Method of producing pyrochlore type oxide

Abstract

To provide a method of producing a pyrochlore type oxide, capable of making the particles of a pyrochlore type oxide finer.SOLUTION: The method of producing a pyrochlore type oxide, comprises mixing step S30 of mixing a composite oxide at least including a cation other than an alkali metal cation and an alkali metal compound including an alkali metal cation, and calcination step S40 of heating the mixture including the composite oxide and the alkali metal compound at a specified temperature to form a pyrochlore type oxide. When the composite oxide includes an alkali metal cation, the composition ratio of the alkali metal cation in the composite oxide is below the composition ratio of the alkali metal cation in the alkali metal compound. The mixture includes the alkali metal compound in the amount exceeding the stoichiometric ratio with respect to the pyrochlore type oxide. In the calcination step, the alkali metal compound is liquefied by heating at a specified temperature.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present disclosure relates to a method for producing a pyrochlore-type oxide. [Background technology]

[0002] Non-Patent Document 1 describes a method for producing a pyrochlore-type oxide by combining a precursor made of a composite oxide containing Li with LiF and LaF. 3 It is described that the raw materials consisting of these are mixed, and when the precursor contains Ta, the mixture is fired at 1200°C. [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] 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 Summary of the Invention [Problem to be solved by the invention]

[0004] However, in the above-mentioned conventional manufacturing method, the sintering temperature is high, so the particles of the pyrochlore-type oxide produced grow together and become coarse. For example, when using a pyrochlore-type oxide as a solid electrolyte for a secondary battery, it is desirable to make the thickness of the solid electrolyte as thin as possible in order to reduce resistance, and it is desirable to make the particles of the pyrochlore-type oxide fine.

[0005] In view of the above, an object of the present disclosure is to provide a method for producing a pyrochlore-type oxide capable of reducing the size of pyrochlore-type oxide particles. [Means for solving the problem]

[0006] In order to achieve the above object, one aspect of the present disclosure is a method for producing a pyrochlore oxide containing multiple cations including an alkali metal cation, comprising a mixing step (S30) and a calcination step (S40). In the mixing step, a complex oxide containing at least cations other than an alkali metal cation is mixed with an alkali metal compound containing an alkali metal cation. In the calcination step, the mixture containing the complex oxide and the alkali metal compound is heated at a predetermined temperature to produce a pyrochlore oxide. When the complex oxide contains an alkali metal cation, the composition ratio of the alkali metal cation in the complex oxide is less than the composition ratio of the alkali metal cation in the alkali metal compound. The mixture contains an alkali metal compound in a stoichiometric ratio exceeding that of the pyrochlore oxide. In the calcination step, the alkali metal compound is liquefied by heating at a predetermined temperature.

[0007] This allows the pyrochlore oxide to be produced by a solid-liquid reaction in which a liquid-phase alkali metal compound reacts with a solid-phase composite oxide. In the solid-liquid reaction, the pyrochlore oxide can be produced at a lower temperature than in the solid-phase reaction, and the particles of the pyrochlore oxide can be made fine by lowering the firing temperature.

[0008] The reference characters in parentheses for each of the above components indicate the corresponding relationship with the specific means described in the embodiments described later. [Brief description of the drawings]

[0009] [Figure 1] 1 is a cross-sectional view showing a configuration of a secondary battery according to an embodiment. [Diagram 2] FIG. 1 is a diagram showing the crystal structure of a pyrochlore-type oxide. [Diagram 3]1 is a diagram showing a process for producing a pyrochlore type oxide. FIG. [Figure 4] This is an SEM image of a pyrochlore-type oxide. [Diagram 5] 1 is a table showing the types and particle sizes of precursors and Li compounds in Examples and Comparative Examples. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[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 a negative electrode current collector 11, a negative electrode 12, a positive electrode current collector 13, a positive electrode 14, and a solid electrolyte 15. The solid electrolyte 15 corresponds to the solid electrolyte for secondary batteries.

[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 in which charging and discharging are performed by lithium ions moving between the negative electrode 12 and the positive electrode 14 via the solid electrolyte 15.

[0013] A laminate including these negative electrode 12, positive electrode 14, and solid electrolyte 15 is provided between negative electrode current collector 11 and positive electrode current collector 13. Negative electrode current collector 11 and negative electrode 12 are in contact. Positive electrode current collector 13 and positive electrode 14 are in contact. Negative electrode current collector 11 and 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 may be any material that can be used as a negative electrode active material for a lithium ion battery, 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. As the positive electrode 14, for example, 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 ), and ternary positive electrode materials (NMC) mainly composed of nickel, manganese, and cobalt can be used.

[0017] The solid electrolyte 15 has ion conductivity and is capable of moving lithium ions between the negative electrode 12 and the positive electrode 14. In order to reduce the resistance of the secondary battery 10, it is desirable that the solid electrolyte 15 be formed as thin as possible.

[0018] The solid electrolyte 15 is an oxide-based solid electrolyte having the composition formula "Aa 2-α Ab (1+α) / 3 B 2 O 7-β X γ In order to make solid electrolyte 15 as thin as possible, it is desirable for the particle diameter of the pyrochlore oxide constituting solid electrolyte 15 to be 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 the pyrochlore oxide is the length of the largest diameter part of the particle, and can also be called the maximum diameter or the major axis. In this 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.

[0020] The geometric shape of the 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 most frequent value estimated by assuming that the distribution of the maximum diameters of the measured particles is a log-normal distribution is obtained as the particle diameter.

[0021] In the above composition formula, O is an oxygen atom, and Aa, Ab, B, and X represent any element or group. Aa, Ab, and B are different types of cations, and O and X are different types of anions. Aa is an alkali metal cation. Pyrochlore-type oxides contain multiple cations in their composition, consisting of the alkali metal cation Aa and multiple cations Ab and B other than the alkali metal cation Aa. In other words, pyrochlore-type oxides contain multiple cations in their composition, including the alkali metal cation Aa.

[0022] As shown in FIG. 2, the solid electrolyte 15 having the pyrochlore structure is composed of BO 6 It has a crystal structure that consists of a three-dimensional network of octahedra. 6 In the formula, the cation B is located at the center, and O is located at the vertex. 6 It shares a vertex with BO 6 In the three-dimensional network consisting of these, 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. The cation Aa can be Mg or H other than the alkali metal. That is, 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. At least one of La, Ce, Nd, and Sm can be used as the lanthanoid represented by Ab. 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 made of lanthanoid, and a part of the lanthanoid constituting Ab may be replaced with an alkaline earth metal (Ca, Mg, Sr, etc.). In the solid electrolyte 15 of this embodiment, in the above composition formula, the pyrochlore structure in which α is 0.6<α<2.0 and β is 0<β≦1 contains a lanthanoid, which is considered to cause defects in the crystal structure and improve the ionic conductivity. In this embodiment, La is used as Ab.

[0027] The solid electrolyte 15 of the present embodiment has a general pyrochlore structure represented by the composition formula "A 2 B 2 O 7 In the above, cation A is a composite cation made of lithium metal and lanthanide. This is believed to contribute to the improvement of the ionic conductivity of 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 the group 13 to 15 elements. B forms an octahedron surrounded by six O atoms in a 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 be substituted for the O atoms that constitute the pyrochlore structure. X has a different electronegativity and polarizability from 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 within the range of 0<γ≦1, and at least a part of the O atoms that constitute the pyrochlore structure is 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 a part of the O atoms constituting the pyrochlore structure is replaced with an anion having a different electronegativity and polarizability from the O atom, resulting in the crystal having lattice defects. It is believed that the ionic conductivity of the solid electrolyte 15 of this embodiment is improved due to the defect structure being included in the pyrochlore structure.

[0031] In the solid electrolyte 15 of this embodiment, a defect structure is formed in which some of Aa and Ab are missing. The composition formula of a general pyrochlore structure is "A 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 that 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] In addition, the solid electrolyte 15 of the present embodiment is a composite anion compound having a pyrochlore structure containing a plurality of anions such as O and X, and BO 6 Since the coordinated octahedral structure contains the anion represented by X, 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. Therefore, it is considered that the solid electrolyte 15 of this embodiment has high ion conductivity when used under an electric field such as in a battery.

[0034] In addition, since α, β, and γ in the above composition formula affect lattice defects and ionic conductivity, it is desirable to use them within appropriate ranges. If the values ​​of α, β, and γ are large, the defect concentration in the crystal lattice will increase, but if they exceed a certain amount, the concentration of the alkali metal represented by Aa will decrease and the ionic conductivity will decrease. For this reason, 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, the cation Aa is Li, the cation Ab is La, the cation B is Ta, and the anion X is F, with α = 0.75, β = 1, and γ = 1. In the following, "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 a mixing step and a firing step.

[0037] (1st 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 the cation Ab, and the tantalum source is a raw material for the cation B. The lanthanum source and the tantalum source may be oxides, carbonates, fluorides, acetates, chlorides, hydroxides, and the like. In this embodiment, La is used as the lanthanum source. 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 process) Next, La 2 O 3 , Ta 2 O 5 The first firing step S20 is a calcination step in which the mixture is calcined. In the first firing step S20, the mixture is heated in an air atmosphere or an inert atmosphere to form the precursor La. 0.29TaO 3 The mixture is baked.

[0039] The heating time in the first firing step S20 is preferably 1 to 20 hours, and more preferably 5 to 10 hours. The heating temperature in the first firing step S20 is preferably within a range of 400°C to 1400°C, and more preferably within a 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. The precursor La 0.29 TaO 3 is a composite oxide containing multiple 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 which is the cation Ab and Ta which is the cation B. In other words, the precursor La 0.29 TaO 3 is a composite oxide containing multiple cations Ab and B other than the alkali metal cation Aa in its composition.

[0042] The precursor does not essentially contain alkali metal cations, but may contain trace amounts of alkali metal cations. When 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 process) Next, a lithium source and a fluorine source were prepared as the raw materials for the target compound LLTOF, and these were used as the 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 can be an oxide, a carbonate, a fluoride, an acetate, a chloride, a 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) of 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 A solid-liquid reaction takes place in which the following react: 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 a range of 1.5 to 3 times the stoichiometric ratio (molar ratio) to the target compound. If the amount of LiF is too small, the amount of LiF in the liquid phase may become insufficient due to the decrease in LiF accompanying the progress of the reaction in the second firing step, and the solid-liquid reaction may not be able to be maintained. Therefore, the amount of LiF is preferably 1.5 times or more the stoichiometric ratio. If the amount of LiF is too large, a lot of LiF remains when the target compound is produced, 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 to the target compound.

[0046] (Second firing process) Next, the precursor La 0.29 TaO 3 The second firing step S40 is a firing step in which the mixture of LiF and the lithium source Li is fired. The second firing step S40 can be regarded as a firing step. In the second firing step S40, La 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 sinter the target compound LLTOF.

[0047] In the second firing step S40, LiF is melted by heating and becomes liquid phase. That is, in the second firing step S40, liquid phase LiF and solid phase La 0.29 TaO 3 The target compound LLTOF is produced by a solid-liquid reaction.

[0048] The mixture contains an excess of LiF relative to the target compound, so that even if the amount of LiF decreases as the reaction proceeds, there will still be enough liquid LiF 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 within a range of 500°C to 1000°C, and more preferably within a range of 650°C to 850°C.

[0050] In the second calcination step S40, since calcination at a high temperature increases the particle size of the product, it is desirable to lower the heating temperature to reduce the particle size of the product. Furthermore, when calcination is performed at a high temperature, Li and F are likely to volatilize, and the composition of the product is likely to deviate from the target compound LLTOF. For this reason, the heating temperature in the second calcination step S40 is desirably 1000°C or less, and more desirably 850°C or less.

[0051] Furthermore, if the heating temperature in the second firing step S40 is too low, the reactivity decreases, the reaction rate slows down, and it takes a long time to generate the pyrochlore oxide. Therefore, in order to prevent the reaction rate from slowing down, the heating temperature in the second firing 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 TaO 3 A eutectic reaction occurs between the precursor La and LiF. 0.29 TaO 3 Since it does not contain Li, a eutectic reaction easily occurs between LiF and the alloy.

[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 firing step S40 is set to a temperature lower than the melting point of LiF (848°C). Specifically, the heating temperature in the second firing step S40 is set to 700°C.

[0054] In the second firing step S40, La 0.29 TaO 3The target compound LLTOF is produced by calcining 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 calcination step S40, the liquid phase LiF and the solid phase La 0.29 TaO 3 The solid-liquid reaction allows the reaction to proceed at a lower temperature than in a solid-state reaction, which makes it possible to reduce the particle size of the pyrochlore-type oxide LLTOF.

[0055] Through the above process, the composition formula "Li 1.25 La 0.58 Ta 2 O 6 It is possible to obtain crystals of a pyrochlore-type oxide represented by the formula "F".

[0056] In addition, La 2 O 3 , Ta 2 O 5 By changing the mixture ratio of LiF, the composition formula "Li 2-α La (1+α) / 3 Ta 2 O 7-β F γ The pyrochlore structure crystal represented by La 2 O 3 , Ta 2 O 5 By changing the mixing ratio of LiF and LiF, the α, β, and γ in the composition formula can be adjusted. In addition, part of the material sublimes during firing. 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 and second firing steps.

[0057] FIG. 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, and 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, and 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 The precursor Li was 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. LiF and LaF 3 was the chemical equivalent to the target compound LLTOF.

[0059] As shown in Fig. 4, in the comparative example, the particle size of the pyrochlore-type oxide is significantly larger than 10 µm, whereas in this embodiment, a pyrochlore-type oxide with a particle size of several µm or less is obtained. In this manner, in this embodiment, the particles of the pyrochlore-type oxide can be made fine. By using the pyrochlore-type 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 with reference to examples and comparative examples shown in Fig. 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 chemical equivalent amount to the target compound is fired by a solid-phase reaction.

[0061] The precursors in Examples 1 to 6 and Comparative Examples 1 and 2 are composite 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 Li 2 CO 3 It 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 these 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), and as a result, the pyrochlore phase was confirmed in Examples 1 to 6 and Comparative Example 1, but not in Comparative Example 2. That is, in Examples 1 to 6, which used a liquid phase reaction, a 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, a pyrochlore-type oxide was obtained at a high temperature of 1200°C, but not 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] The particle size of Comparative Example 1, which was fired at a temperature of 1200°C, was 20.6 μm, whereas the particle size of Examples 1 to 6, which were fired at a temperature of 1000°C or less, was 7.5 μm or less, which was significantly smaller. Furthermore, the particle size of Examples 1 to 3, 5, and 6, which were fired at a temperature of 700°C, was smaller than that of Example 4, which was fired at a temperature of 1000°C. In Comparative Example 2, which was fired at a temperature of 700°C, a particle size of 1.0 μm was obtained, but the target compound was not produced due to the solid-phase reaction being carried out at a low temperature as described above.

[0066] In the embodiment described above, a mixture of a composite oxide that is a precursor of a pyrochlore type oxide and does not contain an alkali metal compound and an alkali metal compound in an amount exceeding the stoichiometric ratio to the pyrochlore type oxide is heated and fired to generate a pyrochlore type oxide. This liquefies the alkali metal compound, and the pyrochlore type oxide can be generated by a solid-liquid reaction in which the liquid phase alkali metal compound reacts with the solid phase composite oxide. In the solid-liquid reaction, the pyrochlore type oxide can be generated at a lower temperature than in the solid phase reaction, and the particles of the pyrochlore type oxide can be made fine by lowering the firing temperature. By using the fine pyrochlore type oxide 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.

[0067] Furthermore, the eutectic reaction between the solid-phase complex 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 of the mixture of the complex oxide and the alkali metal compound is set lower than the melting point of the alkali metal compound. Because the alkali metal compound contained in the mixture melts at a temperature lower than its melting point due to the eutectic reaction, the alkali metal compound can be melted and a solid-liquid reaction can be carried out even if the firing temperature is lower than the melting point of the alkali metal compound. As a result, the pyrochlore type oxide can be produced at a lower firing temperature, and the particles of the pyrochlore type oxide can be made finer.

[0068] (Other embodiments) 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. In addition, the means disclosed in each of the above-described embodiments may be appropriately combined within a practicable range.

[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 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 method for producing a pyrochlore type oxide disclosed in this specification has the following features. (Item 1) A method for producing a pyrochlore type oxide containing a plurality of cations including an alkali metal cation, comprising the steps of: A mixing step (S30) of mixing a composite oxide containing at least a cation other than the alkali metal cation with an alkali metal compound containing the alkali metal cation; A calcination step (S40) of heating the mixture containing the composite oxide and an alkali metal compound at a predetermined temperature to produce the pyrochlore type oxide; Equipped with When the composite oxide contains the 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 includes an excess of the alkali metal compound relative to the pyrochlore oxide in a stoichiometric ratio; In the calcination step, the alkali metal compound is liquefied by heating at the predetermined temperature. (Item 2) 2. The method for producing a pyrochlore type oxide according to item 1, wherein the predetermined temperature is within a range of 500°C to 1000°C. (Item 3) 3. The method for producing a pyrochlore type oxide according to item 1 or 2, wherein the predetermined temperature is a temperature lower than the melting point of the alkali metal compound. (Item 4) 4. The method for producing a pyrochlore type oxide according to any one of items 1 to 3, wherein the primary particle size of the pyrochlore type oxide is within a range of 0.01 μm to 10 μm. (Item 5) 5. The method for producing a pyrochlore type 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) 6. The method for producing a pyrochlore type oxide according to any one of items 1 to 5, wherein the composite oxide is a perovskite type oxide. (Item 7) the pyrochlore type oxide is produced using a plurality of raw materials each containing the plurality of cations, 7. The method for producing a pyrochlore type oxide according to any one of items 1 to 6, 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 8) 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 for a secondary battery (15). [Explanation of symbols]

[0071] 15 Solid electrolyte S30 2nd mixing process (mixing process) S40 Second firing process (firing process)

Claims

1. A method for producing a pyrochlore type oxide containing a plurality of cations including an alkali metal cation in its composition, comprising the steps of: A mixing step (S30) of mixing a composite oxide containing at least a cation other than the alkali metal cation with an alkali metal compound containing the alkali metal cation; A calcination step (S40) of heating the mixture containing the composite oxide and an alkali metal compound at a predetermined temperature to produce the pyrochlore type oxide; Equipped with When the composite oxide contains the 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 includes an excess of the alkali metal compound relative to the pyrochlore oxide in a stoichiometric ratio; In the calcination step, the alkali metal compound is liquefied by heating at the predetermined temperature.

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

4. 2. The method for producing a pyrochlore type oxide according to claim 1, wherein the primary particle size of the pyrochlore type oxide is within a 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. the pyrochlore type oxide is produced using a plurality of raw materials each containing the plurality of cations, 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.

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

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