Fluoride ion conductors and fluoride ion batteries

Abstract

This disclosure provides a fluoride ion conductor with high ionic conductivity, and a fluoride ion battery containing such a fluoride ion conductor. [Solution] The fluoride ion conductor of this disclosure is represented by the following compositional formula (1): A 1-x AE x M2F 7+x … (1) (In formula (1) above, A = Na, K, Rb, Cs, or a combination thereof; AE = Ca, Sr, Ba, or a combination thereof; M = Sc, Y, Ln (Ln is a lanthanide element), Al, Ga, In, or a combination thereof; and 0

Description

[Technical Field]

[0001] This disclosure relates to fluoride ion conductors and fluoride ion batteries. [Background technology]

[0002] As disclosed in Non-Patent Documents 1-4, fluorides of various compositions are known. These can be used as fluoride ion conductors.

[0003] For example, BaR2F8 (R: rare earth element) is 10 at 800°C. -3 S·cm -1 Non-patent document 1 discloses that it exhibits ionic conductivity of the order of magnitude.

[0004] Furthermore, Non-Patent Document 2, for example, discloses a KIn2F7-type KYb2F7, which has a structure similar to that disclosed in Non-Patent Document 1. However, Non-Patent Document 2 does not disclose the ionic conductivity. [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] NI Sorokin et al., “Anion Transport in BaR2F8 Crystals at Elevated Temperatures”, Russian Journal of Electrochemistry, Vol.38, No.5, 2002, pp.522-525 [Non-Patent Document 2] Y. LE FUR et al., Structure cristalline de la phase β-KYb2F7, JOURNAL of solid state CHEMISTRY Vol.35, 1980, pp.29-33 [Non-Patent Document 3] K.E.D. Wapenaar et al., CONDUCTIVITY ENHANCEMENT IN Bal-xLaxF2+x SOLID ELECTROLYTES, Solid State lonics No.5, 1981, pp.637-640

Non-Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0006] There is room for improvement in the ionic conductivity of fluoride ion conductors.

[0007] The present disclosure aims to provide a fluoride ion conductor having high ionic conductivity and a fluoride ion battery including such a fluoride ion conductor as a solid electrolyte for a fluoride ion battery.

Means for Solving the Problems

[0008] The present inventors have found that the above problems can be solved by the following means. <Aspect 1> A fluoride ion conductor represented by the following compositional formula (1): A 1-x AE x M2F 7+x … (1) (In the above formula (1), A = Na, K, Rb, Cs, or a combination thereof; AE = Ca, Sr, Ba, or a combination thereof; M = Sc, Y, Ln (Ln is a lanthanoid element), Al, Ga, In, or a combination thereof; and 0 < x < 1). <Aspect 2> The fluoride ion conductor according to Aspect 1, represented by the following compositional formula (2): A 1-x Ba x M2F 7+x … (2) (In the above formula (2), A = K, Rb, or a combination thereof; M = Yb, Lu, or a combination thereof; and 0.025 ≦ x ≦ 0.9). <Aspect 3> The fluoride ion conductor according to Aspect 1, represented by the following compositional formula (3): A 1-x Ba x Er2F 7+x … (3) (In the above formula (3), A = K, Rb, or a combination thereof; and 0.2 ≦ x ≦ 0.8). <Aspect 4> The fluoride ion conductor according to Aspect 1, represented by the following compositional formula (4): K 1-x Ba x Lu2F 7+x … (4) (In the above formula (4), 0.025 ≦ x ≦ 0.5). <Aspect 5> A fluoride ion battery comprising the fluoride ion conductor according to any one of Aspects 1 to 4 as a solid electrolyte for a fluoride ion battery. [Advantages of the Invention]

[0009] According to the present disclosure, it is possible to provide a fluoride ion conductor having high ionic conductivity, and a fluoride ion battery containing such a fluoride ion conductor as a solid electrolyte for a fluoride ion battery. [Brief Description of the Drawings]

[0010] [Figure 1] Figure 1 is a schematic cross-sectional view showing an example of a fluoride-ion battery according to this disclosure. [Figure 2] Figure 2 shows the X-ray diffraction spectrum of K0.9Ba0.1Yb2F7.1. [Figure 3] Figure 3 shows the X-ray diffraction spectrum of K1-xBaxYb2F7+x (0≦x≦1). [Figure 4] Figure 4 shows the X-ray diffraction spectrum of K0.9Ba0.1M2F7.1 (M=Sc, Lu, Yb, Tm, and Er). [Figure 5] Figure 5 shows the X-ray diffraction spectrum of Rb0.65Ba0.35M2F7.35 (M=Dy, Y, and Er). [Figure 6] Figure 6 shows the X-ray diffraction spectrum of Rb1-xBaxEr2F7+x(0.2≦x≦0.8). [Figure 7] Figure 7(a) shows the crystal structure of K0.975Ba0.025Yb2F7.025, and Figure 7(b) shows the crystal structure of K0.8Ba0.2Yb2F7.2. [Figure 8] Figure 8(a) shows the crystal structure of KYb2F7 as a comparative example, and Figure 8(b) shows the crystal structure of BaYb2F8 as a comparative example. [Figure 9] Figure 9 shows the LSV curves for K0.9Ba0.1Yb2F7.1 and K0.9Ba0.1Lu2F7.1. [Modes for carrying out the invention]

[0011] The embodiments of this disclosure will be described in detail below. However, this disclosure is not limited to the embodiments described below, and can be implemented in various modified forms within the scope of the essence of the disclosure.

[0012] Fluoride ion conductors The fluoride ion conductor of this disclosure is represented by the following compositional formula (1): A 1-x AE xM2F 7+x … (1) (In the above formula (1), A = Na, K, Rb, Cs, or any combination thereof; AE = Ca, Sr, Ba, or any combination thereof; M = Sc, Y, Ln (where Ln is a lanthanide element), Al, Ga, In, or any combination thereof; and 0 <x<1)。

[0013] The Disclosing Parties have unexpectedly discovered that the ionic conductivity of a fluoride ion conductor can be improved by substituting a portion of a predetermined monovalent alkali metal element represented by "A" in AM2F7, a KIn2F7-type structure as disclosed in Non-Patent Document 2, with a predetermined divalent alkaline earth metal element.

[0014] The reason for this is presumed to be as follows, although this is not intended to be bound by any theory: That is, by substituting a portion of the predetermined alkali metal element represented by "A" in AM2F7 with a predetermined alkaline earth metal element with a higher valence, A 1-x AE x M2F 7+x (A: a specified alkali metal element, AE: a specified alkaline earth metal element) can be generated. 1-x AE x M2F 7+x Therefore, it is thought that excess fluoride ions are introduced to maintain electrical neutrality. Since these excess fluoride ions exist at relatively unstable sites in the crystal structure, they are easily moved to adjacent sites, and thus the ionic conductivity of the fluoride ion conductor is thought to improve.

[0015] The elements constituting the fluoride ion conductor of this disclosure will be described below.

[0016] In formula (1) representing the fluoride ion conductor of the present disclosure, A is an alkali metal element that is Na, K, Rb, Cs, or a combination thereof, and AE is an alkaline earth metal element that is Ca, Sr, Ba, or a combination thereof. As described above, the ion conductivity of the fluoride ion conductor is improved by substituting a part of "A" with "AE".

[0017] In "M" of formula (1), the "lanthanoid element" represented by "Ln" is La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu.

[0018] In formula (1) representing the fluoride ion conductor of the present disclosure, x satisfies 0 < x < 1. That is, in the fluoride ion conductor of the present disclosure, at least a part of "A" is substituted by "AE", and not all of "A" is substituted by "AE". x may be 0.01 or more, 0.025 or more, 0.05 or more, 0.075 or more, 0.1 or more, 0.2 or more, 0.25 or more, 0.3 or more, or 0.35 or more, and may be 0.9 or less, 0.8 or less, 0.75 or less, 0.7 or less, 0.6 or less, 0.5 or less, 0.4 or less, 0.35 or less, 0.3 or less, 0.25 or less, 0.2 or less, or 0.1 or less.

[0019] The fluoride ion conductor of the present disclosure may be represented by the following compositional formula (2): A 1-x Ba x M2F 7+x … (2) (In the above formula (2), A = K, Rb, or a combination thereof; M = Yb, Lu, or a combination thereof; and 0.025 ≤ x ≤ 0.9).

[0020] In formula (2), A may particularly be K.

[0021] In equation (2), x may be 0.025 or greater, 0.05 or greater, 0.075 or greater, or 0.1 or greater, and may be 0.75 or less, 0.7 or less, 0.6 or less, 0.5 or less, 0.4 or less, 0.35 or less, 0.3 or less, 0.25 or less, 0.2 or less, or 0.1 or less. When x is within the above range, the ionic conductivity of the fluoride ion conductor is further improved. The reason for this is thought to be that such fluoride ion conductors have a small amount of impurities in their structure, although this is not intended to be constrained by any theory.

[0022] The fluoride ion conductor of this disclosure may be represented by the following compositional formula (3): A 1-x Ba x 2F 7+x … (3) (In the above formula (3), A = K, Rb, or any combination thereof; and (0.2 ≤ x ≤ 0.8).

[0023] In equation (3), A may be Rb in particular.

[0024] In equation (3), x may be 0.30 or greater, 0.35 or greater, or 0.4 or greater, and may be 0.6 or less, 0.5 or less, 0.4 or less, or 0.35 or less. When x is within the above range, the ionic conductivity of the fluoride ion conductor is further improved. The reason for this is thought to be that such fluoride ion conductors have a small amount of impurities in their structure, although this is not intended to be constrained by any theory.

[0025] The fluoride ion conductors of this disclosure may be represented by the following compositional formula (4): K 1-x Ba x Lu2F 7+x … (4) (In equation (4) above, 0.025 ≤ x ≤ 0.5).

[0026] The fluoride ion conductor represented by equation (4) not only exhibits high ionic conductivity but also has a wide potential window and therefore high reduction resistance. This is thought to be due to the use of Lu as "M" instead of an easily reduced element such as Yb, although this is not intended to be constrained by any theory.

[0027] In equation (4), x may be 0.05 or greater, 0.075 or greater, or 0.1 or greater, and may be 0.4 or less, 0.35 or less, 0.3 or less, 0.25 or less, 0.2 or less, or 0.1 or less.

[0028] The fabrication of the fluoride ion conductor described herein can be confirmed by X-ray diffraction (XRD). Specifically, this can be confirmed by the presence or absence of the following peaks when the material is measured under an argon atmosphere using a Smartlab (manufactured by Rigaku Corporation) with a CuKα1 source, under the following conditions: measurement range 10-60°, scan speed 1.5° / min, and measurement interval 0.01°: A single peak at 16±2°, 21±2°, 30.5±2°, 33.5±2°, and 35±2°; and Four peaks in the range of 26±2° to 28±2°.

[0029] The method for producing the fluoride ion conductor of this disclosure is not particularly limited. For example, a method of mixing raw material compounds and then calcining them in an inert gas atmosphere is exemplified.

[0030] The raw material compounds are not particularly limited. For example, in the case of a fluoride ion conductor where A=K, AE=Ba, and M=Yb in formula (1), examples of raw material compounds include potassium fluoride (KF), barium fluoride (BaF2), and ytterbium fluoride (YbF3). Such raw material compounds may be prepared by conventional methods or may be commercially available products.

[0031] The method of mixing the raw material compounds is not particularly limited, and they can be mixed, for example, using an agate mortar. The mixing time is not particularly limited, and may be, for example, 10 minutes or more, 20 minutes or more, or 30 minutes or more, and may be 3 hours or less, 2 hours or less, 1 hour or less, or 30 minutes or less.

[0032] The inert gas used during firing may be, for example, argon gas or nitrogen gas. The firing temperature is not particularly limited and may be, for example, 500°C or higher, 700°C or higher, 800°C or higher, or 900°C or higher, and may be 1300°C or lower, 1100°C or lower, 1000°C or lower, or 900°C or lower. The firing time is not particularly limited and may be, for example, 10 hours or more, 11 hours or more, or 12 hours or more, and may be 15 hours or less, 14 hours or less, 13 hours or less, or 12 hours or less.

[0033] The form of the fluoride ion conductor is not particularly limited and may be, for example, a compacted powder or a sintered body. A compacted powder can be obtained by pressurizing the obtained fluoride ion conductor. A sintered body can be obtained by further heating the compacted powder under a vacuum or inert gas atmosphere.

[0034] Fluoride-ion batteries As illustrated in Figure 1, the fluoride ion battery 1 of the present disclosure includes the fluoride ion conductor of the present disclosure as a solid electrolyte for the fluoride ion battery. For details of the fluoride ion conductor of the present disclosure, please refer to the above description relating to the fluoride ion conductor of the present disclosure.

[0035] As illustrated in Figure 1, the fluoride ion battery 1 of this disclosure may have a negative electrode current collector layer 10, a negative electrode active material layer 20, an electrolyte layer 30, a positive electrode active material layer 40, and a positive electrode current collector layer 50 in this order.

[0036] The fluoride ion conductor of this disclosure may be included as a solid electrolyte in at least one of the layers: the positive electrode active material layer, the electrolyte layer, and the negative electrode active material layer. For example, the fluoride ion conductor of this disclosure may be included in the negative electrode active material layer and the electrolyte layer, or it may be included only in the electrolyte layer.

[0037] If the fluoride ion conductor of this disclosure is included in the electrolyte layer, the fluoride ion battery of this disclosure may be a solid-state battery. If the fluoride ion conductor of this disclosure is included in the positive electrode active material layer and / or the negative electrode active material layer, the fluoride ion battery of this disclosure may be a liquid-system battery containing an electrolyte as the electrolyte layer, or it may be a solid-state battery having a solid electrolyte layer as the electrolyte layer. In this disclosure, "solid-state battery" means a battery that uses at least a solid electrolyte as the electrolyte, and therefore a solid-state battery may use a combination of a solid electrolyte and a liquid electrolyte as the electrolyte. Furthermore, the solid-state battery of this disclosure may be an all-solid-state battery, i.e., a battery that uses only a solid electrolyte as the electrolyte.

[0038] The elements constituting the fluoride-ion battery of this disclosure are described below.

[0039] <Negative electrode current collector layer> Examples of materials for the negative electrode current collector layer include stainless steel (SUS), copper, nickel, iron, titanium, platinum, and carbon.

[0040] Examples of negative electrode current collector layer shapes include foil-like, mesh-like, and porous shapes.

[0041] <Negative electrode active material layer> The negative electrode active material layer contains a negative electrode active material and may optionally contain a solid electrolyte, a conductive additive, and a binder.

[0042] The thickness of the negative electrode active material layer is not particularly limited and can be adjusted as appropriate depending on the battery configuration.

[0043] (Negative electrode active material) The negative electrode active material is typically a compound that defluorinated during charging. Examples of negative electrode active materials include elemental metals, alloys, and fluorides of metal oxides. Examples of metallic elements contained in negative electrode active materials include La, Ca, Al, Eu, Li, Si, Ge, Sn, In, V, Cd, Cr, Fe, Zn, Ga, Ti, Nb, Mn, Yb, Zr, Sm, Ce, Mg, and Pb. In particular, MgF z AlF z LaF z CeF z CaF z , or PbF z This is acceptable. Note that the above z is a real number greater than 0.

[0044] (solid electrolyte) The solid electrolyte may be the fluoride ion conductor of this disclosure, one commonly used as a solid electrolyte in a fluoride ion battery, or a combination thereof. If neither the electrolyte layer nor the positive electrode active material layer contains the fluoride ion conductor of this disclosure, the negative electrode active material layer may contain the fluoride ion conductor of this disclosure as the solid electrolyte.

[0045] (Conductive additive) Examples of conductive additives include carbon materials. Examples of carbon materials include carbon black such as acetylene black, Ketjen black, furnace black, and thermal black, as well as graphene, fullerene, and carbon nanotubes.

[0046] (binder) Examples of binders include fluorine-based binders such as polyvinylidene fluoride (PVdF) and polytetrafluoroethylene (PTFE).

[0047] <Electrolyte layer> If the fluoride-ion battery of this disclosure is a liquid-type battery, the electrolyte layer may consist of, for example, an electrolyte solution and an optional separator.

[0048] (electrolyte) The electrolyte may contain, for example, a fluoride salt and an organic solvent.

[0049] (Separator) The separator material is not particularly limited as long as it has a composition that can withstand the operating range of fluoride-ion batteries.

[0050] If the fluoride ion battery of this disclosure is a solid-state battery, the electrolyte layer may include, for example, a solid electrolyte. In this case, the electrolyte layer may optionally contain a binder.

[0051] (solid electrolyte) The solid electrolyte may be the fluoride ion conductor of this disclosure, one commonly used as a solid electrolyte in a fluoride ion battery, or a combination thereof. If neither the negative electrode active material layer nor the positive electrode active material layer contains the fluoride ion conductor of this disclosure, the electrolyte layer may contain the fluoride ion conductor of this disclosure as a solid electrolyte. In this case, the fluoride ion conductor of this disclosure may form the electrolyte layer as a single solid electrolyte.

[0052] (binder) For details regarding the binder, refer to the above description concerning the negative electrode active material layer in this disclosure.

[0053] <Cathode active material layer> The positive electrode active material layer of this disclosure comprises a positive electrode active material and may optionally include a solid electrolyte, a conductive additive, and a binder.

[0054] The thickness of the positive electrode active material layer is not particularly limited and can be adjusted as appropriate depending on the battery configuration.

[0055] (Cathode active material) The positive electrode active material is typically an active material that defluorinates during discharge. Examples of positive electrode active materials include elemental metals, alloys, metal oxides, and fluorides thereof. Examples of metallic elements contained in positive electrode active materials include Cu, Ag, Ni, Co, Pb, Mn, Au, Pt, Rh, V, Os, Ru, Fe, Cr, Bi, Nb, Sb, Ti, Sn, and Zn.

[0056] (solid electrolyte) The solid electrolyte may be the fluoride ion conductor of this disclosure, one commonly used as a solid electrolyte in a fluoride ion battery, or a combination thereof. If neither the negative electrode active material layer nor the electrolyte layer contains the fluoride ion conductor of this disclosure, the positive electrode active material layer may contain the fluoride ion conductor of this disclosure as its solid electrolyte.

[0057] (Conductive additives and binders) For conductive additives and binders, refer to the above description regarding the negative electrode active material layer in this disclosure.

[0058] <Positive electrode current collector layer> Examples of materials for the positive electrode current collector layer include lead, stainless steel (SUS), aluminum, nickel, iron, titanium, platinum, and carbon.

[0059] Examples of positive electrode current collector shapes include foil-like, mesh-like, and porous shapes. [Examples]

[0060] Example 1-1 <Preparation of evaluation samples> (Synthesis of fluoride ion conductors) Potassium fluoride (KF), barium fluoride (BaF2), and ytterbium fluoride (YbF3) (all manufactured by Aldrich) 0.975 Ba 0.025 Yb2F 7.025The powder was weighed to achieve the desired composition and mixed in an agate mortar for 30 minutes. Under an argon (Ar) atmosphere, the mixed powder was placed in a copper (Cu) tube, and both ends of the Cu tube were folded with pliers to seal the Cu tube containing the mixed powder. The Cu tube was fired in a 900°C annular furnace for 12 hours. After natural cooling, the Cu tube was opened in a glove box under an Ar atmosphere to obtain the fluoride ion conductor powder.

[0061] (Preparation of compacted powder) The obtained fluoride ion conductor powder was subjected to uniaxial compression molding at 340 MPa to obtain a compacted powder.

[0062] <evaluation> (X-ray diffraction measurement) X-ray diffraction (XRD) measurements were performed on the fluoride ion conductor powder obtained by synthesis using a Smartlab (manufactured by Rigaku Corporation) with a CuKα1 source under the following conditions: Atmosphere...Ar atmosphere Measurement range: 10-60° Scan speed: 1.5° / min Measurement interval: 0.01°.

[0063] (Ionic conductivity) Gold powder was pressure-molded onto the top and bottom of the compacted fluoride ion conductor to attach gold electrodes, and the ionic conductivity was measured by AC impedance measurement under the following conditions: Frequency range: 7MHz to 0.5Hz Applied voltage: 100mV Atmosphere...under an argon current Temperature: 25°C to 400°C.

[0064] Examples 1-2 to 1-14, Example 2-1, Examples 3-1 to 3-7, Examples 4-1 and 4-2, Comparative Examples 1-1 and 1-2, and Reference Examples 2-1 and 2-2 Except for changing the composition as shown in Tables 1-4, the fluoride ion conductor powders and compacts for each example were prepared and evaluated in the same manner as in Example 1-1. The other raw materials used were as follows, all manufactured by Aldrich: RbF, ScF3, LuF3, TmF3, ErF3, DyF3, YF3, and ErF3.

[0065] Tables 1-4 show the composition, ionic conductivity, and state of the generated phase for each example of the fluoride ion conductor. The state of the generated phase was confirmed by XRD spectroscopy, as shown in Figures 2-6.

[0066] [Table 1]

[0067] [Table 2]

[0068] [Table 3]

[0069] [Table 4]

[0070] As shown in Figures 2-6, the fluoride ion conductors of the examples were found to have the following peaks: A single peak at 16±2°, 21±2°, 30.5±2°, 33.5±2°, and 35±2°; and Four peaks in the range of 26±2° to 28±2°.

[0071] As shown in Tables 1-4, the fluoride ion conductors in the examples in which a predetermined alkali metal element was substituted with a predetermined alkaline earth metal element exhibited higher ionic conductivity than the fluoride ion conductors in the comparative examples.

[0072] This is thought to be because, by substituting some of the predetermined alkali metal elements in the fluoride ion conductor with predetermined alkaline earth metal elements with higher valencies, an excess of fluoride ions is introduced to maintain electrical neutrality, and the fluoride ions become more easily diffused.

[0073] The crystal structures of the fluoride ion conductors of Examples 1-1 and 1-5, and the crystal structures of the fluoride ion conductors of Comparative Examples 1-1 and 1-2 are shown in Figures 7(a) and 7(b), and Figures 8(a) and 8(b), respectively.

[0074] As shown in Tables 1 and 3, it was confirmed that when the generated phase is a single phase of the KIn2F7 type, that is, when the amount of impurities is so small that it cannot be detected, the ionic conductivity tends to be high.

[0075] Examples 5-1 to 5-3 <Preparation of evaluation samples> (Fabrication of sintered bodies) Except for changing the composition as shown in Table 5, the fluoride ion conductor powders for each example were synthesized in the same manner as in Example 1-1, and then compacted powders were prepared. The obtained compacted powders were wrapped in platinum (Pt) foil and sealed in a quartz ampoule under vacuum. The ampoule was heated at 800°C for 12 hours to obtain a sintered body.

[0076] <evaluation> (X-ray diffraction measurement) The fluoride ion conductor powders of Examples 5-1 to 5-3 were evaluated in the same manner as in Example 1-1. The evaluation results, along with the evaluation results for fluoride ion conductor powders of similar compositions, are shown in Figure 4.

[0077] (Ionic conductivity) Pt electrodes were deposited on the top and bottom of the obtained fluoride ion conductor sintered body by sputtering, and the ionic conductivity was measured by AC impedance measurement under the following conditions: Frequency range: 100MHz to 100Hz Applied voltage: 10mV-100mV Atmosphere...under an argon current Temperature: 25℃ to 200℃

[0078] (Potential window) Lead fluoride (PbF2) (manufactured by Aldrich) and acetylene black (AB) (manufactured by Denka Co., Ltd.) were weighed in a mass ratio of 95:5. These were mixed in a ball mill at 600 rpm for 3 hours. This yielded a counter electrode mixture.

[0079] A battery for evaluation was fabricated by laminating Pt foil, a sintered body of the above-mentioned fluoride ion conductor as the electrolyte layer, a counter electrode composite layer, Pb foil, and aluminum (Al) foil in this order, and then compacting them into powder.

[0080] For the fabricated evaluation battery, with a cutoff potential of -3V, -50 μA cm -2 Linear sweep voltammetry (LSV) measurements were performed under conditions of 150°C.

[0081] Table 5 shows the composition, ionic conductivity, and state of the generated phase of each example of fluoride ion conductor. For comparison, Table 5 also shows compacts with compositions corresponding to Examples 5-1 and 5-3 (Examples 1-4 and 3-3). Figure 9 shows the LSV curves of the fluoride ion conductors of Examples 5-1 and 5-2.

[0082] [Table 5]

[0083] As shown in Table 5, the sintered body had higher ionic conductivity than the compacted powder. This is thought to be because the contact area between fluoride ion conductors is larger in the sintered body, which facilitates the diffusion of fluoride ions.

[0084] Furthermore, as shown in Figure 9, the fluoride ion conductor of Example 5-2, where M is Lu, had a higher reduction potential and a wider potential window than the fluoride ion conductor of Example 5-1, where M is Yb. In other words, the fluoride ion conductor of Example 5-2 had high reduction resistance. It should be noted that the reduction resistance was evaluated for a sintered fluoride ion conductor, but it is not considered that sintering the fluoride ion conductor contributes to its reduction resistance. Therefore, it is expected that similar evaluation results can be obtained for a compacted fluoride ion conductor as well. [Explanation of Symbols]

[0085] 1. Fluoride-ion battery 10. Negative electrode current collector layer 20 Negative electrode active material layer 30 Electrolyte layer 40 Cathode active material layer 50 Positive electrode current collector layer

Claims

1. A fluoride ion conductor represented by the following compositional formula (1): A １－ｘ AE ｘ M ２ F ７＋ｘ … (1) (In formula (1) above, A = Na, K, Rb, Cs, or any combination thereof; AE = Ca, Sr, Ba, or any combination thereof; M = Sc, Y, Ln (where Ln is a lanthanide element), Al, Ga, In, or a combination thereof; and 0 < x < 1).

2. The fluoride ion conductor according to claim 1, represented by the following compositional formula (2): A １－ｘ Ba ｘ M ２ F ７＋ｘ … (2) (In the above formula (2), A = K, Rb, or any combination thereof; M = Yb, Lu, or any combination thereof; and (0.025 ≤ x ≤ 0.9).

3. The fluoride ion conductor according to claim 1, represented by the following compositional formula (3): A １－ｘ Ba ｘ Er ２ F ７＋ｘ … (3) (In the above formula (3), A = K, Rb, or any combination thereof; and (0.2 ≤ x ≤ 0.8).

4. The fluoride ion conductor according to claim 1, represented by the following compositional formula (4): K １－ｘ Ba ｘ Lu ２ F ７＋ｘ … (4) (In equation (4) above, 0.025 ≤ x ≤ 0.5).

5. A fluoride ion battery comprising the fluoride ion conductor described in any one of claims 1 to 4 as a solid electrolyte for a fluoride ion battery.

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