JPH0239065B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to high-temperature batteries, and aims to extend the life of the battery by providing materials with excellent corrosion resistance for the anode active material container, anode current collector, etc. that are in contact with the anode active material. The principle of this battery was announced by Ford Motor Company in the United States in 1966, and it is also called a sodium-sulfur battery because it uses sodium and sulfur as active materials. As shown in Figure 1, its structure consists of 1 cathode terminal, 2 cathode active material (sodium), 3 solid electrolyte tube with sodium ion conductivity (e.g. beta alumina), and 4 carbon or graphite felt. impregnated anode active material (e.g. sulfur or sodium polysulfide), 5
It consists of an anode active material container that also serves as an anode current collector.
An insulator 8 (for example, alpha alumina) for electrically insulating a lid that airtightly seals the cathode chamber and the anode chamber is bonded to the upper open end of the solid electrolyte tube with a glass 9. The operating temperature is the melting point of the bipolar active material,
The temperature is usually 300 to 350°C in consideration of the conductivity of the solid electrolyte. The battery reaction is as shown in the equation below. 2Na + 3S discharge -→ ←- Rechargeable Na ₂ S ₃ high-temperature batteries are being developed in various countries as high-performance secondary batteries for power load adjustment and electric vehicle power supplies, but one of the key points in their development is their durability. There is a need for an anode active material container and an anode current collector with excellent sodium polysulfide properties. Sodium polysulfide has a very strong corrosive effect, and sulfides are generated when the anode active material container, anode current collector, etc. are corroded. As corrosion progresses, sulfur in the anode active material is consumed due to sulfidation, and the amount of sulfur that acts as an active material decreases, reducing battery capacity and reaching the end of its life.
Further, if corrosion progresses locally, holes may form in the anode active material container, etc., and the active material may flow out to the outside, resulting in the end of the service life. The performance required for the anode active material container, anode current collector, etc. is that it must be corrosion resistant to the severely corrosive anode active material and have mechanical strength.
Materials with excellent workability are desirable. Furthermore, good electrical conductivity is also required when performing anode current collection. In general, SUS 304, molybdenum, chromium, and the iron-chromium-silicon alloy developed by the present inventors are being considered as anode active material containers, anode current collectors, etc., but SUS304 corrodes quickly and As shown in the figure, the battery capacity rapidly decreases with a small number of charge/discharge cycles. In the case of molybdenum, during a charge/discharge cycle in which the discharge is terminated in a partially discharged state where the anode active material composition is around Na ₂ S ₅ , corrosion as a whole is small, and as shown in Figure 3, there is no decrease in battery capacity. Although it is rare, localized corrosion, so-called pitting corrosion, occurs, resulting in a short lifespan. Furthermore, during a charge/discharge cycle in which the anode active material composition is discharged to a fully discharged state near Na ₂ S ₃ , the progress of corrosion is accelerated, as shown in Figure 4, compared to the partial discharge shown in Figure 3. As a result, the battery capacity decreases rapidly. Chromium has little corrosion overall in the Na ₂ S ₅ to _{Na 2} S ₃ range, but it does cause pitting corrosion and is extremely brittle, making it nearly impossible to process into containers etc. with current technology. It does not satisfy the requirements. The present inventors first discovered that iron is a metal material that has corrosion resistance and excellent workability for anode active materials.
Although we proposed a chromium-silicon alloy, we discovered that a high-purity iron-chromium alloy is even more superior in terms of battery capacity changes. The preferable range of the iron-chromium alloy is 40 to 60% Cr, 0.02% or less of C, 0.015% or less of N, and the balance is substantially iron. When Cr exceeds the upper limit, workability is poor, and when it is below the lower limit, corrosion resistance is poor. When C and N exceed the upper limits, local corrosion such as intergranular corrosion and pitting corrosion tends to occur, and workability is impaired. Corrosivity is further improved by adding one or more of Ti, Nb, and Zr in a total amount of 0.05 to 1% to the above iron-chromium alloy. Ti,
Addition of Nb and Zr improves corrosion resistance by suppressing grain boundary precipitation of chromium carbonitride, and also improves ductility and toughness. However, if added in excess of the upper limit,
Intermetallic compounds containing Ti, Nb, and Zr are formed, which impairs ductility and toughness. As mentioned above, the anode active material is made up of S (sulfur) or _Na2S5 , which has _a high ratio of S to Na, due to discharge.
Although the composition changes to Na ₂ S ₃ , the corrosion caused by Na ₂ S ₃ is the most severe, and the anode active material container, anode current collector, etc. must have sufficient corrosion resistance against Na ₂ S ₃ . The results of a comparative corrosion resistance test between the alloy of the present invention and conventional metal materials are shown below. 1 mm x 8 mm x 30 mm of the alloy of the present invention shown in Table 1
The specimen was vacuum sealed in a Pyrex glass tube with 15 g of Na ₂ S ₃ and subjected to a corrosion test at 350° C. for 384 hours. The corrosion thickness was determined from the weight loss, surface area, and specific gravity of the test piece. The presence or absence of pitting corrosion was also examined using a microscope. The results are shown in Table 2.

【table】

【table】

[Table] From these results, the alloys of the present invention have extremely superior Na ₂ S ₃ resistance in terms of corrosion thickness and local corrosion, except for reference example Alloy 4, when compared with the reference example alloys. Next, the alloys of the present invention (Table 1 1, 2, 3, 4,
5 and 6), an anode active material container serving as an anode current collector as shown in 5 in FIG. 1 was prepared, a sodium-sulfur battery was assembled, and a charge/discharge cycle test was conducted. The test temperature was 350°C, and the discharge was performed for 3 hours at a rate of current (55 mA/cm ² as the surface current density of the anode active material container) for 5 hours, and after a 10 minute break, the discharge amount reached 100% in 5 hours.
The charging/discharging conditions are such that one cycle consists of charging an amount of electricity and resting for 10 minutes. Changes in battery capacity with respect to cycles of batteries using each anode active material container up to 450 cycles and an investigation of the anode active material container after the test were conducted. FIG. 6 shows changes in battery capacity with respect to charge/discharge cycles of batteries using positive electrode active material containers made of alloy samples 1, 2, 3, 4, 5, and 6. Molybdenum and iron-chromium under similar test conditions
This is a good change in battery capacity compared to the results of batteries using a silicon alloy as the anode active material container (see FIGS. 3 and 5). Furthermore, the surface of the container 5 that was in contact with the anode active material after the test had no abnormalities in any of the alloy samples of the present invention, and had excellent corrosion resistance against the anode active material. FIG. 7 shows a photograph of alloy sample 2 as an example. On the other hand, when the surface of the molybdenum container battery that was in contact with the anode active material was examined, local corrosion had occurred as shown in FIG. Next, a battery having the structure shown in FIG. 9 was created. In the sodium-sulfur battery shown in FIG. 9, a solid electrolyte tube is filled with an anode active material 14, and the cathode and anode are reversed from those in the battery shown in FIG. The same reference numerals as in FIG. 1 have the same names, so explanations will be omitted. At the terminal 15 (anode current collector) inside the solid electrolyte tube,
In this case, corrosion resistance is required. Since the cathode container 10 is only in contact with sodium 12, it is made of SUS.
304 has sufficient corrosion resistance. Other configurations are first
It is similar to the battery shown in the figure. In the anode current collector 15 of the battery shown in FIG. 8, only the surface in contact with the anode active material was covered with a thin film made of an alloy sample according to the present invention (alloy sample 5 in Table 1).
In this experiment, a 0.2 mm thick pipe was made from alloy sample 5, and an iron rod was fitted into the pipe. , middle and bottom circumferences were welded using an electron beam. The lower end of the pipe was made slightly longer to prevent the anode active material from coming into contact with the iron inside, and was crushed into a wire and then welded to completely protect the iron rod from the cathode active material. The high temperature battery shown in Figure 9 was subjected to a 450 cycle charge/discharge test under the same conditions as the above test.
The results were exactly the same as those of the above test, including the capacity characteristics and the surface condition after the test. As explained above, the present invention suppresses corrosion of the anode active material container, anode current collector, etc. caused by the anode active material,
It aims to extend the life of high-temperature batteries, and its industrial value is extremely large. Note that the present invention relates to a metal material used for a portion in contact with an anode active material, and is not limited to its structure, method of use, or method of coating.

[Brief explanation of drawings]

Figure 1 is a vertical cross-sectional view of a high-temperature battery, Figure 2 is a SUS
Figure 3 is a graph of the capacity characteristics of a battery using molybdenum as the anode active material container. Figure 4 is a graph of the partial discharge cycle capacity characteristics of a battery using molybdenum as the anode active material container. FIG. 5 is a graph of the full discharge cycle capacity characteristics of the battery used. FIG. 5 is a graph of the partial discharge cycle capacity characteristic of a battery using an iron-chromium-silicon alloy as the anode active material container. FIG. A graph of the partial discharge cycle capacity characteristics of the battery used in the positive electrode active material container, Figure 7a shows the characteristics of the positive electrode active material after the partial charge/discharge test of the battery using the positive electrode active material container prepared with Alloy Sample 2. Figure 7b is a micrograph of the metallographic structure on the surface, and Figure 8a is a micrograph of the metallographic structure of the cross section. Figure 8a is a photomicrograph of the anode active material after a partial charge/discharge test of a battery using a cathode active material container made of molybdenum. FIG. 8b is a microscopic photograph of the metallographic structure of the surfaces in contact with each other, FIG. 8b is a microscopic photograph of the metallographic structure of a cross section, and FIG. 9 is a longitudinal cross-sectional view of a battery with the inside of the solid electrolyte tube serving as an anode. DESCRIPTION OF SYMBOLS 1... Cathode terminal, 2... Cathode active material, 3... Solid electrolyte tube, 4... Anode active material, 5... Anode active material container that also serves as an anode current collector, 6... Anode lid, 7...
...Cathode lid, 8...Insulation ring, 9...Glass, 1
0... Cathode active material container, 12... Cathode active material, 1
4... Anode active material, 15... Anode terminal (current collector).

Claims (1)

[Claims] 1. At least the surface of the anode active material container and the anode current collector in contact with the anode active material is made of 40 to 60% Cr and 0.02% C.
The following is a high-temperature battery characterized by using an iron-chromium alloy containing 0.015% or less of N and the remainder being substantially iron. 2. At least the surface of the anode active material container and anode current collector in contact with the anode active material is made of 40 to 60% Cr and 0.02% C.
Hereinafter, it is characterized by using an iron-chromium alloy containing 0.015% or less of N, and further containing 0.05 to 1% of any one or more of Ti, Nb, and Zr, and the balance being substantially iron. high temperature battery.