JP2001172073A - beta-ALUMINA ELECTROLYTE AND METHOD FOR PRODUCING THE SAME - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a β-alumina electrolyte used as a solid electrolyte and provide a method for producing the same. SOLUTION: This method for producing the β-alumina electrolyte is characterized by forming a mixture containing 100-1,000 ppm SiO2, 100-500 ppm CaO or K2O and the rest essentially consisting of an aluminum starting material and sodium starting material, and burning the mixture under a burning condition which contains a step of performing a heat treatment for crystallizing its impurities at a grain boundary glass phase after attaining its maximum temperature.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a beta-alumina electrolyte used as a secondary battery such as a sodium-sulfur battery and a sodium-molten salt battery or a solid electrolyte such as an alkali metal thermoelectric conversion battery, which operates using sodium ions as a carrier. And its manufacturing method.

[0002]

2. Description of the Related Art Beta alumina electrolyte has high sodium ion conductivity and is therefore used as an electrolyte for various batteries using sodium ions as a carrier. Since this electrolyte occupies a considerable part of the internal resistance of the battery, a dense sintered body having high conductivity and high strength is preferable. The beta alumina includes β alumina (theoretical composition Na ₂ O · 11Al ₂ O ₃ ) and β ″ alumina (theoretical composition Na ₂ O
• There are two types of crystal forms, 5.3Al ₂ O ₃ ). Since β "alumina has higher conductivity and shows high performance as a battery, practically, β" alumina or β "alumina and β
A mixture of alumina is frequently used.

[0003] It is known that impurities contained in a beta-alumina electrolyte adversely affect its conductivity and durability. Therefore, generally, starting materials having high cost and high purity are used. In order to reduce the cost of the raw material, there is a need for a method capable of producing a beta-alumina electrolyte having conductive properties equivalent to those of the related art, even if a raw material having low purity is used.

On the other hand, in Japanese Patent No. 2135003, the content of trace components in beta-alumina is adjusted to SiO ₂ <200 ppm and Fe ₂ O ₃ <1000 p.
pm, CaO <100 ppm, K ₂ O <200 ppm, and SiO ₂ + Fe ₂ O ₃ + CaO
By specifying the total amount of + K ₂ O to be 100 ppm or more, it is possible to improve the electrical conductivity of the sintered body after the initial and long-term energization. However, SiO ₂ , CaO, K as impurities
_The problem is that the amount of ₂ O is limited to a very small amount.

[0005]

Accordingly, it has been an object to provide a beta-alumina electrolyte having high conductivity even if a raw material containing more impurities is used, and a method for producing the same.

[0006]

Means for Solving the Problems The inventors of the present invention added SiO ₂ , CaO, and K ₂ O to those using high-purity starting materials in order to analyze the state of impurities in beta alumina. A beta-alumina sintered body was prepared, and the structure was observed.From the results, it was determined that all of them existed as a glass phase at the grain boundary.Therefore, SiO ₂ , CaO, and K ₂ O Is considered to be a cause of deterioration of the characteristics of the beta-alumina (mainly a decrease in conductivity) and a decrease in durability. When β-alumina containing these grain boundary glass phases was subjected to crystallization, Fig. 1
As shown in (a), the glass phase 2 at the grain boundary generated between the particles 1 is crystallized at the grain boundary triple point as shown in FIG. Thereby, the impurity phase at the interface is aggregated, and finally, as shown in FIG.
A structure in which a phase in which impurities were crystallized at the grain boundary triple point was dispersed, and beta alumina having excellent durability was obtained without deterioration of conductive properties even when a raw material containing impurities was used.
The present invention is based on this finding.

[0007] The above-mentioned problems, SiO ₂ of 100~1000ppm, 100~5
The problem is solved by providing a beta-alumina electrolyte containing 00 ppm CaO or K ₂ O, and the balance substantially consisting of an aluminum starting source and a sodium starting source, and a method for producing the same.

Further, according to the production method of the present invention, SiO ₂
000 ppm, CaO, when manufacturing a beta alumina using a raw material containing 100~500ppm a K ₂ O, after reaching the maximum temperature of sintering conditions, to hold at 600 to 1100 ° C. is the crystallization temperature region of the glass This is a method for producing a beta alumina electrolyte, which is a feature of the present invention.

[0009] A preferred production method of the present invention, the SiO ₂ 100
~1000ppm, CaO, when manufacturing a beta alumina using a raw material containing 100~500ppm a K ₂ O, Ti as a nucleating agent for the glass
O ₂ or ZrO ₂ or both is a method for producing a beta-alumina electrolyte, characterized by containing 100 to 500 ppm.

[0010] A further preferred manufacturing method of the present invention, SiO ₂
The 100 to 1000 ppm, CaO, when manufacturing a beta alumina using a raw material containing 100~500ppm a K ₂ O, the TiO ₂ or ZrO ₂ or both contain 100~500ppm as nucleating agent of the glass, sintering conditions A method for producing a beta-alumina electrolyte, characterized in that after reaching the maximum temperature, the temperature is maintained at 600 to 1100 ° C., which is the glass crystallization temperature range.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, preferred embodiments of a beta alumina electrolyte and a method for producing the same according to the present invention will be described in detail, but the present invention is not limited thereto. Method for producing a beta-alumina electrolyte of the present invention, the SiO ₂ in the starting material 100
～1000Ppm, CaO, using a raw material containing 100 to 500 ppm K ₂ O, respectively, can be produced, for example, following steps. First, in the step, an aluminum starting material and a sodium starting material are mixed to prepare a raw material powder.

In this step, the aluminum starting material is preferably aluminum oxide. As the sodium starting material, sodium salts such as sodium carbonate and sodium nitrate can be used. The mixing ratio of the aluminum starting material and the sodium starting material is Al ₂ O
Preferably, the molar ratio of ₃ / Na ₂ O is in the range of 5 to 7.

The aluminum starting material and the sodium starting material may be mixed in a powder form. However, the aluminum starting material and the sodium starting material are mixed with water, and if necessary, a dispersing agent is added and wet-mixed. Is desirable. Further, it is preferable to use a nucleating agent depending on the subsequent crystallization treatment. In this case, it is preferable to mix the titanium starting material and / or zirconium simultaneously with the starting material. As the titanium starting material and the zirconium starting material, it is desirable to use a liquid such as a metal alkoxide, but a solid powder such as an oxide can also be used.

The nucleating agent is added in an amount of 100 to 500 in terms of oxide in 100 parts by weight of the beta alumina electrolyte after sintering.
It is preferable to be in the range of ppm by weight. This nucleating agent is 1
If the amount is less than 00 ppm by weight, uniform mixing becomes difficult and no improvement in conductivity is observed.In addition, if the amount exceeds 500 ppm by weight, no further effect is obtained, and the addition of a large amount is not preferable because it causes an increase in cost. .

In the step, an aluminum starting material,
After mixing the sodium starting material and the nucleating agent used as required, the raw material powder is dried and pulverized as necessary, and then calcined at 1200 to 1350 ° C. In this step, if the calcining temperature is lower than 1200 ° C., the β ”conversion ratio of the calcined powder (the ratio of β” alumina in the whole crystal) is low, so that the β ”conversion ratio of the sintered body is low and the conductivity is low. If the temperature falls below 1350 ° C., the β ″ conversion rate decreases with the scattering of sodium, and the sintering of the calcined powder proceeds, so that pulverization becomes difficult or requires a long time, which is not preferable.

Since the calcined powder obtained in this way has grown in grains, it is necessary to grind the powder to improve sinterability. It is preferable to add a magnesium starting material during the pulverization of the calcined powder and to mix the pulverized powder at the same time as the pulverization. This pulverization / mixing is desirably a wet pulverization / mixing by adding a magnesium starting material and water to form a slurry. It is preferable to use magnesium salts such as magnesium oxide and magnesium nitrate as the magnesium starting material. The proportion of the magnesium starting material to be added is preferably in the range of 1.5 to 4.5 parts by weight based on 100 parts by weight of the calcined powder.

The mixture obtained by adding the magnesium starting material to the calcined powder and wet-milling and mixing as described above is formed into a predetermined shape. When the mixture is obtained in the form of a slurry, molding is easier by drying and granulating with a spray dryer.

Finally, in the step, the compact obtained in the calcination step is heated to 1560 to 1640 ° C. and sintered to obtain a beta alumina sintered body (beta alumina electrolyte).
In this step, when performing crystallization heat treatment of impurities, after reaching the maximum temperature during sintering,
It is necessary to hold for 1 to 5 hours in the range of 11100 ° C.
If the temperature is lower than 600 ° C. or higher than 1100 ° C., crystal nuclei are hardly generated and the impurity phase is hardly crystallized, which is not desirable. Also, if the holding time is less than 1 hour, crystallization does not proceed sufficiently, and even if it exceeds 5 hours, the effect does not change,
No further heat treatment is required.

[0019]

The present invention will be described in more detail with reference to the following Examples, but it should not be construed that the invention is limited thereto.

Example 1 High-purity aluminum oxide (SiO ₂ , CaO, K ₂ O
ppm), and SiO ₂ ,
Beta alumina to which CaO and K ₂ O were added was prepared. As a manufacturing method, first, aluminum oxide and sodium carbonate
The mixture was weighed so that the molar ratio of Al ₂ O ₃ / Na ₂ CO ₃ became 6, a dispersant was added, and the mixture was wet-mixed with a ball mill for 20 hours. At this time, SiO ₂ , CaO and K ₂ O were simultaneously added.

A low-purity aluminum oxide containing 300 ppm of SiO ₂ and CaO each as a raw material was also used. After concentrating the obtained slurry with a rotary evaporator,
It was dried all day and night with a dryer at ℃. Crush this dried product,
After passing through a 500 μm sieve, it was calcined. 200 ° C for calcination
/ Hour, hold at 1250 ° C for 2 hours, and 200 ° C /
It was assumed that the temperature dropped in time.

Since the obtained calcined powder grows in grains, it is necessary to grind the powder to improve the sinterability. At the time of this pulverization, 4 wt% of magnesium oxide and a dispersant were added and wet-mixed in a ball mill for 45 hours. A granulated powder was prepared from this slurry by a spray dryer. Using the granulated powder, a sample for conductivity measurement was uniaxially molded into a mold of 4 × 4 × 20 mm by a mold, and then kept at a pressure of 1.5 t / cm ² for 2 minutes by a cold isostatic press (CIP). Molded.

The obtained molded body was heated at a heating rate of 5 ° C./min, and kept at 1640 ° C. for 30 minutes.
After maintaining at 100 ° C. for 4 hours, the temperature was lowered to produce a sintered body. The conductivity was measured by baking a platinum electrode on a 3 × 3 × 15 mm test piece and using an AC two-terminal method. The measurement temperature is 300 ° C.

[0024]

[Table 1] Table 1 shows the impurity content of the alumina raw material used in Example 1, the crystallization conditions, and the conductivity of the obtained beta alumina electrolyte.

COMPARATIVE EXAMPLE 1 The sintering conditions were the same as in the Example except that the crystallization heat treatment was not performed, the temperature was raised at a rate of 5 ° C./min, the temperature was maintained at 1640 ° C. for 30 minutes, and the temperature was lowered at 5 ° C./min. A sintered body was produced in the same manner as in 1. Table 2 shows the composition of the sample.

[0026]

[Table 2] Table 2 shows the impurity content of the alumina raw material used in Comparative Example 1 and the conductivity of the obtained beta alumina electrolyte.

According to Table 1, for a sample containing 100 to 1000 ppm of SiO ₂ or 100 to 500 ppm of CaO or K ₂ O and holding at 600 to 1100 ° C. after reaching the maximum temperature as a sintering condition, The rate was 0.21 to 0.24 S / cm, which was almost the same as that of the sample to which no impurity was added (Sample No. 1). In addition, the sample using the low-purity raw material containing 300 ppm of SiO ₂ and CaO each had almost the same value as the sample using the high-purity raw material (sample No. 1) by the crystallization heat treatment. On the other hand, as shown in Table 2, those containing impurities and not subjected to the crystallization heat treatment have a decreased electrical conductivity of 0.15 to 0.18 S / cm.

Example 2 Using the same raw materials as in Example 1, beta-alumina was added to which SiO ₂ , CaO, K ₂ O and TiO ₂ or ZrO ₂ were added so as to have the contents shown in Table 3. As a production method, when mixing aluminum oxide and sodium carbonate, TiO ₂ or ZrO ₂ is added as an alkoxide, and firing conditions are 1640 ° C.
The procedure is the same as in Example 1 except that the temperature is kept for 30 minutes.

[0029]

[Table 3] Table 3 shows the impurity content of the alumina raw material used in Example 2, the nucleating agent addition amount, and the conductivity of the obtained beta alumina electrolyte.

According to Table 3, 100 to 1000 ppm of SiO ₂ or 100 to 500 ppm of CaO or K ₂ O, and TiO ₂ or ZrO ₂
In the sample added with 100 to 500 ppm, the conductivity is 0.22 to 0.24 S
/ Cm, which is almost the same value as the sample to which no impurity is added (Sample No. 1 in Table 1). In addition, the sample using a low-purity raw material containing 300 ppm of SiO ₂ and CaO each had a value substantially equivalent to that of a sample using a high-purity raw material (sample No. 1 in Table 1) due to the addition of TiO ₂ .

Example 3 Using the same raw materials as in Example 1, beta-alumina was prepared by adding SiO ₂ , CaO, K ₂ O and TiO ₂ or ZrO ₂ so as to have the contents shown in Table 4. The production method is the same as that of Example 1 except that when mixing aluminum oxide and sodium carbonate, TiO ₂ or ZrO ₂ is added as an alkoxide.

[0032]

[Table 4] Table 4 shows the impurity content of the alumina raw material used in Example 3, the nucleating agent addition amount, the heat treatment conditions, and the conductivity of the obtained beta alumina electrolyte.

According to Table 4, 100 to 1000 ppm of SiO ₂ or 100 to 500 ppm of CaO or K ₂ O, and TiO ₂ or ZrO ₂
100 to 500 ppm, and after reaching the maximum temperature as a sintering condition, the sample held at 600 to 1100 ° C has a conductivity of 0.23 to
0.25S / cm, with no impurities added (Table 1
Of the sample No. 1).

[0034]

According to the present invention, a beta-alumina electrolyte having high conductivity can be obtained by performing a heat treatment for crystallizing impurities in a grain boundary glass phase, even if a raw material containing more impurities than before is used. be able to. Further, according to the method for producing beta-alumina of the present invention, even when a low-purity raw material containing impurities is used, a beta-alumina sintered body having the same conductivity as the high-purity raw material can be obtained, and the production cost of beta-alumina can be reduced. Can be lowered.

[Brief description of the drawings]

FIG. 1 is a model diagram of grain boundary glass phase crystallization, in which (a) shows that glass is generated between particles, (b) shows that an impurity phase at the interface is aggregated by crystallization, and (c) ) Indicates that the impurities are crystallized.

[Explanation of symbols]

 1 Particle 2 Glass phase 3 Crystallization treatment of grain boundary phase

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Yoshimi Yashima 1-8-1 Koura, Kanazawa-ku, Yokohama-shi, Kanagawa Prefecture Inside the Basic Research Laboratory, Mitsubishi Heavy Industries, Ltd. 8-1 Mitsubishi Heavy Industries, Ltd. Basic Technology Research Laboratories (72) Inventor Yuichiro Murakami 12 Nishikicho, Naka-ku, Yokohama-shi, Kanagawa Prefecture F-term within Ryobi Engineering Co., Ltd. 4G030 AA04 AA07 AA08 AA16 AA17 AA36 AA37 AA67 BA03 CA01 CA05 GA27 5G301 CA02 CA12 CA17 CA25 CA28 CA30 CD01 CD10 CE02 5H029 AJ06 AJ14 AM15 CJ02 CJ08 CJ28 DJ17 EJ06 HJ14

Claims (10)

[Claims] 1. 100-1000 ppm SiO ₂ , 100-500 ppm CaO
Alternatively, a beta-alumina electrolyte containing K ₂ O and a balance substantially consisting of an aluminum starting material and a sodium starting material. 2. The beta alumina electrolyte according to claim 1, wherein the aluminum starting material is aluminum oxide. 3. The beta alumina electrolyte according to claim 1, wherein the sodium starting material is sodium carbonate. 4. A beta alumina electrolyte according to claim 1, wherein the mixture comprises a glass nucleating agent. 5. The glass nucleating agent is 100 to 500 ppm.
5. The beta-alumina electrolyte of claim 4, wherein the electrolyte is TiO ₂ or ZrO ₂ or both. 6. 100-1000 ppm SiO ₂ , 100-500 ppm CaO
Alternatively, in forming and firing a mixture containing K ₂ O and the balance substantially consisting of an aluminum starting material and a sodium starting material, after reaching a maximum temperature as a sintering condition, a heat treatment for crystallizing impurities in a grain boundary glass phase is performed. Performing a process for producing a beta-alumina electrolyte. 7. The method according to claim 6, wherein a nucleating agent for glass is added to the mixture. 8. The glass nucleating agent is 100 to 500 ppm.
The method according to claim 7, wherein TiO ₂ or ZrO ₂ or both are used. 9. The method according to claim 6, wherein the maximum temperature is 1600 to 1650 ° C. 10. The heat treatment for crystallizing the impurities in the grain boundary glass phase is a heat treatment in which after reaching the maximum temperature, the impurities are maintained at a crystallization temperature range of 600 to 1100 ° C. for 1 to 5 hours in the glass phase. The method according to any one of claims 6 to 9, wherein: