Ion conductor, secondary battery, and method for manufacturing ion conductor

Abstract

Disclosed is an ion conductor which includes a plurality of cations, a plurality of anions, and vacancies in a crystal, wherein an isotropic atomic displacement parameter of an anion having the largest isotropic atomic displacement parameter among the plurality of anions is 2 Å2 or more. The plurality of cations includes conductive ions which are able to be conducted in the crystal and non-conductive ions which are not conducted in the crystal, and at least one of a conductive ion, a non-conductive ion, and a vacancy is present in a conduction site through which cations are conducted. The sum of the occupancy rates of the conductive ions, the non-conductive ions, and the vacancies in the conduction site is 100%, the occupancy rate of the conductive ions is within the range of 10% to 70%, the occupancy rate of the non-conductive ions is within the range of 10% to 50%, and the occupancy rate of the vacancies is within the range of 8% to 50%.

Description

Ion conductor, secondary battery, and method for manufacturing ion conductor CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is based on Japanese Patent Application No. 2023-183093, filed on October 25, 2023, the contents of which are incorporated herein by reference.

[0002] The present disclosure relates to an ion conductor, a secondary battery, and a method for producing an ion conductor.

[0003] In recent years, from the viewpoint of improving safety, all-solid-state batteries in which the entire battery is solidified using a solid electrolyte have attracted attention as lithium ion batteries that do not use organic solvents as electrolytes. Patent Document 1 proposes that some of the cations contained in the crystalline structure of the solid electrolyte be replaced with cations having a different ionic radius to improve ionic conductivity.

[0004] Japanese Patent Application Laid-Open No. 2012-224520

[0005] However, in the above-mentioned prior art, the ionic conductivity is 2×10 －４ S / cm to 7 x 10 －４ The improvement is only about 200 S / cm, and a sufficient improvement in ionic conductivity is not obtained.

[0006] In view of the above, an object of the present disclosure is to improve the ionic conductivity of an ionic conductor.

[0007] In order to achieve the above object, one aspect of the present disclosure provides an ionic conductor containing a plurality of cations, a plurality of anions, and vacancies in a crystal, wherein the isotropic atomic displacement parameter of an anion having the largest isotropic atomic displacement parameter is 2 Å or less. ２ That's all. The multiple cations include conductive ions that can conduct through the crystal and non-conductive ions that do not conduct through the crystal, and at least one of conductive ions, non-conductive ions, and vacancies exists in the conductive sites where the cations conduct. The sum of the occupancy rates of the conductive ions, non-conductive ions, and vacancies in the conductive sites is 100%. In the conductive sites, the occupancy rate of the conductive ions is in the range of 10% to 70%, the occupancy rate of the non-conductive ions is in the range of 10% to 50%, and the occupancy rate of the vacancies is in the range of 8% to 50%.

[0008] According to the present disclosure, the isotropic atomic displacement parameter of the anion having the largest isotropic atomic displacement parameter is 2 Å. ２ This reduces the activation energy required for conductive ions to hop between adjacent anions. Furthermore, by adjusting the occupancy rates of conductive ions, non-conductive ions, and vacancies in the cation conduction sites within a specific range, the number of non-conductive ions bonded to anions can be adjusted, ensuring a conduction path for conductive ions. These improvements can improve the ionic conductivity of the ionic conductor of the present disclosure.

[0009] FIG. 1 is a cross-sectional view showing the configuration of a secondary battery according to an embodiment; FIG. 2 is a diagram showing the crystal structure of a pyrochlore oxide; FIG. 3 is a diagram for explaining the isotropic atomic displacement parameters of anions contained in a pyrochlore oxide; FIG. 4 is a diagram for explaining the occupancy rates of conductive ions, non-conductive ions, and vacancies in a pyrochlore oxide; FIG. 5 is a diagram showing a manufacturing process for a pyrochlore oxide; and FIG. 6 is a diagram for explaining the ionic conductivity of ionic conductors of Examples and Comparative Examples.

[0010] Hereinafter, an embodiment in which the ion conductor 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 an anode 12 and a cathode 14.

[0011] As shown in FIG. 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 .

[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 ), ternary positive electrode material mainly composed of nickel, manganese and cobalt (LiNi ｘ Mn ｙ Co ｚ O ２ :NMC) and the like can be used.

[0017] The solid electrolyte 15 allows lithium ions to move between the negative electrode 12 and the positive electrode 14. In other words, the solid electrolyte 15 is an ion conductor having a structure that allows cations to be conducted. In this embodiment, a pyrochlore oxide having a pyrochlore crystal structure is used as the ion conductor that constitutes the solid electrolyte 15. The pyrochlore oxide may be composed entirely of a crystalline structure, or may be composed of a composite that includes an amorphous structure in part of its crystalline structure, like glass ceramics.

[0018] The pyrochlore oxide used as an ion conductor has the composition formula "Aa ２－α Ab （１＋α）／３ B ２ O ７－β X γ " 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 each represent a different type of cation, and O and X each represent a different type of anion. Pyrochlore oxides contain multiple cations, multiple anions, and vacancies within the crystal. The multiple cations include conductive ions that can conduct electricity through the crystal and non-conductive ions that do not conduct electricity through the crystal.

[0019] As shown in FIG. 2, the pyrochlore oxide is 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.

[0020] 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.

[0021] An alkali metal cation can be used as the cation Aa. The alkali metal represented by Aa can be any of Li, Na, K, Rb, and Cs. Mg or H, other than alkali metals, can 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 this embodiment, Li is used as Aa. The composition ratio (2-α) of Aa is within the range of 0<(2-α)<1.4.

[0022] 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.

[0023] 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 pyrochlore oxide of this embodiment has a pyrochlore structure in the above composition formula, where 0.6<α<2.0, 0<β≦1, and 0<γ≦1. The inclusion of a lanthanoid in the pyrochlore structure generates defects in the crystal structure, which is thought to improve ionic conductivity. In this embodiment, La is used as Ab.

[0024] The pyrochlore oxide of this embodiment has the general composition formula "A ２ B ２ O ７ " Cation A in the above is a composite cation made of lithium metal and a lanthanide. This is thought to contribute to the improvement of the ionic conductivity of the pyrochlore-type oxide.

[0025] 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, Nb is used as B.

[0026] 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.

[0027] The pyrochlore oxide 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 that have different electronegativity and polarizability from the O atoms. It is believed that the pyrochlore oxide of this embodiment has improved ionic conductivity due to the presence of the defect structure in the pyrochlore structure.

[0028] In the pyrochlore oxide 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 pyrochlore oxide of this embodiment, the composition ratios of Aa and Ab are "2-α" and "(1+α) / 3," respectively, and since 0.6<α<2.0, the sum of the composition ratios of Aa and Ab is less than 2. In other words, in the crystal structure of the pyrochlore oxide of this embodiment, at least a portion of Aa and Ab is missing, forming vacancies. The composition ratio corresponding to the missing portions (vacancies) of Aa and Ab is (2α-1) / 3.

[0029] At least one of cations Aa, Ab, and vacancies exists at the A site of the pyrochlore oxide of this embodiment. The A site of the pyrochlore oxide is a conductive site for cations, and the cations Aa are conductive ions that conduct through the crystal, while the cations Ab are non-conductive ions that do not conduct through the crystal.

[0030] 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.

[0031] Furthermore, the pyrochlore oxide of this embodiment is a complex anion compound in which a plurality of anions such as O and X are contained in the 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 is believed that this is why the pyrochlore-type oxide of this embodiment has high ionic conductivity when used in an electric field such as a battery.

[0032] 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.

[0033] In this embodiment, the solid electrolyte 15 is “LiLa ０．６６ Ta ２ O ６ In other words, Li is used as the cation Aa, La is used as the cation Ab, Ta is used as the cation B, and F is used as the anion X, with α = 1, β = 1, and γ = 1. Hereinafter, "LiLa ０．６６ Ta ２ O ６ "LLTOF" is also referred to as "LLTOF."

[0034] Next, the isotropic atomic displacement parameter of the anion contained in the pyrochlore oxide of this embodiment will be described with reference to Fig. 3. The isotropic atomic displacement parameter is an index of the ease of movement of atomic positions in a crystal.

[0035] The pyrochlore oxide of this embodiment contains O and F as anions. The isotropic atomic displacement parameters of the anions contained in the pyrochlore oxide vary depending on the type and ratio of the constituent elements. In the example shown in FIG. 3, the isotropic atomic displacement parameter of O is 1.20 Å. ２ and the isotropic atomic displacement parameter of F is 6.70 Å ２ is.

[0036] In solid electrolytes such as pyrochlore oxides, the conductive ions, Li, diffuse by hopping between cation-conducting sites within the crystal. The smaller the activation energy of hopping, the higher the ionic conductivity.

[0037] As shown in Figure 3, when Li hops between adjacent anions, the larger the isotropic atomic displacement parameter of the anion, the larger the displacement of the anion and the easier it is for Li to move. As a result, the activation energy for hopping decreases and ionic conductivity improves.

[0038] Therefore, among the multiple anions contained in the pyrochlore-type oxide, at least the anion with the largest isotropic atomic displacement parameter has an isotropic atomic displacement parameter of 2 Å. ２ It is desirable that the isotropic atomic displacement parameter of the anion is 2 Å or more.２ When the isotropic atomic displacement parameter of the anion having a large isotropic atomic displacement parameter is 5 Å or more, the activation energy for Li hopping is reduced, and the ionic conductivity of the pyrochlore-type oxide is sufficiently improved. ２ In this case, the effect of further improving the ionic conductivity of the pyrochlore-type oxide can be obtained.

[0039] Furthermore, among the multiple anions contained in the pyrochlore oxide, it is desirable that the ratio of the isotropic atomic displacement parameter of the anion with the largest isotropic atomic displacement parameter to the isotropic atomic displacement parameter of the anion with the smallest isotropic atomic displacement parameter be equal to or greater than 4. When the ratio of the isotropic atomic displacement parameter of the anion with the largest isotropic atomic displacement parameter to the isotropic atomic displacement parameter of the anion with the smallest isotropic atomic displacement parameter is equal to or greater than 4, the effect of sufficiently improving the ionic conductivity of the pyrochlore oxide can be obtained.

[0040] Next, the occupancy rates of conductive ions, non-conductive ions, and vacancies in pyrochlore-type oxides will be explained using Figure 4. Figure 4 shows the conductive sites of cations centered around the anion F in a pyrochlore-type oxide crystal.

[0041] In Fig. 4, the vertices of the octahedron with F at the center are Li conduction sites where Li can diffuse. Either a conducting ion, Li, a non-conducting ion, La, or a vacancy exists in the Li conduction site. In Fig. 4, solid circles represent Li or La, and dashed circles represent vacancies.

[0042] In Fig. 4, vacancies without an x ​​mark are Li metastable sites, and Li present in an adjacent Li conductive site can move there. After the Li migrates, the Li conductive site becomes a vacancy, and Li can move there from the Li conductive site adjacent to the vacancy.

[0043] In Fig. 4, (1) shows the case where no La is bonded to F, (2) shows the case where one La is bonded to F, and (3) shows the case where two La are bonded to F. In Fig. 4, the arrows indicate the Li conduction pathways present at the far left of the figure.

[0044] As shown in FIG. 4(1), when La is not bonded to F, many metastable sites exist around Li, and many Li conduction paths exist.

[0045] According to the study by the present inventors, it was found that when La exists in the Li conductive site, the metastable site adjacent to La disappears. In other words, as the amount of La bonded to F increases, the number of Li conductive paths decreases, and the ionic conductivity decreases.

[0046] As shown in Figure 4(2), when one La atom is bonded to F, three metastable sites adjacent to La disappear, and the Li conduction path decreases. Furthermore, as shown in Figure 4(3), when two La atoms are bonded to F, all metastable sites adjacent to F disappear. In other words, when two or more La atoms are bonded to one F, that F can no longer contribute to the conduction of Li.

[0047] In pyrochlore oxides, if the content of La in the composition is too high, the ionic conductivity decreases. On the other hand, pyrochlore oxides must contain a certain proportion of La in order to satisfy electrical neutrality and maintain a stable crystal structure while containing vacancies in the structure for ionic conduction. Therefore, in order to obtain good ionic conduction in pyrochlore oxides, it is necessary to adjust the occupancy rates of conducting ions, non-conducting ions, and vacancies at the Li conductive sites in the crystal to appropriate proportions.

[0048] In this embodiment, when the total occupancy of the conductive ions, non-conductive ions, and vacancies at the Li conductive sites in the composition of the pyrochlore oxide is taken as 100%, the occupancy of the conductive ions is set to a range of 10% to 70%, the occupancy of the non-conductive ions is set to a range of 10% to 50%, and the occupancy of the vacancies is set to a range of 8% to 50%. By ensuring that the occupancies of the conductive ions, non-conductive ions, and vacancies at the Li conductive sites satisfy the above-mentioned numerical ranges, it is possible to adjust the number of non-conductive ions that bond to anions, and to ensure conduction paths for the conductive ions.

[0049] The above-mentioned isotropic atomic displacement parameters and occupancies can be obtained by X-ray diffraction measurements and Rietveld refinement. ２－α Ab （１＋α）／３ B ２ O ７－β X γ The initial values ​​of the isotropic atomic displacement parameters, fractional coordinates, and occupancy of each element are taken from the reference [Cyrille Galven et al., Eur. J. Inorg. Chem. 33, pp5272-5283 (2010)]. Aa, Ab, B, and X correspond to La, Li, Nb, and F, respectively, in the reference. The initial occupancy values ​​used are the raw material mixing ratios for Ab and B, 6 for O, and 1 for F. The initial occupancy value of Aa is set to a value that satisfies charge compensation. X-ray diffraction measurements are preferably performed by the Debye-Scherrer method using synchrotron X-rays. When the Debye-Scherrer method is used, X-ray absorption correction is performed.

[0050] In Rietveld analysis, before refining the fractional coordinates, isotropic atomic displacement parameters, and occupancy rates, the values ​​of the scale factor, background parameter, shift parameter, profile parameter, and lattice constant are refined. This refinement process may be performed in stages as appropriate. Next, the occupancy rates are refined. After that, the occupancies of Aa, Ab, and X are refined one by one in order. Then, the occupancy values ​​are fixed, and the isotropic atomic displacement parameters of O and X are refined.

[0051] 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.

[0052] (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.

[0053] (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. ０．３３ TaO ３ The mixture is baked.

[0054] 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.

[0055] In the first firing step S20, La ２ O ３ and Ta ２ O５ In the first baking step S20, La is reacted as a precursor of the target compound. ０．３３ TaO ３ In the first firing step, the precursor La ０．３３ TaO ３ In addition to the raw material La ２ O ３ and Ta ２ O ５ may remain unreacted.

[0056] Precursor La ０．３３ TaO ３ is a perovskite-type oxide. ０．３３ 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 ０．３３ 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.

[0057] 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.

[0058] (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 ０．３３ TaO ３A 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.

[0059] In the second mixing step, LiF is mixed with the precursor La at a predetermined ratio. ０．３３ 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. ０．３３ 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.

[0060] 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.

[0061] (Second baking step) Next, precursor La ０．３３ TaO ３ The 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.０．３３ TaO ３ The mixture of LiF and LLTOF is heated at a predetermined temperature to calcinate the target compound LLTOF.

[0062] 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. ０．３３ TaO ３ The target compound LLTOF is produced by a solid-liquid reaction.

[0063] 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.

[0064] 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.

[0065] 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.

[0066] 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.

[0067] In the second firing step S40, La ０．３３ TaO ３ A eutectic reaction occurs between the precursor La and LiF. ０．３３ TaO ３Since the material does not contain Li, a eutectic reaction easily occurs between the material and LiF.

[0068] 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.

[0069] In the second firing step S40, La ０．３３ 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. ０．３３ 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.

[0070] By the above steps, the composition formula "LiLa ０．６６ Ta ２ O ６ According to the production method of this embodiment, it is possible to obtain a pyrochlore oxide crystal represented by "F". The production method of this embodiment makes it possible to obtain a pyrochlore oxide that satisfies the above-mentioned ranges of the isotropic atomic displacement parameters of anions and the ranges of the occupancies of conductive ions, non-conductive ions, and vacancies.

[0071] 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 " can be obtained. 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.

[0072] Next, the relationship between the isotropic atomic displacement parameter, the occupancy rate of conductive ions, non-conductive ions and vacancies, and the ionic conductivity will be described using the example and comparative example shown in FIG.

[0073] Examples 1 to 3 and Comparative Examples 1 and 2 are different oxide-based ionic conductors. Examples 1 to 3 and Comparative Example 1 are pyrochlore-type oxides, and Comparative Example 2 is a garnet-type oxide. Example 1 is LiLa ０．６６ Ta ２ O ６ F, Example 2 is Li １．２５ La ０．５８ Nb ２ O ６ F, Example 3 is Li １．２５ La ０．５８ Ta ２ O ６ F, Comparative Example 1 is LiCaTaO ６ F, Comparative Example 2 is Li ７ La ３ Zr ２ O １２ The conductive ions in Examples 1 to 3 and Comparative Examples 1 and 2 are Li, the non-conductive ions in Examples 1 to 3 and Comparative Example 2 are La, and the non-conductive ions in Comparative Example 1 are Ca. Comparative Example 2 contains only O as the anion. Comparative Example 2 has two types of cation conductive sites, and these are indicated as (i) and (ii) in the columns for the occupancy rates of conductive ions and vacancies in FIG. 6.

[0074] The isotropic atomic displacement parameters and the occupancies of conducting ions, non-conducting ions, and vacancies in Examples 1 to 3 and Comparative Examples 1 and 2 were obtained by the above-mentioned X-ray diffraction measurement and Rietveld analysis. In the X-ray diffraction measurement, when the wavelength of the synchrotron X-ray was 0.068 nm and the inner diameter of the sample capillary was 0.18 mm, the Li １．２５ La ０．５８ Nb ２ O６ The product of the linear absorption coefficient of F and the capillary inner diameter, X μＲ When the wavelength of the synchrotron X-ray is 0.067 nm and the inner diameter of the sample capillary is 0.18 mm, Li １．２５ La ０．５８ Ta ２ O ６ F and LiLa ０．６６ Ta ２ O ６ The product of the linear absorption coefficient of F and the capillary inner diameter, X μＲ is set to 1.7.

[0075] As shown in FIG. 6, in the ionic conductors of Comparative Examples 1 and 2, the isotropic atomic displacement parameter of the anion with the largest isotropic atomic displacement parameter (F in Comparative Example 1 and O in Comparative Example 2) is 2 Å. ２ In contrast, in the ionic conductors of Examples 1 to 3, the isotropic atomic displacement parameter of the anion (F) having the largest isotropic atomic displacement parameter is less than 2 Å. ２ Furthermore, in the ionic conductors of Examples 1 to 3, the isotropic atomic displacement parameter of the anion (F) having the largest isotropic atomic displacement parameter is 5 Å. ２ That's all.

[0076] In the ionic conductor of Comparative Example 1, the ratio of the isotropic atomic displacement parameter of the anion (F) with a large isotropic atomic displacement parameter to the isotropic atomic displacement parameter of the anion (O) with a small isotropic atomic displacement parameter is less than 4. The ionic conductor of Comparative Example 2 does not contain multiple anion species, and the ratio of the isotropic atomic displacement parameters is not obtained. In contrast, in the ionic conductors of Examples 1 to 3, the ratio of the isotropic atomic displacement parameter of the anion (F) with a large isotropic atomic displacement parameter to the isotropic atomic displacement parameter of the anion (O) with a small isotropic atomic displacement parameter is 4 or more.

[0077] In addition, the occupancies of conductive ions, non-conductive ions, and vacancies in the ionic conductors of Examples 1 to 3 are within the ranges of 10% to 70%, 10% to 50%, and 8% to 50%, respectively. On the other hand, the ionic conductors of Comparative Examples 1 and 2 do not satisfy the above-mentioned ranges for the occupancies of conductive ions, non-conductive ions, and vacancies.

[0078] The ionic conductors of Examples 1 to 3 satisfy both the above-mentioned ranges of the isotropic atomic displacement parameters and the ranges of the occupancies of conducting ions, non-conducting ions, and vacancies. In contrast, the ionic conductors of Comparative Examples 1 and 2 do not satisfy either the above-mentioned ranges of the isotropic atomic displacement parameters or the ranges of the occupancies of conducting ions, non-conducting ions, and vacancies.

[0079] As shown in FIG. 6, the ionic conductivity of the ionic conductors of Examples 1 to 3 was 1×10 －３ S / cm, and Example 2 is 5×10 －３ S / cm, and Example 3 is 1.5×10 －３ In contrast, the ionic conductivity of the ionic conductors of Comparative Examples 1 and 2 was 6×10 －５ S / cm, and Comparative Example 2 is 4×10 －４ That is, the ionic conductivities of the ionic conductors of Examples 1 to 3 are two orders of magnitude higher than that of the ionic conductor of Comparative Example 1, and are one order of magnitude higher than that of the ionic conductor of Comparative Example 2.

[0080] In addition, a pyrochlore-type oxide (Li １．２５ La ０．５８ Nb ２ O ６ F) has an ionic conductivity of 2 × 10 －３ S / cm. When producing pyrochlore-type oxides by solid-state reaction, firing at high temperatures of 1000°C or higher is required, which is thought to cause some of the Li or F to volatilize during firing. Furthermore, firing at high temperatures tends to generate impurities in the pyrochlore-type oxides.

[0081] In contrast, in this embodiment, the pyrochlore-type oxide is produced by a solid-liquid reaction, so that the pyrochlore-type oxide can be produced at a lower temperature than the solid-phase reaction, and the pyrochlore-type oxide can satisfy the above-mentioned ranges of the isotropic atomic displacement parameters and the ranges of the occupancies of the conductive ions, non-conductive ions, and vacancies. Therefore, the ionic conductivity (5×10) of the pyrochlore-type oxide of Example 2 produced by the solid-liquid reaction was －３S / cm) is the ionic conductivity (2 × 10 －３ It is considered that the viscosity is higher than that of the conventional viscoelasticity (S / cm).

[0082] In the ionic conductor of the present embodiment described above, the isotropic atomic displacement parameter of the anion having the largest isotropic atomic displacement parameter is 2 Å. ２ As a result, the activation energy when conductive ions hop between adjacent anions can be reduced. Furthermore, in the ionic conductor of this embodiment, the cation conduction sites have an occupancy rate of conductive ions in the range of 10% to 70%, an occupancy rate of non-conductive ions in the range of 10% to 50%, and an occupancy rate of vacancies in the range of 8% to 50%. This makes it possible to adjust the number of non-conductive ions bound to anions, and ensure a conduction path for conductive ions.

[0083] According to the ionic conductor that satisfies the above conditions, 10 －３ High ionic conductivity on the order of S / cm can be obtained. By using an ionic conductor with such high ionic conductivity as the solid electrolyte 15 of the secondary battery 10, the output density of the secondary battery 10 can be improved.

[0084] In this embodiment, a pyrochlore-type oxide is produced by heating and calcining a mixture of a composite oxide (precursor of the pyrochlore-type oxide) that 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 process liquefies the alkali metal compound, and the pyrochlore-type oxide is produced by a solid-liquid reaction in which the liquid alkali metal compound reacts with the solid composite oxide. The solid-liquid reaction produces the pyrochlore-type oxide at a lower temperature than the solid-phase reaction, allowing for a lower calcination temperature. This production method allows for the production of a pyrochlore-type oxide that satisfies the above-mentioned numerical ranges for the isotropic atomic displacement parameter and occupancy, thereby improving the ionic conductivity of the pyrochlore-type oxide.

[0085] 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, a pyrochlore-type oxide can be produced at a lower firing temperature.

[0086] 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.

[0087] For example, in the above embodiment, the ionic conductor of the present disclosure is applied to a solid electrolyte of a lithium ion battery, but the ionic conductor of the present disclosure may also be applied to other types of secondary batteries. Specifically, when K is used as the cation Aa of the ionic conductor, the ionic conductor can be used as a solid electrolyte for a potassium ion battery. Furthermore, when Na is used as the cation Aa of the ionic conductor, the ionic conductor can be used as a solid electrolyte for a sodium ion battery.

[0088] Furthermore, in the above embodiment, an example has been described in which the ionic conductor of the present disclosure is applied to a pyrochlore oxide, but the ionic conductor of the present disclosure can also be applied to ionic conductors other than pyrochlore oxides.

[0089] The features of the ionic conductor, secondary battery, and method for manufacturing the ionic conductor disclosed in this specification are as follows: (Item 1) An ionic conductor containing a plurality of cations, a plurality of anions, and vacancies in a crystal, wherein the isotropic atomic displacement parameter of the anion having the largest isotropic atomic displacement parameter of the plurality of anions is 2 Å. ２The ionic conductor is characterized in that the plurality of cations include conductive ions that can be conducted in the crystal and non-conductive ions that do not conduct in the crystal, at least one of conductive ions, non-conductive ions and vacancies is present in the conduction sites where the cations conduct, the sum of the occupancies of the conductive ions, the non-conductive ions and the vacancies in the conduction sites is 100%, the occupancy of the conductive ions is within the range of 10% to 70%, the occupancy of the non-conductive ions is within the range of 10% to 50%, and the occupancy of the vacancies in the conduction sites is within the range of 8% to 50%. (Item 2) An ionic conductor in which the anion with the largest isotropic atomic displacement parameter among the plurality of anions has an isotropic atomic displacement parameter of 5 Å or less. ２An ionic conductor according to item 1, wherein the ratio of the isotropic atomic displacement parameter of the anion having the largest isotropic atomic displacement parameter to the isotropic atomic displacement parameter of the anion having the smallest isotropic atomic displacement parameter among the plurality of anions is 4 or more. (Item 4) An ionic conductor according to any one of items 1 to 3, wherein the elements constituting the anions include at least one selected from O, F, Cl, Br, I, S, OH, and P. (Item 5) An ionic conductor according to any one of items 1 to 4, wherein the elements constituting the conduction ions include at least one selected from Li, Na, K, Rb, Cs, Mg, and H. (Item 6) An ionic conductor according to any one of items 1 to 5, wherein the ionic conductor has a pyrochlore-type crystal structure. (Item 7) A secondary battery comprising: a solid electrolyte (15) for a secondary battery made of the ion conductor according to any one of items 1 to 6; and a positive electrode (14) and a negative electrode (12) disposed so as to sandwich the solid electrolyte for a secondary battery. (Item 8) A method for producing an ionic conductor according to any one of Items 1 to 6, wherein the conductive ions are alkali metal cations, the method 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, a composition ratio of the alkali metal cations in the complex oxide is less than a composition ratio of the alkali metal cations in the alkali metal compound, and the mixture contains the alkali metal compound in an amount exceeding a stoichiometric ratio with respect to the pyrochlore oxide, and the calcination step liquefies the alkali metal compound by heating at the predetermined temperature.

[0090] 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. An ionic conductor containing a plurality of cations, a plurality of anions, and vacancies in a crystal, wherein the isotropic atomic displacement parameter of an anion having the largest isotropic atomic displacement parameter among the plurality of anions is 2 Å or less. ２ the plurality of cations include conductive ions capable of conducting through the crystal and non-conductive ions that do not conduct through the crystal, at least one of the conductive ions, the non-conductive ions and the vacancies is present at a conductive site where the cations are conducted, the sum of the occupancy rates of the conductive ions, the non-conductive ions and the vacancies at the conductive site is 100%, and the occupancy rate of the conductive ions is within a range of 10% to 70%, the occupancy rate of the non-conductive ions is within a range of 10% to 50%, and the occupancy rate of the vacancies is within a range of 8% to 50% at the conductive site.

2. Among the plurality of anions, the anion having the largest isotropic atomic displacement parameter is 5 Å. ２ The ionic conductor according to claim 1 .

3. The ionic conductor according to claim 1, wherein, among said plurality of anions, the ratio of the isotropic atomic displacement parameter of the anion having the largest isotropic atomic displacement parameter to the isotropic atomic displacement parameter of the anion having the smallest isotropic atomic displacement parameter is 4 or more.

4. The ionic conductor according to claim 1, wherein the elements constituting the anion include at least one selected from the group consisting of O, F, Cl, Br, I, S, OH and P.

5. The ionic conductor according to claim 1, wherein the elements constituting the conductive ions include at least one selected from the group consisting of Li, Na, K, Rb, Cs, Mg and H.

6. The ionic conductor according to claim 1, which has a pyrochlore type crystal structure.

7. A secondary battery comprising: a solid electrolyte for secondary batteries (15) made of the ion conductor according to any one of claims 1 to 6; and a positive electrode (14) and a negative electrode (12) disposed so as to sandwich the solid electrolyte for secondary batteries.

8. A method for producing an ionic conductor as defined in any one of claims 1 to 6, wherein the conductive ions are alkali metal cations, the method 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 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 with respect to the pyrochlore type oxide, and in the calcination step, the alkali metal compound is liquefied by heating at the predetermined temperature.

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