JP2001151562A - Beta-alumina solid electrolyte and its production process - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a β-alumina solid electrolyte which has excellent ionic conductivity, high mechanical strengths and denseness and consists of fine and homogeneous crystals and also has no defects throughout its whole bag-shaped body and can be produced at a low cost with simple production process stages, and also to provide a production process of the β-alumina solid electrolyte. SOLUTION: This solid electrolyte has <=2.5 Ω.cm electrical resistivity at 350 deg.C, >=300 MPa radial crushing strength, >=200MPa fracture strength under internal hydraulic pressure and a >=3.23 g/cm3 density. This production process involves blending an alumina raw material, an Na compound and a stabilizer such as MgO, Mg0-based compound, Li20 or Li20-based compound, in their respective prescribed blending ratios to obtain the objective β-alumina solid electrolyte.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a beta-alumina solid electrolyte having excellent characteristics such as low electric resistance and high strength, and a method for producing the same. More specifically, the present invention relates to a high-density, radial compression strength and internal water pressure strength. TECHNICAL FIELD The present invention relates to a beta-alumina solid electrolyte having high Na content and excellent Na ion conductivity, and a method for producing the same.

[0002]

BACKGROUND ART A beta-alumina solid electrolyte is mainly used as a diaphragm of a sodium-sulfur battery (hereinafter, referred to as a “NaS battery”). Beta-alumina solid electrolyte has the property that sodium ion conductivity is extremely high, while beta-alumina crystals are hexagonal plate-shaped crystals, and the direction in which sodium ions conduct is perpendicular to the C axis. And has the property of having anisotropy.

[0003] A method for producing a solid electrolyte tube of a NaS battery is as follows: α-alumina, a Na compound, and materials such as MgO or Li ₂ O as a stabilizer are mixed at an appropriate ratio and then calcined to obtain Aluminized and pulverized. Next, after granulating the obtained pulverized raw material, it is formed into a tubular shape and fired to obtain a beta-alumina solid electrolyte tube.

In such a conventional manufacturing method, the raw material is calcined in advance and converted into beta-alumina, and then formed into a tubular shape by pressing in the thickness direction. The plate-like beta-alumina crystals are oriented, and as a result, the electrical resistance of the beta-alumina solid electrolyte tube in the NaS battery increases, causing a problem that the output of the battery decreases and the power loss during charging increases.

A major part of the internal resistance of a NaS battery is occupied by a solid electrolyte tube, and many studies have been conducted to reduce the resistance of a beta-alumina solid electrolyte tube. The solid electrolyte tube of the NaS battery is required to have not only electric resistance but also characteristics such as high strength, denseness, and fine crystal of the structure. As a manufacturing method for solving this requirement, a manufacturing method disclosed in Japanese Patent Application Laid-Open No. H11-12028 has been proposed. This manufacturing method, adding a seed crystal to the raw material powder, the dispersion, forming a Li ₂ O / Na ₂ O weight ratio of beta-alumina solid electrolyte of the seed crystal and sintering of the raw material powder Li ₂ O / Na ₂ This is a production method characterized by increasing the O weight ratio.

In this publication, the resistivity at 330 ° C. is 1.5 to 2.5 Ωcm, and the radial crushing strength is 250
To 350 MPa, the characteristics of the beta-alumina solid electrolyte described in this publication are as described in Table 1 of the publication, and the resistivity is 2.
The crushing strength is 4 Ωcm or more and the crushing strength is 270 MPa or less. There is no description of a beta alumina solid electrolyte having both lower resistivity and higher radial crushing strength. Therefore, the beta alumina solid electrolyte disclosed in this publication has a resistivity of 2.4 to 3.5 Ωcm and a radial crushing strength of 230.
It is interpreted as a beta alumina solid electrolyte having properties in the range of ~ 250 MPa.

Further, the method proposed in this publication goes through a calcining step of converting the raw material into beta-alumina, so that the steps are complicated and the cost is unavoidable. Furthermore,
A step of adjusting the Li ₂ O / Na ₂ O weight ratio is also added, and problems such as complicated steps and high cost are increased.

The present inventor has previously described Japanese Patent Application Laid-Open No. 7-27274.
No. 9 proposed a method of directly producing a beta-alumina solid electrolyte without going through a calcining step of converting the raw material into beta-alumina. This production method uses an alumina source, a magnesium source, and a sodium source to produce a beta alumina solid electrolyte, using magnesium-aluminum spinel as a magnesium source,
This is a production method in which each raw material is mixed, granulated, molded, and then fired to obtain a beta-alumina solid electrolyte without calcining the raw material. The properties of the beta-alumina solid electrolyte obtained by this method have a resistivity of 4.0Ω.
cm or less, the internal water pressure breaking strength is 150 MPa or more, and the density is 3.20 g / cm ³ or more.

[0009]

Problems to be Solved by the Invention The use of NaS batteries using large cells has been increasing in order to reduce the manufacturing cost of cells per unit battery capacity. As the size of the cell increases, the size of the beta-alumina solid electrolyte tube used also increases, but the current density in the current-carrying part of the beta-alumina solid electrolyte tube increases, resulting in a decrease in energy efficiency due to voltage loss of the NaS battery. . Further, since strength as a structural body is required, there is a limit to thinning. Accordingly, there is a need for a beta-alumina solid electrolyte having low resistance and high strength characteristics that are even better than the characteristic levels of beta-alumina solid electrolytes that have been developed so far, and a method for producing the same.

The present invention has been made in view of the above problems, and aims at (1) to be excellent in ionic conductivity (low in resistivity), and (2).
High mechanical strength (high radial compression strength, etc.)
(3) dense, (4) fine and homogeneous crystals (no coarse crystals), (5) no defects throughout the bag-shaped solid electrolyte tube (high internal water pressure fracture strength). (6) An object of the present invention is to provide a beta-alumina solid electrolyte which satisfies that the manufacturing process is simple and the manufacturing cost is low, and a manufacturing method thereof.

[0011]

According to the present invention, 35
The resistivity at 0 ° C. is 2.5 Ωcm or less, and the radial crushing strength is 3
00MPa or more, internal water pressure fracture strength 200MPa or more,
And a beta-alumina solid electrolyte having a density of 3.23 g / cm ³ or more.

Further, according to the present invention, an alumina raw material and N
a, a manufacturing method of preparing a beta alumina solid electrolyte by mixing MgO or a compound thereof as a stabilizer, or Li ₂ O or a compound thereof at a predetermined ratio, wherein the alumina raw material as the alumina source is: A method for producing a beta-alumina solid electrolyte is provided, wherein the alumina raw material is an alumina produced by subjecting the object to be fired to have a Na ₂ O content of 0.1% by weight or less in the process of producing the alumina raw material. .

In the production method of the present invention, the alumina raw material is preferably α-alumina produced by the Bayer method. Further, in the production method of the present invention, the alumina raw material is preferably α-alumina, and the specific surface area after the main firing is preferably in the range of 1 to 7 m ² / g. Also, an alumina raw material and a Na compound, and M
gO or its compound, or Li ₂ O or a compound formulated at a predetermined ratio, after mixing and grinding, and after molding granulation, followed by firing, it is preferable not to perform calcination of the raw material. Furthermore, the stabilizing agent is magnesium - aluminum spinel or Li ₂ O · nAl ₂ O
It is preferably ₃ .

In the present invention, beta-alumina is β-Al ₂ O ₃ (Na ₂ O · 11Al ₂ O ₃ ), β ″
_{-Al 2 O 3 (Na 2 O} · 5Al 2 O 3), β "'- Al 2 O 3
And the like, and particularly refers to those having a large β ″ -Al ₂ O ₃ content, that is, a so-called β ″ conversion ratio of 95% or more.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail. In the present invention, first, when producing a beta-alumina solid electrolyte, an alumina raw material as an alumina source is subjected to a final firing in which Na ₂ O content is 0.1% by weight or less in the manufacturing process. By using the alumina produced in this way, beta alumina having extremely good properties as a solid electrolyte was obtained.

The present inventor has previously described Japanese Patent Application Laid-Open No. 7-27274.
As a result of measuring and investigating the solid electrolyte manufactured by the manufacturing method disclosed in Japanese Patent Publication No. 9, it was found that beta alumina was oriented although it was small. As a result of repeated studies from various angles for further improvement, α
It was found that beta-alumina orientation was caused by alumina orientation. The present inventor focused on this point, and found that the Na ₂ O content was 0.1% in the production process of the alumina raw material.
Alumina produced by calcining an object to be calcined at not more than 10% by weight has a low orientation. By using this alumina as a raw material and producing it by the production method disclosed in Japanese Patent Application Laid-Open No. 7-272749, extremely high characteristics can be obtained. A good beta alumina solid electrolyte tube could be obtained.

Next, a method for producing an alumina raw material will be described. Alpha alumina produced by the Bayer method is relatively inexpensive and most widely used as a raw material for alumina. FIG. 1 shows the manufacturing flow. Bauxite is used as a raw material and dissolved by caustic soda and steam to form a supersaturated solution of sodium aluminate. Impurities in bauxite, such as iron oxide, titanium oxide, and silica, are separated and removed from the solution as undissolved residues (red mud).

After red mud separation, a step of producing low-soda aluminum hydroxide, a step of producing general alumina products or low-soda alumina products widely used as refractory raw materials, etc. It is roughly divided into the manufacturing process. The alumina used in the present invention is, for example, α-alumina obtained by main-baking low-soda aluminum hydroxide having a Na ₂ O content of 0.1% by weight or less obtained in the step of producing low-soda aluminum hydroxide. is there.

As another example, a specific alumina obtained in a process for producing low-soda alumina can be used as the alumina raw material of the present invention. That is, after separation of red mud, aluminum hydroxide is crystallized, washed with water to produce aluminum hydroxide having a Na ₂ O content of 0.2% by weight or more, and then calcined to form transitional alumina (intermediate alumina). It is formed and washed with an acid to form an intermediate alumina having a Na ₂ O content of 0.1% by weight or less, which is finally calcined at a temperature of 1200 to 1500 ° C. to produce α-alumina. Α-alumina thus produced can be used as the alumina raw material of the present invention.

However, even in the case of low-soda alumina, the above-mentioned aluminum hydroxide having a Na ₂ O content of 0.2% by weight or more is used as a material to be calcined, and a soda removal agent is added thereto.
Α-alumina obtained by main firing at 200 to 1500 ° C. is Na
Α-alumina having a ₂ O content of 0.1% by weight or less,
It cannot be used as the alumina raw material of the present invention. Even if such alumina is used, the effect of the present invention cannot be obtained.

[0021] Also, as another example, the content of Na ₂ O is the sintering of 0.2 wt% or more of aluminum hydroxide as the baked product, Na the α-alumina obtained was prepared acid washed ₂ Α-alumina having an O content of 0.1% by weight or less cannot be used as the alumina raw material of the present invention. Even if such alumina is used, the effect of the present invention cannot be obtained.
Also, general alumina products widely used generally cannot be used.

Alumina as an alumina source in the present invention is obtained by calcining aluminum hydroxide having an Na ₂ O content of 0.1% by weight or less, intermediate alumina, or at a temperature lower than the main calcining in the process of producing the alumina raw material. This is α-alumina produced by subjecting the object to be fired, such as alumina, to main firing. Here, the meaning of the main firing means a step of performing a heat treatment at the highest temperature in the production process of the alumina raw material. When the alumina raw material is produced, heat treatment may be further performed at a temperature lower than the main firing after the main firing, which does not mean firing in the final heat treatment step. Further, in the alumina used in the present invention, Na _{2 in} α-alumina after the main firing is used.
The O content is not implied. This defines the content of Na ₂ O in the material to be fired immediately before the main firing.

The method for reducing the Na ₂ O content of the object to be fired to 0.1% by weight or less is not particularly limited. Of the alumina by the Bayer method, a method used in a method for producing alumina called low-soda alumina may be used. In general, a method of controlling the crystallization of aluminum hydroxide to reduce soda, a method of calcining aluminum hydroxide lightly and a method of washing Na ₂ O component appearing on the surface with water, or a method such as chlorine or boron compound A method of adding a soda removing agent that reacts with Na ₂ O to generate a low-boiling compound, followed by baking.

The important point of the alumina used in the present invention is that the material to be fired having the Na ₂ O content of 0.1% by weight or less by these methods is calcined or calcined until the material to be fired is obtained. Α-alumina obtained by firing (ie, main firing) at the highest temperature higher than the temperature in the process is used as an alumina raw material.

[0025] Also, specific surface area after the firing of the α-alumina particles preferably 1~7m ² / g, further, 2 to 4 m ²
/ G is preferred. The specific surface area of the α-alumina particles after the main baking is generally used because the α-alumina particles after the main baking form secondary particles. Therefore, the pulverization treatment is performed after the main baking. By producing an increase. The specific surface area after the main baking here means the specific surface area of the secondary particles after the main baking before performing the pulverizing treatment. The average particle diameter of a mixture obtained by mixing and pulverizing an alumina raw material, a Na compound, and a magnesium-aluminum spinel as a stabilizer at a predetermined ratio required for beta-alumina was 0.8.
μm or less, more preferably 0.6 μm or less.

An alumina raw material, a Na compound, and a magnesium-aluminum spinel as a stabilizer are mixed in a predetermined ratio required for beta-alumina, mixed and pulverized, granulated by a spray dryer, and added to a tubular shape. A method for producing a beta-alumina solid electrolyte tube by calcining and then calcining the raw material without calcining the raw material, wherein the alumina raw material has a Na ₂ O content of 0.1% by weight or less. By using α-alumina obtained by firing the product, the obtained beta-alumina solid electrolyte has a resistivity at 350 ° C. of 2.5 Ωcm or less, a radial crushing strength of 300 MPa or more, and an internal water pressure strength of 200 MPa.
These have extremely excellent properties as described above.

In general, the crystal orientation of a molded product is affected by the particle shape. The α-alumina used in the present invention has a low crystal orientation and is improved, and the orientation of particles generated at the time of press-molding into a tubular shape is suppressed, and as a result, beta-alumina obtained by firing It is presumed that the solid electrolyte tube has been improved to a state where there is almost no crystal orientation in the thickness direction. It is considered that this resulted in the extremely excellent characteristics as described above.

Since the beta-alumina particles calcined using the α-alumina raw material according to the present invention have a shape that is difficult to be oriented at the time of pressure molding like the raw material, the beta-alumina particles obtained by calcining the raw material are re-used. The same effect can be expected in a manufacturing method in which granulation is performed after pulverization and firing is performed after molding.

As the alumina raw material used in the present invention,
Production methods other than the Bayer method, for example, the thermal decomposition method of ammonium alum, the calcination method of a hydrolyzate of an organic aluminum salt, the underwater spark discharge method of high-purity aluminum, the thermal decomposition method of ammonium aluminum carbonate, ethylene chloride Alumina produced by methods such as the hydrin method and the gas phase oxidation method of anhydrous aluminum chloride can also be used if cost is ignored.

In the production method of the present invention, the magnesia-alumina spinel used is preferably a magnesia-alumina molar ratio (MgO / Al ₂ O ₃ ) of 1 or more and MgO-rich, preferably 1.0 to 1.5. Is more preferable. As the sodium compound as a sodium source, a conventionally known compound such as sodium carbonate can be used, but it is preferable to use sodium hydrogen carbonate or sodium oxalate. Although sodium hydrogen carbonate and sodium oxalate may be used alone, they can be used as a mixture with sodium carbonate.

In the granulation step, the average particle size is usually 50 to 10
A granulated product is prepared so as to have a thickness of 0 μm. Molding is 1.5 to
The molding is performed at a pressure of n / cm ² or more, preferably 2.0 ton / cm ² or more, to produce a molded body having a density of 1.9 g / cm ³ or more. In addition, the maximum temperature for firing is 1580-
Set in the range of 1650 ° C.

[0032]

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples.

(Examples 1 to 8, Comparative Examples 1 to Comparative Examples
3) In Example 1, hydroxyl purified by the Bayer method
By controlling the precipitation conditions of aluminum chloride, Na_TwoO content
Yielded 0.07% by weight of aluminum hydroxide. this
The specific surface area obtained by main firing is 3m^Two/ G of α-alumina
Alumina; 71% by weight, sodium compound
Material: 15% by weight, magnesium-aluminum spine
Blended to be 14% by weight and wet ball mill
Until the average particle size of the slurry reaches 0.5 μm.
I combined. The average diameter of this slurry is measured with a spray dryer.
A granulated powder of 60 μm is prepared and is pressed for 2 tons by hydrostatic pressing.
on / cm ^TwoA molded product in the shape of a bag tube was produced at a pressure of.
With the molded body contained in a sheath made of MgO,
Bake at 1600 ° C for 60 minutes, outer diameter 45mm, length 35
0mm, 1.5mm thickness beta alumina solid electrolyte tube
Was prepared. Characteristics of the obtained beta alumina solid electrolyte tube
The properties are shown in Table 1.

In Example 2, as shown in Table 1, Na ₂
It was manufactured under the same conditions as in Example 1 except that aluminum hydroxide having an O content different from that of Example 1 was used. Similarly, in Examples 3 to 8, as shown in Table 1, aluminum hydroxide having a different Na ₂ O content from that of Example 1 was used, and α-alumina having a specific surface area different from that after the main firing was used. Except for this, it was manufactured under the same conditions as in Example 1. For comparison, as Comparative Example 1 to Comparative Example 3, as shown in Table 1, the Na ₂ O content was 0.2% by weight.
It was manufactured under the same conditions as in Example 1 except that the above aluminum hydroxide was used.

From Table 1, it is preferable to use α-alumina having a specific surface area of α-alumina of ² to 4 m ² / g after the main sintering as a raw material, since a beta-alumina solid electrolyte tube having particularly excellent characteristics can be obtained. .

(Examples 9 to 16, Comparative Examples 4 to 6) In Examples 9 to 16, hydroxylated Na ₂ O having a content of 0.2% by weight or more purified by the Bayer method was used. Aluminum is calcined (or calcined) at various temperatures such that α-alumina is not formed to form an intermediate alumina having a specific surface area shown in Table 1, and the obtained intermediate alumina is subjected to water washing or acid washing. 0.05 as described in Table 1.
Alternatively, an intermediate alumina having a Na ₂ O content of 0.03% by weight was used, and α-alumina having a specific surface area as shown in Table 1 obtained by sintering these at various temperatures was used as a raw material. Is manufactured by the same method as in Example 1. The same applies to Comparative Examples 4 to 6.

(Examples 17 to 24, Comparative Examples 7 to 9) In Examples 17 to 24, the aluminum hydroxide (Na ₂ O content 0.2% by weight) purified by the Bayer method was used. Was fired at a temperature at which α-alumina was formed.
Α-alumina having various specific surface areas as shown in Table 1 was obtained.
These were adjusted to the Na ₂ O content (0.05 or 0.03% by weight) as shown in Table 1 by washing with water or acid washing, and then baked. It was produced in the same manner as in Example 1 except that α-alumina having a specific surface area as shown in Table 1 was used as a raw material. Comparative Examples 7 to
The same applies to Comparative Example 8.

The electric resistance, the internal water pressure strength, and the radial crushing strength were measured as follows.

(Method of Measuring Electric Resistance) The electric resistance was determined as a value at 350 ° C. using a Na / Na current test apparatus shown in FIG. In FIG. 2, the Na / Na electric current test apparatus includes a tubular beta-alumina sintered body (beta-alumina tube) 1 to be measured, insulating supports 2 and 3 made of α-alumina, a stainless steel electrode 4, A container 7 and a beta alumina tube 1
The molten aluminum 8 at 350 ° C. was supplied thereinto, and a constant current was passed between the electrode outlets 5 and 6 to determine the electrical resistivity of the beta alumina tube 1 to be measured as the specific resistance.

(Method of Measuring Inner Water Pressure Strength) Water pressure is applied to the inner wall of a tubular beta-alumina sintered body (beta-alumina tube) via a rubber tube, and the water pressure value at which the beta-alumina tube breaks and the dimensions of the beta-alumina tube Was used to measure the internal water pressure strength.

(Measurement method of radial crushing strength) A test piece obtained by cutting a tubular beta-alumina tube into a ring shape was used.
The radial crushing strength was measured from the value at which a load was applied in the radial direction from the outer peripheral side and destroyed and the dimensions of the test piece.

[0042]

[Table 1]

Comparative Example 10 In Comparative Example 10, aluminum hydroxide (Na ₂ O content: 0.2% by weight or more) purified by the Bayer method was calcined, and the specific surface area was 3 m ² / g.
Was obtained. This is converted to N by acid cleaning.
After the a ₂ O content was adjusted to 0.05% by weight, heat treatment was performed at a temperature lower than that of the main firing. It was manufactured in the same manner as in Example 1 except that the α-alumina thus obtained was used as a raw material. Table 2 shows the characteristics of the obtained beta alumina solid electrolyte tube. Comparative Example 10 shows that even when α-alumina obtained by heat-treating α-alumina having a low Na ₂ O content was used as a raw material,
This shows that the effect is not obtained if the content of Na ₂ O in the object to be fired in the main firing step of α-alumina is high. The method for measuring various characteristic values is the same as in Table 1.

[0044]

[Table 2]

As is apparent from the above results, the beta alumina sintered body manufactured within the scope of the present invention is:
Good properties such as electrical resistance and internal water pressure strength,
In the above, it can be seen that the beta-alumina sintered body manufactured under the more preferable conditions described above has further excellent properties such as electric resistance and internal water pressure strength.

[0046]

As described above, according to the present invention, in the process of producing an alumina raw material, an object to be fired having a Na ₂ O content of 0.1% by weight or less is fully fired according to the present invention. Since the alumina produced by the above method is used, the beta-alumina solid electrolyte obtained in the present invention has a very small beta-alumina crystal orientation, and has properties such as electric resistance and internal water pressure strength. It is very good.

[Brief description of the drawings]

FIG. 1 is a flowchart showing a method for producing a Bayer method alumina raw material.

FIG. 2 is a configuration diagram showing an example of a Na / Na conduction test apparatus for obtaining electric resistance.

[Explanation of symbols]

1 ... beta alumina tube, 2,3 ... insulating support, 4 ... electrode, 5,6 ... electrode outlet, 7 ... container, 8 ... molten sodium.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Seiki Kawaguchi 2-56, Sudacho, Mizuho-ku, Nagoya, Aichi Prefecture Inside Nihon Insulators Co., Ltd. No. 56 F Term in Japan Insulators Co., Ltd. (Reference) 4G030 AA02 AA03 AA07 AA36 BA03 BA20 CA02 5G301 CA02 CA12 CD01 CE02 5H029 AJ02 AJ11 AM12 AM15 CJ02 CJ08 EJ05

Claims (6)

[Claims] 1. The resistivity at 350 ° C. is 2.5 Ωcm.
Hereinafter, the radial crushing strength is 300 MPa or more, and the internal water pressure breaking strength is
200MPa or more, and the density is 3.23g / cm ^Threethat's all
A beta-alumina solid electrolyte, characterized in that: 2. A process for preparing a beta-alumina solid electrolyte by mixing an alumina raw material and a Na compound and MgO or a compound thereof as a stabilizer, or Li ₂ O or a compound thereof at a predetermined ratio, to obtain a beta-alumina solid electrolyte. A beta-alumina solid, characterized in that the alumina raw material as a source is alumina produced by subjecting a material to be calcined having a Na ₂ O content of 0.1% by weight or less to main firing in a production process of the alumina raw material. Manufacturing method of electrolyte. 3. The method according to claim 2, wherein said alumina raw material is α-alumina produced by a Bayer method.
3. The method for producing a beta-alumina solid electrolyte according to item 1. 4. The production of a beta alumina solid electrolyte according to claim 2, wherein the alumina raw material is α-alumina, and the specific surface area after main firing is in a range of 1 to 7 m ² / g. Method. 5. An alumina raw material and a Na compound, and MgO or a compound thereof as a stabilizer, or Li ₂ O or a compound thereof are mixed at a predetermined ratio, mixed and pulverized, granulated, formed, and then fired. The method for producing a beta-alumina solid electrolyte according to claim 2, wherein the calcining of the raw material is not performed. 6. The method for producing a beta alumina solid electrolyte according to claim 5, wherein the stabilizer is magnesium-aluminum spinel or Li ₂ O.nAl ₂ O ₃ .