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

Abstract

In the ion conductor crystal containing a plurality of cations, a plurality of anions, and holes, the isotropic atomic displacement parameter of the anion having the largest isotropic atomic displacement parameter among the plurality of anions is 2 A 2 The plurality of cations includes conductive ions that can conduct in the crystal and non-conductive ions that do not conduct in the crystal, and at least any one of the conductive ions, the non-conductive ions, and the holes is present in the conductive site of the cation conduction. The sum of the occupancy of the conductive ions, the non-conductive ions, and the holes in the conductive site is 100%, the occupancy of the conductive ions is 10% to 70%, the occupancy of the non-conductive ions is 10% to 50%, and the occupancy of the holes is 8% to 50%.

Description

[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. Technical Field

[0002] This disclosure relates to ionic conductors, secondary batteries, and methods for manufacturing ionic conductors. Background Technology

[0003] In recent years, from the perspective of improving safety, lithium-ion batteries that do not use organic solvents as electrolytes, i.e., all-solid-state batteries that use solid electrolytes to solidify the entire battery, have attracted much attention. Patent document 1 proposes a method to improve ionic conductivity by replacing some of the cations contained in the crystal structure of a solid electrolyte with cations of different ionic radii.

[0004] [Existing Technical Documents]

[0005] [Patent Literature]

[0006] Patent Document 1: Japanese Patent Application Publication No. 2012-224520 Summary of the Invention

[0007] However, in the aforementioned prior art, the ionic conductivity is only 2 × 10⁻⁶. -4 The S / cm was increased to approximately 7 × 10 -4 The ionic conductivity was around S / cm, but not sufficiently improved.

[0008] In view of the above, the purpose of this disclosure is to improve the ionic conductivity of ionic conductors.

[0009] To achieve the above objectives, one aspect of this disclosure is an ionic conductor, which is an ionic conductor containing multiple cations, multiple anions, and holes in a crystal, wherein among the multiple anions, the anion with the largest isotropic atomic displacement parameter has an isotropic atomic displacement parameter of 2 Å. 2 The above describes the process. The multiple cations include conductive ions capable of conducting within the crystal and non-conductive ions that do not conduct within the crystal. The conductive sites for cation conduction contain at least one of the following: conductive ions, non-conductive ions, and holes. The sum of the occupancy rates of conductive ions, non-conductive ions, and holes at each conductive site is 100%. Within each conductive site, the occupancy rate of conductive ions is in the range of 10% to 70%, the occupancy rate of non-conductive ions is in the range of 10% to 50%, and the occupancy rate of holes is in the range of 8% to 50%.

[0010] According to this disclosure, the isotropic atomic displacement parameter of the anion with the largest isotropic atomic displacement parameter is 2 Å. 2The above methods can reduce the activation energy of conductive ions jumping between adjacent anions. Furthermore, by adjusting the occupancy rates of conductive ions, non-conductive ions, and holes at the conductive sites of cations within a specific range, the number of non-conductive ions binding to anions can be adjusted, ensuring the conductive pathway of conductive ions. Therefore, the ionic conductivity of the ionic conductor of this disclosure can be improved. Attached Figure Description

[0011] [ Figure 1 This is a cross-sectional view showing the configuration of the secondary battery according to the embodiment.

[0012] [ Figure 2 The diagram shows the crystal structure of pyrochlore-type oxides.

[0013] [ Figure 3 A diagram used to illustrate the isotropic atomic displacement parameters of anions contained in pyrochlore-type oxides.

[0014] [ Figure 4 [A diagram used to illustrate the conductive ions, non-conductive ions, and hole occupancy in pyrochlore-type oxides.]

[0015] [ Figure 5 The diagram shows the manufacturing process of pyrochlore-type oxides.

[0016] [ Figure 6 A graph illustrating the ionic conductivity of the ionic conductors in the embodiments and comparative examples. Detailed Implementation

[0017] The following description, using accompanying drawings, illustrates embodiments of applying the ion conductor of this disclosure to a solid electrolyte for secondary batteries. The secondary battery 10 of this embodiment is a lithium-ion battery that is charged and discharged by the movement of lithium ions between the negative electrode 12 and the positive electrode 14.

[0018] like Figure 1 As shown, the secondary battery 10 includes a negative current collector 11, a negative electrode 12, a positive current collector 13, a positive electrode 14, and a solid electrolyte 15.

[0019] A solid electrolyte 15 is sandwiched between the positive electrode 14 and the negative electrode 12. The negative electrode 12 is in contact with the solid electrolyte 15. The positive electrode 14 is in contact with the solid electrolyte 15. 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.

[0020] A laminate comprising the negative electrode 12, the positive electrode 14, and the solid electrolyte 15 is disposed between the negative electrode current collector 11 and the positive electrode current collector 13. The negative electrode current collector 11 is in contact with the negative electrode 12. The positive electrode current collector 13 is in contact with the positive electrode 14. The negative electrode current collector 11 and the positive electrode current collector 13 are connected via the laminate.

[0021] The negative electrode current collector 11 and the positive electrode current collector 13 can be made of any material suitable for use as current collectors in lithium-ion batteries. In this embodiment, Cu is used as the negative electrode current collector 11 and Al is used as the positive electrode current collector 13.

[0022] 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 carbon-based negative electrode materials, oxide-based negative electrode materials, metal-based negative electrode materials, etc. In this embodiment, a lithium-based negative electrode material or a silicon-based negative electrode material is used.

[0023] The cathode material constituting the cathode 14 can be any material suitable for use as a cathode active material in lithium-ion batteries. Examples of cathode 14 include cobalt-based cathode materials (LiCoO2), nickel-based cathode materials (LiNiO2), manganese-based cathode materials (LiMn2O4), iron phosphate-based cathode materials (LiFePO4), and ternary cathode materials with nickel / manganese / cobalt as the main components (LiNiO2). x Mn y Co z O2 (NMC), etc.

[0024] The solid electrolyte 15 enables lithium ions to move between the negative electrode 12 and the positive electrode 14. That is, the solid electrolyte 15 is an ionic conductor with a structure capable of conducting cations. In this embodiment, a pyrochlore-type oxide with a pyrochlore-type crystal structure is used as the ionic conductor constituting the solid electrolyte 15. The pyrochlore-type oxide may consist entirely of a crystalline structure, or it may be formed as a composite material, such as a glass ceramic, in which an amorphous structure is included within a portion of the crystalline structure.

[0025] Pyrochlore-type oxides used as ionic conductors have a compositional formula starting with "Aa" 2-α Ab (1+α) / 3 B2O 7-β X γ The structure of pyrochlore is represented by the symbol "". In the above composition, 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-type oxide crystals contain multiple cations, multiple anions, and vacancies. Among the multiple cations are conductive ions that can conduct within the crystal and non-conductive ions that do not conduct within the crystal.

[0026] like Figure 2 As shown, pyrochlore-type oxides have a crystal structure forming a three-dimensional network of octahedrons composed of BO6. In BO6, O is arranged at the vertices with the cation B as the center, sharing vertices with adjacent BO6 ions. A hexagonal tunnel structure is formed within the three-dimensional network composed of BO6, containing cation A and anion X.

[0027] In the above composition formula, 0.6 < α < 2.0, 0 < β ≤ 1, and 0 < γ ≤ 1. Changes in α cause changes in the composition ratio of Aa and Ab, while changes in β cause changes in the composition ratio of O and X.

[0028] An alkali metal cation Aa can be used. The alkali metal represented by Aa can be any one 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 of Li, Na, K, Rb, Cs, Mg, and H. In this embodiment, Li is used as Aa. The composition ratio (2-α) of Aa is in the range of 0 < (2-α) < 1.4.

[0029] The cation Ab contains at least one lanthanide element. As the lanthanide element represented by Ab, at least one of La, Ce, Nd, and Sm can be used. In this embodiment, La is used as Ab. The composition ratio of Ab (1+α) / 3 is in the range of 0.53 < (1+α) / 3 < 1.

[0030] The basic structure of cationic Ab is composed of lanthanides, and some of the lanthanides constituting Ab can also be replaced by alkali metals (Ca, Mg, Sr, etc.). It is believed that in the pyrochlore-type oxide of this embodiment, by including lanthanides in the pyrochlore structure with 0.6 < α < 2.0, 0 < β ≤ 1, and 0 < γ ≤ 1 in the above composition, defects are generated in the crystal structure, thereby increasing the ionic conductivity. In this embodiment, La is used as Ab.

[0031] In the pyrochlore-type oxide of this embodiment, cation A in the general formula "A2B2O7" is a composite cation using lithium metal and lanthanide elements. This is believed to contribute to improving the ionic conductivity of the pyrochlore-type oxide.

[0032] Cation B is a metallic cation distinct from Aa and Ab, and is a transition metal or a metal selected from Groups 13 to 15. In crystals, B forms an octahedron surrounded by six oxygen atoms. The transition metal represented by B can be a Group 4 or Group 5 transition metal; more specifically, it can be at least one of Nb, Ta, Ti, Zr, Hf, and V. The Group 13 element represented by B can be Al, Ga, or In; the Group 14 element can be Ge or Sn; and the Group 15 element can be Sb or Bi. In this embodiment, Nb is used as B.

[0033] Anion X is an anion capable of replacing O atoms constituting the pyrochlore structure. The electronegativity and polarizability of X differ from those of 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 constituting the pyrochlore structure are replaced by X. In this embodiment, F is used as X.

[0034] The pyrochlore-type oxide of this embodiment has a defect structure containing lattice defects in the crystal because a portion of the O atoms constituting the pyrochlore structure are replaced by anions with electronegativity and polarizability different from those of the O atoms. It is believed that the ionic conductivity of the pyrochlore-type oxide of this embodiment is improved due to the presence of defect structures within the pyrochlore structure.

[0035] In the pyrochlore-type oxide of this embodiment, a portion of Aa and Ab are missing as defect structures. The typical pyrochlore structure has the formula "A₂B₂O₇", with an A cation composition ratio of 2. In contrast, in the pyrochlore-type oxide of this embodiment, the Aa and Ab composition ratios are "2-α" and "(1+α) / 3", respectively, and 0.6 < α < 2.0, therefore the sum of the Aa and Ab composition ratios is less than 2. That is, in the crystal structure of the pyrochlore-type oxide of this embodiment, at least a portion of Aa and Ab is missing, forming a vacancy. Furthermore, the composition ratio corresponding to the missing portions (vacancy) of Aa and Ab is (2α-1) / 3.

[0036] In the pyrochlore-type oxide of this embodiment, at least one of the following is present in the A site: cation Aa, cation Ab, or vacancy. The A site of the pyrochlore-type oxide is a cation conduction site, cation Aa is a conduction ion that conducts within the crystal, while cation Ab is a non-conducting ion that does not conduct within the crystal.

[0037] In addition to deviations in composition ratios, defect structures can also be formed in the above composition formulas by making the sum of the valences of the cations composed of Aa, Ab, and B and the anions composed of O and X negative.

[0038] Furthermore, the pyrochlore-type oxide of this embodiment is a complex anionic compound containing multiple anions such as O and X in the pyrochlore structure. Since the BO6 coordination octahedral structure contains anion represented by X, the alkali metal Aa can be located in the central part of the space with the BO6 coordination octahedron without being close to it. Therefore, the pyrochlore-type oxide of this embodiment is considered to have high ionic conductivity when used under an electric field such as a battery.

[0039] Furthermore, since α, β, and γ in the above composition affect lattice defects and ionic conductivity, they are preferably used within appropriate ranges. When the values ​​of α, β, and γ are large, the defect concentration in the lattice increases, but beyond a certain amount, the concentration of alkali metal (denoted as Aa) decreases, and the ionic conductivity also decreases. Therefore, ideally, α should be controlled 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.

[0040] In this embodiment, LiLa was used as the solid electrolyte 15. 0.66 "Ta₂O₆F" represents a pyrochlore-type oxide. That is, Li is used as the cation Aa, La as the cation Ab, Ta as the cation B, and F as the anion X, with α=1, β=1, and γ=1. The following "LiLa"... 0.66 Ta2O6F is also known as "LLTOF".

[0041] Next, using Figure 3 This section describes the isotropic atomic displacement parameters of the anions contained in the pyrochlore-type oxide of this embodiment. The isotropic atomic displacement parameters are indicators of the ease with which atoms can move within a crystal.

[0042] In the pyrochlore-type oxide of this embodiment, O and F are included as anions. The isotropic atomic shift parameters of the anions contained in the pyrochlore-type oxide vary depending on the types and proportions of the constituent elements. Figure 3 In the example shown, the isotropic atomic displacement parameter of O is 1.20 Å. 2 The isotropic atomic displacement parameter of F is 6.70 Å. 2 .

[0043] In solid electrolytes such as titanate oxides, lithium diffuses as a conductive ion via hopping between cation conduction sites within the crystal. The lower the activation energy of the hopping, the higher the ionic conductivity.

[0044] like Figure 3As shown, when lithium hops between adjacent anions, the larger the isotropic atomic displacement parameter of the anion, the greater the displacement of the anion, and the easier it is for lithium to move. As a result, the activation energy of the hopping decreases, and the ionic conductivity increases.

[0045] Therefore, among the multiple anions contained in pyrochlore-type oxides, ideally, the anion with the larger isotropic atomic shift parameter should have an isotropic atomic shift parameter of 2 Å. 2 That's all. When the isotropic atomic displacement parameter of the anion is 2 Å... 2 The above results in a reduction of the activation energy during Li hopping and a significant increase in the ionic conductivity of pyrochlore-type oxides. Furthermore, when the isotropic atomic displacement parameter of the anion with a large isotropic atomic displacement parameter is at least 5 Å... 2 At that time, the ionic conductivity of pyrochlore oxides was further improved.

[0046] Furthermore, among the multiple anions contained in pyrochlore-type oxides, ideally, the ratio of the isotropic atomic shift parameter of the anion with the largest isotropic atomic shift parameter to that of the anion with the smallest isotropic atomic shift parameter should be set to 4 or more. When the ratio of the isotropic atomic shift parameter of the anion with the largest isotropic atomic shift parameter to that of the anion with the smallest isotropic atomic shift parameter is 4 or more, the ionic conductivity of the pyrochlore-type oxide is sufficiently improved.

[0047] Next, using Figure 4 This indicates the occupancy rates of conductive ions, non-conductive ions, and holes in pyrochlore-type oxides. Figure 4 The conductive sites of cations centered on the anion F are shown in the crystals of pyrochlore-type oxides.

[0048] exist Figure 4 In this model, the vertices of the octahedron centered at F are Li conduction sites where Li can diffuse. These Li conduction sites can contain any of the following: conducting ions (Li), non-conducting ions (La), or holes. Figure 4 In the diagram, solid circles represent Li or La, and dashed circles represent holes.

[0049] exist Figure 4 In the diagram, holes not marked with an × represent metastable sites for Li, and Li atoms located at adjacent Li conduction sites can move. After a Li atom moves, the Li conduction site becomes a hole, and Li atoms can then move from the adjacent Li conduction sites.

[0050] exist Figure 4In the diagram, (1) shows the case where La is not bound to F, (2) shows the case where one La is bound to F, and (3) shows the case where two Las are bound to F. Figure 4 In the diagram, the arrow indicates the conduction path of Li located in the rear left corner.

[0051] like Figure 4 As shown in (1), when La is not bound to F, there are many metastable sites around Li and many Li conduction paths.

[0052] According to the inventors' research, it was found that when La is present at Li conduction sites, the metastable sites adjacent to La disappear. That is, the more La bound to F, the fewer Li conduction paths there are, and the lower the ionic conductivity.

[0053] like Figure 4 As shown in (2), when one La combines with F, the three metastable sites adjacent to La disappear, and the Li conduction path is reduced. Furthermore, as... Figure 4 As shown in (3), when two La atoms combine with F, all metastable positions adjacent to F disappear. That is, when one F atom combines with two or more La atoms, the F atom cannot contribute to the conduction of Li.

[0054] In pyrochlore oxides, excessively high La content leads to decreased ionic conductivity. Conversely, for ionic conductivity, the structure of pyrochlore oxides requires the inclusion of holes, and a certain proportion of La is necessary to maintain electroneutrality and crystal stability. Therefore, to achieve good ionic conductivity in pyrochlore oxides, the occupancy rates of conducting ions, non-conducting ions, and holes at Li conductive sites must be adjusted to appropriate proportions.

[0055] In this embodiment, in the composition of the pyrochlore oxide, when the sum of the occupancy rates of conducting ions, non-conducting ions, and holes in the Li conductive sites is set to 100%, the occupancy rate of conducting ions is set to a range of 10% to 70%, the occupancy rate of non-conducting ions is set to a range of 10% to 50%, and the occupancy rate of holes is set to a range of 8% to 50%. By ensuring that the occupancy rates of conducting ions, non-conducting ions, and holes in the Li conductive sites meet the above numerical ranges, the number of non-conducting ions that bind to anions can be adjusted, and the conduction path of conducting ions can be ensured.

[0056] The aforementioned isotropic atomic displacement parameters and occupancy rates can be obtained through X-ray diffraction and Rietveld analysis. 2-α Ab （1+α） / 3 B2O 7-β X γThe initial values ​​of the isotropic atomic displacement parameters and fractional coordinates of each element, as well as the initial values ​​of the occupancy, were adopted from the reference [Cyrille Galven al., Eur. J. Inorg. Chem. 33, pp5272-5283(2010)]. Aa, Ab, B, and X correspond to La, Li, Nb, and F in the reference, respectively. As initial values ​​of occupancy, Ab and B were based on the mixing ratio of the raw materials, O was 6, and F was 1. The initial value of the occupancy of Aa was set to satisfy the charge compensation value. X-ray diffraction was preferably performed using the Debye-Scherrer method with radioactive X-rays. When using the Debye-Scherrer method, X-ray absorption correction was performed.

[0057] Before refining the fractional coordinates, isotropic atomic displacement parameters, and occupancy, Rietveld analysis refines the values ​​of the scale factor, background parameters, displacement parameters, profile parameters, and lattice constant. This refinement process can also be performed in stages. Next, the occupancy is refined. Then, the occupancy of Aa, Ab, and X is refined sequentially. Afterward, with the occupancy values ​​fixed, the isotropic atomic displacement parameters of O and X are refined.

[0058] Next, use Figure 3 The manufacturing method of the solid electrolyte 15 according to this embodiment will be described. In the manufacturing method of 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 sequentially. The second mixing step S30 and the second firing step S40 correspond to the mixing step and the firing step, respectively.

[0059] (First mixing process)

[0060] In the first mixing step S10, multiple raw materials, each containing one of the multiple cations contained in the target compound LLTOF, are mixed to obtain a mixture. The multiple 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. Oxides, carbonates, fluorides, acetates, chlorides, hydroxides, etc., can be used as the lanthanum source and tantalum source. In this embodiment, La2O3 is used as the lanthanum source, and Ta2O5 is used as the tantalum source. In the first mixing step, the particles of La2O3 and Ta2O5 are mixed in a predetermined ratio.

[0061] (First firing process)

[0062] Next, the first firing step S20 is performed, in which the mixture of La2O3 and Ta2O5 is fired. The first firing step S20 can be considered a pre-firing step. In the first firing step S20, the mixture is heated under atmospheric or inactive atmosphere to refract the precursor La. 0.33TaO3 was fired.

[0063] The heating time for the first firing step S20 is ideally set to 1 to 20 hours, and more ideally to 5 to 10 hours. The heating temperature for the first firing step S20 is ideally set to the range of 400℃ to 1400℃, and more ideally to the range of 500℃ to 800℃.

[0064] In the first calcination step S20, a solid-phase reaction occurs between La₂O₃ and Ta₂O₅ in the solid state. Through the first calcination step S20, La₂O₃, as a precursor to the target compound, is generated. 0.33 TaO3. In the first firing process, besides La as a precursor... 0.33 In addition to TaO3, sometimes the raw materials La2O3 and Ta2O5 may also remain in an unreacted state.

[0065] Precursor La 0.33 TaO3 is a pyrochlore-type oxide. The precursor La... 0.33 TaO3 is a complex oxide containing multiple cations, including at least cations other than alkali metal cations. In this embodiment, the precursor does not contain an alkali metal cation corresponding to cation Aa (Li in this embodiment), but instead contains La as cation Ab and Ta as cation B. That is, the precursor La... 0.33 TaO3 is a composite oxide whose composition contains at least several cations Ab and B, in addition to the alkali metal cation Aa.

[0066] The precursor is essentially free of alkali metal cations, but may contain trace amounts. When the precursor contains alkali metal cations, the proportion of alkali metal cations in the precursor is lower than that in LiF.

[0067] (Second mixing process)

[0068] Next, the second mixing step S30 is performed, in which lithium and fluorine sources are prepared as raw materials for the target compound LLTOF, and then mixed with the precursor La. 0.33 A mixture is obtained by reacting TaO3. The lithium source is the raw material for the cation Aa, and the fluorine source is the raw material for the anion X. As the lithium source, oxides, carbonates, fluorides, acetates, chlorides, hydroxides, etc., can be used. In this embodiment, LiF is used as both the lithium source and the fluorine source. LiF is an alkali metal compound containing an alkali metal cation.

[0069] In the second mixing step, LiF is mixed with the precursor La at a specified ratio. 0.33A mixture is obtained by mixing TaO3. This mixture contains an amount of LiF exceeding the stoichiometric ratio (molar ratio) relative to the target compound LLTOF. That is, the mixture contains an excess of LiF relative to the target compound. Therefore, in the second calcination step following the second mixing step, LiF melts and liquefies, resulting in a reaction between the liquid phase LiF and the solid phase La. 0.33 The reaction involves a solid-liquid mixture of TaO3 and TaO3. The stoichiometric ratio refers to the ratio of the molar number of LiF as a reactant to the molar number of LLTOF as the target compound.

[0070] Ideally, the amount of LiF should be set within the range of 1.5 to 3 times the stoichiometric ratio (molar ratio) of the target compound. If the amount of LiF is too small, there is a concern that the liquid phase will be insufficient due to the reduction of LiF accompanying the reaction in the second calcination step, making it impossible to maintain the solid-liquid reaction. Therefore, the amount of LiF is ideally more than 1.5 times the stoichiometric ratio. If the amount of LiF is too large, a large amount of LiF will remain after the formation of the target compound, increasing impurities. Furthermore, if the amount of LiF is too large, the composition of the product is prone to deviating from the target compound LLTOF. Therefore, the amount of LiF is ideally less than 3 times the stoichiometric ratio. In this embodiment, the amount of LiF is set to 2 times the stoichiometric ratio of the target compound.

[0071] (Second firing process)

[0072] Next, the second firing process S40 is carried out, in which the precursor La is fired... 0.33 A mixture of TaO3 and lithium source LiF is calcined. The second calcination step S40 can be considered the main calcination step. In the second calcination step S40, La... 0.33 A mixture of TaO3 and LiF is heated at a predetermined temperature in an atmospheric or inactive atmosphere to calcine the target compound LLTOF.

[0073] In the second firing step S40, LiF is melted by heating to become a liquid phase. That is, in the second firing step S40, the liquid phase LiF reacts with the solid phase La. 0.33 The target compound LLTOF is generated through a solid-liquid reaction of TaO3.

[0074] The mixture contains an excess of LiF relative to the target compound. Therefore, even if the LiF decreases as the reaction proceeds, a sufficient amount of liquid LiF for the solid-liquid reaction can be ensured.

[0075] The heating time for the second firing step S40 is ideally set to 1 to 20 hours, and more ideally to 1 to 10 hours. The heating temperature for the second firing step S40 is ideally set to the range of 500℃ to 1000℃, and more ideally to the range of 650℃ to 850℃.

[0076] In the second firing step S40, the particle size of the product increases due to high-temperature firing. Therefore, it is ideal to lower the heating temperature to reduce the particle size of the product. Furthermore, at higher firing temperatures, Li and F tend to volatilize, and the composition of the product may deviate from the target compound LLTOF. Therefore, it is ideal to set the heating temperature of the second firing step S40 to below 1000°C, and more ideally, below 850°C.

[0077] Furthermore, if the heating temperature of the second firing step S40 is too low, the reactivity will decrease, the reaction rate will slow down, and the formation of pyrochlore oxide will take longer. Therefore, to suppress the slowing down of the reaction rate, it is ideal to set the heating temperature of the second firing step S40 to 500°C or higher, and even more ideally, to 650°C or higher.

[0078] La occurs in the second firing process S40 0.33 The eutectic reaction of TaO3 and LiF. The precursor La in this embodiment... 0.33 Since TaO3 does not contain Li, it readily undergoes a eutectic reaction with LiF.

[0079] LiF melts at a temperature below its melting point (848°C) through a eutectic reaction. Therefore, in this embodiment, the heating temperature of the second firing step S40 is set to a temperature below the melting point of LiF (848°C). Specifically, the heating temperature of the second firing step S40 is set to 700°C.

[0080] In the second firing process S40, La is subjected to... 0.33 TaO3 and LiF are calcined to produce the target compound LLTOF. 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 reacts with the solid phase La... 0.33 The solid-liquid reaction of TaO3 can be carried out at a lower temperature than that of the solid-phase reaction. Therefore, the particle size of LLTOF, which is a pyrochlore-type oxide, can be reduced.

[0081] Through the above procedures, we can obtain the compositional formula "LiLa". 0.66The crystal of pyrochlore-type oxide, represented by "Ta2O6F", can be obtained according to the manufacturing method of this embodiment. Pyrochlore-type oxides satisfying the above-mentioned range of isotropic atomic displacement parameters of anions and the range of conduction ions, non-conducting ions, and hole occupancy rates can be obtained.

[0082] Furthermore, by changing the mixing ratio of La2O3 and Ta2O5 and the mixing ratio of LiF in the above manufacturing process, it is possible to obtain a product with the composition "Li 2-α La (1+α) / 3 Ta2O 7-β F γ The figure represents a pyrochlore crystal structure. The α, β, and γ components in the composition can be adjusted by changing the mixing ratios of La₂O₃, Ta₂O₅, and LiF. Furthermore, during firing, a portion of the material sublimates. Therefore, α, β, and γ can also be adjusted by changing the firing conditions, furnace atmosphere, and furnace size in the first and second firing processes.

[0083] Next, using Figure 6 The examples and comparative examples shown illustrate the relationship between isotropic atomic displacement parameters, the occupancy rates of conducting ions, non-conducting ions, and holes, and ionic conductivity.

[0084] Examples 1-3 and Comparative Examples 1 and 2 are ionic conductors of different oxide systems. Examples 1-3 and Comparative Example 1 are pyrochlore-type oxides, and Comparative Example 2 is a garnet-type oxide. Example 1 is LiLa. 0.66 Ta2O6F, Example 2 is Li 1.25 La 0.58 Nb2O6F, Example 3 is Li 1.25 La 0.58 Ta₂O₆F, Comparative Example 1 is LiCaTaO₆F, Comparative Example 2 is Li₇La₃Zr₂O 12 In Examples 1-3 and Comparative Examples 1 and 2, the conducting ion was Li; in Examples 1-3 and Comparative Example 2, the non-conducting ion was La; and in Comparative Example 1, the non-conducting ion was Ca.

[0085] Comparative Example 2 contains only O as an anion. Comparative Example 2 has two types of cation conduction sites. Figure 6 In the column for conducting ions and hole occupancy, the conduction sites of each cation are represented as (i) and (ii).

[0086] The isotropic atomic displacement parameters and the occupancy rates of conducting ions, non-conducting ions, and holes in Examples 1-3 and Comparative Examples 1 and 2 were obtained through the above-described X-ray diffraction measurements and Rietveld analysis. In the X-ray diffraction measurements, when the wavelength of the X-ray radiation was set to 0.068 nm and the inner diameter of the sample capillary was set to 0.18 mm, Li...1.25 La 0.58 The product of the X-ray absorption coefficient of Nb₂O₆F and the inner diameter of the capillary, X μR The value is 0.8. When the wavelength of the X-ray radiation is 0.067 nm and the inner diameter of the sample capillary is 0.18 mm, Li... 1.25 La 0.58 Ta2O6F and LiLa 0.66 The product of the X-ray absorption coefficient of Ta₂O₆F and the inner diameter of the capillary, X μR It is 1.7.

[0087] like Figure 6 As shown, 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 less than 2 Å. 2 In this regard, among the ionic conductors of Examples 1-3, the isotropic atomic displacement parameter of the anion (F) with the largest isotropic atomic displacement parameter is 2 Å. 2 That's all. Furthermore, in the ionic conductors of Examples 1-3, the isotropic atomic displacement parameter of the anion (F) with the largest isotropic atomic displacement parameter is 5 Å. 2 above.

[0088] Furthermore, in the ionic conductor of Comparative Example 1, the ratio of the isotropic atomic shift parameter of the anion (F) with a larger isotropic atomic shift parameter to the isotropic atomic shift parameter of the anion (O) with a smaller isotropic atomic shift parameter is less than 4. The ionic conductor of Comparative Example 2 does not contain multiple types of anions, making it impossible to obtain a ratio of isotropic atomic shift parameters. In contrast, in the ionic conductors of Examples 1-3, the ratio of the isotropic atomic shift parameter of the anion (F) with a larger isotropic atomic shift parameter to the isotropic atomic shift parameter of the anion (O) with a smaller isotropic atomic shift parameter is 4 or more.

[0089] Furthermore, the occupancy rates of conducting ions, non-conducting ions, and holes in the ionic conductors of Examples 1-3 are in the ranges of 10%-70%, 10%-50%, and 8%-50%, respectively. On the other hand, the ionic conductors in Comparative Examples 1 and 2 do not meet the above-mentioned numerical ranges for the occupancy rates of conducting ions, non-conducting ions, and holes.

[0090] The ion conductors of Examples 1-3 satisfy both the range of isotropic atomic displacement parameters and the range of occupancy rates of conducting ions, non-conducting ions, and holes described above. In contrast, the ion conductors of Comparative Examples 1 and 2 do not satisfy both the range of isotropic atomic displacement parameters and the range of occupancy rates of conducting ions, non-conducting ions, and holes described above.

[0091] like Figure 6As shown, the ionic conductivity of the ionic conductors in Examples 1-3 are as follows: Example 1 has a conductivity of 1×10⁻⁶. -3 S / cm, Example 2 is 5×10 -3 S / cm, Example 3 is 1.5×10 -3 S / cm. In contrast, the ionic conductivity of the ionic conductors in Comparative Examples 1 and 2 is as follows: Comparative Example 1 is 6 × 10⁻⁶. -5 S / cm, Comparative Example 2 is 4×10 -4 S / cm. That is, the ionic conductivity of the ionic conductors in Examples 1 to 3 is two digits higher than that of the ionic conductor in Comparative Example 1 and one digit higher than that of the ionic conductor in Comparative Example 2.

[0092] Furthermore, pyrochlore-type oxides (Li) with the same composition as those in Example 2 were obtained through solid-state reaction. 1.25 La 0.58 The ionic conductivity of Nb₂O₆F is 2 × 10⁻⁶. -3 S / cm. When preparing pyrochlore-type oxides via solid-state reaction, calcination is required at temperatures above 1000℃, suggesting that some Li or F will volatilize during calcination. Furthermore, calcination at high temperatures easily generates impurities in pyrochlore-type oxides.

[0093] In contrast, in this embodiment, since the pyrochlore-type oxide is produced via a solid-liquid reaction, it can be produced at a lower temperature than that of a solid-phase reaction, and the pyrochlore-type oxide can satisfy the range of isotropic atomic displacement parameters and the occupancy range of conductive ions, non-conductive ions, and holes described above. Therefore, the ionic conductivity (5 × 10⁻⁶) of the pyrochlore-type oxide of Example 2 produced via a solid-liquid reaction is considered to be... -3 The S / cm ratio is higher than that of pyrochlore-type oxides of the same composition produced by solid-state reaction (2 × 10⁻⁶). -3 S / cm).

[0094] In the ionic conductor described above, the isotropic atomic displacement parameter of the anion with the largest isotropic atomic displacement parameter is 2 Å. 2 That's all. Therefore, the activation energy of conductive ions jumping between adjacent anions can be reduced. Furthermore, in this embodiment, the ionic conductor has a conductive ion occupancy rate of 10% to 70% at the cation conduction sites, a non-conductive ion occupancy rate of 10% to 50%, and a hole occupancy rate of 8% to 50%. This allows for adjustment of the number of non-conductive ions binding to anions, ensuring the conduction path of conductive ions.

[0095] Based on the ionic conductors that meet the above conditions, 10 can be obtained. -3High ionic conductivity on the order of S / cm. Using this ionic conductor with high ionic conductivity as the solid electrolyte 15 of the secondary battery 10 can improve the output density of the secondary battery 10.

[0096] Furthermore, in this embodiment, pyrochlore-type oxides are generated by heating and firing a mixture of a composite oxide (which is a precursor of pyrochlore-type oxides and does not contain alkali metal compounds) and an amount of alkali metal compound exceeding the stoichiometric ratio relative to the pyrochlore-type oxide. Thus, pyrochlore-type oxides can be generated through a solid-liquid reaction, in which the alkali metal compound is liquefied and the liquid-phase alkali metal compound reacts with the solid-phase composite oxide. In the solid-liquid reaction, pyrochlore-type oxides can be generated at a lower temperature than in a solid-phase reaction, thereby reducing the firing temperature. This manufacturing method yields pyrochlore-type oxides that satisfy the numerical range of the aforementioned isotropic atomic displacement parameters and occupancy rates, and improves the ionic conductivity of the pyrochlore-type oxides.

[0097] Furthermore, the solid-phase composite oxide undergoes a eutectic reaction with the liquid-phase alkali metal compound, causing the alkali metal compound to melt at a temperature below its original melting point. Therefore, in this embodiment, the firing temperature of the mixture of the composite oxide and the alkali metal compound is set below the melting point of the alkali metal compound. Since the eutectic reaction causes the alkali metal compound contained in the mixture to melt at a temperature below its melting point, the alkali metal compound can still melt and undergo a solid-liquid reaction even when the firing temperature is set below its melting point. As a result, pyrochlore-type oxides can be generated at a lower firing temperature.

[0098] This disclosure is not limited to the embodiments described above, and various modifications can be made without departing from the spirit of this disclosure. Furthermore, the means disclosed in the above embodiments can be appropriately combined within the scope of what is feasible.

[0099] For example, in the above embodiments, the ion conductor of this disclosure is applied to the solid electrolyte of lithium-ion batteries, but it can also be applied to different types of secondary batteries. Specifically, when K is used as the cation Aa in the ion conductor, the ion conductor can be used as a solid electrolyte for potassium-ion batteries. Furthermore, when Na is used as the cation Aa in the ion conductor, the ion conductor can be used as a solid electrolyte for sodium-ion batteries.

[0100] Furthermore, although the examples of applying the ionic conductor of this disclosure to pyrochlore-type oxides have been described in the above embodiments, the ionic conductor of this disclosure is also applicable to ionic conductors other than pyrochlore-type oxides.

[0101] The following demonstrates the features of the ionic conductor, secondary battery, and manufacturing method of the ionic conductor disclosed in this specification.

[0102] (Project 1)

[0103] An ionic conductor, which is an ionic conductor containing multiple cations, multiple anions, and holes in a crystal, wherein...

[0104] Among the plurality of anions, the anion with the largest isotropic atomic displacement parameter has an isotropic atomic displacement parameter of Å2. 2 above,

[0105] The plurality of cations includes conductive ions that can conduct in the crystal and non-conductive ions that do not conduct in the crystal.

[0106] The cation conduction sites contain at least one of the following: conducting ions, non-conducting ions, and holes.

[0107] The sum of the occupancy rates of the conducting ions, the non-conducting ions, and the holes in the conducting site is 100%.

[0108] In the said conduction site, the occupancy rate of the conducting ions is in the range of 10% to 70%, the occupancy rate of the non-conducting ions is in the range of 10% to 50%, and the occupancy rate of the holes is in the range of 8% to 50%.

[0109] (Project 2)

[0110] According to the ion conductor described in Project 1, among the plurality of anions, the anion with the largest isotropic atomic displacement parameter has an isotropic atomic displacement parameter of 5 Å. 2 above.

[0111] (Project 3)

[0112] According to the ion conductor described in Project 1 or 2, wherein, among the plurality of anions, 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 more.

[0113] (Project 4)

[0114] The ionic conductor according to any one of items 1 to 3, wherein the element constituting the anion comprises at least one selected from O, F, Cl, Br, I, S, OH and P.

[0115] (Project 5)

[0116] The ion conductor according to any one of items 1 to 4, wherein the element constituting the conductive ion comprises at least one selected from Li, Na, K, Rb, Cs, Mg and H.

[0117] (Project 6)

[0118] The ionic conductor according to any one of items 1 to 5 has a pyrochlore-type crystal structure.

[0119] (Project 7)

[0120] A secondary battery, comprising:

[0121] A solid electrolyte (15) for a secondary battery comprising the ion conductor described in any one of items 1 to 6, and

[0122] The positive electrode (14) and negative electrode (12) are provided in such a way as to clamp the solid electrolyte of the secondary battery.

[0123] (Project 8)

[0124] A method for manufacturing an ionic conductor, which is the method for manufacturing an ionic conductor as described in any one of items 1 to 6, wherein,

[0125] The conducting ion is an alkali metal cation.

[0126] The method includes:

[0127] A mixing step (S30) involves mixing a composite oxide containing at least cations other than the alkali metal cation with an alkali metal compound containing the alkali metal cation.

[0128] A firing process (S40) in which a mixture comprising the composite oxide and an alkali metal compound is heated at a specified temperature to produce the pyrochlore-type oxide.

[0129] When the composite oxide contains the alkali metal cation, the proportion of the alkali metal cation in the composite oxide is less than the proportion of the alkali metal cation in the alkali metal compound.

[0130] The mixture contains an amount of the alkali metal compound in a stoichiometric ratio relative to the pyrochlore-type oxide.

[0131] In the firing process, the alkali metal compound is liquefied by heating at the specified temperature.

[0132] This disclosure is based on embodiments, but should be understood as not being limited to the described embodiments or structures. This disclosure also includes various modifications and equivalent variations. Furthermore, while various combinations and arrangements are shown in this disclosure, other combinations and arrangements containing only one element, more elements, or fewer elements also fall within the scope and spirit of this disclosure.

Claims

1. An ionic conductor, comprising a plurality of cations, a plurality of anions, and holes in a crystal, characterized in that, Among the plurality of anions, the anion with the largest isotropic atomic displacement parameter has an isotropic atomic displacement parameter of 2 Å. 2 above, The plurality of cations includes conductive ions that can conduct in the crystal and non-conductive ions that do not conduct in the crystal. The cation-conducting sites contain at least one of the conducting ion, the non-conducting ion, and the hole. The sum of the occupancy rates of the conducting ions, the non-conducting ions, and the holes in the conducting site is 100%. In the said conduction site, the occupancy rate of the conducting ions is in the range of 10% to 70%, the occupancy rate of the non-conducting ions is in the range of 10% to 50%, and the occupancy rate of the holes is in the range of 8% to 50%.

2. The ion conductor according to claim 1, characterized in that, Among the various anions, the anion with the largest isotropic atomic displacement parameter has an isotropic atomic displacement parameter of 5 Å. 2 above.

3. The ion conductor according to claim 1, characterized in that, Among the plurality of anions, 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 more.

4. The ion conductor according to claim 1, characterized in that, The elements constituting the anion include at least one selected from O, F, Cl, Br, I, S, OH and P.

5. The ion conductor according to claim 1, characterized in that, The elements constituting the conductive ion include at least one selected from Li, Na, K, Rb, Cs, Mg and H.

6. The ion conductor according to claim 1, characterized in that, It has a pyrochlore-type crystal structure.

7. A secondary battery, characterized in that, have: A solid electrolyte (15) for a secondary battery comprising the ion conductor according to any one of claims 1 to 6, and The positive electrode (14) and negative electrode (12) are provided in such a way as to clamp the solid electrolyte of the secondary battery.

8. A method for manufacturing an ionic conductor, as described in any one of claims 1 to 6, characterized in that, The conducting ion is an alkali metal cation. The method includes: A mixing step (S30) involves mixing a composite oxide containing at least cations other than the alkali metal cation with an alkali metal compound containing the alkali metal cation. A firing process (S40) in which a mixture comprising the composite oxide and an alkali metal compound is heated at a specified temperature to produce the pyrochlore-type oxide. When the composite oxide contains the alkali metal cation, the proportion of the alkali metal cation in the composite oxide is less than the proportion of the alkali metal cation in the alkali metal compound. The mixture contains an amount of the alkali metal compound in a stoichiometric ratio relative to the pyrochlore-type oxide. In the firing process, the alkali metal compound is liquefied by heating at the specified temperature.

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