JP7523903B2 - Oxide-Ion Conductors - Google Patents

Description

The present invention relates to an oxide ion conductor. The present invention also relates to a solid electrolyte junction including the oxide ion conductor. The present invention further relates to an electrochemical element including the solid electrolyte junction.

Oxide ion conductors are materials that have attracted attention as functional ceramics that can be used in various electrochemical elements, such as solid electrolytes for batteries, such as solid electrolyte fuel cells, ion batteries, and air batteries, as well as sensors and separation membranes. One type of oxide ion conductor is known to be an apatite-type oxide ion conductor, which is a material that exhibits oxide ion conductivity by the movement of interstitial oxygen. Patent Document 1 describes lanthanum silicate, represented by _La9.33+x ( _Si4.7B1.3 ) _{O26 +z,} as one type of apatite-type oxide ion conductor.

As one electrochemical element using an oxide ion conductor, Non-Patent Document 1 proposes a carbon dioxide gas sensor. In this carbon dioxide gas sensor, a lithium ion conductive material, for example lithium carbonate, is used as a gas detection electrode arranged on the surface of the oxide ion conductor.

International Publication No. 2016/111110

Sensors and Actuators B24-25 (1995) 260-265

The inventors' investigations have revealed that when the lanthanum silicate described in Patent Document 1 is used as the oxide ion conductor in the carbon dioxide gas sensor described in Non-Patent Document 1, the lanthanum silicate deteriorates. For example, it has been found that the lanthanum silicate becomes powdered.

The object of the present invention is therefore to improve oxide ion conductors, and more specifically, to provide an oxide ion conductor that is less susceptible to degradation when used in combination with an electrode containing a lithium ion conductor.

The present invention provides an oxide ion conductor that contains rare earth elements, silicon (Si), and oxygen (O), and has an apatite-type crystal structure in which a 004 diffraction peak is observed at 2θ = 51.9° ± 0.9° in an X-ray diffraction pattern measured by a powder X-ray diffractometer using CuKα1 radiation.

The present invention also provides a solid electrolyte junction comprising the oxide ion conductor and an electrode disposed on its surface.

The present invention further provides an electrochemical element having the above-mentioned solid electrolyte junction.

The present invention provides an oxide ion conductor that is less susceptible to degradation when used in combination with an electrode containing a lithium ion conductor.

FIG. 1 is an X-ray diffraction chart of the oxide ion conductors obtained in the examples and comparative examples.

The present invention will be described below based on its preferred embodiments. The oxide ion conductor of the present invention is a type of solid electrolyte. The oxide ion conductor of the present invention preferably contains rare earth elements, silicon (Si), and oxygen (O), and is generally a substance in the category of rare earth silicates. The rare earth elements include one or more elements selected from the group consisting of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, and Lu. The oxide ion conductor of the present invention may contain one or more elements selected from the group consisting of Be, Mg, Ca, Sr, and Ba in addition to the rare earth elements, silicon (Si), and oxygen (O).

The oxide ion conductor of the present invention preferably has an oriented apatite-type crystal structure. "Orientation" means that the crystal has an orientation axis, and the oxide ion conductor of the present invention preferably has c-axis orientation.

When the oxide ion conductor of the present invention has a c-axis orientation, characteristic diffraction peaks are observed on the (002), (004), and (006) planes when the oxide ion conductor is measured by an X-ray diffraction (hereinafter also referred to as "XRD") device. In particular, the oxide ion conductor of the present invention is characterized by the diffraction angle (2θ) at which the diffraction peak derived from the (004) plane is observed, compared with the lanthanum silicates known so far. In detail, in the XRD pattern measured by a powder XRD device using CuKα1 radiation, the 004 diffraction peak is preferably observed at a position of 2θ = 51.9° ± 0.9°. The inventor's research has revealed that the oxide ion conductor in which the 004 diffraction peak is observed at this position is more suppressed in deterioration than the lanthanum silicates known so far, even when used in combination with a lithium ion conductor. To make this advantage even more pronounced, it is more preferable that the 004 diffraction peak be observed at 2θ = 51.9° ± 0.7°, and even more preferable that it be observed at 51.9° ± 0.6°. If multiple peaks are observed within 51.9° ± 0.9°, the position of the peak with the highest intensity is determined as the 004 diffraction peak.

In addition to the 004 diffraction peak, the oxide ion conductor of the present invention also exhibits peaks derived from the (00n) plane (n represents a positive integer), such as the 002 diffraction peak and the 006 diffraction peak, and the positions of these diffraction peaks are shifted to the high angle side, unlike those of previously known lanthanum silicates. In the present invention, the oxide ion conductor is identified by focusing on the 004 diffraction peak, which has a high diffraction intensity and is easy to determine the peak position.

The inventors' investigations have revealed that in order for the oxide ion conductor to exhibit a 004 diffraction peak at the above-mentioned position, it is effective to adjust the composition of the oxide ion conductor. As described above, the oxide ion conductor of the present invention preferably contains a rare earth element, silicon (Si), and oxygen (O), and it is particularly preferable that the oxide ion conductor of the present invention contains at least lanthanum (La) as a rare earth element. It is also particularly preferable that the oxide ion conductor of the present invention contains yttrium (Y).

A particularly preferred oxide ion conductor in the present invention is represented by the formula (1): _{A9.33+x- aYa} [ _{Si6.00-yMy ] O26.00+z} .
In formula (1), A is one or more elements selected from the group consisting of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu, Be, Mg, Ca, Sr, and Ba, and includes at least La.
M is one or more elements selected from the group consisting of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Ga, Zr, Ta, Nb, B, Zn, Sn, W and Mo.
x is a number equal to or greater than -1.33 and equal to or less than 1.50.
y is a number equal to or greater than 0.00 and equal to or less than 3.00.
z is a number equal to or greater than −5.00 and equal to or less than 5.20.
a is a number equal to or greater than 0.6 and equal to or less than 2.6.

In formula (1), La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Be, Mg, Ca, Sr, and Ba listed as A are elements that have in common that they are lanthanoids or alkaline earth metals that can become positively charged ions and form an apatite-type hexagonal crystal structure. Among these, from the viewpoint of further increasing the oxide ion conductivity, it is preferable that A is one or a combination of two or more selected from the group consisting of La, Nd, Ba, Sr, Ca, and Ce, and contains at least La.

The M element in formula (1) is preferably one or more elements selected from the group consisting of B, Ge, Zn, W, Sn, and Mo. Among these, it is more preferable that the M element is one or more elements selected from the group consisting of B, Ge, and Zn, in terms of high orientation and high productivity.

In formula (1), x is preferably -1.00 or more and 1.00 or less, more preferably 0.00 or more and 0.70 or less, and even more preferably 0.45 or more and 0.65 or less, from the viewpoint of further improving the degree of orientation and oxide ion conductivity of the oxide ion conductor.

In terms of filling the positions of the Si elements in the apatite-type crystal lattice, y in formula (1) is preferably 0.40 or more and less than 1.00, more preferably 0.40 or more and 0.90 or less, and even more preferably 0.80 or less, particularly 0.70 or less, and even more preferably 0.50 or more and 0.70 or less.

From the viewpoint of maintaining electrical neutrality within the apatite-type crystal lattice, z in formula (1) is preferably -5.00 or more and 3.61 or less, more preferably -3.00 or more and 2.00 or less, and more preferably -2.00 or more and 1.50 or less, and even more preferably -1.00 or more and 1.00 or less.

In formula (1), the ratio of the number of moles of A to the number of moles of Si, in other words (9.33+x-a)/(6.00-y) in formula (1), is preferably 1.40 or more and 3.00 or less, and more preferably 1.50 or more and 2.00 or less, from the viewpoint of maintaining the spatial occupancy rate in the apatite-type crystal lattice.

As is clear from formula (1), the oxide ion conductor of the present invention corresponds to an oxide ion conductor described in Patent Document 1 (International Publication WO 2016/111110) in which a part of the A site is replaced with an yttrium (Y) element. The amount of replacement with the yttrium element is represented by the value of a in formula (1), and as described above, the value is preferably 0.6 to 2.6. By replacing a part of the A site represented by formula (1) with an yttrium element, the oxide ion conductor of the present invention is one in which the 004 diffraction peak is observed in the above-mentioned diffraction angle range in the XRD measurement. And, even if the oxide ion conductor in which the 004 diffraction peak is observed in such a diffraction angle range is used in combination with a lithium ion conductor, its deterioration is effectively suppressed. The addition of the yttrium element to an oxide ion conductor made of lanthanum silicate is described, for example, in International Publication WO 2017/018149. However, this document only describes the use of yttrium element at the M site in the above formula (1), and does not describe substituting a portion of the A site with yttrium element. Whether yttrium element substitutes a portion of the A site or is present at the M site can be determined by analyzing the results of XRD measurements or by composition analysis using ICP atomic emission spectroscopy.

Specific examples of _the oxide ion conductor represented by formula (1) include, but _are not limited to, _La8.6Y1.1 ( _{Si5.3B0.7 ) O26.7 , La8.0Y1.7 ( Si5.3B0.7} ) _O26.7 , and _{La7.5Y2.2 ( Si5.3B0.7} ) _O26.7 .

The reason why degradation such as pulverization does not occur even when the oxide ion conductor of the present invention is used in combination with a lithium ion conductor has not yet been fully elucidated, but the present inventor believes that the reason is as follows. When the lanthanum silicate described in Patent Document 1 comes into contact with a lithium ion conductor, for example lithium carbonate, the silicon in the lanthanum silicate reacts with the lithium in the lithium carbonate to produce lithium silicate (Li ₄ SiO ₄ ). At the same time, lanthanum oxide (La ₂ O ₃ ) is also produced. Since lanthanum oxide is a substance with very high hygroscopicity, it easily reacts with water in the air to produce lanthanum hydroxide (La(OH) ₃ ). It is believed that this series of reactions causes the lanthanum silicate to deteriorate and pulverize.
In contrast, when the oxide ion conductor of the present invention comes into contact with lithium carbonate, yttrium reacts more readily with lithium than silicon in the oxide ion conductor, so that silicon is less likely to be extracted from the oxide ion conductor, and yttrium is extracted instead. As a result, the state of lithium silicate having an apatite-type crystal structure is maintained, and the generation of lanthanum hydroxide is suppressed, which in turn suppresses pulverization.

In a preferred embodiment, the oxide ion conductor of the present invention can have a degree of orientation measured by the Lotgering method, i.e., a Lotgering degree of orientation of 0.6 or more, preferably 0.8 or more, and more preferably 0.9 or more. In order to achieve a Lotgering degree of orientation of 0.6 or more, it is advantageous to manufacture the oxide ion conductor by the method described below. However, the method is not limited to this method.

In a preferred embodiment, the oxide ion conductor of the present invention can have an oxide ion conductivity of 10 ⁻⁴ S/cm or more at 500° C., preferably 10 ⁻³ S/cm or more, and more preferably 10 ⁻² S/cm or more. In order to achieve an oxide ion conductivity of 10 ⁻⁴ S/cm or more at 500° C., it is preferable that the Lotgering orientation degree is 0.6 or more. However, the present invention is not limited to such a method.

In a preferred embodiment, the oxide ion conductor of the present invention can have a transport number of 0.8 or more, preferably 0.9 or more, and more preferably 0.95 or more. In order to achieve a transport number of 0.8 or more, it is preferable that the purity of the compound represented by the above formula (1) is 90% or more. However, the method is not limited to this.

Next, a preferred method for producing the oxide ion conductor of the present invention will be described. The oxide ion conductor of the present invention is a compound represented by formula (2): A2 _{+xSiO5 +z} (wherein A is one or more elements selected from the group consisting of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu, Be, Mg, Ca, Sr, and Ba, and contains at least La. In the formula, x is -1 to 1, and z is -2 to 2), and Y2 _{+xSiO5 +z} The oriented apatite structure is preferably produced by a method including a step (referred to as a "gas-solid diffusion step") of heating a precursor obtained by firing a mixture of a compound represented by the formula (where x is -1 to 1, and z is -2 to 2) and an M element (where M is one or more elements selected from the group consisting of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Ga, Zr, Ta, Nb, B, Zn, Sn, W, and Mo) in a gas phase containing the M element (where M is one or more elements selected from the group consisting of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Ga, Zr, Ta, Nb, B, Zn, Sn, W, and Mo) to cause the precursor to have an oriented apatite structure by reaction between the M element and the precursor. As long as this production method includes the gas-solid diffusion step, it is optional to add other steps.

According to the present production method, not only can an oxide ion conductor having an apatite-type structure in which the crystals are oriented in one direction be obtained, but also the occurrence of cracks in the crystals can be suppressed, making it possible to produce an oriented apatite-type oxide ion conductor with a larger area.
The M element (cation) in the gas phase reacts with the precursor from the surface of the precursor to form an oriented apatite composite oxide, and the reaction at the interface between the precursor and the generated apatite phase progresses, so that the entire precursor can be made into a composite oxide having an oriented apatite-type crystal structure. Therefore, the oxide ion conductor of the present invention can be successfully produced by this production method.

The precursor used in this manufacturing method is a calcined product of A2 ₊ xSiO5 _{+z and Y2+xSiO5+z. The ratio of A2+xSiO5+ z and Y2+ xSiO5 + z is adjusted} to be the atomic ratio of the A element and Y contained in the oxide ion conductor represented by formula (1). _{A2+xSiO5 +z} can _be manufactured by calcining a mixture of _A2O3 and SiO2 _{. Y2+xSiO5 +z} can be manufactured by calcining _a mixture of _Y2O3 and _SiO2 .

The mixture of A2 _{+xSiO5 +z} and Y2 _{+xSiO5 +z} is fired, for example, by heating the molded body of the mixture in the air. The firing temperature is preferably set to 1200°C or more and 1800°C or less, more preferably set to 1400°C or more and 1700°C or less. The firing time is preferably set to 1.0 hours or more and 5.0 hours or less, more preferably set to 2.0 hours or more and 4.0 hours or less. The precursor thus obtained may be an oriented body or a non-oriented body.

The obtained precursor is subjected to a gas-solid diffusion process to diffuse element M into the precursor, obtaining the desired oxide ion conductor. As element M in the gas phase penetrates into the crystals via the surface of the precursor, the crystals become oriented. Therefore, the orientation direction can be controlled by masking part of the surface of the precursor.

The M element may be any element that can become gaseous at 1000°C or higher at which the precursor changes to an apatite-type crystal structure and can obtain the necessary vapor pressure. Here, the "necessary vapor pressure" means a vapor pressure that can move in a gaseous state in the atmosphere and can proceed with the reaction by diffusing at grain boundaries or within grains from the surface of the precursor to the inside. From this point of view, the M element may be one or more elements selected from the group consisting of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Ga, Zr, Ta, Nb, B, Zn, Sn, W, and Mo. These M elements react with the precursor surface and are introduced into the Si site to form an oriented apatite structure sintered body. If the M element is, for example, B, compounds containing the M element may include B ₂ O ₃ , H ₃ BO ₃ , LaBO ₃ , and LaB _6. Amorphous bodies such as borosilicate glass may also be used. On the other hand, when the M element is Zn, ZnO, Zn metal _{, Zn2SiO4} , etc. can be used. When it is Ge, _GeO2 , Ge metal, etc. can be used. When it is W, _WO3 , _WO2 , W metal, etc. can be used. When it is Sn, _SnO2 , SnO, Sn metal, etc. can be used. When it is Mo, _MoO2 , _MoO3 , _MoSi2 , Mo metal, etc. can be used.

The gas phase containing the M element may contain any of ions containing the M element, steam containing the M element, and gas containing the M element. For example, it may be a gas phase containing steam containing the M element and oxygen. Therefore, the heating atmosphere at this time, i.e., the atmosphere inside the container containing the M element, may be any of the air atmosphere, a vacuum state, an oxidizing atmosphere, a reducing atmosphere, and an inert atmosphere.

In the gas-solid diffusion step, a specific method for heating the precursor in a gas phase containing the M element can be, for example, a method in which the precursor and a compound containing the M element are placed in a container, such as a sealed container or a container with a lid, and heated. In detail, the compound containing the M element is vaporized, and the atmosphere in the container becomes a gas phase atmosphere containing the M element, and the M element is reacted with the surface of the precursor. However, the method is not limited to this method.
The term "container" in the gas-solid diffusion process means a container that limits the space required to obtain the above-mentioned "necessary vapor pressure," and examples of the container include a reaction tube, a chamber, a sagger with a lid, etc., although it is not limited to these.

As an example of this manufacturing method, a precursor obtained by firing a mixture of _La2SiO5 and _{Y2SiO5 and B2O3 powder} are placed in the same alumina container with a lid _and heated at 1200°C to 1700°C to vaporize _{the B2O3 powder} , and the atmosphere in the container becomes _a gas phase atmosphere containing B element, and c-axis oriented apatite _{La9.33+x-aYa ( Si4.7B1.3} ) _O26+z in which B is substituted at _the Si site can be synthesized.

The heating temperature in the gas-solid diffusion process (i.e., the set temperature of the heating furnace) is preferably 1000°C or higher, more preferably 1100°C or higher, and even more preferably 1200°C or higher. There is no particular upper limit to the heating temperature, but it is understood that the upper limit temperature is around 1700°C, at which the apatite-type crystal structure can be maintained.

A solid electrolyte junction can be obtained by disposing electrodes on the surface of the oxide ion conductor obtained in this manner. A pair of electrodes is usually used. The shape of the oxide ion conductor is not limited. For example, it can be flat or cylindrical. When the oxide ion conductor has a flat shape, for example, a pair of electrodes can be disposed on the same plate surface, or an electrode can be disposed on each plate surface. When the oxide ion conductor has a cylindrical shape, for example, electrodes are usually disposed on the inner and outer circumferential surfaces.

Various electrochemical elements can be obtained using the solid electrolyte junction. Examples of electrochemical elements include various gas sensors such as carbon dioxide gas sensors and oxygen sensors, fuel cells, oxygen separation membranes, and water electrolysis.

When the solid electrolyte junction is used as a carbon dioxide gas sensor, a lithium ion conductor, for example, lithium carbonate, can be used as at least the sensing electrode of the pair of electrodes. When lanthanum silicate, a hitherto known oxide ion conductor, is used in combination with lithium carbonate, the lanthanum silicate deteriorates, but the oxide ion conductor of the present invention suppresses the deterioration even when used in combination with lithium carbonate.

The solid electrolyte assembly can also be used as a cell of a fuel cell (SOFC). In this case, for example, a fuel gas is supplied to the anode electrode of the solid electrolyte assembly, and an oxidant (air, oxygen, etc.) is supplied to the cathode electrode to operate the solid electrolyte assembly at 350 to 1000°C. At the cathode electrode, oxygen atoms that have received electrons become O ^2- ions. The O ^2- ions reach the anode electrode via the oxide ion conductor, where they combine with hydrogen and release electrons. This generates electricity.

The solid electrolyte assembly can also be used as an oxygen separation membrane. In this case, as in the case of use as a fuel cell (SOFC), air is supplied to the cathode electrode and the assembly is operated at 350 to 1000°C. At the cathode electrode, oxygen atoms that have received electrons become O ^2- ions. The O ^2- ions reach the anode electrode via the oxide ion conductor, where ^they release electrons and combine with each other to generate oxygen molecules. As a result, only the oxygen molecules pass through the oxide ion conductor and are separated from other gases in the atmosphere.

In each of the above applications, the thickness of the oxide ion conductor is preferably 0.01 μm or more and 1000 μm or less, and more preferably 0.1 μm or more and 500 μm or less, from the viewpoint of suppressing electrical resistance and manufacturing stability. The electrodes used in each of the above applications are preferably porous. The electrode material may be any known material for the application, and the thickness is preferably about 0.01 μm or more and 70 μm or less.

The present invention will be described in more detail below with reference to examples. However, the scope of the present invention is not limited to such examples. Unless otherwise specified, "%" means "% by mass."

Example 1
_La2O3 and _SiO2 were mixed in _a molar ratio of 1:1, and ethanol was added and mixed in a ball mill. This mixture was dried, crushed in a mortar, and fired in an air atmosphere at 1650°C for 3 hours using a platinum crucible. Ethanol was then added to the fired product and it was pulverized in a planetary ball mill to obtain a first pre-fired powder.
Separately from this operation, _Y2O3 and _SiO2 were mixed in _a molar ratio of 1:1, ethanol was added, and the mixture was mixed in a ball mill. This mixture was dried, pulverized in a mortar, and fired in an air atmosphere at 1650°C for 3 hours using a platinum crucible. Next, ethanol was added to this fired product, and it was pulverized in a planetary ball mill to obtain a second pre-fired powder.
Ethanol was added to the mixture of the first pre-calcined powder and the second pre-calcined powder, and mixed in a ball mill. The mixing ratio of the two was set so that the ratio of La and Y contained in the target oxide ion conductor was the value shown in Table 1 below. The obtained mixed powder was placed in a 20 mmφ molding machine and pressed from one direction to perform uniaxial molding, and then further cold isostatic pressing (CIP) was performed at 700 MPa for 1 minute to form a pellet. The pellet was heated in air at 1650°C for 3 hours to obtain a precursor.
800 mg of the obtained precursor and 140 mg of _B2O3 powder were placed in a sagger with a lid and heated in air at 1550°C (furnace _atmosphere temperature) for 50 hours using an electric furnace to generate _B2O3 vapor in the sagger and react _{the B2O3 vapor} with _the precursor. In this way, the desired oxide ion conductor was obtained.

[Examples 2 and 3]
The first pre-calcined powder and the second pre-calcined powder used in producing the precursor in Example 1 were mixed so that the ratio of La to Y contained in the target oxide ion conductor was the value shown in the following Table 1. Except for this, an oxide ion conductor was obtained in the same manner as in Example 1.

Comparative Example 1
An oxide ion conductor was obtained in the same manner as in Example 1, except that only the first pre-calcined powder was used in the production of the precursor, and the second pre-calcined powder was not used.

[Comparative Examples 2 and 3]
The first pre-calcined powder and the second pre-calcined powder used in producing the precursor in Example 1 were mixed so that the ratio of La to Y contained in the target oxide ion conductor was the value shown in the following Table 1. Except for this, an oxide ion conductor was obtained in the same manner as in Example 1.

〔evaluation〕
The oxide ion conductors obtained in the examples and comparative examples were subjected to XRD measurement by the following method to determine the angle of the 004 diffraction peak. The degree of orientation was also measured by the following method. Furthermore, the oxide ion conductivity was measured by the following method. Furthermore, the stability after disposing an electrode made of lithium carbonate was evaluated by the following method. The results are shown in Table 1 below and FIG. 1.

[XRD Measurement]
The XRD measurement device used was a RINT-TTRIII manufactured by Rigaku Corporation. The radiation source used was CuKα1 radiation (λ=1.5418 Å). Furthermore, measurements were performed in the range of 2θ=10° to 80° under the following conditions.

[Degree of orientation]
The degree of orientation of the c-axis was calculated by the Lotgering method using the following formula: The degree of orientation f was calculated from formula (3) using the ratio ρ of the sum of all peak intensities obtained in XRD measurements of the oxide ion conductors obtained in the examples and comparative examples to the sum of peak intensities attributable to the (002) plane and the (004) plane.
f=(ρ-ρ ₀ )/(1-ρ ₀ ) (1)
In the formula, ρ ₀ : theoretical value
ρ ₀ = ΣI ₀ (00l)/ΣI ₀ (hkl)
ρ: Measured value
ρ=ΣI(00l)/ΣI(hkl)

[Oxide ion conductivity]
A platinum film having a thickness of 150 nm was formed by sputtering on both sides of the oxide ion conductor obtained in the examples and comparative examples to form electrodes, thereby obtaining a solid electrolyte joint. The solid electrolyte joint was placed in a heating furnace, the temperature in the furnace was set to 500° C., and complex impedance analysis was performed using an impedance measuring device at a frequency range of 0.1 Hz to 32 MHz. The oxide ion conductivity (S/cm) of each solid electrolyte joint was calculated from the total resistance component (intragranular resistance + grain boundary resistance).

〔Stability〕
A 5 wt % lithium carbonate paste dispersed in α-terpineol containing ethyl cellulose was applied with a brush to one side of the oxide ion conductor obtained in the examples and comparative examples. The oxide ion conductor was then fired in air at 750°C for 1 hour to form an electrode made of lithium carbonate, thereby obtaining a solid electrolyte joint. This solid electrolyte joint was left to stand for 5 days in an environment of 25°C and 30% RH or higher, and the presence or absence of deterioration of the oxide ion conductor was evaluated according to the following criteria.
◯: The shape of the sintered body is visually maintained, and electrodes can be formed on the surface of the solid electrolyte.
×: Visually, the shape of the sintered body was not maintained and it had changed to a powder-like state, so that an electrode could not be formed on the electrolyte surface.

As is clear from the results shown in Table 1, the oxide ion conductors obtained in the respective examples have apatite-type crystal structure oriented along the c-axis, and show a 004 diffraction peak in a specific range of diffraction angles. The inventors have confirmed from the results of XRD that Y contained in the oxide ion conductors exists at La sites.
It is apparent that the oxide ion conductors obtained in the examples have oxide ion conductivity equivalent to that of the oxide ion conductors of the comparative examples, and are stable against lithium carbonate.

Claims (4)

An oxide ion conductor containing a rare earth element, silicon (Si) element, and oxygen (O) element, in which a 004 diffraction peak is observed at 2θ=51.9°±0.9° in an X-ray diffraction pattern measured by a powder X-ray diffractometer using CuKα1 radiation, and which has an apatite-type crystal structure,
Formula (1): A _9.33+x-a Y _a [Si _6.00-y M _y ]O _26.00+z (wherein A is one or more elements selected from the group consisting of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu, Be, Mg, Ca, Sr, and Ba, and contains at least La;
M is one or more elements selected from the group consisting of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Ga, Zr, Ta, Nb, B, Zn, Sn, W, and Mo, and contains at least B.
The oxide ion conductor represented by the formula (1) is an oxide ion conductor having a molecular weight of 1.00 or more and a molecular weight of 1.50 or less. ... 2. The oxide ion conductor according to claim 1, which has a degree of orientation of 0.6 or more as measured by the Lotgering method. 3. A solid electrolyte joint comprising the oxide ion conductor according to claim 1 or 2 and an electrode disposed on a surface thereof. An electrochemical device comprising the solid electrolyte junction according to claim 3 .

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