JPH0492855A - Production of beta-alumina electrolyte raw material - Google Patents

Abstract

PURPOSE:To produce a beta-alumina electrolyte raw material composed of nearly single phase with easy low-temperature calcination process by adding acetic acid to a mixed solution containing alkoxides of Al and Na at a specific ratio and hydrolyzing with a methanol-diluted water to obtain a gel. CONSTITUTION:A mixed solution of an aluminum alkoxide and a sodium alkoxide is prepared at an Al:Na atomic ratio of (6-10):1. The mixed solution is mixed with acetic acid and then hydrolyzed in one stage by adding methanol- diluted water in an amount not less than the amount necessary for hydrolysis, preferably about <=4 times the necessary amount. The gel produced by this process is aged, dried at about 70 deg.C and calcined at a low temperature (about <=1200 deg.C, preferably about 1000-1100 deg.C). A single-phase beta-alumina electrolyte raw material composed of fine particles having diameter of about <=0.5mum can be produced by this process.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to beta-alumina used as a solid electrolyte diaphragm in sodium-sulfur batteries, sodium-molten salt batteries, alkali metal thermoelectric conversion batteries, etc. that operate using sodium ions as carriers. Concerning a method for producing electrolyte raw materials.

[Conventional technology]

Beta-alumina electrolytes have high sodium ion conductivity and are therefore used as electrolytes in various batteries that use n/a) IJium ions as carriers. Since this electrolyte accounts for most of the internal resistance of the battery, a dense sintered body with low resistance and high strength is desirable, and because it contains sodium, which easily volatizes at high temperatures during sintering, such as 1700°C. It is preferable to sinter at as low a temperature as possible. Beta alumina also has β-alumina (theoretical composition:
There are two types of crystal forms: Na2O・11^1203) and β'-alumina (theoretical composition: Na2O・5.3A]203), and although the β type has a higher specific resistance, it is said to be more durable. ing. Beta alumina in the present invention refers to β-alumina.

There are two conventional methods for producing β-alumina raw materials. One method is to mix powders of alumina oxide and sodium carbonate and calcinate them at about 1,200 to 1,300°C to obtain a powder that exhibits a mixed crystalline phase of β-alumina and β'-alumina. One method is to perform two-stage hydrolysis of an alkoxide solution of aluminum and sodium, and then calcining the resulting gel at 1,200 to 1,550°C to form β-alumina, as shown in Japanese Patent Application Laid-open No. 55-90470. There are two methods for obtaining a powder exhibiting a mixed phase of β'-alumina. By sintering these powders at about 1700°C, a sintered body of β-alumina phase (sometimes containing some β'-alumina) can be obtained.

[Problem to be solved by the invention]

The present invention aims to provide a method for producing a β-alumina electrolyte raw material using the alkoxide solution of luminium and sodium as a starting material using the conventional method described above. (This is a method)
After partially hydrolyzing IJ alumina alkoxide, it is mixed with aluminum alkoxide and then further hydrolyzed, a complicated two-step hydrolysis method. - Shows a mixed phase of alumina and does not show a single crystalline phase.

In view of the above-mentioned state of the art, the present invention aims to provide a manufacturing method that is simple, has no problems in industrially handling raw materials, and can obtain a single-phase zo-β-alumina raw material powder in low-temperature calcination. .

In particular, low-temperature sintering is very important in terms of preventing evaporation of Na, not changing the composition, or reducing the power of the sintering furnace. For low-temperature sintering, it is necessary to reduce the particle size of the calcined powder particles to a certain extent, and the above-mentioned Japanese Patent Laid-Open No. 5
The calcined powder obtained by the two-stage hydrolysis method of alkoxides of Publication No. 5-90470 satisfies this condition with a diameter of about 0.18 μm, but as mentioned above, the method is a complicated two-stage hydrolysis method. It also has the disadvantage of exhibiting a mixed phase of β-alumina and β'-alumina.

Conventionally, in the preparation of fine particles by hydrolysis of alkoxides, the so-called one-step hydrolysis method in which water is directly added to the alkoxide mixed solution without using the two-step hydrolysis method described above has been used to prepare aluminum hydroxide (boehmite). ) was said to precipitate, and a calcined powder with a high alumina content could be obtained even after firing.

In consideration of the above facts, the present invention aims to select the conditions for one-stage hydrolysis and to obtain a high-quality raw material powder for β-alumina electrolyte by calcining the obtained gel.

[Means to solve the problem]

In the present invention, a mixed solution of aluminum alkoxide and sodium alkoxide prepared at a ^1:Na atomic ratio of 6 to 10:1 is diluted with methanol under conditions of addition of acetic acid, and water is added in an amount greater than the amount necessary for hydrolysis. This is a method for producing a β-alumina electrolyte raw material, which is characterized in that it is hydrolyzed in one step under the addition conditions of , and the resulting gel is dried and then calcined at a low temperature.

The reason why the atomic ratio of aluminum alkoxide and sodium alkoxide is specified to be 6 to 10:1 in the present invention is that β-alumina cannot be obtained if it deviates from this range.

In order to carry out hydrolysis in one step, the solution concentration of aluminum alkoxide is preferably 0.62 mol/A or less. This is because if the concentration is higher than this, the agglomeration will be severe and no fine particles will be produced, and if the agglomeration occurs, it will be difficult to handle the powder afterwards. Also, the amount of water for hydrolysis is required to be larger than the amount of water for hydrolysis, but unless it is less than four times the amount required for hydrolysis, flocculation will start, so you should not add more water than that. do not have. Also, water for hydrolysis should be diluted with methanol. Otherwise, agglomeration will become severe.

Acetic acid is prepared by adding CH, CD to a low concentration solution of aluminum alkoxide, diluting twice the amount of water with methanol and diluting it 10 times.
In gel calcined products with different DH/^1 ratios, CH, CD
When DH/AI=0.1, there is the most β phase and the least λ phase.

[Effect]

The β-alumina electrolyte raw material of the present invention can be obtained by hydrolyzing a mixed solution of aluminum and sodium alkoxide in one step, and then calcining the dried gel at a low temperature of 1200°C or lower, preferably at 1000°C or higher and 1100°C or lower. can get. At low temperatures below 1200℃, the single phase ratio (^
When the l/Na ratio is up to 8, there is a single phase, and when it exceeds 8, other phases are mixed), so there is almost no volatilization of the Naz[] component and there is no change in the charging composition and calcined powder composition, and grain growth is Because it is small, it is 0.5 μm or less, more specifically 0.05 to 0.2
Very fine particles of μm size can be obtained.

Since the calcined powder thus obtained is almost single-phase fine particles, it can be easily sintered at low temperatures (it has the advantage of suppressing the volatilization of Na2O as much as possible).

In addition to alkoxides of aluminum and sodium, alkoxides of elements that change into oxides upon firing, such as lithium alkoxide or magnesium alkoxide, may be added.

[Example] Next, the present invention will be explained in more detail using actual examples.

The solubility of aluminum isopropoxide and sodium ethoxide in various solvents (methanol, ethanol, n- and l-propanol, n and 1-butanol, xylene and toluene) was determined. , methanol was selected as the solvent for sodium ethoxide, respectively.

Note that both alkoxide solutions were filtered using filter paper (5C) to remove insoluble residues and hydrolyzates, and then the solution concentrations were determined.

0.29 and 0.62 for solutions of aluminum
mol/j! Two kinds of solutions were prepared, and a solution of 0.34 mol/β was prepared for the IJium solution. The person's 1:Na ratio is 6.7, 8.8.5.9°10
The solutions were mixed so that 1.

For selection of hydrolysis conditions, Al:Na=8.
5 and acetic acid (CH3COOH) to prevent the preferential formation of aluminum hydroxide (boehmite).
was added to the solution. The amount added is CH3COO1l
: Al ratio 0. 0.01. It was set to 0.1. Then, 2 times and 4 times the theoretical amount of water required for hydrolysis was added directly to the mixed solution in a biurette, and 10 times more water was added with methanol.
0.5-/min, respectively, and 2. Or
was added at an addition rate of nll/min.

That is, conditions were selected from a total of 24 conditions: ^1 solution concentration (2 conditions), acetic acid concentration (3 conditions), amount of water added (2 conditions), and presence or absence of water dilution (2 conditions).

First, the aluminum solution concentration is low (0.29 mol/
In a system where the solution was mixed with a sodium solution, the sol prepared by changing the above 12 conditions (amount of acetic acid, amount of water added, dilution ratio of water) was transferred to a Teflon vat and left overnight. , aging was performed. Next, it was dried in a dryer at 70° C. day and night, and the white gel was calcined to obtain calcined powder. Aluminum solution is 0.62 mol#!
The result was that the gel aggregated a little more easily in the 0.29 mol/J system than in the 0.29 mol/J system.

- As an example, the acetic acid amount ratio ([
:)13CDDH/AI) = 0.01, calcination temperature and crystals when calcination time is constant (10 hours) for a system diluted 10 times with methanol and added with twice the theoretical amount of water The phase ratio was determined by X-ray diffraction and is shown in FIG.

As can be seen from Figure 1, the β-alumina phase begins to form at 1000°C, and the β-alumina phase (NaaAI2MDsx
++) and γ-alumina (Al2O2), the ratio of β-alumina phase reaches its maximum at 1100°C, and the ratio of α-alumina phase reaches its maximum at 1200°C or higher, which is thought to be due to evaporation of Na.
-Alumina (8120,) started to form.

From the results shown in Figure 1, the calcination conditions were fixed at 1100°C for 10 hours, and the proportion of the crystal phase was determined under the other 12 conditions (amount of acid, amount of water added, dilution ratio of water). Shown in the figure.

(A) in Figure 2 is when only twice the theoretical amount of water is added, (B) is when twice the theoretical amount of water is diluted 10 times with methanol, and (C) (D) shows the case where only 4 times the theoretical amount of water was added, and (D) shows the case where 4 times the theoretical amount of water was diluted 10 times with methanol and added. In FIG. 2, N indicates the C)13[1'0 (IH/^1 ratio), and all the phases other than the λ phase indicate the β-alumina phase. Also, ■ to ■ indicate the experiment numbers.

From Figure 2, except for the condition (C) (only water added 4 times the theoretical value), there is a mixed phase of β-alumina and λ-alumina, but in (B) there is some aggregation, and other conditions （＾）、(
Among the three systems B) and (D), the (D) system also shows some aggregation, so the conditions of experiment number ■ (CH3CO[lH/AI ratio The standard conditions were a mixed solution in which acetic acid was added so that the ratio of 0.01 to 0.01 was diluted 10 times with methanol, and the amount of the solution was twice the theoretical value.

According to the results of scanning electron microscope observation of the calcined powder prepared under these standard conditions, the particle size of the calcined powder obtained was approximately 0.
．． It is about 1 μm. Other slightly agglomerated calcined powders were also observed, and the size of the secondary particles was constant at approximately 0.1 μm.

Next, the Al:Na ratio was set to 6.7, 8, 8.5, 9.10.
Figure 3 shows the ratio of the crystal phase of the calcined powder obtained by hydrolyzing, drying and calcining mixed solutions prepared at six different concentrations under the above standard conditions.

From FIG. 3, a single β-alumina phase was observed when the I:Na ratio was up to 6.7.8, and a mixed phase of a β-alumina phase and some λ-alumina phase was observed when the I:Na ratio was 8 or more. Further, β-alumina is a hexagonal crystal, and the lattice constant of the obtained calcined powder was determined, and the a-axis and C-axis are shown in FIGS. 1 and 5, respectively. It was found that the lattice constant of the calcined powder thus obtained was almost independent of the Al:Na ratio.

In addition, the crystallite diameter of the calcined powder obtained in the same manner was set to 5herrer.
It was obtained from the O formula, and as shown in Figure 6, this was calculated from Al:
The number of people remained constant at approximately 270 regardless of the Na ratio.

In this way, when the Al:Na ratio is in the range of 6 to 10:1, the particle size of the calcined powder (0.1 μm) and the crystallite size (270)
and lattice constant (a-axis: 5.55, C-axis: 22.5
5) showed almost constant values, and fine and homogeneous calcined powder was obtained. In addition, in a sample with an AI:Na ratio of 10:1, λ-
Approximately 25% alumina phase was observed, but -shaft molding (100%
kg/cat), then subjected to CIP treatment, molded into pellets, embedded in powder of the same composition, and sintered at 1650°C for 10 minutes, a β single phase was obtained, which solved the practical problem with the crystalline phase. Naturally, when the Al:Na ratio was 8 or less, a β single-phase sintered body was obtained.

In addition, the calcined powder (AI/Na = 7) prepared this time and the calcined powder (AI/Na = 7) showing a mixed phase of β-alumina and β' alumina obtained by the usual solid phase method were 1,650℃, which is 50℃ lower than 1,700℃, which is said to be the sintering temperature of
When sintered for 0 minutes, a relative density of 93.0% for the former and 90.2% for the latter was obtained, confirming that the powder obtained in the present invention has an improved sintered density. Ta.

〔Effect of the invention〕

As explained above, according to the present invention, it is possible to obtain a raw material for a β-alumina electrolyte that is single-phase and has a small particle size at a low temperature. In addition, by using such a raw material, sintering can be performed at a lower temperature than that obtained by conventional methods, which prevents volatilization of the Na2O component or abnormal grain growth (Duplex structure). I can do it. Those with a structure free of abnormal grain growth are generally said to have excellent durability. Furthermore, since a mixed solution of sodium and aluminum alkoxides is used as a starting material, sodium and aluminum are mixed at the atomic level. Therefore, the usual solid phase method (A1203 and Na2O
The Na
There is little unevenness of components, which is thought to be a factor in improving durability.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the relationship between the calcination temperature and the ratio of the crystalline phase in an example of the present invention, FIG. 2 is a diagram showing the relationship between the hydrolysis conditions and the ratio of the crystalline phase after calcination, and FIG. Figure 3 is a diagram showing the relationship between the Al:Na ratio of calcined powder and the proportion of crystalline phase;
Figure 4 is a chart showing the lattice constant (a-axis) of calcined powder, Figure 5 is a chart showing the lattice constant (c-axis) of calcined powder, and Figure 6 is a chart showing the crystallite diameter of calcined powder. It is.

Claims (1)

[Claims] A mixed solution of aluminum alkoxide and sodium alkoxide prepared such that the Al:Na atomic ratio is 6 to 10:1 is diluted with methanol under the conditions of addition of acetic acid and the addition of more than the amount of water required for hydrolysis. A method for producing a β-alumina electrolyte raw material, which comprises performing hydrolysis in one step, drying the resulting gel, and then calcining it at a low temperature.