Method for producing pyrochlore-type oxide

Abstract

This method for producing a pyrochlore-type oxide is provided with: a mixing step (S30) for mixing a composite oxide that contains at least a cation other than an alkali metal cation, and an alkali metal compound that contains an alkali metal cation; and a firing step (S40) for generating a pyrochlore-type oxide by heating the mixture containing the composite oxide and the alkali metal compound at a prescribed temperature. If the composite oxide contains an alkali metal cation, the composition ratio of the alkali metal cation in the composite oxide is less than the composition ratio of the alkali metal cation in the alkali metal compound. The mixture contains the alkali metal compound in an amount exceeding the stoichiometric ratio thereof to the pyrochlore-type oxide. In the firing step, the alkali metal compound is liquefied by heating at the prescribed temperature.

Description

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

[0001] This application is based on Japanese Patent Application No. 2023-183094, 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 oxide containing multiple cations, including alkali metal cations, comprising a mixing step and a calcination step. In the mixing step, a composite oxide containing at least cations other than alkali metal cations is mixed with an alkali metal compound containing alkali metal cations. In the calcination step, the mixture containing the composite oxide and the alkali metal compound is heated at a predetermined temperature to produce a pyrochlore oxide. When the composite oxide contains alkali metal cations, the composition ratio of the alkali metal cations in the composite oxide is less than the composition ratio of the alkali metal cations in the alkali metal compound. The mixture contains an alkali metal compound in an amount exceeding the stoichiometric ratio relative to the pyrochlore oxide. In the calcination step, the alkali metal compound is liquefied by heating at a predetermined temperature.

[0008] This allows the production of pyrochlore-type oxides through a solid-liquid reaction in which a liquid alkali metal compound reacts with a solid composite oxide. The solid-liquid reaction produces pyrochlore-type oxides at lower temperatures than the solid-state reaction, and lowering the firing temperature allows for finer particles of the pyrochlore-type oxides.

[0009] It is a cross-sectional view showing the configuration of a secondary battery according to an embodiment. It is a diagram showing the crystal structure of a pyrochlore-type oxide. It is a diagram showing a manufacturing process of a pyrochlore-type oxide. It is an SEM image of a pyrochlore-type oxide. It is a table showing the types and particle sizes of precursors and Li compounds of Examples and Comparative Examples.

[0010] Hereinafter, an embodiment in which the pyrochlore oxide of the present disclosure is applied to a solid electrolyte for a secondary battery will be described with reference to the drawings. The secondary battery 10 of this embodiment is a lithium-ion battery in which charging and discharging are performed by the movement of lithium ions between the negative electrode 12 and the positive electrode 14.

[0011] 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.

[0012] 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. The positive electrode 14 and the solid electrolyte 15 are in contact. The negative electrode 12 and the positive electrode 14 are connected via the solid electrolyte 15. The secondary battery 10 of this 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.

[0013] 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.

[0014] 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 this embodiment, Cu is used as the negative electrode current collector 11, and Al is used as the positive electrode current collector 13.

[0015] 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 this embodiment, a lithium-based negative electrode material or a Si-based negative electrode material is used.

[0016] 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.

[0017] 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.

[0018] 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 this embodiment has a primary particle diameter in the range of 0.01 μm to 10 μm.

[0019] The particle diameter of a 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 this embodiment, the mode (peak value) of the particle diameter distribution is taken as the particle diameter. The particle diameter of a pyrochlore oxide can be obtained as follows.

[0020] 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.

[0021] 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.

[0022] As shown in FIG. 2, the solid electrolyte 15 having the pyrochlore structure is made of BO６ 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.

[0023] 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.

[0024] 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. In other words, the cation Aa contains at least one selected from Li, Na, K, Rb, Cs, Mg, and H. In this embodiment, Li is used as Aa. The composition ratio (2-α) of Aa is within the range of 0<(2-α)<1.4.

[0025] 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 this 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.

[0026] 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 this embodiment has a pyrochlore structure in which α is 0.6<α<2.0 and β is 0<β≦1 in the above composition formula. This is thought to result in defects in the crystal structure, improving the ionic conductivity. In this embodiment, La is used as Ab.

[0027] The solid electrolyte 15 of this embodiment has a general pyrochlore structure and a composition formula of "A ２ B２ 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.

[0028] 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 this embodiment, Ta is used as B.

[0029] 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 this embodiment, F is used as X.

[0030] The solid electrolyte 15 of this 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 this embodiment is improved due to the defect structure included in the pyrochlore structure.

[0031] In the solid electrolyte 15 of this 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 this 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 this 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.

[0032] 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.

[0033] The solid electrolyte 15 of this embodiment is a composite anion compound in which a pyrochlore structure contains multiple anions such as O and X, 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 this embodiment has high ionic conductivity when used under an electric field such as in a battery.

[0034] 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.

[0035] In this embodiment, the solid electrolyte 15 is “Li １．２５ La ０．５８ Ta ２ O ６In other words, Li is used as the cation Aa, La as the cation Ab, Ta 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 "LLTOF" is also referred to as "LLTOF."

[0036] Next, a method for producing the solid electrolyte 15 of this embodiment will be described with reference to Fig. 3. In the method for producing the solid electrolyte 15, a first mixing step S10, a first firing step S20, a second mixing step S30, and a second firing step S40 are performed in this order. The second mixing step S30 and the second firing step S40 correspond to the mixing step and the firing step.

[0037] (First Mixing Step) In the first mixing step S10, a mixture is obtained by mixing a plurality of raw materials each containing a plurality of cations contained in the target compound LLTOF. The plurality of raw materials mixed in the first mixing step S10 include a lanthanum source and a tantalum source. The lanthanum source is a raw material for cation Ab, and the tantalum source is a raw material for cation B. The lanthanum source and the tantalum source may be oxides, carbonates, fluorides, acetates, chlorides, hydroxides, etc. In this embodiment, the lanthanum source is La ２ O ３ , Ta as a tantalum source ２ O ５ In the first mixing step, La ２ O ３ and Ta ２ O ５ The particles are mixed in a predetermined ratio.

[0038] (First firing step) Next, La ２ O ３ , Ta ２ O ５ The first firing step S20 is performed to fire the mixture. The first firing step S20 can be regarded as a pre-firing step. In the first firing step S20, the mixture is heated in an air atmosphere or an inert atmosphere to form the precursor La. ０．２９ TaO ３ The mixture is baked.

[0039] The heating time in the first baking step S20 is preferably 1 to 20 hours, more preferably 5 to 10 hours. The heating temperature in the first baking step S20 is preferably in the range of 400°C to 1400°C, more preferably in the range of 500°C to 800°C.

[0040] In the first firing step S20, La ２ O ３ and Ta ２ O ５ In the first baking step S20, La is reacted as a precursor of the target compound. ０．２９ TaO ３ In the first firing step, the precursor La ０．２９ TaO ３ In addition to the raw material La ２ O ３ and Ta ２ O ５ may remain unreacted.

[0041] Precursor La ０．２９ TaO ３ is a perovskite-type oxide. ０．２９ TaO ３ is a composite oxide containing a plurality of cations, and contains at least cations other than alkali metal cations. The precursor of this embodiment does not contain an alkali metal cation (Li in this embodiment) corresponding to the cation Aa, but contains La as the cation Ab and Ta as the cation B. That is, the precursor La ０．２９ TaO ３ is a composite oxide containing a plurality of cations Ab and B other than at least the alkali metal cation Aa in its composition.

[0042] The precursor essentially does not contain alkali metal cations, but may contain trace amounts of alkali metal cations, and if the precursor contains alkali metal cations, the composition ratio of the alkali metal cations in the precursor is less than the composition ratio of the alkali metal cations in LiF.

[0043] (Second Mixing Step) Next, a lithium source and a fluorine source are prepared as raw materials for the target compound LLTOF, and these are mixed with a precursor La ０．２９ TaO ３ A second mixing step S30 is performed in which the lithium source is a raw material for the cation Aa, and the fluorine source is a raw material for the anion X. The lithium source may be an oxide, carbonate, fluoride, acetate, chloride, hydroxide, or the like. In this embodiment, LiF is used as the lithium source and the fluorine source. LiF is an alkali metal compound containing an alkali metal cation.

[0044] In the second mixing step, LiF is mixed with the precursor La at a predetermined ratio. ０．２９ TaO ３ The mixture contains LiF in an amount exceeding the stoichiometric ratio (molar ratio) relative to the target compound LLTOF. In other words, the mixture contains an excessive amount of LiF relative to the target compound. Therefore, in the second firing step performed after the second mixing step, the LiF melts and liquefies, resulting in a mixture of liquid LiF and solid La. ０．２９ TaO ３ The stoichiometric ratio is the ratio of the number of moles of the reactant LiF to the number of moles of the target compound LLTOF.

[0045] The amount of LiF is preferably within the range of 1.5 to 3 times the stoichiometric ratio (molar ratio) relative to the target compound. If the amount of LiF is too small, the decrease in LiF associated with the progress of the reaction in the second firing step may result in insufficient liquid phase LiF, making it impossible to maintain the solid-liquid reaction. Therefore, the amount of LiF is preferably 1.5 times or more the stoichiometric ratio. If the amount of LiF is too large, a large amount of LiF remains during the production of the target compound, resulting in a large amount of impurities. Furthermore, if the amount of LiF is too large, the composition of the product is likely to deviate from the target compound LLTOF. Therefore, the amount of LiF is preferably 3 times or less the stoichiometric ratio. In this embodiment, the amount of LiF is set to twice the stoichiometric ratio relative to the target compound.

[0046] (Second baking step) Next, precursor La ０．２９ TaO ３The second firing step S40 is performed to fire the mixture of the lithium source LiF and the lithium source Li. The second firing step S40 can be regarded as a firing step. In the second firing step S40, the mixture of the lithium source LiF and the lithium source Li is fired in an air atmosphere or an inert atmosphere. ０．２９ TaO ３ The mixture of LiF and LLTOF is heated at a predetermined temperature to calcinate the target compound LLTOF.

[0047] In the second firing step S40, LiF is melted by heating and becomes a liquid phase. That is, in the second firing step S40, the liquid phase LiF and the solid phase La are melted. ０．２９ TaO ３ The target compound LLTOF is produced by a solid-liquid reaction.

[0048] The mixture contains an excess amount of LiF relative to the target compound, so even if the amount of LiF decreases as the reaction proceeds, a sufficient amount of liquid-phase LiF can be secured for the solid-liquid reaction.

[0049] The heating time in the second baking step S40 is preferably 1 to 20 hours, and more preferably 1 to 10 hours. The heating temperature in the second baking step S40 is preferably in the range of 500°C to 1000°C, and more preferably in the range of 650°C to 850°C.

[0050] In the second firing step S40, firing at a high temperature increases the particle size of the product, so it is desirable to lower the heating temperature to reduce the particle size of the product. Furthermore, firing at a high temperature tends to volatilize Li and F, which can lead to deviations in the composition of the product from the target compound LLTOF. Therefore, the heating temperature in the second firing step S40 is desirably 1000°C or lower, and more desirably 850°C or lower.

[0051] Furthermore, if the heating temperature in the second baking step S40 is too low, the reactivity decreases, the reaction rate slows down, and it takes a long time to produce the pyrochlore oxide. Therefore, in order to prevent the reaction rate from slowing down, the heating temperature in the second baking step S40 is preferably 500°C or higher, and more preferably 650°C or higher.

[0052] In the second firing step S40, La ０．２９ TaO３ A eutectic reaction occurs between the precursor La and LiF. ０．２９ TaO ３ Since the material does not contain Li, a eutectic reaction easily occurs between the material and LiF.

[0053] Due to the eutectic reaction, LiF melts at a temperature lower than its natural melting point (848°C). For this reason, in this embodiment, the heating temperature in the second baking step S40 is set to a temperature lower than the melting point of LiF (848°C). Specifically, the heating temperature in the second baking step S40 is set to 700°C.

[0054] In the second firing step S40, La ０．２９ TaO ３ The target compound LLTOF is produced by firing LiF and LiF. If impurities such as LiF remain in the product, they can be separated by washing with water or a solvent. In the second firing step S40, the liquid phase LiF and the solid phase La are separated. ０．２９ TaO ３ The solid-liquid reaction of reacting the above allows the reaction to proceed at a lower temperature than in a solid-state reaction, which allows the particle size of the pyrochlore-type oxide LLTOF to be reduced.

[0055] 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".

[0056] In the above manufacturing process, La ２ O ３ , Ta ２ O ５ By changing the mixing ratio of LiF, the composition formula "Li ２－α La （１＋α）／３ Ta ２ O ７－β F γ It is possible to obtain a crystal with a pyrochlore structure represented by La ２ O ３ , Ta ２ O ５By changing the mixing ratio of Li and LiF, the α, β, and γ in the composition formula can be adjusted. Furthermore, when the material is fired, a portion of the material sublimes. Therefore, the α, β, and γ can be adjusted by changing the firing conditions, the firing furnace atmosphere, and the firing furnace size in the first firing step and the second firing step.

[0057] 4 shows SEM images of the pyrochlore oxides of this embodiment and the comparative example. In this embodiment, the pyrochlore oxide was produced by a solid-liquid reaction, while in the comparative example, the pyrochlore oxide was produced by a solid-phase reaction. The pyrochlore oxides of this embodiment and the comparative example are LLTOF. The scale of the SEM image of this embodiment is 1 μm, while the scale of the SEM image of the comparative example is 10 μm.

[0058] 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. ３ was the chemical equivalent to the target compound LLTOF.

[0059] 4, in the comparative example, the particle size of the pyrochlore oxide is significantly larger than 10 μm, whereas in this embodiment, a pyrochlore oxide having a particle size of several μm or less is obtained. In this manner, in this embodiment, the particles of the pyrochlore oxide are made finer. By using the pyrochlore oxide of this 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.

[0060] Here, the particle size of the pyrochlore oxide of this embodiment will be described using examples and comparative examples shown in Figure 5. In examples 1 to 6, a mixture of a precursor not containing Li and a Li compound in an amount exceeding the stoichiometric ratio to the target compound is fired by a solid-liquid reaction. In comparative examples 1 and 2, a mixture of a precursor containing Li and a Li compound in a chemically equivalent amount to the target compound is fired by a solid-phase reaction.

[0061] The precursors of Examples 1 to 6 and Comparative Examples 1 and 2 are complex oxides containing multiple cations, and are perovskite-type oxides. ０．２９ TaO ３ The precursors of Comparative Examples 1 and 2 were Li ０．５ La ０．５ Ta ２ O ６ The Li compound in Examples 1 to 4 and Comparative Examples 1 and 2 was LiF, the Li compound in Example 5 was LiF and LiCl, and the Li compound in Example 6 was LiF and LiCl. ２ CO ３ is.

[0062] The stoichiometric ratio (molar ratio) of the Li compound to the target compound is 1.5 in Example 1, 2.0 in Examples 2, 4 to 6, 3.0 in Example 3, and 1.0 in Comparative Examples 1 and 2. In Example 5, the stoichiometric ratio of each of LiF and LiCl is 1.0, and the total stoichiometric ratio is 2.0. Similarly, in Example 6, the stoichiometric ratio of LiF and Li ２ CO ３ The stoichiometric ratio of each of the above is 1.0, and the total stoichiometric ratio is 2.0. The firing temperatures of Examples 1 to 3, 5, and 6 and Comparative Example 2 are 700°C, the firing temperature of Example 5 is 1000°C, and the firing temperature of Comparative Example 1 is 1200°C.

[0063] The product phases of the compounds obtained in Examples 1 to 6 and Comparative Examples 1 and 2 were evaluated by X-ray diffraction (XRD). As a result, a pyrochlore phase was confirmed in Examples 1 to 6 and Comparative Example 1, but not in Comparative Example 2. In other words, in Examples 1 to 6, which used a liquid-phase reaction, pyrochlore-type oxide was obtained at a low temperature of 1000°C or less. In contrast, in Comparative Examples 1 and 2, which used a solid-phase reaction, pyrochlore-type oxide was obtained at a high temperature of 1200°C, but pyrochlore-type oxide could not be obtained at a low temperature of 700°C.

[0064] The primary particle diameters of the obtained compounds were 0.4 μm in Example 1, 1.0 μm in Example 2, 3.0 μm in Example 3, 7.5 μm in Example 4, 2.5 μm in Example 5, 2.3 μm in Example 6, 20.6 μm in Comparative Example 1, and 1.0 μm in Comparative Example 2.

[0065] Comparative Example 1, where the firing temperature was 1200°C, had a particle diameter of 20.6 μm, while Examples 1 to 6, where the firing temperature was 1000°C or less, had particle diameters of 7.5 μm or less, which were significantly smaller. Furthermore, Examples 1 to 3, 5, and 6, where the firing temperature was 700°C, had smaller particle diameters than Example 4, where the firing temperature was 1000°C. In Comparative Example 2, where the firing temperature was 700°C, a particle diameter of 1.0 μm was obtained, but as described above, the target compound was not produced due to the low temperature at which the solid-state reaction was carried out.

[0066] In the embodiment described above, a pyrochlore-type oxide is produced by heating and firing a mixture of a composite oxide, which is a precursor of the pyrochlore-type oxide and does not contain an alkali metal compound, and an alkali metal compound in an amount exceeding the stoichiometric ratio relative to the pyrochlore-type oxide. This liquefies the alkali metal compound, and a solid-liquid reaction is carried out in which the liquid alkali metal compound reacts with the solid composite oxide to produce the pyrochlore-type oxide. The solid-liquid reaction produces the pyrochlore-type oxide at a lower temperature than the solid-state reaction, and lowering the firing temperature allows the particles of the pyrochlore-type oxide to be refined. Using the refined pyrochlore-type oxide as the solid electrolyte 15 of the secondary battery 10 allows the solid electrolyte 15 to be made thinner, thereby reducing the resistance of the secondary battery 10.

[0067] Furthermore, the eutectic reaction between the solid-phase composite oxide and the liquid-phase alkali metal compound causes the alkali metal compound to melt at a temperature lower than its inherent melting point. For this reason, in this embodiment, the firing temperature for the mixture of the composite oxide and the alkali metal compound is set lower than the melting point of the alkali metal compound. Because the eutectic reaction causes the alkali metal compound contained in the mixture to melt at a temperature lower than its melting point, the alkali metal compound can be melted and a solid-liquid reaction can occur even if the firing temperature is lower than the melting point of the alkali metal compound. As a result, the pyrochlore oxide can be produced at a lower firing temperature, and the particles of the pyrochlore oxide can be made finer.

[0068] 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.

[0069] 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.

[0070] 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, comprising: a mixing step (S30) of mixing a complex oxide containing at least cations other than the alkali metal cations with an alkali metal compound containing the alkali metal cations; and a calcination step (S40) of heating the mixture containing the complex oxide and the alkali metal compound at a predetermined temperature to produce the pyrochlore oxide, wherein if the complex oxide contains the alkali metal cations, the composition ratio of the alkali metal cations in the complex oxide is less than the composition ratio of the alkali metal cations in the alkali metal compound, the mixture contains the alkali metal compound in an amount greater than the stoichiometric ratio relative to the pyrochlore oxide, and the calcination step liquefies the alkali metal compound by heating at the predetermined temperature. (Item 2) A method for producing a pyrochlore oxide according to Item 1, wherein the predetermined temperature is within a range of 500°C to 1000°C. (Item 3) A method for producing a pyrochlore oxide according to Item 1 or 2, wherein the predetermined temperature is lower than the melting point of the alkali metal compound. (Item 4) A method for producing a pyrochlore oxide according to any one of Items 1 to 3, wherein the primary particle diameter of the pyrochlore oxide is within the range of 0.01 μm to 10 μm. (Item 5) A method for producing a pyrochlore oxide according to any one of Items 1 to 4, wherein the composite oxide does not contain the alkali metal cation in its composition. (Item 6) A method for producing a pyrochlore oxide according to any one of Items 1 to 5, wherein the composite oxide is a perovskite-type oxide. (Item 7) A method for producing a pyrochlore oxide according to any one of Items 1 to 6, wherein the pyrochlore oxide is produced using a plurality of raw materials each containing the plurality of cations, and each of the plurality of raw materials is at least one selected from the group consisting of fluoride, acetate, chloride, hydroxide, carbonate, and oxide.(Item 8) The method for producing a pyrochlore-type oxide according to any one of Items 1 to 7, wherein the pyrochlore-type oxide is a solid electrolyte (15) for a secondary battery.

[0071] 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 type oxide containing multiple cations including alkali metal cations in its composition, comprising: a mixing step (S30) of mixing a complex oxide containing at least cations other than the alkali metal cations with an alkali metal compound containing the alkali metal cation; and a calcination step (S40) of heating the mixture containing the complex oxide and the alkali metal compound at a predetermined temperature to produce the pyrochlore type oxide, wherein when the complex oxide contains the alkali metal cations, the composition ratio of the alkali metal cations in the complex oxide is less than the composition ratio of the alkali metal cations in the alkali metal compound, the mixture contains the alkali metal compound in an amount exceeding the stoichiometric ratio relative to the pyrochlore type oxide, and in the calcination step, the alkali metal compound is liquefied by heating at the predetermined temperature.

2. A method for producing a pyrochlore type oxide according to claim 1, wherein the predetermined temperature is within the range of 500°C to 1000°C.

3. The method for producing a pyrochlore type oxide according to claim 1, wherein the predetermined temperature is lower than the melting point of the alkali metal compound.

4. The method for producing a pyrochlore type oxide according to claim 1, wherein the primary particle size of the pyrochlore type oxide is within the range of 0.01 μm to 10 μm.

5. The method for producing a pyrochlore type oxide according to claim 1, wherein the composite oxide does not contain the alkali metal cation in its composition.

6. The method for producing a pyrochlore type oxide according to claim 1, wherein the composite oxide is a perovskite type oxide.

7. A method for producing a pyrochlore-type oxide as described in claim 1, wherein the pyrochlore-type oxide is produced using a plurality of raw materials each containing the plurality of cations, and each of the plurality of raw materials is at least one selected from the group consisting of fluorides, acetates, chlorides, hydroxides, carbonates and oxides.

8. A method for producing a pyrochlore type oxide according to any one of claims 1 to 7, wherein the pyrochlore type oxide is a solid electrolyte (15) for a secondary battery.

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