JP7672202B2 - Lithium ion conductor - Google Patents

Description

The present invention relates to a lithium ion conductor, and more specifically, to a novel lithium ion conductor made of an oxide having a composition represented by the general formula: ALi6 _+s ( _{X1- tZt} )O6 _+δ (wherein A is a monovalent metal element, X is a pentavalent metal element, and Z is a tetravalent metal element).

Lithium ion conductors are solid electrolyte materials that constitute all-solid-state lithium ion batteries. In order to increase the output of a battery, high conductivity is required for the solid electrolyte. Currently, sulfides such as Li ₁₀ GeP ₂ S ₁₂ (LGPS) are known as lithium ion conductors that exhibit high conductivity at room temperature. However, sulfides have practical problems such as deterioration of characteristics due to decomposition of materials 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. 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.

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, KLi ₆ BiO ₆ has recently been proposed as a promising material (Non-Patent Documents 1 and 2). Although experimental verification has not yet been performed, a conductivity of 6 mS/cm at room temperature has been predicted by first-principles molecular dynamics calculations. However, the predicted conductivity of KLi ₆ BiO ₆ is lower than that of LGPS (10 mS/cm at room temperature), and oxide materials with higher conductivity are desired.

Y. Zhang et al., Nature Comm. 10, 5260(2019) X.He et al., Adv. Energy Mater., 1902078(2019)

The problem that the present invention aims to solve is to provide a new lithium ion conductor that exhibits high conductivity at room temperature.

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/r _X1 ≦15…(2)
|r _X1 −r _zq |×100/r _X1 ≦15…(3)
however,
_r is the ionic radius of the most abundant element _X1 in 6-coordination among the elements X occupying the X site;
_rXp is the ionic radius of the p-th (p≧2) element Xp other than the element _X1 among the elements X occupying the X _site in 6-coordination;
r _Zq is the ionic radius of the qth (q≧1) element _Zq in 6-coordination among the elements Z occupying the X site.
The term "X site" refers to a site that can be occupied by the element X and the element Z in the crystal structure.

The oxide having the composition represented by formula (1) exhibits high conductivity at room temperature. This is because all oxides having the composition represented by formula (1)
(a) in the presence of excess lithium, lithium ions diffuse to link existing sites within the crystal; and
(b) The lithium ion diffusion barrier ( _Emig ) is lower than _that of _KLi6BiO6 ;
It is thought that.

The conductivity of a conventional lithium ion conductor. 1 is a list of elements selected as substitution elements for KLi ₆ (Ta/Bi)O ₆ . FIG. 1 is a diagram showing the conductivity of various oxides obtained by first-principles molecular dynamics calculations. FIG. 1 is a diagram showing the diffusion trajectory of lithium ions obtained by first-principles molecular dynamics calculations. FIG. 2 illustrates the change in energy along the diffusion path of lithium ions. FIG. 13 is a diagram showing the conductivity of an oxide with various amounts of excess lithium and an oxide doped with excess lithium and Zr simultaneously, which are obtained from first-principles molecular dynamics calculations.

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.

[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.
"Crystal structure similar to space group R3_m" refers to a crystal structure that is not strictly a crystal structure belonging to space group R3_m, but can be regarded as the same. In a crystal structure belonging to space group R3_m, Li is usually located at the 18f site, and the 9d site is empty (see FIG. 4). Ion 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 belonging to space group R3_m", but in this application, it is treated as a "crystal structure similar to space group R3_m".

1.2. Composition
The lithium ion conductor according to the present invention has a composition represented by the following formula (1) (excluding KLi ₆ BiO ₆ ).
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.

[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. These elements are suitable as the element A because they can occupy the 3b site and oxides containing these elements exhibit relatively high ionic conductivity. The lithium ion conductor may contain either K or Rb, or may contain both.

[1.2.2. Li]
As described above, in a crystal structure 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.

[1.2.3. Element X]
The element X is an element located at the 3a site (six-coordination site) (hereinafter, also simply referred to as the "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 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.

[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 in order 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" includes semimetals such as Si and Ge.

[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 neutral condition cannot be maintained, and there is a risk of phase separation or Li precipitation occurring. Therefore, s must be less than 0.5.

[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.

[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. In addition, 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=s-t. δ is −0.25≦δ≦+0.25 from the upper limits of s and t.

[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 is the ionic radius of the most abundant element _X1 in 6-coordination among the elements X occupying the X site;
_rXp is the ionic radius of the p-th (p≧2) element Xp other than the element _X1 among the elements X occupying the X _site in 6-coordination;
r _Zq is the ionic radius of the qth (q≧1) element _Zq in 6-coordination among the elements Z occupying the X site.
The term "X site" refers to a site that can be occupied by the element X and the element Z in the crystal structure.

The element _X1 represents the element that is contained most abundantly among the elements X occupying the X site. When there are a plurality of elements X that are contained most abundantly, the element _X1 represents the element that has the largest ionic radius among them.
The element _Xp and the element _Zq , which satisfy the formulas (2) and (3), both have an ionic radius close to that of the element _X1 , and therefore can substitute for the element _X1 . In addition, since the element _Zq has a smaller valence than the element X, when the element _Zq is doped, an excess of lithium ions is introduced into the 9d site. Therefore, a material containing the element _Zq may exhibit higher conductivity than a material not containing the element _Zq .

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

In order to substitute the element _X1 with the element _Xp and/or the element _Zq , it is preferable that the difference in ionic radius between the element _Xp and the element _Zq in 6-coordination with the element _X1 is 15% or less (see Hume-Rothery's law, References 3 to 6). The difference in ionic radius is preferably 12% or less, 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

For example, when the element _X1 is V5 ⁺ ( _rX1 = 0.53),
(a) Examples of the element _Xp (p≧2) include As5 ⁺ ( _rX2 =0.46) and Sb5 ⁺ ( _rX3 =0.6).
(b) Examples of elements _Zq (q≧1) include Ni4 ⁺ ( _rZ1 =0.48), Se4 ⁺ ( _rZ2 =0.50), Mn4+ (rZ3 =0.53), Co4 ⁺ ( _rZ4 =0.53), Ge4 ⁺ ( _rZ5 =0.53), Cr4 ⁺ ( _rZ6 =0.55), V4 ⁺ ( _rZ7 =0.58), ^{Fe4+ (} _rZ8 =0.585), Rh4 ⁺ ( _rZ9 =0.60), Ti4 ⁺ ( _rZ10 = _0.605 ), Pd4 ⁺ ( _rZ11 =0.615), Ru4 ⁺ ( _rZ12 =0.62), etc.
Table 1 below shows an example of the combination of elements.

1.3. Specific Examples
The lithium ion conductor according to the present invention preferably has the following composition.
[1.3.1. Specific Example 1: K-based oxide]
A first specific example is KLi ₆ VO ₆ , KLi ₆ NbO ₆ , KLi ₆ TaO ₆ , KLi ₆ AsO ₆ , or KLi ₆ SbO _6. Hereinafter, these are also collectively referred to as "K-based oxides".
Among the compositions represented by formula (1), the K-based compounds have the advantages of high ionic conductivity, thermodynamic stability, etc. Among them, KLi ₆ NbO ₆ , KLi ₆ TaO ₆ , and KLi ₆ SbO ₆ have the advantages of low toxicity, low electronic conduction due to a large band gap, etc.

[1.3.2. Specific Example 2: Rb-based oxide]
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, _low electronic conduction due to a large band gap, _etc.

[1.3.3. Specific Example 3: KZ-based oxide]
A third specific example is KLi ₆ BiO ₆ or a K-based oxide in which a part of X (+5 valence) is replaced with Z (+4 valence). Hereinafter, these are collectively referred to as "K-Z-based oxides". The element Z preferably satisfies the above formula (3).
Among the compositions represented by formula (1), the KZ oxides have the advantages of high ionic conductivity and thermodynamic stability.

[1.3.4. Specific Example 4: Rb-Z-based oxide]
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-mentioned formula (3).
Among the compositions represented by formula (1), the Rb-Z based compounds have the advantages of high ionic conductivity and thermodynamic stability.

[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 rather than 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.

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.

[2. Method for producing 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 raw materials to obtain a predetermined composition;
(b) calcining the raw material mixture under predetermined conditions;
(c) The calcined powder is appropriately pulverized, and then the calcined powder is molded and sintered to produce the powder.
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.

[3. Action]
The conductivity of conventional lithium ion conductors is shown in Figure 1. Currently, sulfides such as _Li10GeP2S12 (LGPS ₎ are known as lithium ion conductors that exhibit high conductivity at room temperature. However, sulfides have practical problems such as deterioration _of properties due to decomposition of materials 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.

In contrast, the oxide represented by formula (1) has a higher conductivity calculated by first-principles molecular dynamics calculation than that of the previously reported material, KLi ₆ BiO _6. This is because all oxides having 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) The lithium ion diffusion barrier ( _Emig ) is lower than _that of _KLi6BiO6 ;
It is thought that.

1. Introduction
Recently, the present inventors have developed a unique method for rapid evaluation of lithium ion conductivity (Reference 7). Using this method, we screened all oxide materials registered in the Inorganic Crystal Structure Database (ICSD). As a result, KLi ₆ TaO ₆ , KLi ₆ IrO ₆ , and KLi ₆ BiO ₆ were found to have high predicted conductivity. These materials have already been synthesized and their crystal structures have been elucidated (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)

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 _band gap of zero and to be metallic, and therefore has been excluded as a candidate for an ion conductor.
Therefore, we carried out element substitution on KLi ₆ (Ta/Bi)O ₆ to search for new materials that exhibit high conductivity. Figure 2 shows a list of elements selected as substitution elements for KLi ₆ (Ta/Bi)O ₆ .

In searching for materials, candidates were selected that met the following criteria:
(1) The material has a low formation energy ΔHf of the compound based on the oxide raw materials of the constituent elements, and is free from soft mode vibrations and has high dynamic stability based on lattice vibration calculations.
(2) The lithium ion conductivity at room temperature, as determined by first-principles molecular dynamics calculations, is higher than that of the previously reported KLi ₆ BiO _6. In Non-Patent Document 2, the conductivity of KLi ₆ BiO ₆ 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 material has a band gap (E _g ) of 2 eV or more as calculated by the highly accurate HSE method. Materials that satisfy this condition are likely to be ionic conductors rather than materials that conduct electrons/holes.

2. Results
2.1. Conductivity at room temperature
Table 2 shows the predicted conductivity σ at room temperature (300 K) 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" represents the 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 diffusion barrier for lithium ions;
"E _a " represents the activation energy obtained from first-principles molecular dynamics calculations (FPMD).
FIG. 3 shows the conductivity of various oxides obtained from first-principles molecular dynamics calculations.

From Table 2 and FIG. 3, the following can be seen.
(1) Among the materials shown in Table 2, all of the materials other than KLi ₆ BiO ₆ (hereinafter collectively referred to as "proposed materials") have a higher conductivity than KLi ₆ BiO ₆ .
(2) All of _the proposed materials have a band gap of 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 a higher conductivity than the material with A = Rb. However, when using it in an application device, it is necessary to select the material taking into consideration the usage conditions and the combination with the electrode material.

2.2. Ion Conduction Pathways
The diffusion path of lithium ions was identified by first-principles molecular dynamics calculations. Figure 4 shows the diffusion trajectory of lithium ions obtained by 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 to connect sites (18f sites and 9d sites) present in the crystal.
The change in energy along the diffusion path of lithium ions is shown in Figure 5. The reason why the proposed materials shown in Table 2 show high conductivity is believed to be because their diffusion barrier ( _Emig ) values are smaller than that of _KLi6BiO6 ( _0.13 eV), and the energy change accompanying the diffusion of lithium ions is small.

From the above, it was understood why the proposed material has high ionic conductivity in the presence of _excess lithium: _ALi6XO6 exhibits high conductivity because the change in potential energy felt by lithium ions is small and lithium ions can easily diffuse.
In addition, the proposed material in which the X site is Ta, Nb, etc. exhibits higher ionic conductivity than the proposed material in which the X site is Bi. This is thought to be because the ionic radius of Ta, Nb, etc. is smaller than that of Bi, so that the connectivity of the diffusion path of lithium ions is increased and the diffusion barrier is reduced.

2.3. Effect of dopants
Appropriate doping is required to stabilize the excess lithium in the proposed material, which has the general formula _ALi6XO6 . The proposed material containing a dopant Z has the general formula ALi6 _+s (X1 _{-tZt ) O6+δ} , where the dopant _Z was selected according to the well-known Hume-Rothery law.

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 (Y-Z) lithium ions are introduced in excess 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 = (Y-Z) x 100/Z.

The following can be seen from FIG.
(1) The conductivity of _KLi6TaO6 and _{KLi6 +s} (Ta1 _{- tZrt} ) _O6 at room temperature is higher 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) The conductivity of KLi _6+s (Ta _1-t Zr _t )O ₆ at room temperature is lower than that of KLi ₆ TaO _6. This is believed 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 KLi ₆ BiO ₆ 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 believed that the higher the concentration of excess lithium, the higher the conductivity, as in the case of KLi ₆ TaO ₆ .

The above describes the embodiments of the present invention in detail, but the present invention is not limited to the above embodiments, and various modifications are possible without departing from the gist of the present invention.

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

Claims (4)

A lithium ion conductor having the following configuration:
(1) The lithium ion conductor has a composition represented by the following formula (1) (excluding KLi ₆ Bi O ₆ and KLi _6+s (Ta _1-t Z _t )O _6+δ ) .
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) 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 is the ionic radius of the most abundant element _X1 in 6-coordination among the elements X occupying the X site;
_rXp is the ionic radius of the p-th (p≧2) element Xp other than the element _X1 among the elements X occupying the X _site in 6-coordination;
r _Zq is the ionic radius of the qth (q≧1) element _Zq in 6-coordination among the elements Z occupying the X site.
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. The lithium ion conductor according to claim 1, wherein the band gap calculated using the HSE method is 2 eV or more. (a) K-based oxides such as KLi ₆ VO ₆ , KLi ₆ NbO ₆ , KLi ₆ AsO ₆ , or KLi ₆ SbO ₆ ;
(b) _Rb -based oxides such _{as RbLi6VO6} , _RbLi6NbO6 , _{RbLi6TaO6 , RbLi6AsO6} , _{RbLi6SbO6 , or RbLi6BiO6 ;}
(c) KLi 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) The lithium ion conductor according to claim 1 or 2, which contains 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 any one of claims 1 to 3, which is used as a bulk, a powder, a nanoparticle, or a thin film.

Images