oxide was not obtained at a hydrothermal synthesis temperature of 130°C.

[0065] The primary particle diameters 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. The primary particle diameters in Examples 1 to 7 were measured using SEM-EDX in order to distinguish between the precursor and the target compound.

[0066] In Examples 1 to 7, pyrochlore-type oxides with particle sizes of 3.0 μm or less were obtained. In Examples 1 to 7, the particle size of the pyrochlore-type oxide was smaller when the hydrothermal synthesis temperature was lower. Furthermore, in Examples 1 and 2, where the hydrothermal synthesis temperature was the same, the particle size of the pyrochlore-type oxide was larger in Example 2, which had a greater amount of excess Li than in Example 1. This is thought to be because the reactivity improved as the amount of excess Li increased, resulting in a larger particle size.

[0067] In the first embodiment described above, a precursor that is a complex oxide with a corundum structure is produced by hydrothermal synthesis in the first heating step S11. In the hydrothermal synthesis, the produced precursor reacts to produce a pyrochlore-type oxide. In the hydrothermal synthesis reaction, the pyrochlore-type oxide can be produced at a lower temperature than in a solid-state reaction, and by lowering the heating temperature, the particles of the pyrochlore-type oxide can be refined. By using the refined pyrochlore-type oxide as the solid electrolyte 15 of the secondary battery 10, the solid electrolyte 15 can be made thinner, thereby reducing the resistance of the secondary battery 10.

[0068] In the first embodiment, the corundum-structure composite oxide produced as a precursor in the first heating step S11 is fired in the second heating step S13 to produce a pyrochlore-type oxide. In this way, a pyrochlore-type oxide can be obtained from a precursor obtained by hydrothermal synthesis. Furthermore, the heating temperature in the second heating step S13 is higher than that in the hydrothermal synthesis in the first heating step, which allows the particle size of the pyrochlore-type oxide to be increased as needed.

[0069] In addition, in the first embodiment, the hydrothermal synthesis is performed using an alkaline mixed solution, which can improve the reactivity of the hydrothermal synthesis and the yield of the precursor.

[0070] Second Embodiment Next, a second embodiment of the present disclosure will be described. In this second embodiment, the description of the same parts as in the first embodiment will be omitted, and only the different parts will be described.

[0071] Fig. 6 shows a method for producing the solid electrolyte 15 of the second embodiment. Fig. 6 shows a method for producing an LLTOF. In the method for producing the solid electrolyte 15 of the second embodiment, as in the first embodiment, a first mixing step S20, a first heating step S21, a second mixing step S21, and a second heating step S23 are performed in this order. The first mixing step S20 and the first heating step S21 correspond to the mixing step and the heating step.

[0072] In the first heating step 21 of the second embodiment, a solid-liquid synthesis method is used as a liquid phase method. In the solid synthesis method, a low-melting-point alkali metal compound (a Li compound in the second embodiment) melts to form a molten liquid, and the liquid Li compound reacts with a solid transition metal compound (a Ta compound in the second embodiment), thereby generating a precursor of a corundum structure.

[0073] (First Mixing Step) In the first mixing step S20, the starting materials LiF and La(OH) ３ , Ta ２ O ５ The Li compound is mixed to obtain a mixture. The Li compound is, for example, LiF, LiOH, Li ２ CO ３ The amount of Li supplied as the Li compound is an excess amount exceeding the stoichiometry of the target compound LLTOF. Note that La oxide or fluoride, or a raw material via these, may be added to the starting material, or hydroxide may be replaced with these.

[0074] (First Heating Step) Next, the mixture produced in the first mixing step S20 is heated in the first heating step S21. In the first heating step S21, the mixture is heated to a predetermined temperature by a solid-liquid synthesis method in an air atmosphere or an inert atmosphere to generate a precursor. In the solid-liquid reaction in the first heating step S21, the Li compounds including LiF melt and become liquid.

[0075] The heating temperature when performing solid-liquid synthesis in the first heating step S21 is a temperature equal to or higher than the melting point of the Li compound, and is preferably in the range of 500° C. to 1000° C. The heating temperature in solid-liquid synthesis is more preferably in the range of 600° C. to 900° C., for example. The Li compound does not necessarily have to be entirely melted, as long as at least a portion is melted.

[0076] In the first heating step S21, LiTaO having a corundum structure is used as a precursor. ３ and LaF with Tysonite structure ３ is generated.

[0077] In the first heating step S21, a reaction for producing a precursor proceeds by solid-liquid synthesis, and the precursor further reacts to produce a pyrochlore oxide, which is the target compound. That is, in the first heating step S21, the precursor LiTaO produced by solid-liquid synthesis ３ , LaF ３ reacts to produce the target compound LLTOF, and the precursor LiTaO ３ , LaF ３ The solid-liquid synthesis product contains the target compound LLTOF. The starting material Li compound may remain in the solid-liquid synthesis product. In the first heating step S11, by performing solid-liquid synthesis, the reaction can proceed at a lower temperature than in a solid-phase reaction, and the particle size of LLTOF can be reduced.

[0078] In the solid-liquid synthesis in the first heating step S21, heating may be performed only once or may be performed two or more times. When heating is performed twice in the first heating step S21, the heating temperature for the first time can be set lower than the heating temperature for the second time.

[0079] When the heat treatment is performed twice, the product obtained by the first heating is crushed and then the second heating is performed. ３ , LaF ３ The starting material Li compound is also included. If necessary, a Li compound may be added during the second heating.

[0080] In the second heating, the Li compound melts and becomes a liquid phase. By crushing the precursor produced in the first heating, the precursor can be uniformly mixed, and the second heating can promote the reaction to produce the target compound LLTOF.

[0081] In this second embodiment, since the target compound LLTOF is obtained in the first heating step S21, the second mixing step S22 and the second heating step S23 may be performed as needed. In the second mixing step S22, LiF and a Li compound are mixed with 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. Note that the second heating step S23 is the same as in the first embodiment, and therefore a description thereof will be omitted.

[0082] 7 shows an SEM image of the pyrochlore oxide of the second embodiment. In this second embodiment, the precursor produced by solid-liquid reaction is heated to produce the pyrochlore oxide. The pyrochlore oxide of this second embodiment is LLTOF. The scale of the SEM image of this second embodiment is 10 μm.

[0083] 7, the manufacturing method of the second embodiment provides a pyrochlore-type oxide having a particle size of several μm or less. In this way, the second embodiment can reduce the size of the pyrochlore-type oxide particles.

[0084] The particle size of the pyrochlore oxide of this second embodiment will now be described using Examples 8 to 11 and Comparative Example 2 shown in Figure 8. In Example 2, the target compound LLTOF was produced by calcining a precursor produced by solid-liquid synthesis. In Comparative Example 2, no Li compound was added.

[0085] The Li compound in Examples 10 and 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 LiOH. ２ CO ３ The amount of excess Li relative to the target compound was 100 mol % in Examples 8 to 11 and 0 mol % in Comparative Example 2.

[0086] The solid-liquid synthesis temperature in Example 8 and Comparative Example 2 was 600°C, the solid-liquid synthesis temperature in Example 9 was 700°C, and the solid-liquid synthesis temperature in Example 10 was 900°C. In Example 11, heating in solid-liquid synthesis was performed twice, with the first solid-liquid synthesis temperature being 400°C and the second solid-liquid synthesis temperature being 700°C.

[0087] The produced phases of the compounds obtained in Examples 8 to 11 and Comparative Example 2 were evaluated by X-ray diffraction (XRD) to confirm whether or not the pyrochlore phase, the target compound, was produced. As a result, it was confirmed that the pyrochlore phase, the target compound, was at least partially produced in Examples 8 to 11. On the other hand, in Comparative Example 2, the production of the pyrochlore phase, the target compound, was not confirmed at all. In other words, when the amount of excess Li was 0 mol%, no pyrochlore-type oxide was obtained.

[0088] The primary particle diameters 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. Note that the primary particle diameters in Examples 8 to 11 were measured using SEM-EDX in order to distinguish between the precursor and the target compound.

[0089] In Examples 8 to 11, pyrochlore-type oxides with particle sizes of 6.1 μm or less were obtained. In particular, in Examples 8, 9, and 11, where the solid-liquid synthesis temperature was 700° C. or less, pyrochlore-type oxides with particle sizes of 3.1 μm or less were obtained. In other words, the particle size of the pyrochlore-type oxides was smaller when the solid-liquid synthesis temperature was lower.

[0090] In the second embodiment described above, a precursor that is a complex oxide with a corundum structure is produced by solid-liquid synthesis in the first heating step S21. In the solid-liquid synthesis, the produced precursor reacts to produce a pyrochlore-type oxide. In the solid-liquid synthesis reaction, the pyrochlore-type oxide can be produced at a lower temperature than in a solid-phase reaction, and by lowering the heating temperature, the particles of the pyrochlore-type oxide can be made finer.

[0091] The present disclosure is not limited to the above-described embodiments, and various modifications can be made as follows without departing from the spirit of the present disclosure. Furthermore, the means disclosed in the above-described embodiments may be combined as appropriate within the scope of feasibility.

[0092] For example, in the above embodiment, the pyrochlore oxide of the present disclosure is applied to the solid electrolyte of a lithium-ion battery, but the pyrochlore oxide of the present disclosure may also be applied to other secondary batteries. Specifically, when K is used as the alkali metal represented by Aa in the composition formula of the pyrochlore oxide, it can be used as a solid electrolyte for a potassium-ion battery, and when Na is used as the alkali metal represented by Aa in the composition formula, it can be used as a solid electrolyte for a sodium-ion battery.

[0093] In the above embodiment, the hydrothermal synthesis in the first heating step S11 was performed using an alkali metal fluoride (specifically, LiF), but the hydrothermal synthesis may also be performed using an alkali metal compound other than a fluoride (for example, a hydroxide, etc.). In this case, since the precursor produced by the hydrothermal synthesis does not contain the element F, a compound containing the element F can be mixed with the precursor in the second mixing step S12, and then calcined in the second heating step S13, thereby obtaining a pyrochlore-type oxide containing the element F as the anion X.

[0094] The features of the method for producing a pyrochlore oxide disclosed in this specification are as follows: (Item 1) A method for producing a pyrochlore oxide containing multiple cations, including alkali metal cations, in its composition, comprising: a mixing step (S10, S20) of mixing multiple raw materials, each containing the multiple cations; and a heating step (S11, S21) of heating the mixture containing the multiple raw materials at a predetermined temperature by a liquid-phase method to produce a complex oxide with a corundum structure containing at least the alkali metal cations in its composition. (Item 2) A method for producing a pyrochlore oxide according to Item 1, wherein at least one of the multiple raw materials is at least partially dissolved or melted in the liquid-phase method. (Item 3) A method for producing a pyrochlore oxide according to Item 1 or 2, wherein the liquid-phase method is a hydrothermal synthesis method. (Item 4) A method for producing a pyrochlore oxide according to Item 1 or 2, wherein the predetermined temperature is within a range of 150°C to 1000°C. (Item 5) A method for producing a pyrochlore oxide according to any one of Items 1 to 4, wherein the liquid used in the liquid-phase method is a solution or melt of an alkali metal compound containing the alkali metal cation. (Item 6) A method for producing a pyrochlore oxide according to any one of Items 1 to 5, wherein the liquid used in the liquid-phase method is alkaline. (Item 7) A method for producing a pyrochlore oxide according to any one of Items 1 to 6, wherein the pyrochlore oxide produced from the complex oxide is a particle having a particle size on the order of nanometers to micrometers. (Item 8) A method for producing a pyrochlore oxide according to any one of Items 1 to 7, wherein the heating step is a first heating step, and the method further comprises a second heating step (S13, S23) for heating the complex oxide produced in the first heating step to produce the pyrochlore oxide. (Item 9) A method for producing a pyrochlore oxide according to Item 8, wherein the heating temperature in the second heating step is higher than the heating temperature in the first heating step. (Item 10) A method for producing a pyrochlore-type oxide according to any one of Items 1 to 9, wherein each of the plurality of raw materials is at least one selected from the group consisting of fluorides, acetates, chlorides, hydroxides, carbonates, and oxides.(Item 11) The method for producing a pyrochlore-type oxide according to any one of Items 1 to 10, wherein the pyrochlore-type oxide is an electrolyte for a secondary battery (15).

[0095] Although the present disclosure has been described with reference to the embodiments, it is understood that the present disclosure is not limited to the embodiments or structures. The present disclosure also encompasses various modifications and modifications within the scope of equivalents. In addition, although various combinations and forms are shown in the present disclosure, other combinations and forms including only one element, more, or less than one element are also within the scope and spirit of the present disclosure.

Claims

1. A method for producing a pyrochlore oxide containing a plurality of cations including alkali metal cations in its composition, comprising the steps of: a mixing step (S10, S20) of mixing a plurality of raw materials each containing the plurality of cations; a heating step (S11, S21) of heating the mixture containing the plurality of raw materials at a predetermined temperature by a liquid phase method to produce a composite oxide having a corundum structure containing at least the alkali metal cation in its composition; Equipped with The composite oxide is a precursor of the pyrochlore oxide, which is the target compound.

2. The method for producing a pyrochlore oxide according to claim 1, wherein the pyrochlore oxide has a composition formula of Aa 2−α Ab (1+α) / 3 B 2 O 7−β X γ, where Aa is an alkali metal, Ab contains at least a lanthanoid, B is a cation different from Aa and Ab, and X is an anion that can be substituted for an O atom constituting the pyrochlore solid electrolyte, and in the composition formula, α is in the range of 0.6<α<2.0, β is in the range of 0<β≦1, and γ is in the range of 0<γ≦1, and the pyrochlore oxide contains a defect structure.

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

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

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

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

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

8. 2. The method for producing a pyrochlore-type oxide according to claim 1, wherein the pyrochlore-type oxide produced from the composite oxide is in the form of particles having a particle size on the order of nanometers to micrometers.

9. 2. The method for producing a pyrochlore-type oxide according to claim 1, further comprising a second heating step (S13) of heating the composite oxide produced in the first heating step to produce the pyrochlore-type oxide, when the heating step is a first heating step.

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

11. 2. The method for producing a pyrochlore-type oxide according to claim 1, wherein each of the plurality of raw materials is at least one selected from the group consisting of fluorides, acetates, chlorides, hydroxides, carbonates, and oxides.

12. 12. The method for producing a pyrochlore-type oxide according to claim 1, wherein the pyrochlore-type oxide is an electrolyte (15) for a secondary battery.