JPH0765857A - Beta alumina electrolyte - Google Patents

Abstract

PURPOSE:To suppress variation of sintered body characteristics due to difference in production method, alumina material, and sintering conditions easily and without causing any problem on handling of material, toxicity, and characteristics, industrially by specifying the composition of beta alumina. CONSTITUTION:The mole ratio of Al2O3/Na2O on aluminum oxide, sodium oxide, and lithium oxide that are the oxides to form beta alumina is set at 6 to 7. Also the weight ratio of it to the whole amount of Li2O as crystal stabilizing agent is set to 0.5 to 0.75wt.%. By specifying the composition of the beta alumina like this, an electrolyte with microstructure of which conductivity is not lowered and grains are not grown is obtained. Therefore the durability of a battery can be increased without reducing the output of the battery. Also, becuase the production conditions or sintering conditions affect less on the characteristics, the characteristics do not vary even if a slight difference is produced in the sintering temperature or sintering time, and thus a product stable in quality in industrial production can be provided.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to beta-alumina used as a solid electrolyte for secondary batteries such as sodium-sulfur batteries and sodium-molten salt batteries or alkali metal thermoelectric conversion batteries which operate using sodium ions as carriers.

[0002]

2. Description of the Related Art Beta-alumina electrolytes are used as electrolytes for various batteries using sodium ions as carriers because they have high sodium ion conductivity. Since this electrolyte occupies a large part of the internal resistance of the battery, a dense sintered body having low resistance and high strength is desirable. Also, at the time of high temperature for sintering, for example, at 1700 ° C., it contains sodium that easily volatilizes. Therefore, it is preferable to sinter at a temperature as low as possible. Beta-alumina includes β-alumina (theoretical composition Na ₂ O · 11Al ₂ O ₃ ) and β ″-
There are two types of crystal form called alumina (theoretical composition Na ₂ O.5.3Al ₂ O ₃ ), and β ″ -alumina has higher conductivity and higher performance as a battery, so β ″ is practically used.
-Alumina or a mixture of β "-alumina and β-alumina is often used. Furthermore, in practice β"-
Alumina single phase is used.

The theoretical composition of β ″ -alumina is Na₂O
5.3 Al₂O₃, The actual composition is Al₂
O₃/ Na₂The molar ratio of O is as wide as 6-9
There is. In addition, β ″ -alumina decomposes at high temperatures, so
Li as a crystal stabilizer₂Add O about 1.0wt%
is doing. The most common composition is wt%, Al₂O₃:
Na₂O: Li₂O = 90.4: 8.85: 0.75
Yes, Al₂O₃/ Na ₂The molar ratio of O is 6.25
To do.

The conventional method for producing β ″ -alumina is disclosed in Japanese Patent Publication No.
As disclosed in Japanese Patent Publication No. 7-15063, a calcined powder of a mixture of β-alumina and β ″ -alumina obtained by mixing alumina and sodium carbonate and then firing, and alumina and lithium carbonate are then fired. This is a method in which the calcined powder showing the crystal phase of the obtained zeta-alumina (theoretical composition: Li ₂ O.Al ₂ O ₃ ) is mixed again, molded and sintered to obtain β ″ -alumina. This method is called zeta process because it uses zeta-alumina for the calcined powder before mixing,
At present, it is known as the most general method for producing β ″ -alumina. Further, in the above publication, a solution of a water-soluble salt of sodium and lithium (eg, nitrate, sulfate, chloride, etc.) and alumina powder. The method is described in which the slurry is mixed, dried and calcined to obtain a β ″ -alumina sintered body. However, no detailed example is given for this method and it is not clear whether a good electrolyte is obtained.

Further, Japanese Patent Publication No. 55-90470 discloses a method for obtaining beta-alumina by hydrolyzing aluminum, sodium and lithium alkoxides which are soluble in a non-aqueous solvent, followed by a drying / calcining operation and firing. Has been done. Further, as a known method, there is a method in which three kinds of powder raw materials of aluminum, sodium and lithium are simultaneously mixed in a dry or wet manner and then calcined to obtain beta alumina.

Further, the present inventors previously added alumina starting raw material powder, sodium starting raw material powder, and organic lithium soluble in the solvent to a non-aqueous solvent, and directly dried the resulting mixture, followed by sintering. (Japanese Patent Application No. 4-200116) or a method in which the mixture is once calcined and then sintered (Japanese Patent Application No. 4-20).
No. 7610) is proposed. Furthermore, the present inventors mixed and pulverized alumina starting raw material powder and sodium starting raw material powder to prepare a calcined powder, added organic lithium soluble in a solvent to the calcined powder, and dried the resulting mixture. A method of sintering later (Japanese Patent Application No. 4-227814) is also proposed.

[0007]

Most of β ″ -alumina
The general composition is, as described above, in% by weight, Al₂O₃:
Na₂O: Li₂O = 90.4: 8.85: 0.75
Yes, Al₂O₃/ Na ₂The molar ratio of O is 6.25
To do. This composition is determined by both physical properties of conductivity and microstructure.
It is said to be That is, Na₂OA
l₂O₃As shown in the system phase diagram, firing of β ″ -alumina
Since the binding mechanism is liquid phase sintering, Na which is advantageous for conductivity₂O
Grain growth occurs when the amount is increased. Similarly Li₂O amount also increases
Then, the liquidus temperature is lowered, so that grain growth occurs.
Become. When grain growth occurs, conductivity improves but mechanical strength
Will decrease and the durability of the battery will decrease. That
Therefore, the above composition is a contradictory characteristic of both conductivity and strength.
It seems that it was decided from. Also, β ″ -alumina
Regarding, there is no example prepared with a composition other than the above. That
Therefore, the manufacturing method shown below is especially relevant to the control of the amount of lithium.
However, various problems occur.

First, in the most general conventional method disclosed in Japanese Examined Patent Publication No. 57-15063, since the amount of lithium oxide in the product is very small at 0.75% by weight, lithium called zeta-alumina is contained in the alumina. It is intended to finally disperse the calcined powder in β ″ -alumina by using the calcined powder dispersed in 1. In the zeta process of the conventional method, two kinds of calcined powder are prepared and then pulverized and mixed to be calcined. To make a conclusion
The mixing process is performed three times and the calcining process is performed twice, which complicates the process and increases the cost of the product. This method has a problem that uneven diffusion of lithium is likely to occur due to the non-uniform distribution of lithium caused by the diffusion of lithium in the solid phase reaction.

Further, the method using the water-soluble salt described in the above publication is cheaper in cost than the zeta process, but since the amount of the water-soluble salt of sodium is particularly large during calcination. However, not only harmful gases such as nitrogen oxides (NOx) or sulfur oxides (SOx) are generated, but also non-volatile salts (chlorides, etc.) may be generated even at the temperature of calcination or sintering. Further, since the specific examples are not described in the means, the inventors of the present invention conducted an additional test in the previously proposed patent application (Japanese Patent Application No. 4-2.
No. 00116). As a result, it is pointed out that the means using a water-soluble salt is extremely inferior in strength and durability as an electrolyte to be supplied to a battery except for the abnormal grain growth (100 μm or more) and the conductive aspect. did.

In the method of using a soluble alkoxide as the three-component raw material disclosed in Japanese Patent Publication No. 55-90470, the cost of the raw material is considerably high. Again 3
In order to dissolve the components, the content of the components in the solution is about 10% by weight, and judging from the fact that the slurry which uses the ordinary powder is 60 to 90% by weight, the yield of the raw materials including the solvent is poor. Further, this method has a problem that a long aging time is required because the hydrolysis rate is considerably slow.

Further, in the known method in which powder raw materials are used as the three components, as described in the Zeta process, the dispersion due to the solid phase reaction of lithium is poor, and a crystalline phase other than beta-alumina remains, or The problem of abnormal grain growth arises.

Further, Japanese Patent Application No. 4-20011 by the present inventors.
No. 6 presents a method for producing a beta-alumina electrolyte showing a conductivity and strength equal to or higher than those of conventional methods by a simple method. However, in the invention report, the characteristics of the conventional method cannot be exceeded unless the primary particle diameter of the alumina raw material and the sintering conditions for it are optimized, and there is a problem in the degree of freedom in selecting the alumina raw material and the reproducibility of the product characteristics. There is.

Further, the present inventors have previously prepared alumina starting raw material powder, sodium starting raw material powder in a non-aqueous solvent, and lithium starting as an organolithium compound in which a part or the whole amount is dissolved in the non-aqueous solvent. Beta-alumina showing the same or better characteristics as the conventional method by reducing the effect of alumina raw material on the characteristics by a simple method of preparing a slurry by mixing, calcining, crushing, molding and then sintering the slurry. Proposed a method for producing the above electrolyte with good reproducibility (Japanese Patent Application No. 4-207610). However, the calcined powder β ″
-The ratio of alumina (hereinafter, abbreviated as "β" ratio) is 40-
70%, β-alumina and β ″ -obtained by mixing and calcining a conventional zeta-method alumina and sodium carbonate.
The β ″ ratio of the calcined powder of the mixture of alumina is about 90%, which is a low value. According to the above-mentioned invention report of the present inventors, the calcined powder is molded and then sintered. It was shown that the β ″ conversion rate was almost 100%, but it is clear that the higher the β ″ conversion rate is, the better even in the state of calcined powder.

Similarly, the inventors of the present invention first mixed and calcined the aluminum starting material and the sodium starting material to prepare a calcined powder of beta-alumina, and then prepared the calcined powder and the starting material of lithium soluble in the solvent. A method of molding and sintering after mixing has been proposed (Japanese Patent Application No. 4-227814). However, it is undeniable that the degree of freedom in selecting the lithium raw material is small and a cost disadvantage occurs.

Finally, grain growth is invariably partly observed in all processes other than the present invention. In addition, in the manufacturing method of the present inventors, Al ₂ O ₃ : Na ₂ O: Li _{2 in} % by weight is also used.
In the composition of O = 90.4: 8.85: 0.75, partial grain growth could not be prevented for all lots, and partial grain growth was still observed in some lots. From this, it is judged that it is very difficult to completely prevent grain growth with the above composition, considering the temperature distribution of the firing furnace. However, according to the findings of the present inventors, the microstructure of the sintered body has its composition (the amount of Na ₂ O and the amount of Li ₂ O).
It is known that the change can be caused by the above, and it is thought that the tissue control can be performed by optimizing it.

In view of the various problems involved in the production of the above-mentioned conventional β ″ -alumina, the present invention defines the composition of β ″ -alumina clearly, and handles the raw materials easily and industrially. However, there is no problem in toxicity or characteristics, and it is intended to suppress changes in characteristics of the sintered body as much as possible due to differences in not only the manufacturing method but also the alumina raw material and the sintering conditions.

[0017]

The present invention relates to aluminum oxide, sodium oxide and lithium oxide, which are constituent oxides of beta-alumina, in a molar ratio of Al ₂ O ₃ / Na ₂ O of 6 to 7, preferably 6. 4 to 6.6 and Li ₂ O
The present invention relates to a beta-alumina electrolyte characterized in that the weight ratio of the total amount is 0.5 to 0.75 wt%, preferably 0.60 to 0.70 wt%.

That is, according to the present invention, by defining the composition of β ″ -alumina, there is no problem in handling, toxicity or characteristics of the raw material easily and industrially, and not only the production method but also the alumina raw material and the sintering conditions. It is intended to obtain the beta-alumina electrolyte having stable characteristics by suppressing the change in the characteristics of the sintered body due to the difference between the two.

[0019]

By setting the composition of the beta-alumina of the present invention in a region different from that of the conventional composition, the conductivity of the beta-alumina does not decrease as compared with the conventional composition and the electrolyte has a microstructure without grain growth. Therefore, the durability can be improved without lowering the output performance of the battery. Further, since the influence of the manufacturing conditions or the sintering conditions on the characteristics is small, the characteristics do not change even if the sintering temperature or the sintering time occurs to some extent.
It is possible to supply industrially stable products.

[0020]

EXAMPLES Next, the present invention will be described in more detail with reference to specific examples. When a beta-alumina electrolyte is industrially used as a battery, a tube-shaped sintered body with one end sealed is usually used. In order to industrially mass-produce the above-mentioned tube-shaped sintered body, it is obtained by forming a molded body using granulated powder and sintering it. Therefore, in this embodiment, a method of obtaining a beta-alumina electrolyte by calcining a mixed raw material slurry, using granulated powder by a spray dry method using a wet-milled slurry, and sintering the granulated powder will be described.

Example 1 Preparation of β ″ -alumina was carried out according to the procedure disclosed in Japanese Patent Application No. 4-227814 previously proposed by the present inventors, that is, after putting zirconia balls in a pot. Then, a predetermined amount of aluminum oxide, sodium carbonate, an n-butanol solvent and a dispersant (polyethyleneimine type) were added, and the mixture was mixed for 24 hours. After concentrating with an evaporator, it was dried overnight in a drier at 120 ° C. The dried product was crushed to 50
After passing through a 0 μm sieve, it was subjected to calcination. Calcination is 5 ℃
After heating at 1 / min, hold at 1250 ° C for 2 hours and
The temperature was lowered at min. The calcination powder thus obtained had a β ″ conversion rate (a ratio of the β ″ phase to the entire phase) of about 90%.

The obtained calcined powder was mixed with n of lithium butoxide.
-Butanol solution was added, and the slurry concentration was adjusted with a dispersant (polyethyleneimine type) and n-butanol solvent.
Mixed milling for hours was performed. At that time, Li ₂ O = 0.7
While being fixed at 5 wt%, the remaining 99.25 wt% was converted to a molar ratio of Al ₂ O ₃ / Na ₂ O of 5.2 and 6.
25 (conventional value), 6.5 and 7.0 were prepared. The viscosity of the obtained mixed slurry was adjusted to about 100 cp, and then the mixture was subjected to spray drying. The operating conditions were room temperature and the number of rotations of the disk was 14,000. The particle size of the obtained granulated powder was spherical with an average particle size of 80 to 100 μm.

These granulated powders were molded at a uniaxial pressure of 100 kg / cm ² using a circular mold of 20 mmφ, further put into a rubber, and CIP (cold isostatic pressurization) was performed.
A molded body was obtained by holding at a pressure of 5 t / cm ² for 5 minutes.
The obtained molded body is heated at a temperature rising rate of 5 ° C./min to
After holding at 600 ° C. for 10 minutes, annealing treatment was performed at 1450 ° C. for 5 hours to produce a sintered body. The physical properties of crystal phase, density, average particle size and conductivity are measured and sintered by X-ray diffraction, Archimedes method (solvent ethanol), image processing (after hot phosphoric acid etching of polished surface) and AC two-terminal method, respectively. The body was evaluated.

The density and β ″ conversion of the obtained sintered body are shown in FIG.
Shown in. Judging from the value of β "conversion rate, Al ₂ O ₃ / Na
It can be seen that the optimum value of the molar ratio of ₂ O is in the vicinity of 6.5.

Next, the Al ₂ O ₃ / Na ₂ O ratio is set to 6.2.
Sintered bodies were produced by changing to 6.7 (in steps of 0.1) and by the same method as described above. Relative density and β ″ of the obtained sintered body
The conversion rate is shown in FIG. Further, FIG. 3 shows the average particle diameter obtained by image processing and the electric conductivity at 300 ° C. From FIG. 2, the relative densities are all high at 95% or more in this Al ₂ O ₃ / Na ₂ O molar ratio range, but the β ″ conversion ratio is Al ₂ O ₃ / N.
The a ₂ O ratio was 6.4 to 6.6, which was a particularly high value. Moreover, from FIG. 3, it was shown that the conductivity was higher and the average particle size was larger as the Al ₂ O ₃ / Na ₂ O molar ratio was smaller. In the relationship between the average grain size and the microstructure examined in advance, no grain growth was observed when the average grain size was 1.0 μm or less, but partial grain growth was observed from about 1.2 μm.
It has been known that when the thickness is 1.5 μm or more, a vigorous grain growth is exhibited. It is clear that in the sample showing grain growth, the strength is lowered even if the conductivity is increased and the durability is deteriorated. for that reason,
Judging from the average particle size, it is found that the Al ₂ O ₃ / Na ₂ O molar ratio is preferably 6.3 or more. That is, judging from the β ″ conversion rate and the average particle size in FIGS. 2 and 3, respectively, Al
It can be seen that the optimum value of the ₂ O ₃ / Na ₂ O molar ratio is 6.4 to 6.6.

Example 2 With the Al ₂ O ₃ / Na ₂ O ratio fixed at 6.5, the amount of Li ₂ O between 0.25 and 1.
Example 1 was changed at 0 wt% (in increments of 0.25 wt%).
A sintered body was produced by the same operation as above. Density and β ″ at that time
The conversion rate is shown in FIG. The average particle diameter and the value of conductivity at 300 ° C. are shown in FIG. From this, it is understood that when the amount of Li ₂ O is small, the density is low and the electrical conductivity is low, and when the amount of Li ₂ O is increased, the electrical conductivity is improved but the average particle size is increased. From this, the optimum value of the amount of Li ₂ O is 0.5 to
It can be seen that it is between 0.75 wt%.

Next, Li₂O amount of 0.50 to 0.75 wt
% (In increments of 0.05 wt%), same as in Example 1
A sintered body was produced by the operation of. The density and β ″ conversion rate at that time
As shown in FIG. In addition, the average particle size and the conductivity at 300 ° C
Values are shown in FIG. From now on, Li₂Β "when the amount of O is small
It does not become a single phase, has a low conductivity, and Li₂O amount
If the amount is large, the conductivity will be high, but the average particle size may increase.
I understand. That is, Li ₂The optimum amount of O is 0.60 to 0.
It can be seen that it is 70 wt%.

(Example 3) A sintered body having a composition in which the molar ratio of Al ₂ O ₃ / Na ₂ O and the amount of Li ₂ O are 6.5 and 0.65 wt%, respectively, within the optimum composition range of the present invention. Was prepared in the same manner as in Example 1. Furthermore, Japanese Examined Japanese Patent Publication 57-1
According to the most general conventional method disclosed in Japanese Patent Publication No. 5063, the same composition as above and the molar ratio of Al ₂ O ₃ / Na ₂ O and L are obtained.
Sintered bodies having conventional compositions of i ₂ O amounts of 6.25 and 0.75 wt% were produced. 8 and 9 show the density and β ″ conversion rate when the sintering temperature was changed (steps of 20 ° C.) from 1560 to 1640 ° C. in the production of these three kinds of sintered bodies, respectively. The diameter and the electric conductivity at 300 ° C. are shown in FIGS.
8 to 11, it can be seen that the β ″ -alumina having the composition of the present invention has a small change in characteristics due to the sintering temperature in the production method of the present invention. Further, even in the conventional production method (zeta process), It can be seen that the composition of the present invention has smaller characteristic fluctuations on the side of higher sintering temperature than the conventional composition.In particular, in the case of the conventional production method and the conventional composition, the increase of the average particle diameter with the sintering temperature, that is, It is clear that the grain growth is remarkable and the durability of the battery is problematic.
By combining the composition of alumina and the manufacturing method disclosed in the method previously proposed by the present inventors (Japanese Patent Application No. 4-227814), it was possible to suppress the change in characteristics more than the conventional manufacturing method.

[0029]

As described above, according to the composition of beta-alumina of the present invention, as compared with the one prepared by the conventional composition, the beta-alumina having a fine structure without deteriorating the conductivity is conventionally used. It can be prepared by a simpler method than the method and the durability as a battery electrolyte is improved.
Further, even if the sintering conditions are changed, the change in the electrolyte characteristic is small, and the reliability as the battery electrolyte by the industrial manufacturing method is improved.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a roughly changed Al ₂ O ₃ / Na ₂ O molar ratio of a sintered body and a density and a β ″ conversion rate in Example 1 of the present invention.

FIG. 2 is a graph showing the relationship between the slightly changed Al ₂ O ₃ / Na ₂ O molar ratio of the sintered body and the density and β ″ conversion rate in Example 1 of the present invention.

FIG. 3 is a graph showing the relationship between the slightly changed Al ₂ O ₃ / Na ₂ O molar ratio of the sintered body in Example 1 of the present invention, and the average particle diameter and conductivity.

FIG. 4 is a chart showing the relationship between the roughly changed amount of Li ₂ O, the density, and the β ″ conversion rate of the sintered body in Example 2 of the present invention.

FIG. 5 is a chart showing the relationship between the coarsely changed amount of Li ₂ O, the average particle size, and the conductivity of the sintered body in Example 2 of the present invention.

FIG. 6 is a chart showing the relationship between the minutely changed amount of Li ₂ O, the density, and the β ″ conversion rate of the sintered body in Example 2 of the present invention.

FIG. 7 is a chart showing the relationship among the minutely changed amount of Li ₂ O, the average particle diameter, and the conductivity of the sintered body in Example 2 of the present invention.

FIG. 8 is a chart showing a relationship between a sintering temperature and a density of a sintered body in Example 3 of the present invention.

FIG. 9 is a chart showing a relationship between a sintering temperature and a β ″ conversion rate of a sintered body in Example 3 of the present invention.

FIG. 10 is a chart showing a relationship between a sintering temperature and an average particle diameter of a sintered body in Example 3 of the present invention.

FIG. 11 is a chart showing the relationship between the sintering temperature and the electrical conductivity of the sintered body in Example 3 of the present invention.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yasuhiko Mizutani 1-8-1 Koura, Kanazawa-ku, Yokohama, Kanagawa Prefecture Mitsubishi Heavy Industries, Ltd.

Claims (1)

[Claims] 1. Regarding aluminum oxide, sodium oxide and lithium oxide which are constituent oxides of beta-alumina, the molar ratio of Al ₂ O ₃ / Na ₂ O is 6 to 7 and Li _{2 is 2.}
A beta-alumina electrolyte characterized in that the weight ratio of the amount of O to the whole is 0.5 to 0.75 wt%.