JP7803524B2 - Hydride ion conductor, method for producing same, and all-solid-state battery - Google Patents

Description

This disclosure relates to a hydride ion conductor, a method for producing the same, and an all-solid-state battery.

Protons and oxygen ions are used as mobile ions in energy conversion devices such as fuel cells and air batteries. On the other hand, hydride ions are monovalent, have a moderate ionic radius of approximately 1.2 angstroms, and have high polarizability, making them more suitable as mobile ions than protons and oxygen ions. Furthermore, because the redox potential of H ₂ + 2e ⁻ → 2H ⁻ is high at −2.25 V (vs. SHE), they have the potential to create new high-potential energy conversion devices.

Patent Document 1 (JP 2011-204632 A) describes "a hydride ion conductor having a composition represented by the general formula Ln _2-X M _X AH _y O ₃ (wherein Ln represents a trivalent rare earth element, M represents tetravalent Ce or an alkaline earth metal element, and A represents Li or Na. When M is tetravalent Ce, 0<x<0.2 and y=1+x; and when M is an alkaline earth metal element, 0<x<1 and y=1−x)."

Patent Document 2 (JP 2017-098067 A) describes "a hydride ion conductor having a composition represented by the general formula M ¹ _X M ² _y AH _z O _α (wherein M ¹ represents a trivalent rare earth element, M ² represents an alkaline earth metal element or Mg, and A represents Li, Na, Sc, Co, Ni, Cu, Mn, or Fe; 0≦x≦2; 0≦y≦2; x+y=2; 0<z<4; and 1≦α<3 or 3<α<4)."

JP 2011-204632 A Japanese Patent Application Laid-Open No. 2017-098067

The hydride ion conductors described in Patent Documents 1 and 2 exhibit high hydride ion conductivity in the medium temperature range of 300°C to 350°C. There is a demand for hydride ion conductors that exhibit hydride ion conductivity at lower temperatures, preferably around room temperature to 100°C.

The present disclosure aims to provide a hydride ion conductor that has high conductivity at room temperature, a method for producing the same, and an all-solid-state battery that includes the hydride ion conductor and can operate at room temperature.

Lanthanum hydride _LaH3 not only has a fluorite-type structure, which is a framework structure suitable for the diffusion of anions, but also has hydride ions ( ^H- ) occupying octahedral interstitial sites, which is thought to result in high-speed hydride ion conductivity through a quasi-interstitial mechanism. However, _LaH3 is difficult to use as a solid electrolyte because it exhibits electronic conductivity due to the vacancy of ^H- at atmospheric pressure, which results in the formation of donor levels.

The present inventors have found that by substituting a portion of the La ³⁺ in LaH ₃ with alkali metal ions or alkaline earth metal ions that have stronger ionic bonding than La ³⁺ , it is possible to suppress the introduction of vacancies or expand the band gap, thereby suppressing the development of electronic conductivity due to hydride ion deficiency in LaH _3. The present inventors have also found that the alkali metal ion or alkaline earth metal ion substituted product has a hydride ion conductivity at a level that allows it to function as a solid electrolyte at room temperature, and that the alkali metal ion or alkaline earth metal ion substituted product can be used to fabricate an all-solid-state battery that can operate at room temperature.

The present disclosure encompasses the following aspects.
[Aspect 1]
A _{hydride ion conductor} having ^a _{composition represented} by the general formula ^{M1xM2yM3zHαOβ} (wherein ^M1 is a trivalent rare earth element, ^{M2 is} at least one Group 2 element, ^M3 is at least one Group 1 element selected from alkali metals, 0.4≦x<1, 0≦y≦0.6, 0≦z≦0.5, 0.7<x+y+z≦1, 0<y+z≦0.6, 2<α<3, 0≦β<0.3, and 3x+2y+z=α+2β).
[Aspect 2]
2. The hydride ion conductor according to claim 1, wherein the trivalent rare earth element is at least one selected from the group consisting of La, Ce, Nd, Sc, and Y.
[Aspect 3]
3. The hydride ion conductor according to claim 1, wherein the Group 2 element is at least one selected from the group consisting of Mg, Ca, Sr, and Ba.
[Aspect 4]
The hydride ion conductor according to any one of Aspects 1 to 3, wherein the Group 1 element is at least one selected from the group consisting of Na, K, Rb, and Cs.
[Aspect 5]
^A _{method for} producing a _{hydride ion conductor} having a ^composition represented by the general formula ^{M1xM2yM3zHαOβ} (wherein ^M1 is a trivalent rare earth element, ^M2 is at least one Group 2 element, ^M3 is at least one Group 1 element selected from alkali metals, 0.4≦x<1, 0≦y≦0.6, 0≦z≦0.5, 0.7<x+y+z≦1, 0<y+z≦0.6, 2<α<3, 0≦β<0.3, and 3x+2y+z=α+2β),
a hydride containing ^M1 ;
A method for producing a hydride ion conductor, comprising reacting a hydride containing ^M2 , a hydride containing ^M3 , or a combination thereof.
[Aspect 6]
A method for producing a hydride ion conductor according to Aspect 5, wherein the reaction is carried out by a high-pressure synthesis method in the presence of a hydrogen gas generating agent.
[Aspect 7]
6. The method for producing a hydride ion conductor according to claim 5, wherein the reaction is carried out by a mechanochemical method.
[Aspect 8]
8. The method for producing a hydride ion conductor according to claim 7, further comprising performing a hydrogen annealing treatment.
[Aspect 9]
A method for producing a hydride ion conductor according to any one of Aspects 5 to 8, wherein the trivalent rare earth element is at least one selected from the group consisting of La, Ce, Nd, Sc, and Y.
[Aspect 10]
A method for producing a hydride ion conductor according to any one of Aspects 5 to 9, wherein the Group 2 element is at least one selected from the group consisting of Mg, Ca, Sr, and Ba.
[Aspect 11]
A method for producing a hydride ion conductor according to any one of Aspects 5 to 10, wherein the Group 1 element is at least one selected from the group consisting of Na, K, Rb, and Cs.
[Aspect 12]
An all-solid-state battery comprising: a positive electrode containing a hydride ion donor; a negative electrode containing a hydrogen storage metal or a hydrogen storage alloy; and the hydride ion conductor according to any one of Aspects 1 to 4, disposed between the positive electrode and the negative electrode.
[Aspect 13]
13. The all-solid-state battery according to claim 12, wherein the hydride ion donor is a metal hydride.
[Aspect 14]
14. The all-solid-state battery according to claim 12 or 13, wherein the negative electrode comprises a Ti-containing composite active material.

The present disclosure provides a hydride ion conductor that exhibits high conductivity at room temperature. An all-solid-state battery containing this hydride ion conductor can operate at room temperature.

The above description should not be considered as disclosing all embodiments of the present invention or all advantages associated with the present invention.

1 shows powder X-ray diffraction patterns of a hydride ion conductor synthesized by a high-pressure synthesis method and the raw material LaH _3-δ . Powder X-ray diffraction patterns of a hydride ion conductor synthesized by a mechanochemical method and the raw materials LaH _3-δ and SrH ₂ . 1 shows the results of crystal structure analysis of the hydride ion conductor of Example 8 by the Rietveld method using powder X-ray diffraction. FIG. 1 shows the crystal structure of La _0.8 Sr _0.2 H _2.482 O _0.159 . 1 shows the diffuse reflectance spectra of the hydride ion conductors of Examples 7 to 11. 1 is a plot of the band gap versus the amount of Sr in solid solution. Photographs of the hydride ion conductors of Examples 7 to 11 and the raw material LaH _3-δ . 1 is an impedance spectrum at 25° C. of the hydride ion conductor of Example 8. 1 shows Arrhenius plots of hydride ion conductors of Examples 7 to 11 and activation energies of hydride ion diffusion (boxed). 1 shows discharge curves of all-solid-state batteries using the hydride ion conductors of Examples 7 to 11 as solid electrolytes. 1 shows powder X-ray diffraction patterns of a Ti-containing composite active material before and after discharge. 1 shows powder X-ray diffraction patterns of a hydride ion conductor synthesized by a mechanochemical method and the raw materials LaH _3-δ and NaH.

The present invention will now be described in more detail with reference to the drawings to illustrate representative embodiments, but the present invention is not limited to these embodiments.

[Hydride ion conductor]
In one embodiment, the hydride ion conductor has a composition represented by the general formula M ¹ _x M ² _y M ³ _z H _α O _β .

^M1 is a trivalent rare earth element. Examples of the trivalent rare earth element represented by ^M1 include at least one element selected from the group consisting of La, Ce, Nd, Sc, Y, Sm, Eu, and Gd. From the viewpoints of ionic radius and cost, the trivalent rare earth element is preferably at least one element selected from the group consisting of La, Ce, Nd, Sc, and Y, and more preferably La.

^M2 is at least one Group 2 element. From the viewpoint of ionic radius, the Group 2 element is preferably at least one selected from the group consisting of Mg, Ca, Sr, and Ba, and more preferably at least one selected from the group consisting of Sr and Ba.

^M3 is at least one Group 1 element selected from alkali metals. The Group 1 element is preferably at least one selected from the group consisting of Na, K, Rb, and Cs from the viewpoint of ionic radius, and more preferably at least one selected from the group consisting of Na and K from the viewpoint of cost.

x represents the composition ratio of ^M1 , which is an essential constituent atom of the hydride ion conductor, and is 0.4≦x<1. x is preferably 0.5 to 0.9, and more preferably 0.7 to 0.8.

y represents the composition ratio of ^M2 in the hydride ion conductor, and 0≦y≦0.6. y is preferably 0.1 to 0.4, and more preferably 0.15 to 0.3.

When y is 0, the hydride ion conductor has a composition represented by the general formula M ¹ _x M ³ _z H _α O _β . In this embodiment, z is preferably from 0.15 to 0.4, more preferably from 0.2 to 0.3.

z represents the composition ratio of ^M3 in the hydride ion conductor, and 0≦z≦0.5. z is preferably 0 to 0.3, and more preferably 0 to 0.1.

When z is 0, the hydride ion conductor has a composition represented by the general formula M ¹ _x M ² _y H _α O _β . In this embodiment, y is preferably between 0.15 and 0.4, more preferably between 0.2 and 0.3.

x, y and z are represented by the following formulas (1) and (2):
0.7<x+y+z≦1 (1)
0<y+z≦0.6 (2)
With respect to formula (1), it is preferable that 0.8≦x+y+z≦1, and more preferably 0.9≦x+y+z≦1. With respect to formula (2), it is preferable that 0.1≦y+z≦0.4, and more preferably 0.15≦y+z≦0.3.

α represents the composition ratio of hydrogen atoms (H), which are the hydride ion source of the hydride ion conductor, and is in the range of 2<α<3. α is preferably 2.5 to 2.98, and more preferably 2.7 to 2.95.

β represents the composition ratio of oxygen atoms (O) in the hydride ion conductor, and is in the range of 0≦β<0.3. Oxygen atoms (O) are atoms that can be present in the hydride ion conductor due to the raw materials used in synthesizing the hydride ion conductor or the manufacturing conditions. β is preferably 0.2 or less, and more preferably 0.1 or less.

x, y, z, α, and β represent charge compensation of the hydride ion conductor as shown in the following formula (3):
3x+2y+z=α+2β (3)
Satisfy the following.

[Method of manufacturing hydride ion conductor]
The hydride ion conductor can be obtained by reacting a hydride containing ^M1 with a hydride containing ^M2 , a hydride containing ^M3 , or a combination thereof.

Examples of hydrides containing M ¹ include LaH ₃ , CeH ₃ , NdH ₃ , ScH ₃ , YH ₃ , SmH ₃ , EuH ₃ , and GdH _3. Hydrides containing M ¹ may be deficient in hydride ions, in which case the hydride is represented as M ¹ H _3-δ (e.g., LaH _3-δ in the case of lanthanum hydride).

Hydrides containing ^M2 include, for example, _MgH2 , _CaH2 , _SrH2 , and _BaH2 .

Hydrides containing ^M3 include, for example, NaH, KH, RbH, and CsH.

A hydride containing ^M1 , a hydride containing ^M2 , or a hydride containing ^M3 may contain oxygen atoms (O) as impurities when used as a raw material for synthesizing a hydride ion conductor, for example, in a powder state, and the oxygen atoms may be introduced into the hydride ion conductor.

The charging ratio of the hydrides can be adjusted appropriately depending on the composition of the desired hydride ion conductor. Usually, the hydrides are used in a stoichiometric ratio corresponding to the composition of ^M1 , ^M2 , and ^M3 . Before reacting the hydrides, they may be mixed using an agate mortar, a ball mill, or the like to obtain a precursor. Mixing is preferably carried out in an inert gas atmosphere such as argon gas to prevent the incorporation of oxygen from the air. The precursor may be used in powder form or may be formed into pellets before use.

In one embodiment, the reaction is carried out by high-pressure synthesis in the presence of a hydrogen gas generating agent. The reactor used for high-pressure synthesis is not particularly limited. For example, a hydride ion conductor can be synthesized by mixing the hydride or precursor with the hydrogen gas generating agent, encapsulating the mixture in a capsule made of Au, Pt, or rock salt in a cubic anvil-type high-pressure apparatus, and then baking the mixture for 10 minutes to 24 hours at a pressure of 1 GPa to 10 GPa and a temperature of 600°C to 1200°C.

As the hydrogen gas generating agent, for example, a metal hydride such as _NaBH4 or _LiAlH4 , a metal hydroxide such as Ca(OH) ₂ , or a mixture thereof can be used. In one embodiment, a mixture of _NaBH4 and Ca(OH) ₂ , or _LiAlH4 is used as the hydrogen gas generating agent. The hydrogen gas generating agent may be formed into pellets.

In another embodiment, the reaction is carried out by a mechanochemical method. The reactor used for the mechanochemical method is not particularly limited. For example, the hydride or precursor described above can be sealed in a tungsten carbide pot together with tungsten carbide balls, and mixed and milled using a planetary ball mill at a temperature of 15°C to 100°C and a rotation speed of 100 rpm to 1000 rpm for 30 minutes to 24 hours to synthesize a hydride ion conductor.

After synthesizing a hydride ion conductor using the mechanochemical method, a hydrogen annealing treatment may be performed. This hydrogen annealing treatment reduces the hydride ion deficiency in the hydride ion conductor, thereby increasing its hydride ion conductivity. There are no particular restrictions on the reactor used for the hydrogen annealing treatment; the hydrogen annealing treatment can also be performed in the reactor used for the mechanochemical method. For example, the hydrogen annealing treatment can be performed for 1 to 24 hours in a hydrogen gas atmosphere at a temperature of 250°C to 600°C and a pressure of 0.1 to 5 MPa.

The hydride ion conductors disclosed herein can be suitably used in solid-state batteries such as hydride ion secondary batteries and ammonia fuel cells, particularly all-solid-state batteries.

In one embodiment, an all-solid-state battery is provided, comprising: a positive electrode containing a hydride ion donor; a negative electrode containing a hydrogen storage metal or alloy; and the above-described hydride ion conductor disposed between the positive electrode and the negative electrode.

The hydride ion donor contained in the positive electrode may be a material itself that is a hydride or other hydride ion source, or it may be a permeable membrane made of Pd, PdCu alloy, or the like that can convert hydrogen gas supplied from an external source into hydride ions and supply them to the hydride ion conductor.

In one embodiment, the hydride ion donor is a metal hydride. Examples of metal hydrides include hydrides containing ^M1 . The metal hydride is preferably LaH3 _-δ (where δ represents the amount of hydride ion deficiency, and 0≦δ<0.5).

Examples of hydrogen storage metals or alloys contained in the negative electrode include metals with high hydrogen affinity, such as Ti, Zr, Mg, V, and rare earth elements (e.g., La), and alloys containing these metals with high hydrogen affinity and metals with low hydrogen affinity, such as Fe, Ni, Co, Cr, and Cu. Examples of hydrogen storage alloys include rare earth alloys such as _LaNi5 , magnesium alloys such as _Mg2Ni and _Mg2Cu , and titanium alloys such as Ti-Fe, Ti-Mn, Ti-Ni, and Ti-Cu. The hydrogen storage metal is preferably metallic titanium. The hydrogen storage alloy is preferably a titanium alloy.

In one embodiment, the negative electrode comprises a Ti-containing composite active material. The Ti-containing composite active material may comprise metallic titanium or the titanium-based alloy described above and the hydride ion conductor described above. The hydride ion conductor contained in the Ti-containing composite active material may have the same composition as, or a different composition from, the hydride ion conductor that functions as a solid electrolyte between the positive electrode and the negative electrode.

In this embodiment, the Ti-containing composite active material may further contain additives such as a conductive aid and a binder. Examples of conductive aids include carbon materials, metal particles, and conductive polymers. Examples of carbon materials include acetylene black, ketjen black, carbon fiber, carbon nanotubes, and carbon nanofibers. Examples of binders include rubber-based binders such as styrene butadiene rubber (SBR), and fluorine-based binders such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE).

All-solid-state batteries may have a current collector containing SUS, Al, Ni, Ti, carbon, etc. on the positive electrode, and may have a current collector containing SUS, Cu, Ni, carbon, etc. on the negative electrode.

The following examples illustrate specific embodiments of the present disclosure, but the present invention is not limited thereto. All parts and percentages are by weight unless otherwise specified.

1. Synthesis of hydride ion conductors 1
The raw material hydrides used were lanthanum hydride (LaH _3-δ ), strontium hydride (SrH ₂ ), and barium hydride (BaH ₂ ). _{LaH 3-δ} was obtained by firing block metallic lanthanum at 400°C and 0.5 MPa in a hydrogen gas atmosphere for 10 minutes.

Examples 1 and 2
A hydride ion conductor having a composition represented by the general formula M ¹ _x M ² _y H _α O _β (M ¹ =La, M ² =Sr) was synthesized by high-pressure synthesis. Table 1 shows x and y.

LaH3 _-δ and _SrH2 were weighed in a stoichiometric ratio under an argon gas atmosphere. These hydrides were mixed by hand in an agate mortar for 30 minutes to obtain a powdered precursor. The powdered precursor was pelletized and sealed in a gold tube of a cubic anvil-type high-pressure apparatus together with a mixed pellet containing sodium borohydride ( _NaBH4 ) and calcium hydroxide (Ca(OH) ₂ ) as hydrogen gas generating agents. The pellets were then calcined at a pressure of 3 GPa and a temperature of 1000°C for 30 minutes to obtain a polycrystalline powdered hydride ion conductor.

The powder X-ray diffraction patterns of the obtained hydride ion conductor and the raw material LaH3 _-δ are shown in the upper part of Figure 1. The measurement range of X-ray diffraction was 10°≦2θ≦80°. It was observed that the peak shifted to a lower angle as the amount of Sr in solid solution increased.

Examples 3 to 6
A hydride ion conductor having a composition represented by the general formula M ¹ _x M ² _y H _α O _β (M ¹ =La, M ² =Ba) was synthesized by high-pressure synthesis in the same manner as in Example 1. x and y are shown in Table 2.

The powder X-ray diffraction patterns of the obtained hydride ion conductor and the raw material LaH3 _-δ are shown in the lower part of Figure 1. The measurement range of X-ray diffraction was 10°≦2θ≦80°. It was observed that the peak shifted to a lower angle as the amount of Br in solid solution increased.

Examples 7 to 12 and Comparative Examples 1 to 3
A hydride ion conductor having a composition represented by the general formula M ¹ _x M ² _y H _α O _β (M ¹ =La, M ² =Sr) was synthesized by a mechanochemical method.

Stoichiometric amounts of LaH3 _-δ and _SrH2 were weighed in an argon gas atmosphere. These hydrides were sealed in a tungsten carbide pot together with 10 mm diameter tungsten carbide balls and mixed and pulverized for 3 hours using a planetary ball mill at room temperature (25°C) and 800 rpm. Subsequently, hydrogen annealing was performed for 12 hours in a hydrogen gas atmosphere at 400°C and 1 MPa pressure, yielding a polycrystalline powder of hydride ion conductor.

The powder X-ray diffraction patterns of the obtained hydride ion conductor and the raw materials LaH3 _-δ and _SrH2 are shown in Figure 2. The measurement range of X-ray diffraction was 10°≦2θ≦80°. Sr solid solution was formed at least up to y=0.6 (Example 12).

2. Crystal Structure Analysis The crystal structure of the hydride ion conductor was determined by the Rietveld method using powder X-ray diffraction. The results of the crystal structure analysis of the hydride ion conductor of Example 8 are shown in Figure 3. Because trace amounts of oxygen mixed in the inert gas are thought to be adsorbed on the surface of LaH _3-δ , a previously reported crystal structure model containing oxygen was used for analysis. Since the oxygen occupancy was 0.159(4) and charge compensation was established, the composition of the hydride ion conductor of Example 8 was determined to be La _0.8 Sr _0.2 H _2.482 O _0.159 (x = 0.8, y = 0.2, α = 2.482, β = 0.159). The composition of La and Sr was determined as the ratio of the hydride used as the raw material. The crystal structure of _{La 0.8} Sr _0.2 H _2.482 O _0.159 is shown in Figure 4. It is assumed that the oxygen content in the hydride ion conductor changes depending on the synthesis environment.

The results of powder X-ray diffraction Rietveld analysis are shown in Table 4.

3. Bandgap Measurement The bandgap of the hydride ion conductors of Examples 7 to 11 was calculated using ultraviolet-visible-near-infrared absorption spectroscopy. Measurement conditions were room temperature (25°C) and a wavelength range of 175 to 3300 nm. The resulting diffuse reflectance spectra are shown in Figure 5. A plot of the bandgap versus the amount of Sr solid solution is shown in Figure 6. It was confirmed that the lower limit of the energy of absorbed light shifts to the higher energy side with increasing amount of Sr solid solution. The bandgap was 2.14 eV at y = 0.1, and expanded to 2.78 eV at y = 0.5.

Photographs of the hydride ion conductors of Examples 7 to 11 and the starting material LaH _3-δ are shown in Figure 7. The starting material LaH _3-δ was black due to strong absorption caused by a large amount of hydride ion vacancies. In Examples 7 to 11, the color of the hydride ion conductor changed due to the expansion of the band gap and the reduction of hydride ion vacancies.

4. Hydride Ion Conductivity Measurement The hydride ion conductivity of the hydride ion conductors of Examples 7 to 11 was measured using the electrochemical AC impedance method. Symmetric cells were fabricated by sputtering Pt films as hydrogen-blocking electrodes on both sides of a pelletized hydride ion conductor with a diameter of 6 mm and a thickness of 1.1 mm. The diameter of the Pt films was 5 mm. Measurement conditions were an argon atmosphere, an applied voltage of 10 mV, a frequency range of 0.1 Hz to 35 MHz, and a temperature range of 0 to 100°C. The impedance spectrum of the hydride ion conductor of Example 8 at 25°C is shown in Figure 8. The impedance spectrum consisted of a semicircle on the high-frequency side due to the diffusion of hydride ions within the hydride ion conductor and a rise on the low-frequency side due to the electrode blocking hydrogen. Because it was not possible to separate the bulk and grain boundary components, the hydride ion conductivity of the hydride ion conductor was calculated from the resistance value obtained by adding the bulk and grain boundary resistances.

Figure 9 shows the Arrhenius plots and activation energies (framed) for hydride ion diffusion for the hydride ion conductors of Examples 7 to 11. The smaller the amount of Sr in solid solution, the higher the conductivity and the smaller the activation energy for hydride ion diffusion.

5. Battery Reaction Using Hydride Ion Conductors All-solid-state batteries were fabricated using the hydride ion conductors of Examples 7 to 11 as the solid electrolyte, LaH _3-δ as the positive electrode active material, and Ti-containing composite active material as the negative electrode active material, and constant current discharge tests were performed. 150 mg of hydride ion conductor was used as the solid electrolyte. 70 mg of LaH _3-δ was used as the positive electrode active material. The Ti-containing composite active material was prepared by manually mixing 30 parts by mass of titanium metal, 60 parts by mass of the same hydride ion conductor as the solid electrolyte, and 10 parts by mass of carbon conductive additive VGCF (registered trademark) (manufactured by Showa Denko K.K.) in an agate mortar. 10 mg of Ti-containing composite active material was used as the negative electrode active material. Discharge conditions were room temperature (25°C), under argon gas flow, with a discharge current of 50 μA and a 1/67.2 C rate. A discharge test was carried out until the theoretical capacity of Ti was reached, approximately 1120 mAhg ^-1 , and a voltage was generated up to the theoretical capacity. Figure 10 shows the discharge curve together with the assumed reaction formula.

Figure 11 shows powder X-ray diffraction patterns of the Ti-containing composite active material before and after discharge when the hydride ion conductor of Example 8 was used as a solid electrolyte. At 33° to 42°, the Ti peak observed before discharge was not observed after discharge. Furthermore, after discharge, a new titanium hydride TiH ₂ peak was observed around 58°. The powder X-ray diffraction pattern indicates that the discharge reaction in which hydride ions released from the LaH _3-δ positive electrode pass through the hydride ion conductor (solid electrolyte) and are absorbed by Ti at the negative electrode has progressed completely. These results demonstrate that the hydride ion conductor, in which Sr is solid-dissolved in lanthanum hydride LaH _3-δ , exhibits virtually no electronic conductivity and functions as a hydride ion solid electrolyte at room temperature.

6. Synthesis of hydride ion conductors 2
As the hydrides used as raw materials, lanthanum hydride (LaH _3-δ ) and sodium hydride (NaH) used in Example 1 were used.

Example 13
A hydride ion conductor having a composition represented by the general formula M ¹ _x M ³ _z H _α O _β (M ¹ =La, M ³ =Na, x=0.7, z=0.3) was synthesized by a mechanochemical method.

LaH3 _-δ and NaH were weighed in a stoichiometric ratio under an argon gas atmosphere. These hydrides were sealed in a tungsten carbide pot together with 10 mm diameter tungsten carbide balls and mixed and pulverized using a planetary ball mill at room temperature (25°C) and 800 rpm for 3 hours. Subsequently, hydrogen annealing was performed for 12 hours under a hydrogen gas atmosphere at 400°C and 1 MPa pressure, yielding a polycrystalline powder of hydride ion conductor.

The powder X-ray diffraction patterns of the obtained hydride ion conductor (x = 0.7, z = 0.3) and the raw materials LaH _3-δ and NaH are shown in Figure 12. The measurement range of X-ray diffraction was 10°≦2θ≦80°. The powder X-ray diffraction pattern of the synthesized hydride ion conductor showed no peaks derived from NaH, and a peak shift to the lower angle side due to lattice expansion was observed, suggesting that Na was dissolved in LaH _3-δ .

It will be apparent to those skilled in the art that the above-described embodiments and examples can be modified in various ways without departing from the basic principles of the present invention. It will also be apparent to those skilled in the art that various improvements and modifications of the present invention can be made without departing from the spirit and scope of the present invention.

The hydride ion conductors disclosed herein are useful as solid electrolytes for all-solid-state batteries and other applications. Furthermore, the hydride ion conductors disclosed herein have the potential to create new energy devices that utilize hydride ions as charge carriers.

Claims (14)

A _{hydride ion conductor} having ^a _{composition represented} by the general formula ^{M1xM2yM3zHαOβ} (wherein ^M1 is a trivalent rare earth element, ^{M2 is} at least one Group 2 element, ^M3 is at least one Group 1 element selected from alkali metals, 0.4≦x<1, 0≦y≦0.6, 0≦z≦0.5, 0.7<x+y+z≦1, 0<y+z≦0.6, 2<α<3, 0≦β<0.3, and 3x+2y+z=α+2β). The hydride ion conductor according to claim 1, wherein the trivalent rare earth element is at least one selected from the group consisting of La, Ce, Nd, Sc, and Y. The hydride ion conductor according to claim 1 or 2, wherein the Group 2 element is at least one selected from the group consisting of Mg, Ca, Sr, and Ba. The hydride ion conductor according to any one of claims 1 to 3, wherein the Group 1 element is at least one selected from the group consisting of Na, K, Rb, and Cs. ^A _{method for} producing a _{hydride ion conductor} having a ^composition represented by the general formula ^{M1xM2yM3zHαOβ} (wherein ^M1 is a trivalent rare earth element, ^M2 is at least one Group 2 element, ^M3 is at least one Group 1 element selected from alkali metals, 0.4≦x<1, 0≦y≦0.6, 0≦z≦0.5, 0.7<x+y+z≦1, 0<y+z≦0.6, 2<α<3, 0≦β<0.3, and 3x+2y+z=α+2β),
a hydride containing ^M1 ;
A method for producing a hydride ion conductor, comprising reacting a hydride containing ^M2 , a hydride containing ^M3 , or a combination thereof. The method for producing a hydride ion conductor according to claim 5, wherein the reaction is carried out by a high-pressure synthesis method in the presence of a hydrogen gas generating agent. The method for producing a hydride ion conductor according to claim 5, wherein the reaction is carried out by a mechanochemical method. The method for producing a hydride ion conductor according to claim 7, further comprising performing a hydrogen annealing treatment. The method for producing a hydride ion conductor according to any one of claims 5 to 8, wherein the trivalent rare earth element is at least one selected from the group consisting of La, Ce, Nd, Sc, and Y. The method for producing a hydride ion conductor according to any one of claims 5 to 9, wherein the Group 2 element is at least one selected from the group consisting of Mg, Ca, Sr, and Ba. The method for producing a hydride ion conductor according to any one of claims 5 to 10, wherein the Group 1 element is at least one selected from the group consisting of Na, K, Rb, and Cs. An all-solid-state battery comprising a positive electrode containing a hydride ion donor, a negative electrode containing a hydrogen storage metal or a hydrogen storage alloy, and the hydride ion conductor according to any one of claims 1 to 4 disposed between the positive electrode and the negative electrode. The all-solid-state battery according to claim 12, wherein the hydride ion donor is a metal hydride. The all-solid-state battery according to claim 12 or 13, wherein the negative electrode contains a Ti-containing composite active material.