JPH0774093B2 - Beta-alumina solid electrolyte and method for producing the same - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a beta-alumina solid electrolyte that can be used as a solid electrolyte membrane of a sodium-sulfur battery and a method for producing the same, and more specifically, it relates to a sodium oxide, an oxide. The present invention relates to a beta-alumina sintered body substantially composed of aluminum and zinc oxide and a method for producing the same.

(Prior Art) Since a beta-alumina sintered body has high sodium ion conductivity, it serves as a solid electrolyte membrane for separating molten sulfur as an anode material and molten sodium as a cathode material in a sodium-sulfur battery. Used. And in this sodium-sulfur battery, the solid electrolyte shows most of the internal resistance. Therefore, in order to suppress the output reduction during discharging of the battery and the power loss during charging, it is desirable to lower the specific resistance in the operating temperature range of the beta-alumina sintered body which is the solid electrolyte, for example, 300 ° C. Since the life of the sodium-sulfur battery depends on the life of the solid electrolyte, it is desirable that the beta-alumina sintered body has high strength.

(Problems to be Solved by the Invention) Conventionally, as a method for reducing the specific resistance of a beta-alumina sintered body, a method of containing an oxide of a metal ion having a valence of 2 or less, such as lithium oxide, magnesium oxide, or zinc oxide. It has been known. However, only a beta-alumina sintered body containing lithium oxide, magnesium oxide or both has been put into practical use as a solid electrolyte membrane of a sodium-sulfur battery, and a beta-alumina sintered body containing zinc oxide has been put into practical use. Has not been put to practical use. Regarding the beta-alumina sintered body containing zinc oxide, a method of hot press firing is known as a method of lowering the specific resistance (Osaka Institute of Industrial Technology, Quarterly Report, Vol. 26, No. 3, pages 8 to 12 (1975)). However, the hot pressing method is unsuitable for industrial production because the product is limited to a simple shape and it is expensive, and the content of zinc oxide is increased to 6% by weight or more to 1700. Method of firing at a temperature above ℃ (Japanese Patent Publication No.
No.) is known, but it had the drawback of low strength.

The present invention has been made in view of the above circumstances, has no difficulty in handling raw materials for industrial production, has a low specific resistance and a uniform fine microstructure, and is a solid electrolyte membrane for sodium-sulfur batteries. It is an object of the present invention to provide a beta-alumina sintered body which is practically usable as a product and has a long life, and a method for producing the same.

(Means for Solving Problems) The first invention of the present invention relates to a beta-alumina solid electrolyte containing zinc oxide, which is 8 to 10% by weight of sodium oxide and 2 to 5.5% by weight of zinc oxide. , That the balance is a sintered body consisting essentially of aluminum oxide, the size of the crystal particles constituting the sintered body is 150 μm or less in maximum length, and the specific resistance at 300 ° C. is 10 Ω · cm or less. Characterize.

A second invention of the present invention relates to a method for producing a beta-alumina solid electrolyte containing zinc oxide,
Sodium oxide 8 to 10% by weight, zinc oxide 2 to 5.5% by weight, raw material powders are mixed so that the balance substantially consists of aluminum oxide, this mixture is molded, and then 1650 ° C or less, preferably 1500 ° C or more 1650 It is characterized by firing at a temperature of ℃ or less.

(Operation) The beta-alumina sintered body of the present invention is made of sodium oxide,
Raw material powders of zinc oxide and aluminum oxide are mixed at a predetermined ratio, this mixture is molded into a desired shape, and then this molded body is 1650 ° C. or lower, preferably 1500 ° C. or higher and 1650 ° C.
It is obtained by firing at the following temperature. Since zinc oxide added to reduce the specific resistance of the sintered body has a lower hygroscopicity than lithium oxide or magnesium oxide, there is no difficulty in handling the raw material powder, and no special attention is required in production. Note that sodium oxide, zinc oxide, and aluminum oxide may be added in other forms, such as sulfate, as long as they can be converted into oxides by firing.

The beta-alumina sintered body obtained as described above was subjected to a sodium-sodium cell test in order to evaluate the practicality as a solid electrolyte membrane of a sodium-sulfur battery. The sodium-sodium cell test is a test generally performed as a test method for evaluating the life of a solid electrolyte membrane of a sodium-sulfur battery. As a solid electrolyte membrane for sodium-sulfur batteries, the energization amount in the sodium-sodium cell test is 2000 Ah / c.
If it is m ² or more, it can be put to practical use. Test result, 300
Specific resistance at ℃ is 10Ω ・ cm or less, 4-point bending strength is 150M
The beta-alumina sintered body of Pa or more was found to withstand a current of 2000 Ah / cm ^{2 in} a sodium-sodium cell test. Moreover, the specific resistance at 300 ° C is 10 Ω · cm or less, 4
All the beta-alumina sintered bodies having a point bending strength of 150 MPa or more had a bulk density of 3.25 g / cm ³ or more. Although the effect of the bulk density on the solid electrolyte properties is not clear, the higher the bulk density is, the less the penetration of molten sulfur as the anode material and molten sodium as the cathode material into the solid electrolyte membrane, and the longer the solid electrolyte membrane's life. It is believed that In the present invention, when the bulk density was 3.25 g / cm ³ or more, the energization amount in the sodium-sodium cell test was 2000 Ah / cm ² or more.

Further, this sintered body was mirror-polished and then etched with hot phosphoric acid, and the microstructure was observed with an optical microscope. As a result, the above-mentioned sintered body has a structure composed of uniform fine particles of several μm as shown in FIG. 1 (a), or needle-like or plate-like coarse particles and fine particles as shown in FIG. 1 (b). Had an organization consisting of. The size of the coarse particles had a maximum length of 150 μm or less. The term “coarse particles” as used herein means particles having a size of several tens to several hundreds of μm, in which some of the crystal grains constituting the sintered body have undergone abnormal grain growth and coexist with fine particles of several μm. The maximum length means the maximum value in the length direction of fine particles or acicular or plate-like coarse particles. In addition, in the sintered body having a structure composed of uniform fine particles shown in FIG. 1 (a) and the sintered body having a structure composed of coarse particles and fine particles shown in FIG. 1 (b), the former In the sodium-sodium cell test, there was a tendency for the one to endure longer-term energization.

As described above, a sintered body having a structure composed of uniform fine particles of several μm or a sintered body having a structure composed of coarse particles of 150 μm or less and fine particles of several μm is used for a long time in a sodium-sodium cell test. It is not clear why the former sintered body composed of uniform fine particles has a tendency to withstand a longer-time energization, but it can be considered as follows from the nature of the crystal of beta-alumina. That is, beta-alumina crystals have the property of cleaving along the surface through which sodium ions conduct,
When coarse particles exceeding 0 μm exist in the sintered body, it is considered that the destruction of the sintered body starts from these coarse particles. Further, since the resistance of the coarse particles is smaller than that of the surroundings, sodium ions selectively conduct the coarse particles,
That is, as a result of the electric current being concentrated on the coarse particles, it is considered that the sintered body in which the coarse particles are present cannot withstand long-term energization. A beta-alumina sintered body having a specific resistance at 300 ° C. of 10 Ω · cm or less and a crystal grain size of the sintered body having a maximum length of 150 μm or less is composed of
It is obtained by firing at a temperature of 1650 ° C. or lower, preferably 1500 ° C. or higher and 1650 ° C. or lower in a chemical composition range in which 10% by weight, zinc oxide is 2 to 5.5% by weight, and the balance is aluminum oxide.

The reason for defining the range of chemical composition as above is that if sodium oxide is less than 8% by weight, it will not sinter sufficiently, and if it is more than 10% by weight, the specific resistance at 300 ° C will be greater than 10 Ω · cm. is there. Further, when the content of zinc oxide is less than 2% by weight, the specific resistance does not become 10 Ω · cm or less, and when it is more than 5.5% by weight, the crystal grain size of the sintered body is 150 μm.
The bending strength does not exceed 150 MPa.

The reason for limiting the firing temperature to 1650 ° C. or less, preferably 1500 ° C. or more and 1650 ° C. or less is that the firing temperature in the chemical composition range of the present invention is lower than 1500 ° C. and does not sinter sufficiently, and 1650 ° C.
If it is higher, the crystal grain size of the sintered body is 150μ at the maximum length.
This is because a beta-alumina sintered body having a four-point bending strength of 150 MPa or more cannot be obtained.

(Example) Next, the present invention will be described in more detail with reference to actual examples.

Sodium carbonate, zinc oxide and aluminum oxide were blended in the chemical composition ratios shown in Table 1, polyvinyl butyral was added as a binder, and the mixture was pulverized and mixed in a ball mill using acetone as a medium, and granulated to give an outer diameter of 15 mm, Thickness 1m
A blank tube having a length of m and a length of 200 mm and a square plate having a length of 50 mm, a width of 25 mm and a thickness of 7 mm were formed. Then, the bag tube and the square plate were embedded in an alumina container with powder having the same composition as that of the bag, the container was capped, and the container was held at a predetermined temperature for 1 hour for firing. The raw material powder of aluminum oxide used had an average particle size of 0.6 μm.

The bulk density of the bag tube and the square plate of the beta-alumina sintered body thus obtained was measured by the buoyancy method using butanol. Also, cut out a rod-shaped sample from the square plate and
The specific resistance at 0 ° C, the size of the crystal grains and the 4-point bending strength were measured. The specific resistance was measured by a complex impedance method with four terminals eutectic salt of NaNO ₃ -NaNO ₂ as electrodes.
The size of the crystal particles was observed with an optical microscope after mirror polishing the sample and then etching with hot phosphoric acid. 4-point bending strength is JI
It was measured by SR-1601-1981. In addition, a sodium-sodium cell test was performed on the bag sample.
The sodium metal used in the electrode is of nuclear reactor grade, and the temperature is
The temperature was 350 ° C. and the current density was 1 A / cm ² . Table 1 shows the above measurement results. Table 1 also shows results in which the chemical composition and the firing temperature were out of the range of the present invention as comparative examples. In the table, ○ indicates that the amount of electricity passed in the sodium-sodium cell test was 2000 Ah / cm ² or more, and × indicates that the amount of electricity passed was 20 Ah / cm ^2.
This indicates that the sintered body was broken or the specific resistance of the sintered body was greatly increased before reaching 00 Ah / cm ² .

As is clear from Table 1, the examples of the present invention have a specific resistance at 300 ° C. of 10 Ω · cm or less, a crystal grain size of 150 μm or less at the maximum length, and a 4-point bending strength of 150 MPa.
As described above, it can be seen that the sintered body of the example has the characteristic of having a bulk density of 3.25 g / cm ³ and has a long energization life in the sodium-sodium cell test. As shown in Comparative Examples, when the chemical composition or firing temperature deviates from the range of the present invention, sintering becomes insufficient, or characteristics such as high specific resistance or low bending strength are exhibited, and sodium-sodium cell test is performed. Has a shorter life than that of the example.

As described above, according to the present invention, the specific resistance is low,
A long-life beta-alumina solid electrolyte having high bending strength can be obtained. Moreover, there is no difficulty in handling the raw materials in the production of the solid electrolyte, and the production suitable for industrial production can be performed. Further, the solid electrolyte of the present invention is highly useful because it has a small loss during charging and discharging and a long life when used in a sodium-sulfur battery. Further, the beta-alumina solid electrolyte of the present invention can be applied to a molten salt electrolysis device in the electrolysis industry.

[Brief description of drawings]

FIGS. 1 (a) and 1 (b) are substitute photographs showing the particle structure of the beta-alumina solid electrolyte of the present invention observed with an optical microscope.

Continuation of the front page (72) Inventor, Jouichi Umemoto 62, Kaminokiri, Hiromi-cho, Toyota City, Aichi Prefecture

Claims (3)

[Claims] 1. A sodium oxide of 8 to 10% by weight and a zinc oxide of 2.
~ 5.5% by weight, the balance being aluminum oxide substantially, the size of the crystal grains constituting the sintered body is 150 μm or less at the maximum length, and the specific resistance of the sintered body at 300 ℃ Is less than 10 Ω · cm, a beta-alumina solid electrolyte. 2. The beta-alumina solid electrolyte according to claim 1, wherein the sintered body has a four-point bending strength of 150 MPa or more and a bulk density of 3.25 g / cm ³ or more. 3. 8-10% by weight sodium oxide, 2 zinc oxide
~ 5.5 wt%, the raw material powder is mixed so that the balance is substantially aluminum oxide, the mixture is molded, and then baked at a temperature of 1650 ° C. or less, a method for producing a beta-alumina solid electrolyte, .