Ion conductor, secondary battery, and method of producing ion conductor

Abstract

To improve ion conductivity of an ion conductor.SOLUTION: An ion conductor includes in a crystal, multiple cations, multiple anions and holes. Out of the multiple anions, an isotropic atomic displacement parameter of the anion having the largest isotropic atomic displacement parameter is 2Å2 or more. The multiple cations include conduction ions capable of being conducted in a crystal, and non-conduction ions not being conducted in a crystal. In a conduction site in which the cations are conducted, at least any of the conduction ions, the non-conduction ions and the holes exists. The sum of occupancies of the conduction ion, the non-conduction ion and the hole at the conduction site is 100%. The occupancy of the conduction ion is in a range of 10 to 70%, the occupancy of the non-conduction ion is in a range of 10 to 50%, and the occupancy of the holes is a range of 8 to 50%.SELECTED DRAWING: Figure 6

Description

[Technical field]

[0001] The present disclosure relates to an ion conductor, a secondary battery, and a method for producing the ion conductor. [Background technology]

[0002] 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 been attracting attention as lithium-ion batteries that do not use organic solvents as electrolytes. Patent Document 1 proposes replacing some of the cations contained in the crystal structure of the solid electrolyte with cations having different ionic radii to improve ionic conductivity. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2012-224520 A Summary of the Invention [Problem to be solved by the invention]

[0004] However, in the above-mentioned conventional technology, the ionic conductivity is 2×10 -4 S / cm to 7×10 -4 The improvement is merely about S / cm, and sufficient improvement in ionic conductivity is not obtained.

[0005] In view of the above, an object of the present disclosure is to improve the ionic conductivity of an ionic conductor. [Means for solving the problem]

[0006] In order to achieve the above object, one embodiment of the present disclosure provides an ionic conductor containing a plurality of cations, a plurality of anions, and vacancies in a crystal, the anion having the largest isotropic atomic displacement parameter being 2 Å or less. 2That is 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 occupancies of the conductive ions, non-conductive ions, and vacancies in the conductive sites is 100%. In the conductive sites, the occupancy of the conductive ions is within a range of 10% to 70%, the occupancy of the non-conductive ions is within a range of 10% to 50%, and the occupancy of the vacancies is within a range of 8% to 50%.

[0007] According to the present disclosure, the isotropic atomic displacement parameter of the anion with the largest isotropic atomic displacement parameter is 2 Å. 2 This can reduce 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 conductive sites of cations within a specific range, the number of non-conductive ions bonded to anions can be adjusted, and conductive ion paths can be secured. As a result, the ionic conductivity of the ionic conductor of the present disclosure can be improved.

[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] FIG. 2 is a diagram for explaining the isotropic atomic displacement parameters of anions contained in a pyrochlore-type oxide. [Figure 4] FIG. 2 is a diagram for explaining the occupancy rates of conductive ions, non-conductive ions, and vacancies in a pyrochlore-type oxide. [Diagram 5] 1 is a diagram showing a process for producing a pyrochlore type oxide. FIG. [Figure 6]FIG. 2 is a diagram for explaining the ionic conductivity of ionic conductors of examples and comparative examples. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[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 the negative electrode 12 and the positive electrode 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 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 material mainly composed of nickel, manganese and cobalt (LiNi x Mn y Co z O 2 :NMC) etc. can be used.

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

[0018] The pyrochlore oxides used as ion conductors have the formula "Aa 2-α Ab (1+α) / 3 B 2 O 7-β X γ" 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. Pyrochlore oxides contain multiple cations, multiple anions, and vacancies in 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 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.

[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. Any of Li, Na, K, Rb, and Cs can be used as the alkali metal represented by Aa. 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. 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.

[0023] The basic structure of the cation Ab is lanthanoid, and a part of the lanthanoid constituting Ab may be replaced with an alkaline earth metal (Ca, Mg, Sr, etc.). In the pyrochlore oxide of this embodiment, the pyrochlore structure in the above composition formula, where 0.6<α<2.0, 0<β≦1, and 0<γ≦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.

[0024] The pyrochlore oxide of the present embodiment has the general composition formula "A 2 B 2 O 7 In the above, cation A 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 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, Nb is used as B.

[0026] 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.

[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 replaced with anions having 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 defect structure included in the pyrochlore structure.

[0028] In the pyrochlore oxide of this embodiment, a part of Aa and Ab is missing as a defect structure. 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 one of Aa and Ab is missing in part, forming a vacancy. The composition ratio corresponding to the missing parts (vacancies) of Aa and Ab is (2α-1) / 3.

[0029] At least one of cations Aa, Ab, and vacancies exists in the A site of the pyrochlore oxide of this embodiment. The A site of the pyrochlore oxide is a conductive site for cations, the Aa being a conductive ion that conducts through the crystal, and the Ab being a non-conductive ion that does 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] In addition, the pyrochlore oxide of this embodiment is a complex anion compound in which multiple anions such as O and X are contained in the pyrochlore structure, 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 is believed that this is why the pyrochlore oxide of this embodiment has high ionic conductivity when used in an electric field such as a battery.

[0032] 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.

[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. In the following, "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 parameter of the anions contained in the pyrochlore oxide varies depending on the type of constituent elements, the ratio of the constituent elements, etc. 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 It is.

[0036] In solid electrolytes such as pyrochlore oxides, the conductive ions Li diffuse by hopping between cation-conducting sites in 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 becomes, making it easier for Li to move. As a result, the activation energy of 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 or more. 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 at least 5 Å, the activation energy for Li hopping is reduced, and the ionic conductivity of the pyrochlore-type oxide is sufficiently improved. 2 In the case where the content is set to be equal to or greater than this, 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 is 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 4 or greater, 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 Figure 4, the vertices of the octahedron with F at the center are Li conduction sites where Li can diffuse. In the Li conduction sites, there exist either Li, which is a conducting ion, La, which is a non-conducting ion, or a vacancy. In Figure 4, the solid circle represents Li or La, and the dashed circle represents a vacancy.

[0042] In Figure 4, vacancies without crosses are Li metastable sites, and Li existing in adjacent Li conductive sites can move there. After Li moves there, 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 atom is bonded to F, and (3) shows the case where two La atoms are bonded to F. In Fig. 4, the arrows indicate the Li conduction pathway present at the far left of the figure.

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

[0045] According to the study by the present inventors, it was found that when La exists at 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 Fig. 4(2), when one La atom is bonded to F, the three metastable sites adjacent to La disappear, and the Li conduction path decreases. Furthermore, as shown in Fig. 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 Li conduction.

[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 need to contain a certain amount of La in order to satisfy electrical neutrality and keep the crystal structure stable 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 rate of conductive ions, non-conductive ions, and vacancies in the Li conductive sites in the crystal to appropriate ratios.

[0048] In this embodiment, when the total occupancy rate 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 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%. When the occupancy rates of the conductive ions, non-conductive ions, and vacancies at the Li conductive sites satisfy the above-mentioned numerical ranges, the number of non-conductive ions that bind to anions can be adjusted, and the conductive ion conduction paths can be secured.

[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 O7-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 ​​for Ab and B are the raw material mixing ratio, O is 6, and F is 1. The initial occupancy value of Aa is a value that satisfies charge compensation. X-ray diffraction measurements are preferably performed by the Debye-Scherrer method using synchrotron radiation X-rays. When the Debye-Scherrer method is used, X-ray absorption correction is performed.

[0050] In the Rietveld refinement, the values ​​of the scale factor, background parameter, shift parameter, profile parameter, and lattice constant are refined before the refinement of the fractional coordinates, isotropic atomic displacement parameters, and occupancies. This refinement process may be performed in separate steps as appropriate. Then, the occupancies are refined. Then, the occupancies of Aa, Ab, and X are refined one by one in order. Then, the occupancies 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 a mixing step and a firing step.

[0052] (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.

[0053] (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. 0.33 TaO 3 The mixture is baked.

[0054] 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.

[0055] 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. 0.33 TaO 3 In the first firing step, the precursor La 0.33 TaO3 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. The precursor La 0.33 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 0.33 TaO 3 is a composite oxide containing multiple cations Ab and B other than the alkali metal cation Aa in its composition.

[0057] 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.

[0058] (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 0.33 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.

[0059] In the second mixing step, LiF is mixed with the precursor La at a predetermined ratio. 0.33 TaO3 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. 0.33 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.

[0060] 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.

[0061] (Second firing process) Next, the precursor La 0.33 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. 0.33 TaO 3 The mixture of LiF and LLTOF is heated at a predetermined temperature to sinter the target compound LLTOF.

[0062] 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 0.33 TaO 3The target compound LLTOF is produced by a solid-liquid reaction.

[0063] 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.

[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 within a range of 500°C to 1000°C, and more preferably within a range of 650°C to 850°C.

[0065] 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.

[0066] 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.

[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 3 Since it does not contain Li, a eutectic reaction easily occurs between LiF and the alloy.

[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 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.

[0069] In the second firing step S40, La 0.33 TaO 3 The 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 0.33 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.

[0070] Through the above process, the composition formula "LiLa 0.66 Ta 2 O 6 The manufacturing method of this embodiment makes it possible to obtain pyrochlore oxide crystals that satisfy 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 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 5By 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.

[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 with reference to an embodiment and a comparative example shown in FIG.

[0073] Examples 1 to 3 and Comparative Examples 1 and 2 are different oxide-based ion 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 an anion. Comparative Example 2 has two types of cation conductive sites, which 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 conductive ions, non-conductive 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 Li1.25 La 0.58 Nb 2 O 6 The product of the linear absorption coefficient of F and the capillary inner diameter, Xμ R is set to 0.8. When the wavelength of the synchrotron radiation 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, Xμ R 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) having a large isotropic atomic displacement parameter to the isotropic atomic displacement parameter of the anion (O) having a small isotropic atomic displacement parameter is less than 4. The ionic conductor of Comparative Example 2 does not contain a plurality of 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) having a large isotropic atomic displacement parameter to the isotropic atomic displacement parameter of the anion (O) having a small isotropic atomic displacement parameter is 4 or more.

[0077] In the ionic conductors of Examples 1 to 3, the occupancies of conductive ions, non-conductive ions and vacancies 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 numerical 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 the conductive ions, non-conductive ions, and vacancies. In contrast, the ionic conductors of Comparative Examples 1 and 2 do not satisfy both 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.

[0079] As shown in FIG. 6, the ionic conductivity of the ionic conductors of Examples 1 to 3 was 1×10 -3 S / cm, Example 2 is 5×10 -3 S / cm, 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, Comparative Example 2 was 4×10 -4 S / cm. 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, it is necessary to sinter them at high temperatures of over 1000°C, which is thought to cause some of the Li or F to volatilize during sintering. Furthermore, sintering 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 range of isotropic atomic displacement parameters and the range of occupancies of conductive ions, non-conductive ions, and vacancies. Therefore, the ionic conductivity (5×10 -3 S / cm) is the ionic conductivity (2×10 -3 It is considered that the ion exchange rate is higher than that of the ion exchange rate (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 required for conductive ions to hop between adjacent anions can be reduced. Furthermore, in the ionic conductor of this embodiment, the occupancy rate of conductive ions is within a range of 10% to 70%, the occupancy rate of non-conductive ions is within a range of 10% to 50%, and the occupancy rate of vacancies is within a range of 8% to 50% at the conductive sites of the cations. As a result, the number of non-conductive ions bonded to anions can be adjusted, and the conductive paths of the conductive ions can be secured.

[0083] According to the ionic conductor that satisfies the above conditions, 10 -3 A high ionic conductivity of the order of S / cm can be obtained. By using an ionic conductor having 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 mixture of a composite oxide that is a precursor of the pyrochlore oxide and does not contain an alkali metal compound and an alkali metal compound in an amount exceeding the stoichiometric ratio to the pyrochlore oxide is heated and fired to generate the pyrochlore oxide. This allows the alkali metal compound to be liquefied, and the pyrochlore oxide to be generated by a solid-liquid reaction in which the liquid-phase alkali metal compound reacts with the solid-phase composite oxide. The solid-liquid reaction allows the pyrochlore oxide to be generated at a lower temperature than the solid-phase reaction, and the firing temperature can be lowered. This manufacturing method allows the production of a pyrochlore oxide that satisfies the above-mentioned numerical ranges of the isotropic atomic displacement parameter and occupancy rate, and the ionic conductivity of the pyrochlore oxide to be improved.

[0085] 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 oxide can be produced at a lower firing temperature.

[0086] (Other embodiments) The present disclosure is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit and scope of the present disclosure, as described below. In addition, the means disclosed in each of the above-described embodiments may be appropriately combined within the scope of feasibility.

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

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

[0089] The ionic conductor, the secondary battery, and the method for producing the ionic conductor disclosed in this specification have the following features. (Item 1) An ionic conductor containing a plurality of cations, a plurality of anions, and vacancies in a crystal, Among the plurality of anions, the anion having the largest isotropic atomic displacement parameter has an isotropic atomic displacement parameter of 2 Å or less. 2 That's all. The plurality of cations include conductive ions that can be conducted through the crystal and non-conductive ions that do not conduct through the crystal, At least one of a conductive ion, a non-conductive ion, and a vacancy is present at the conductive site where the cation is conducted; the sum of the occupancy rates of the conductive ions, the non-conductive ions and the vacancies in the conductive sites is 100%; An ionic conductor, in which 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% in the conductive sites. (Item 2) Among the plurality of anions, the anion having the largest isotropic atomic displacement parameter has an isotropic atomic displacement parameter of 5 Å. 2 2. The ionic conductor according to item 1, (Item 3) 3. The ionic conductor according to item 1 or 2, wherein, among the plurality of anions, a 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. (Item 4) 4. The ionic conductor according to any one of items 1 to 3, wherein the anion comprises at least one element selected from the group consisting of O, F, Cl, Br, I, S, OH, and P. (Item 5) 5. The ionic conductor according to any one of items 1 to 4, wherein the element constituting the conductive ion includes at least one selected from the group consisting of Li, Na, K, Rb, Cs, Mg and H. (Item 6) 6. An ionic conductor according to any one of items 1 to 5, having a pyrochlore type crystal structure. (Item 7) A solid electrolyte for a secondary battery (15) comprising the ion conductor according to any one of items 1 to 6; a positive electrode (14) and a negative electrode (12) disposed so as to sandwich the solid electrolyte for secondary batteries; A secondary battery comprising: (Item 8) A method for producing an ionic conductor according to any one of items 1 to 6, comprising: the conductive ion is an alkali metal cation; 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. [Explanation of symbols]

[0090] 15 Solid electrolyte S30 2nd mixing process (mixing process) S40 Second firing process (firing process)

Claims

1. An ionic conductor containing a plurality of cations, a plurality of anions, and vacancies in a crystal, Among the plurality of anions, the anion having the largest isotropic atomic displacement parameter is 2 Å ２ That's all. The plurality of cations include conductive ions that can be conducted through the crystal and non-conductive ions that do not conduct through the crystal, At least one of the conductive ion, the non-conductive ion, and the vacancy is present in the conductive site where the cation is conducted; the sum of the occupancy rates of the conductive ions, the non-conductive ions and the vacancies in the conductive sites is 100%; An ionic conductor, wherein 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% in the conductive sites.

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

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

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

8. A method for producing the ionic conductor according to any one of claims 1 to 6, comprising the steps of: the conductive ion is an alkali metal cation; 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.

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