JPH10167823A - Pyrochlore type solid electrolyte - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To significantly improve the conductivity as compared with the conventional one,
Provided is a pyrochlore solid electrolyte. A defect pyrochlore solid electrolyte represented by the following general formula (1) or (2). (Ca _x Ce _1-x ) ₂ Sn ₂ O _7-y
(1) (where x and y are 0.5 <x <0.65 and 0, respectively)
<Y <0.3. ) M ₂ (A _x Sn _1-x ) ₂ O _7-y (2) (where M is an ionic radius of 1. assuming 8-coordination.
A represents a trivalent metal element in the range of 01 to 1.18 angstroms, A represents Na or K, x and y represent 0 <x <0.16 and 0 <y <0.48, respectively. . )

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a pyrochlore type solid electrolyte material having a large amount of oxygen vacancies and a high conductivity.

[0002]

2. Description of the Related Art Pyrochlore-type compounds are crystallographically related to fluorite and are represented by the general formula A ₂ B ₂ O _7.
It is known that the A site has a trivalent metal element and has an oxygen eight-coordinate structure, and the B site has a tetravalent metal element and has an oxygen six-coordinate structure. (FS S. Galasso, Crystal Chemistry of Fine Ceramics, Agne Technology Center, pp. 119-124 (1984)).

As an ionic conductor, Ln ₂ Zr ₂ O ₇ or Ln ₂ Ti _{2 in} which tetravalent Zr or Ti is introduced into a B site.
O ₇ (Ln = Nd, Sm, Gd, Tb, Er) is known (for example, AJ Burggraaf, M.E.
P. van Dijk, M .; J. Verkerk, So
lid State Ionics, 5,519-52
2, (1981) or M.S. P. van Dijk, J. et al.
H. H. ter Maat, G .; Roelofs, H .;
Bosch, G .; M. H. van de Velde,
P. J. Gelling and A. J. Burgg
raaf, Mat. Res. Bull. , 19,114
9-1156, (1984), etc.).

[0004]

Generally, in order to improve ionic conductivity and obtain high conductivity in oxides,
It is necessary to introduce oxygen vacancies into the compound and diffuse oxide ions through the oxygen vacancy sites.

[0005] However, the pyrochlore compound represented by A ₂ B ₂ O ₇ was in a state where a high electrical conductivity could not be obtained because there was no oxygen defect inside the crystal structure as it was. The compounds shown in the prior art are 1
By heat-treating at a temperature of 550 ° C. or higher, the crystal phase transition to a fluorite-like crystal structure, which is a related structure of the pyrochlore structure, is used to change the structure to a pseudo-fluorite structure at a high temperature. It is reported that the conductivity of oxides can be improved by improving the mobility of oxide ions. However, since the pyrochlore compound thus obtained has almost no oxygen vacancies, it has been difficult to sufficiently improve the conductivity.

[0006] The present invention has been made in view of the above problems, and an object of the present invention is to provide a pyrochlore type solid electrolyte having significantly improved conductivity as compared with a conventional one.

[0007]

Means for Solving the Problems The present inventors have made intensive studies to solve the above problems. As a result, in the pyrochlore type compound represented by the general formula A ₂ Sn ₂ O ₇ , the A site is not a trivalent metal element but a divalent C
By using a and tetravalent Ce, a pyrochlore crystal structure can be created by making the average valence trivalent, and by changing the ratio of Ca and Ce, the average atomic valence can be changed. Providing a solid electrolyte material having a lower valence state than valence state by introducing oxygen vacancies inside the crystal structure in order to maintain the charge balance of the entire compound, increasing ionic conductivity, and having high conductivity In the pyrochlore type compound represented by the general formula A ₂ Sn ₂ O ₇ , the A site has an ionic radius of 1.01 Å to 1.18 Å assuming an oxygen 8 coordination. As a trivalent metal element in the range below
By substituting a small amount of Na or K for the n-site to lower the average valence of the Sn site,
It is possible to create a pyrochlore crystal structure, furthermore, oxygen defects are introduced inside the crystal structure in order to maintain the charge balance of the entire compound, ionic conductivity is increased, and provision of a solid electrolyte material having high conductivity is provided. The inventors have found that it is possible and completed the present invention.

That is, the present invention provides a defect pyrochlore type solid electrolyte represented by the following general formula _{(1), (Ca x Ce} 1-x) 2 Sn 2 O 7-y (1) ( wherein, x, y are 0.5 <x <0.65 and 0 respectively
<Y <0.3. ) And a defective pyrochlore type solid electrolyte represented by the following general formula (2): M ₂ (A _x Sn _1-x ) ₂ O _7-y (2) (where M is assumed to be eight-coordination) The ion radius is 1.
Represents a trivalent metal element in a range from 01 Å to 1.18 Å, and A represents Na or K
Where x and y are 0 <x <0.16 and 0 <y, respectively.
<0.48. ).

Next, the present invention will be described in more detail.

The pyrochlore solid electrolyte of the present invention is a defective pyrochlore solid electrolyte represented by the general formula (1) or (2).

In the following general formula (1), (Ca _x Ce _1-x ) ₂ Sn ₂ O _7-y (1) x must satisfy 0.5 <x <0.65. If it is lower than this, the average valence of the site composed of Ca and Ce becomes three or more, the pyrochlore crystal structure itself is hardly formed, and high conductivity cannot be obtained because oxygen defects are not introduced into the crystal structure. On the other hand, if it exceeds this range, the average valence of the site consisting of Ca and Ce becomes trivalent or less, but the amount of oxygen vacancies becomes excessive, and as a result, the stability of the pyrochlore crystal structure decreases, and the pyrochlore crystal structure is reduced. Otherwise, the purity of the product will decrease and the conductivity will also decrease. The value of y in the above formula (1) is a value determined by calculation in order to maintain an electrical balance between the positive charge and the negative charge in the compound. If the value of x in the above formula (1) is determined, It is uniquely determined and is in the range of 0 <y <0.3.

Further, in the following general formula (2), M ₂ (A _x Sn _1-x ) ₂ O _7-y (2) M has an ionic radius of 1.01 when assuming eight-coordinate oxygen.
Represents a trivalent metal element in a range from Angstroms to 1.18 Angstroms, and specific examples of M include Y (1.015), Ho (1.02), and Dy.
(1.03), Gd (1.06), Eu (1.07),
Sm (1.09), Nd (1.12) and La (1.1
8) and the like. Here, the number in parentheses is oxygen 8
This is the ionic radius assuming coordination. If the ionic radius of M falls below or exceeds the above range, it deviates from the appropriate ionic radius in the pyrochlore crystal structure, and the pyrochlore crystal structure cannot be formed.

Further, in the general formula (2), A represents a monovalent alkali metal, and the value of x must be 0 <x <0.16. If the value of x is 0, the amount of oxygen defects cannot be introduced into the pyrochlore crystal structure, and a sufficiently high conductivity cannot be obtained. When the value of x is 0.16 or more, the alkali metal having a large ionic radius can no longer form a solid solution in the Sn site, and precipitates at the grain boundaries in the sintered body as a product, thereby lowering the purity of the product. Decreases conductivity. The value of y in the above formula (2) is a value determined by calculation in order to maintain an electrical balance between the positive charge and the negative charge in the compound. If the value of x in the above formula (2) is determined, It is uniquely determined and is in the range of 0 <y <0.48.

By selecting the composition of the general formula (1) or the general formula (2), a high ionic conductor composed of a single phase of pyrochlore type crystal can be produced.

Although the synthesis method of the present invention is not particularly limited, for example, a method in which an oxide is used as a raw material powder, which is mixed by dry and wet mixing, followed by firing, and a method in which an aqueous solution of an inorganic salt is used as a raw material and a precipitant is used. A precipitate is prepared as a carbonate by using oxalic acid, etc., and the precipitate is filtered, dried and then calcined, or using an alkoxide method using an alkoxide solution as a raw material.
After mixing the liquid phases, a precipitate is produced by a hydrolysis reaction, and the precipitate can be synthesized by using a method of filtering, drying, calcining, and the like.

Although the mechanism of the effects of the present invention has not been sufficiently elucidated, the general formula A ₂ B ₂ O ₇ (A site is composed of a trivalent metal element and B site is composed of a tetravalent metal element) Wherein the A site of the pyrochlore compound represented by the formula is composed of a plurality of divalent or tetravalent elements;
The oxygen vacancy is introduced into the pyrochlore crystal by adjusting the composition ratio of the valence, or the A site is converted into a trivalent metal element by displacing a small amount of an alkali metal element into the B site to form a solid solution. It is considered that by reducing the valence and introducing oxygen defects inside the pyrochlore crystal, oxygen ion conductivity was imparted and the conductivity was improved.

As described above, the present invention relates to a pyrochlore type solid electrolyte into which oxygen vacancies have been introduced, and it is possible to obtain a novel solid electrolyte utilizing the function of oxygen vacancies.

[0018]

EXAMPLES The present invention will be described in more detail with reference to the following Examples, but it should not be construed that the present invention is limited thereto.

Examples 1 to 6 Calcium carbonate powder (made by Kishida Chemical), cerium oxide (made by Kishida Chemical), tin oxide (made by Kishida Chemical), 3 as oxide comprising valent metal element, Y ₂ O ₃ (manufactured by Kanto Chemical) or La
After mixing ₂ O ₃ (manufactured by Kanto Kagaku) and sodium carbonate or potassium carbonate as an alkali metal element in a predetermined ratio, the mixture was calcined at 1000 ° C. for 1 hour in the air. After the obtained powder was molded by a hydrostatic pressure of 500 kg / cm ² ,
It was fired at 400 ° C. for 5 hours to produce a sintered body.

The crystal phase of the product was identified by performing an X-ray diffraction test (CuKα ray) after pulverizing the sintered body into a powder.

Table 1 shows the relationship between the composition and the crystal structure of the obtained sintered body.

[0022]

[Table 1]

The obtained sintered body is coated with a platinum electrode,
After baking the electrodes at 000 ° C., the complex impedance was measured by an AC two-terminal method, and the conductivity was calculated by the following equation.

Conductivity: log σT = log ｛T · Zco
sθ / (l · S ⁻¹ )｝ where logσT is conductivity (showing the temperature characteristic of ionic conductivity as such a logarithm), Z is impedance,
θ is the stagger angle, l is the thickness of the pellet, S is the area of the platinum electrode on the pellet, and T is the measured temperature using the absolute temperature.

Table 2 shows the values of the conductivity of the obtained sintered body from 400 ° C. to 950 ° C.

[0026]

[Table 2]

COMPARATIVE EXAMPLES 1 TO 8 After the ingredients were mixed at a predetermined ratio using the reagents shown in the examples so as to have the following eight chemical formulas, they were temporarily added to the atmosphere at 1000 ° C. for one hour. Baked. After the obtained powder was molded by a hydrostatic pressure of 500 kg / cm ² ,
It was fired at 400 ° C. for 5 hours to produce a sintered body.

However, the ionic radius of Comparative Example 8 assuming the coordination of eight oxygen atoms of Ga is about 0.70 angstroms.

The crystal phase of the product was identified by performing an X-ray diffraction test (CuKα ray) after pulverizing the sintered body into a powder.

Table 3 shows the relationship between the composition and the crystal structure of the obtained sintered body.

[0031]

[Table 3]

Only in Comparative Examples 3 to 5 in Table 3, a pyrochlore single phase was obtained, and in Comparative Examples 1 and 2,
In Comparative Examples 6 to 8, unreacted substances were confirmed in addition to the pyrochlore phase, and no single phase was obtained.

Sintered bodies were prepared for Comparative Examples 3 to 5 in which a single phase was obtained among the obtained compounds, a platinum electrode was applied, and the electrode was baked at 1000 ° C.
The complex impedance was measured by the AC two-terminal method, and the conductivity was determined in the same manner as in the example.

Table 4 shows the values of the electrical conductivity of the obtained sintered body from 300 ° C. to 900 ° C.

[0035]

[Table 4]

The conductivity of Comparative Examples 3 to 5, which are pyrochlore-type compounds whose conductivity can be measured, was about 1 to 2 orders of magnitude lower than those of Examples 1 to 6. .

Claims (2)

[Claims] 1. A defective pyrochlore solid electrolyte represented by the following general formula (1). (Ca _x Ce _1-x ) ₂ Sn ₂ O _7-y (1) (where x and y are 0.5 <x <0.65 and 0, respectively)
<Y <0.3. ) 2. A defect pyrochlore type solid electrolyte represented by the following general formula (2). M ₂ (A _x Sn _1-x ) ₂ O _7-y (2) (where M is an ionic radius of 1. assuming 8-coordination.
Represents a trivalent metal element in a range from 01 Å to 1.18 Å, and A represents Na or K
Where x and y are 0 <x <0.16 and 0 <y, respectively.
<0.48. )