JPH0624149B2 - Beta-alumina solid electrolyte - Google Patents

Description

TECHNICAL FIELD The present invention relates to a beta-alumina solid electrolyte for sodium-sulfur batteries, and in particular, by controlling the crystal orientation ratio of the beta-alumina crystal in the C-axis direction. Relates to a beta-alumina solid electrolyte having excellent mechanical strength and chemical durability and low electric resistance.

[Prior Art] Generally β as a solid electrolyte for sodium-sulfur batteries.
Alumina or β ″ alumina is known, and its application is considered. Especially, β ″ alumina has a high Na ion conductivity (low Na ion conductivity resistivity), so that it can be used in a bag shape or the like. The structure is applied to sodium-sulfur battery.

A sodium-sulfur battery is a beta-alumina solid electrolyte in which molten metal sodium, which is one cathode active material, and molten sulfur, which is an anode active material, are arranged on the other side, and which are both selectively permeable to sodium ions. Isolate, 300 ~
It is a high temperature secondary battery operated at 350 ° C.

The structure of such a sodium-sulfur battery is, for example, the third one.
As shown in the figure, a cylindrical anode container 2 for containing a conductive material 1 for anode such as carbon felt impregnated with molten sulfur S which is an anode active material, an upper end portion of the anode container 2 and, for example, alpha alumina A cathode container 4 that is connected through an insulator ring 3 and stores molten metal sodium Na, and is joined to the inner peripheral portion of the insulator ring 3 and has sodium ions
The base alumina tube 5 has a cylindrical shape with a function of allowing Na ⁺ to selectively pass therethrough. A cathode tube 7 extending downward through the cathode container 4 to near the bottom of the beta-alumina tube 5 is penetratingly supported in the central portion of the upper lid 6 of the cathode container 4.

In the sodium-sulfur battery having the above structure, when discharged, molten metal sodium emits electrons to become sodium ions, which pass through the beta-alumina solid electrolyte and move to the anode side, and sulfur of the anode and the external circuit. It reacts with the electrons that have passed through it to produce sodium polysulfide, generating a voltage of about 2 volts. Contrary to charging, the reaction of forming sodium and sulfur from sodium polysulfide occurs during charging.

In such a sodium-sulfur battery, the beta-alumina electrolyte plays a very important role. Various studies have been conducted for applying this beta alumina to a sodium-sulfur battery. For example, in a sodium-sulfur battery, the efficiency as a battery is determined by the internal resistance. The lower the internal resistance, the higher the efficiency. Generally, it is said that the solid electrolyte contributes about 40% to the internal resistance of the sodium-sulfur battery. There is. Therefore, in the beta-alumina solid electrolyte required for the sodium-sulfur battery,
It is practically essential to reduce the internal resistance.

As a countermeasure against this, conventionally, a search has been made for a composition having high electric conductivity, or an individual crystal has been enlarged by controlling firing conditions.

[Problems to be Solved by the Invention] However, the above-mentioned composition having high electric conductivity is unstable and lacks chemical durability. In addition, there is a problem that the mechanical strength decreases as the size of each crystal grain increases.

[Means for Solving the Problems] Therefore, the present inventor has solved the problems in the above-described conventional beta-alumina solid electrolyte, maintains the characteristics of conventional ceramics, has excellent mechanical strength and chemical durability, and As a result of various studies to develop a solid electrolyte having low resistance, the present invention has been completed.

That is, according to the present invention, there is provided a beta-alumina solid electrolyte for a sodium-sulfur battery, the beta-alumina solid electrolyte having a C-axis crystal orientation ratio controlled in the range of 0.22 to 0.44.

[Operation] The present invention is characterized by controlling the C-axis orientation ratio of the crystals of the beta-alumina solid electrolyte within the range of 0.22 to 0.44.

By controlling the C-axis orientation ratio of the crystals of the beta-alumina solid electrolyte in this manner, excellent properties such as high mechanical strength and chemical durability and low internal resistance of the solid electrolyte can be provided.

Beta-alumina crystal belongs to hexagonal crystal form and
As shown in the schematic diagram of the single crystal of the beta-alumina solid electrolyte in the figure, the in-plane formed by the A axis and the B axis (A
B surface) has a Na ion conductive surface, which is the vertical direction C
It has no conductivity in the axial direction. Further, since the crystal has a cleavability on the Na ion conductive surface (AB surface), the mechanical strength is low with respect to the tension in the C-axis direction, while the water droplets replace Na ions and enter the AB surface. Therefore, it has a characteristic that the chemical durability in the AB plane direction is low.

In addition, as the beta-alumina solid electrolyte constituting the sodium-sulfur battery, a structure of a polycrystalline sintered body obtained by densely sintering a single crystal having these properties is used because of the relationship of the crystal growth direction. ing. When applying a polycrystalline sintered body of beta-alumina crystals having anisotropy as described above to a structure, the mechanical strength, electrical characteristics or chemical durability of the structure are the characteristics of the individual beta-alumina crystals. Is important, as is the crystal orientation of the individual beta-alumina.

The present invention is to obtain a beta-alumina solid electrolyte that maintains the characteristics of conventional ceramics and has a low internal resistance by controlling the directions of these crystals within a certain range.

The orientation of beta-alumina crystals of the polycrystalline structure was measured by the following measuring method.

Formulations of Na ₂ O, MgO, and Al ₂ O ₃ having a constant composition were formed into a cylindrical shape by various molding methods in Examples described later, and sintered under a constant condition to obtain a sintered body of the cylindrical body. . As shown in Fig. 2, a test piece with a width of 1 mm and a length of 40 mm was made in the longitudinal direction of the coaxial cylindrical body from these sintered bodies, and the diameter of the coaxial cylindrical body was changed to exclude the influence of irregularities on the outer surface. The outer surface was polished by several 100 μm so as to be perpendicular to the direction. The relative amount of each crystal plane in the radial direction of the coaxial cylinder was calculated by measuring the diffraction pattern of the outer surface of the test piece prepared by the above method using an X-ray analyzer.

For the measurement, a goniometer type X-ray analyzer was used. CuK
Using characteristic X-ray of α ₁ , acceleration voltage 35kv, cathode current 20mA
The diffraction pattern was recorded on a chart. The measurement results were obtained by actually measuring the peak heights of the crystal planes forming a constant angle (0,33.3,60,90 °) with the C axis of the beta-alumina crystal, and expressing the proportion occupied by each crystal plane as a relative value. Table 1 shows a method of converting the relative amount of each crystal face of the beta-alumina crystal.

In the present invention, the C-axis orientation rate means C in Table 1 above.
It is the relative height of the total peak height of H ₁ that coincides with the C axis when the angle between the axis and the crystal plane becomes 0. That is, a high C-axis orientation ratio means that the resistance of the beta alumina tube is high because the non-conductive surface of the beta alumina crystal faces the Na ion conductive direction of the beta alumina tube.

Beta alumina described in the present invention means β-alumina (Na
₂ O · 11Al ₂ O ₃ ) and β ″ -alumina (Na ₂ O / 5Al ₂ O ₃ ),
It contains β-alumina and β'-alumina.

[Examples] Hereinafter, the present invention will be described in more detail based on Examples, but the present invention is not limited to these Examples.

(Examples 1 to 29 and Comparative Examples 1 to 11) The orientation rate of the beta-alumina crystals of the sintered body is considered to be due to the arrangement of the primary particles when forming the molded body,
Various sintered bodies were prepared by changing the size of the primary particles and the pressing method during molding.

[Β ″ -Alumina Polycrystalline Structure Forming Method A] As raw materials, Al ₂ O ₃ source is α-Al ₂ O ₃ , Na ₂ O source is Na ₂ CO ₃ , and
A predetermined amount of MgO was measured as the MgO source, and wet mixing was carried out in a ball mill at a water content of 50% for 30 minutes to prepare a β ″ -alumina composition formulation.

After the mixed slurry was dried, it was packed in a brick crucible and calcined for 2 hours in an air atmosphere having a maximum temperature of 1100 to 1400 ° C using an electric furnace. Wet grinding of the powder after calcination was carried out in a ball mill, and slurry having various average particle sizes was prepared by changing the grinding time.

Here, in order to change the crystal orientation ratio of the β ″ -alumina solid electrolyte of the sintered body, the maximum calcination temperature of the β ″ -alumina composition preparation and the mixing and grinding time of the calcined powder were changed. Table 2 shows the average particle size in the slurry of the calcined powder obtained by changing the maximum calcining temperature and the mixing and grinding time. The average particle size is shown by the particle size of 50% by weight.

Then, 2% of ethylene glycol was added as a binder and granulated with a spray drier. The obtained granulated powder was loaded into a rubber mold having a predetermined shape and pressure-molded with a rubber press at a pressing force of 2 t / cm ² to obtain a molded body having a predetermined shape.

The compact was fired at a maximum temperature of 1500 ° C. as a firing condition a, a maximum temperature of 1575 ° C. as a firing condition b, and a maximum temperature of 1650 ° C. as a firing condition c to obtain a sintered body having a predetermined shape.

As a raw material - [beta "molding method B alumina polycrystalline structure, the the Al ₂ O ₃ source _{α-Al 2 O 3, Na} 2 O source Na ₂ C
A predetermined amount of MgO was measured as a source of O ₃ and MgO, and wet blending was performed in a ball mill at a water content of 50% for 30 minutes to prepare a β ″ -alumina composition formulation.

After the mixed slurry is dried, it is packed in a high-purity alumina brick crucible and the maximum temperature is 1100 to 140 using an electric furnace.
Calcination was performed for 2 hours in an air atmosphere at 0 ° C. Wet grinding of the powder after calcination was carried out in a ball mill, and slurry having various average particle sizes was prepared by changing the grinding time.

Here, in order to change the β ″ crystal orientation ratio of the sintered body, β ″
-The maximum calcining temperature of the alumina composition formulation and the pressure pouring pressure of the calcined ground sludge were changed. Table 3 shows the average particle size in the slurry of the calcined powder obtained by changing the maximum calcination temperature and the mixing and grinding time. The average particle size is shown by the particle size of 50% by weight.

Then, 2% of ethylene glycol was added to adjust the water content of the slurry to 40% by weight, and the mixture was pressure-cast into a breathable resin mold at a pressure of 10 to 30 kg / cm ² to prepare a molded product having a predetermined shape.

After drying this molded body, a maximum temperature of 150 was set as the firing condition a.
The sintered body was fired at 0 ° C., a maximum temperature of 1575 ° C. as the firing condition b, and a maximum temperature of 1650 ° C. as the firing condition c to obtain a sintered body having a predetermined shape.

Each characteristic of the sintered body obtained by the above-mentioned molding methods A and B was measured. The measurement results are shown in Table 4.

In Table 4, the methods for measuring Na ion conductivity resistivity, mechanical strength, and water adsorption amount were as follows. The C-axis crystal orientation ratio is as described above.

In addition, in the judgment column in Table 4, ○ indicates a combination that can maintain the mechanical strength and the amount of adsorbed water at the conventional level and decrease the Na ion conductivity resistivity, and Δ indicates that the mechanical strength and the adsorbed amount of the water are the same. A combination that deteriorates in comparison but decreases in Na ion conduction resistivity, and a cross represents a combination in which Na ion conduction resistivity does not decrease.

[Measurement Method of Na Ion Conductivity Resistivity] The Na ion conduction resistivity in the radial direction of the bag-shaped tubular body, that is, the low pressure cylindrical body sintered under various firing conditions was measured by the above method. The open end of the sintered body was cut to prepare a bottomed cylindrical test body having a length of 180 mm.

Keep the specimen at a high temperature of about 350 ° C in an inert atmosphere,
Metal sodium was filled to a depth of 100 mm on the inner and outer surfaces, electrodes for energization and voltage terminals were placed in the metal sodium, and a direct current of 10 A was supplied from a constant current generator to supply the inner and outer surfaces of the bottomed cylindrical body. The voltage difference was measured and the Na ion conductive resistivity was determined. Further, the Na ion conduction resistivity can be obtained from the resistance by this four-terminal method and the shape factors such as surface area and thickness. The measurement was performed by raising and lowering the temperature in the range of 300 to 400 ° C.

[Measurement Method of Mechanical Strength] The radial crushing strength of the coaxial cylindrical body obtained by sintering the molded body obtained by the above method under various sintering conditions was measured. 10 to 1 width from the sintered body
A 5 mm annular test piece is cut and processed. After edge treatment of the cut surface, it was sufficiently dried and a compression test was performed. In the measurement, the breaking load P was obtained at a loading speed of 0.5 mm / min, and the stress was converted according to the following formula 1.

Radial crushing strength σ = P (Dd) / (2ld ² ) ... 1, where D is the outer diameter of the test piece, d is the thickness of the test piece, and 1 is the width of the test piece.

The measurement result of the thick ring strength was expressed as a relative value with the average value of the conventional example being 100.

[Amount of Moisture Adsorption] The open end of the sintered body was cut in the same manner as the test body used for the measurement of the Na ion conductive resistivity to prepare a test body of a bottomed cylindrical body having a length of 180 mm.

After measuring the weight of the bottomed cylindrical test body dried in air at 800 ° C. for 2 hours, it was left in a constant temperature and humidity bath at 50 ° C. and a relative temperature of 80% for 200 hours. Then, the weight of the test body was measured again, and the weight increase per unit surface area was measured.

From the above Examples and Comparative Examples, Examples 1-29 in which the C-axis crystal orientation ratio is in the range of 0.22 to 0.44 as a result of considering the mechanical strength, the performance of the amount of adsorbed water and the Na ion conductive resistivity.
Is clearly preferable.

[Effect of the Invention] The beta-alumina solid electrolyte for sodium-sulfur batteries of the present invention has a crystal orientation ratio of 0.2 in the C-axis direction of the polycrystalline structure.
By controlling so as to be in the range of 2 to 0.44, the solid electrolyte has excellent internal properties such as low internal resistance and high mechanical strength and chemical durability, and is industrially useful.

[Brief description of drawings]

FIG. 1 is a schematic diagram of a single crystal of beta-alumina solid electrolyte,
FIG. 2 is a test piece obtained by longitudinally cutting a beta-alumina cylinder, and FIG. 3 is a schematic cross-sectional view of a sodium-sulfur battery. 1 ... Anode, 5 ... Beta-alumina tube, 7 ... Cathode tube.

Claims (1)

[Claims] 1. A beta-alumina solid electrolyte for sodium-sulfur batteries, which has a C-axis crystal orientation ratio controlled in the range of 0.22 to 0.44.