Lithium ion conductor

Abstract

To provide a novel lithium ion conductor showing high conductivity at room temperature.SOLUTION: A lithium ion conductor has a composition represented by general formula: ALi6+s(X1-tZt)O6+δ (where KLi6BiO6 is excluded). In the general formula, A (positive monovalence) represents K and / or Rb, X (positive pentavalence) represents one or more elements selected from the group consisting of V, Nb, Ta, As, Sb and Bi, Z (positive tetravalence) represents one or more elements selected from the group consisting of Si, Ni, Se, Mn, Co, Ge, Cr, V, Fe, Rh, Ti, Pd, Ru, Ir, Pt, Re, Os, Mo, W, Nb, Ta, Sn, Hf, Zr, Tb, Pb, Pr and Ce, 0≤s<0.5 and 0≤t<0.5 are satisfied, and δ is a value that maintains electrical neutrality. In the lithium ion conductor, the ionic radiuses of constituent elements satisfy a predetermined relationships.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present invention relates to a lithium ion conductor, more specifically, to a lithium ion conductor having the general formula: ALi 6+s (X 1-t Z t )O 6+δ The present invention relates to a novel lithium ion conductor made of an oxide having a composition represented by the following formula: (wherein A is a monovalent metal element, X is a pentavalent metal element, and Z is a tetravalent metal element). [Background technology]

[0002] Lithium ion conductors are solid electrolyte materials that make up all-solid-state lithium ion batteries. To increase the output of a battery, the solid electrolyte must have high conductivity. Currently, the only lithium ion conductor that shows high conductivity at room temperature is Li 10 GeP2S 12 However, sulfides have practical issues such as degradation of properties due to material decomposition and generation of hydrogen sulfide. On the other hand, many lithium ion conductors made of oxides, which are more stable than sulfides, are also being investigated. However, the conductivity of the oxides reported so far is only about 1 mS / cm at room temperature, and materials with higher conductivity are desired.

[0003] In recent years, the development of materials informatics technology has been promoted worldwide as a means of accelerating the search for materials. Attempts have also been made to search for lithium ion conductors using materials informatics techniques, and as a result, KLi6BiO6 has recently been proposed as a promising material (Non-Patent Documents 1, 2). Although experimental verification has not yet been performed, a conductivity of 6 mS / cm@room temperature has been predicted by first-principles molecular dynamics calculations. However, the predicted conductivity of KLi6BiO6 is lower than that of LGPS (10 mS / cm@room temperature), and oxide materials with higher conductivity are desired. [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] Y. Zhang et al., Nature Comm. 10, 5260(2019) [Non-Patent Document 2] X.He et al., Adv. Energy Mater., 1902078(2019) Summary of the Invention [Problem to be solved by the invention]

[0005] An object of the present invention is to provide a novel lithium ion conductor that exhibits high conductivity at room temperature. [Means for solving the problem]

[0006] In order to solve the above problems, the lithium ion conductor according to the present invention has the following configuration. (1) The lithium ion conductor has a composition represented by the following formula (1) (excluding KLi6BiO6). Ali 6+s (X 1-t Z t )O 6+δ …(1) however, A (+1 valence) is K and / or Rb; X (+5 valence) is one or more elements selected from the group consisting of V, Nb, Ta, As, Sb, and Bi; Z (+4 valence) is one or more elements selected from the group consisting of Si, Ni, Se, Mn, Co, Ge, Cr, V, Fe, Rh, Ti, Pd, Ru, Ir, Pt, Re, Os, Mo, W, Nb, Ta, Sn, Hf, Zr, Tb, Pb, Pr, and Ce; 0≦s<0.5, 0≦t<0.5, δ is the value at which electrical neutrality is maintained. (2) The lithium ion conductor satisfies the following formulas (2) and (3). |r X1 -r Xp |×100 / rX1 ≦15 …(2) |r X1 -r zq |×100 / r X1 ≦15 …(3) however, r X1 is the ionic radius of the most abundant element X1 in 6-coordination among the elements X occupying the X site, r Xp is the pth (p≧2) element X other than the element X1 among the elements X occupying the X site. p Ionic radius in 6-coordination of r Zq is the qth (q≧1) element Z among the elements Z occupying the X site. q Ionic radius of 6-coordination. The term "X site" refers to a site that can be occupied by the element X and the element Z in a crystal structure. Effect of the Invention

[0007] The oxides having the composition represented by formula (1) exhibit high conductivity at room temperature. This is because all oxides having the composition represented by formula (1) have the following properties: (a) In the presence of excess lithium, lithium ions diffuse to link existing sites within the crystal; and (b) Lithium ion diffusion barrier (E mig ) is lower than that of KLi6BiO6, It is thought that. [Brief description of the drawings]

[0008] [Figure 1] The conductivity of a conventional lithium ion conductor. [Diagram 2] This is a list of elements selected as replacement elements for KLi6(Ta / Bi)O6. [Diagram 3] FIG. 1 is a diagram showing the conductivity of various oxides obtained by first-principles molecular dynamics calculations. [Figure 4] FIG. 1 is a diagram showing the diffusion trajectory of lithium ions obtained by first-principles molecular dynamics calculations. [Diagram 5] FIG. 2 illustrates the change in energy along the diffusion path of lithium ions. [Figure 6] FIG. 13 is a diagram showing the conductivity of oxides with different amounts of excess lithium and oxides doped with excess lithium and Zr simultaneously, which are obtained from first-principles molecular dynamics calculations. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] An embodiment of the present invention will be described in detail below. [1. Lithium ion conductor] The lithium ion conductor according to the present invention is made of an oxide having the following structure.

[0010] [1.1. Crystal structure] The lithium ion conductor according to the present invention has a crystal structure belonging to space group R3_m (No. 166) or a crystal structure similar thereto. A "crystal structure similar to space group R3_m" refers to a crystal structure that does not strictly belong to space group R3_m, but can be regarded as the same. In a crystal structure that belongs to space group R3_m, Li is usually located at the 18f site, and the 9d site is empty (see FIG. 4). Ionic conduction occurs when excess lithium ions enter all or part of the empty 9d site. A crystal structure in which excess lithium ions enter the 9d site is not strictly a "crystal structure that belongs to space group R3_m," but in the present application, it is treated as a "crystal structure similar to space group R3_m."

[0011] [1.2. Composition] The lithium ion conductor according to the present invention has a composition represented by the following formula (1) (excluding KLi6BiO6). Ali 6+s (X 1-t Z t )O 6+δ …(1) however, A (+1 valence) is K and / or Rb; X (+5 valence) is one or more elements selected from the group consisting of V, Nb, Ta, As, Sb, and Bi; Z (+4 valence) is one or more elements selected from the group consisting of Si, Ni, Se, Mn, Co, Ge, Cr, V, Fe, Rh, Ti, Pd, Ru, Ir, Pt, Re, Os, Mo, W, Nb, Ta, Sn, Hf, Zr, Tb, Pb, Pr, and Ce; 0≦s<0.5, 0≦t<0.5, δ is the value at which electrical neutrality is maintained.

[0012] [1.2.1. Element A] The element A is an element located at the 3b site of a crystal structure belonging to the space group R3_m. In the present invention, the element A is a monovalent metal element and is composed of K or Rb. Any of these elements can occupy the 3b site, and oxides containing these elements exhibit relatively high ionic conductivity, so they are suitable as the element A. The lithium ion conductor may contain either K or Rb, or may contain both.

[0013] [1.2.2. Li] As described above, in crystal structures belonging to the space group R3_m, Li is usually located at the 18f site and the 9d site is vacant. It is believed that ionic conduction occurs when excess lithium ions enter all or part of the vacant 9d site.

[0014] [1.2.3. Element X] The element X is an element located at the 3a site (six-coordinate site) (hereinafter, simply referred to as "X site") of a crystal structure belonging to the space group R3_m. In the present invention, the element X is a pentavalent metal element, and is composed of V, Nb, Ta, As, Sb, or Bi. Any of these elements can occupy the 3a site, and oxides containing these elements all exhibit relatively high ionic conductivity, so that they are suitable as the element X. The lithium ion conductor may contain any one of these elements X, or may contain two or more of them. When the lithium ion conductor contains two or more elements X, the ionic radii of the elements X must satisfy the conditions described below.

[0015] [1.2.4. Element Z] The element Z is a dopant that substitutes for the element X. In the present invention, the element Z is a tetravalent metal element, and is composed of Si, Ni, Se, Mn, Co, Ge, Cr, V, Fe, Rh, Ti, Pd, Ru, Ir, Pt, Re, Os, Mo, W, Nb, Ta, Sn, Hf, Zr, Tb, Pb, Pr, or Ce. When the pentavalent element X is replaced by the tetravalent element Z, excess lithium ions are easily introduced into the 9d site to maintain electrical neutrality. As a result, an oxide containing the element Z may exhibit higher conductivity than an oxide not containing the element Z. The lithium ion conductor may contain any one of these elements Z, or may contain two or more of them. When the lithium ion conductor contains the element Z, the ionic radius of the element Z must satisfy the conditions described below. In the present invention, the term "metal element" also includes metalloids such as Si and Ge.

[0016] [1.2.5.s] s is correlated with the number of lithium ions occupying the 9d site. The lithium ion conductor according to the present invention formally exhibits relatively high conductivity even when s is zero. In general, the larger s is, the more lithium ions occupy the 9d site, and the higher the conductivity may be. As shown in the examples, s is preferably 0.125 or more, more preferably 0.33 or more. On the other hand, if s becomes too large, the charge neutrality condition cannot be maintained, and there is a risk of phase separation or Li precipitation occurring. Therefore, s must be less than 0.5.

[0017] [1.2.6.t] t represents the amount of element Z substituting element X. The lithium ion conductor according to the present invention exhibits relatively high conductivity even when it does not contain element Z. That is, t may be zero. However, when t is zero, it may be difficult to introduce excess lithium (i.e., to make s>0). On the other hand, when t is greater than zero, it is relatively easy to introduce excess lithium, and the larger t is, the higher the conductivity is. As shown in the examples, t is preferably 0.125 or more, more preferably 0.33 or more. On the other hand, if t is too large, element Z may not entirely substitute for element X and may precipitate as a different phase. Therefore, t must be less than 0.5.

[0018] [1.2.7. δ] When no dopant (element Z) is included, δ is ideally zero. However, in reality, even when no dopant is included, excess lithium ions may enter the 9d site. Also, when a dopant (element Z) with a smaller valence than element X is added, excess lithium ions usually enter the 9d site so that electrical neutrality is maintained. In the case of an oxide represented by formula (1), it can be formally expressed as 2δ=st. δ is -0.25≦δ≦+0.25 due to the upper limits of s and t.

[0019] [1.2.8. Ionic radius] The lithium ion conductor according to the present invention satisfies the following formulas (2) and (3). |r X1 -r Xp |×100 / r X1 ≦15 …(2) |r X1 -r zq |×100 / r X1 ≦15 …(3) however, r X1 is the ionic radius of the most abundant element X1 in 6-coordination among the elements X occupying the X site, r Xp is the pth (p≧2) element X other than the element X1 among the elements X occupying the X site. pIonic radius in 6-coordination of r Zq is the qth (q≧1) element Z among the elements Z occupying the X site. q Ionic radius of 6-coordination. The term "X site" refers to a site that can be occupied by the element X and the element Z in a crystal structure.

[0020] The element X1 represents the most abundant element among the elements X occupying the X site. When there are multiple elements X with the most abundant elements, the element X1 represents the element with the largest ionic radius among them. Element X satisfies formula (2) and formula (3) p and element Z q Since the ionic radius of each element is close to that of element X1, they can substitute for element X1. q Since element Z has a smaller valence than element X, q When doped with element Z, excess lithium ions are introduced into the 9d sites. q Materials containing element Z q In some cases, it may exhibit higher conductivity than a material that does not contain

[0021] The ionic radius of each element is described in, for example, References 1 and 2 below. [Reference 1] http: / / pmsl.planet.sci.kobe-u.ac.jp / ~seto / ?page_id=51 [Reference 2]Shannon et al., Acta A 32(1976)751

[0022] element p and / or element Z q To replace element X1 with element X p and element Z q In each of the elements X1 and X2, the difference in ionic radius between them in 6-coordination is preferably 15% or less (see Hume-Rothery's law, References 3 to 6). The difference in ionic radius is preferably 12% or less, and more preferably 10% or less. [Reference 3] W. Hume-Rothery and HM Powell, Z. Krist., 91(1935)23 [Reference 4] W. Hume-Rothery, Atomic Theory for Students of Metallurgy, The Institute of Metals, London, 1969 (fifth reprint) [Reference 5] W. Hume-Rothery, RW Smallman and CW Hawoth, The Structure of Metals and Alloys, The Institute of Metals, London, 1969 [Reference 6] http: / / ja.wikipedia.org / wiki / Hume-Rothery_Law

[0023] For example, element X1 is V 5+ (r X1 = 0.53), (a) Element X p For example, As 5+ (r X2 =0.46), Sb 5+ (r X3 =0.6), etc. (b) Element Z q (q≧1) is, for example, Ni 4+ (r Z1 =0.48), Se 4+ (r Z2 =0.50), Mn 4+ (r Z3 = 0.53), Co 4+ (r Z4 =0.53), Ge 4+ (r Z5 =0.53), Cr 4+ (r Z6 =0.55), V 4+ (r Z7 =0.58), Fe 4+ (r Z8 =0.585), Rh 4+ (r Z9 =0.60), Ti4+ (r Z10 =0.605), Pd 4+ (r Z11 =0.615), Ru 4+ (r Z12 =0.62). Table 1 below shows an example of the combination of elements.

[0024] [Table 1]

[0025] [1.3. Specific examples] The lithium ion conductor according to the present invention preferably has the following composition. [1.3.1. Example 1: K-based oxides] A first specific example is KLi6VO6, KLi6NbO6, KLi6TaO6, KLi6AsO6, or KLi6SbO6. Hereinafter, these are collectively referred to as "K-based oxides." Among the compositions represented by formula (1), K-based compounds have the advantages of high ionic conductivity, thermodynamic stability, etc. Among them, KLi6NbO6, KLi6TaO6, and KLi6SbO6 have the advantages of low toxicity and low electronic conduction due to a large band gap.

[0026] [1.3.2. Example 2: Rb-based oxides] A second specific example is RbLi6VO6, RbLi6NbO6, RbLi6TaO6, RbLi6AsO6, RbLi6SbO6, or RbLi6BiO6. Hereinafter, these are collectively referred to as "Rb-based oxides." Among the compositions represented by formula (1), Rb-based compounds have the advantages of high ionic conductivity, thermodynamic stability, etc. Among them, RbLi6NbO6, RbLi6TaO6, and RbLi6SbO6 have the advantages of low toxicity and low electronic conduction due to a large band gap.

[0027] [1.3.3. Example 3: KZ-based oxides] The third specific example is KLi6BiO6 or K-based oxides in which a part of X (+5 valence) is replaced with Z (+4 valence). Hereinafter, these are collectively referred to as "KZ-based oxides". The element Z preferably satisfies the above formula (3). Among the compositions represented by formula (1), the KZ-based oxides have the advantages of high ionic conductivity and thermodynamic stability.

[0028] [1.3.4. Example 4: Rb-Z oxides] A fourth specific example is an Rb-based oxide in which a portion of X (+5 valence) contained in the Rb-based oxide is replaced with Z (+4 valence). Hereinafter, these are collectively referred to as "Rb-Z-based oxides." The element Z preferably satisfies the above formula (3). Among the compositions represented by formula (1), the Rb-Z compounds have advantages such as high ionic conductivity and thermodynamic stability.

[0029] [1.4. Band gap] In the present invention, the "band gap" refers to a value calculated using the HSE (Heyd Scuseria Ernzerhof) method. If the band gap is too small, electron / hole conduction becomes dominant instead of ionic conduction. Therefore, the band gap is preferably 2 eV or more. The band gap is preferably 3 eV or more, and more preferably 4 eV or more.

[0030] [1.5. Shape] The shape of the lithium ion conductor according to the present invention is not particularly limited, and an optimal shape can be selected depending on the purpose. Specifically, the lithium ion conductor according to the present invention may be used in a bulk state, or in a powder, nanoparticle, or thin film state.

[0031] [2. Manufacturing method of lithium ion conductor] The lithium ion conductor according to the present invention can be produced by various methods. For example, bulk lithium ion conductors are (a) mixing the raw materials to obtain a desired composition; (b) calcining the raw material mixture under specified conditions; (c) The calcined powder is crushed appropriately and then molded and sintered. It can be produced by the above method. The production conditions are not particularly limited, and it is preferable to select the optimum conditions depending on the desired composition. The same applies to the production of powder, nanoparticles, or thin film lithium ion conductors, and they can be produced using known methods.

[0032] [3. Effect] Figure 1 shows the conductivity of conventional lithium ion conductors. Currently, Li 10 GeP2S 12 However, sulfides have practical issues such as degradation of properties due to material decomposition and generation of hydrogen sulfide. On the other hand, many lithium ion conductors made of oxides, which are more stable than sulfides, have been investigated, but the ionic conductivity of the oxides reported so far is insufficient.

[0033] In contrast, the oxide represented by formula (1) has a higher conductivity calculated by first-principles molecular dynamics calculation than the previously reported material KLi6BiO6. This is because all oxides with the composition represented by formula (1) have (a) In the presence of excess lithium, lithium ions diffuse to link existing sites within the crystal; and (b) Lithium ion diffusion barrier (E mig ) is lower than that of KLi6BiO6, It is thought that. EXAMPLES

[0034] [1. Introduction] Recently, the inventors of the present application have developed a unique method for evaluating lithium ion conductivity at high speed (Reference 7). Using this method, they screened all oxide materials registered in the Inorganic Crystal Structure Database (ICSD). As a result, KLi6TaO6, KLi6IrO6, and KLi6BiO6 were found to have high predicted conductivity. These are materials that have already been synthesized and their crystal structures have been clarified (References 8-10). [Reference 7] A. France-Lanord et al., Sci. Rep. 9, 15123(2019) [Reference 8] KLi6TaO6: W. Scheld et al., ZAAC 619, 337(1993) [Reference 9] KLi6IrO6: P. Kroeshell et al., ZAAC 619, 537(1986) [Reference 10] KLi6BiO6: R. Hubenthal et al., Acta Chem. Scandinavia 45, 805(1991)

[0035] Among these, as mentioned above, KLi6BiO6 has been disclosed as a promising lithium ion conductor in Non-Patent Documents 1 and 2. On the other hand, KLi6IrO6 has been found to have a zero band gap and to be metallic, and therefore has been excluded as a candidate for an ion conductor. Therefore, we carried out element substitution on KLi6(Ta / Bi)O6 to search for new materials that exhibit high conductivity. Figure 2 shows a list of elements selected as substitution elements for KLi6(Ta / Bi)O6.

[0036] In searching for materials, the candidates were those that met the following conditions: (1) The material has a low formation energy ΔHf of the compound based on the oxide raw materials of the constituent elements, and, based on calculations of lattice vibration, has no soft mode vibration and high dynamic stability. (2) The lithium ion conductivity at room temperature, as determined by first-principles molecular dynamics calculations, is higher than that of the previously reported KLi6BiO6. In Non-Patent Document 2, the conductivity of KLi6BiO6 at room temperature predicted by first-principles molecular dynamics calculations is 5.7 mS / cm, which is almost identical to the calculated value (4 mS / cm) by the present inventors, thereby cross-confirming the reliability of the method. (3) The band gap (E g ) is 2 eV or more. Materials that meet this condition are likely to be ionic conductors rather than electron / hole conducting materials.

[0037] [2. Results] [2.1. Conductivity at room temperature] Table 2 shows the predicted conductivity σ at room temperature (300K) of a new oxide-based lithium ion conductor discovered by first-principles molecular dynamics calculation, in which the amount of excess lithium (s) is 1 / 8 = 0.125. "V" stands for volume per atom, "E g (PBE)" represents the band gap calculated using the PBE correlation potential, "E g "(HSE)" represents the band gap calculated by the HSE method, "E mig (Li + )" represents the lithium ion diffusion barrier, "E a " represents the activation energy obtained from first-principles molecular dynamics calculations (FPMD). FIG. 3 shows the conductivity of various oxides calculated from first-principles molecular dynamics calculations.

[0038] From Table 2 and Figure 3, the following can be seen: (1) Among the materials shown in Table 2, all of the materials other than KLi6BiO6 (hereinafter collectively referred to as the “proposed materials”) have higher conductivity than KLi6BiO6. (2) The band gaps of all the proposed materials are 2 eV or more. In particular, the band gaps of the proposed materials other than KLi6BiO6 and RbLi6BiO6 are 3 eV or more, making it difficult for electronic conduction to occur. (3) Among the proposed materials, the material with A = K has higher conductivity than the material with A = Rb. However, when using them in application devices, it is necessary to select materials taking into consideration the conditions of use and combination with electrode materials.

[0039] [Table 2]

[0040] [2.2. Ion conduction pathway] The diffusion path of lithium ions was identified by first-principles molecular dynamics calculations. Figure 4 shows the diffusion trajectory of lithium ions obtained from first-principles molecular dynamics calculations. In Figure 4, the shaded area represents the diffusion path of lithium ions. Figure 4 reveals that in the presence of excess lithium, lithium atoms diffuse in such a way that they connect sites (18f sites and 9d sites) present in the crystal. Figure 5 shows the change in energy along the diffusion path of lithium ions. The high conductivity of the proposed materials shown in Table 2 is due to their diffusion barrier (E mig ) is smaller than that of KLi6BiO6 (0.13 eV), which is thought to be because the energy change due to the diffusion of lithium ions is small.

[0041] From the above, we understand the reason why the proposed material has high ionic conductivity in the presence of excess lithium. That is, ALi6XO6 exhibits high conductivity because the change in potential energy felt by lithium ions is small, allowing lithium ions to diffuse easily. In addition, the proposed materials in which the X site is Ta, Nb, etc. exhibit higher ionic conductivity than the proposed materials in which the X site is Bi. This is thought to be because the ionic radii of Ta, Nb, etc. are smaller than that of Bi, which increases the connectivity of the diffusion paths of lithium ions and reduces the diffusion barrier.

[0042] [2.3. Effect of dopants] In order to stabilize the excess lithium in the proposed material represented by the general formula: ALi6XO6, appropriate doping is required. The proposed material containing the dopant Z has the general formula: ALi 6+s (X 1-t Z t )O 6+δ where the dopant Z is chosen according to the well-known Hume-Rothery law.

[0043] Figure 6 shows the conductivity of oxides with different amounts of excess lithium, and oxides doped with excess lithium and Zr simultaneously (when t=s=0.125), obtained from first-principles molecular dynamics calculations. In Figure 6, "X%, Y / Z" indicates that when (YZ) lithium ions are excessively introduced into a unit lattice consisting of Z Li atoms before doping, and the number of Li atoms contained in the unit lattice becomes Y, the amount of lithium ions increases by X%. The relationship between X, Y, and Z is X=(YZ)×100 / Z.

[0044] The following can be seen from Figure 6: (1)KLi6TaO6 and KLi 6+s (Ta 1-t Zr t )O6 has a higher conductivity at room temperature than that of KLi6BiO6 (estimated conductivity: 4 mS / cm at room temperature). (2) The higher the concentration of excess lithium, the higher the conductivity of KLi6TaO6. (3)KLi 6+s (Ta 1-t Zr t The conductivity of KLi6TaO6 at room temperature is lower than that of KLi6TaO6. This is thought to be because Zr at the Ta site reduces the mobility as an impurity in ion conduction. However, the conductivity is 5 mS / cm, which is higher than the conductivity of the known material KLi6BiO6 when only the same amount of excess lithium is introduced (4 mS / cm, see Table 2). In addition, when element Z is simultaneously doped at the Bi site, it is thought that the higher the concentration of excess lithium, the higher the conductivity will be, as in the case of KLi6TaO6.

[0045] Although the embodiment of the present invention has been described in detail above, the present invention is not limited to the above embodiment, and various modifications are possible without departing from the gist of the present invention. [Industrial Applicability]

[0046] The lithium ion conductor according to the present invention can be used as a solid electrolyte material for lithium ion batteries.

Claims

1. A lithium ion conductor having the following configuration: (1) The lithium ion conductor has a composition represented by the following formula (1) (wherein KLi 6 BiO 6 and KLi 6+s (Ta 1-t Z t )O 6+δ ). AL+ 6+s (X 1-t Z t )O 6+δ …(1) however, A (+1 valence) is K and / or Rb; X (+5 valence) is one or more elements selected from the group consisting of V, Nb, Ta, As, Sb, and Bi; Z (+4 valence) is one or more elements selected from the group consisting of Si, Ni, Se, Mn, Co, Ge, Cr, V, Fe, Rh, Ti, Pd, Ru, Ir, Pt, Re, Os, Mo, W, Nb, Ta, Sn, Hf, Zr, Tb, Pb, Pr, and Ce; 0≦s<0.5, 0≦t<0.5, δ is the value at which electrical neutrality is maintained. (2) In the case where the lithium ion conductor contains two or more elements X, the lithium ion conductor satisfies the following formula (2): In the case where the lithium ion conductor contains an element Z, the lithium ion conductor satisfies the following formula (3). |r X1 -r Xp |×100 / r X1 ≦15 …(2) |r X1 -r zq |×100 / r X1 ≦15 …(3) however, r X1 is the element X that is the most abundant among the elements X that occupy the X site. 1 Ionic radius in 6-coordination of r Xp is the element X among the elements X occupying the X site. 1 pth (p≧2) element X other than p Ionic radius in 6-coordination of r Zq is the qth (q≧1) element Z among the elements Z occupying the X site. q Ionic radius of 6-coordination. The term "X site" refers to a site that can be occupied by the element X and the element Z in the crystal structure. (3) The lithium ion conductor has a crystal structure belonging to a space group R3_m, or a crystal structure in which excess lithium ions occupy all or a part of the 9d sites of the crystal structure belonging to the space group R3_m.

2. 2. The lithium ion conductor according to claim 1, wherein the band gap calculated by the HSE method is 2 eV or more.

3. (a) KLi 6 V.O. 6 , K.L.I. 6 NbO 6 , K.L.I. 6 AsO 6 , or KLi 6 SbO 6 A K-based oxide comprising (b) RbLi 6 V.O. 6 , RbLi 6 NbO 6 , RbLi 6 TaO 6 , RbLi 6 AsO 6 , RbLi 6 SbO 6 , or RbLi 6 BiO 6 Rb-based oxide consisting of (c) K.L.I. 6 BiO 6 or a K-Z-based oxide in which a part of the X (+5 valence) contained in the K-based oxide is replaced with the Z (+4 valence); or (d) an Rb-Z-based oxide in which a part of the X (+5 valence) contained in the Rb-based oxide is replaced with the Z (+4 valence); The lithium ion conductor according to claim 1 or 2, comprising:

4. 4. The lithium ion conductor according to claim 1, which is used as a bulk, a powder, a nanoparticle, or a thin film.

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