JPS586276B2 - sodium-sulfur battery - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a high-performance sodium-sulfur battery that improves sodium utilization and simplifies battery manufacturing.

In a sodium-sulfur battery, molten sodium, the cathode active material (cathode chamber), is placed on one side, and molten sulfur, the anode active material, is placed on the other side, through a non-porous bottomed solid electrolyte tube that allows only sodium ions to pass through. It is a high-temperature battery that operates at 300 to 450°C.

The electrochemical reaction associated with discharging and charging is 2N a + discharge. xsiNa2SX.

That is, during discharge, sodium in the cathode chamber A liberates electrons and becomes sodium ions, which pass through the solid electrolyte tube partition wall such as β-alumina and enter the anode chamber B. At the same time, they react with the molten sulfur in the anode chamber B, forming polysulfur. A discharge product called sodium Na2Sx is generated, and the discharge product composition is Na2 82
- When it reaches B, the discharge ends.

As sodium decreases during discharge, the sodium liquid level in the solid electrolyte tube decreases.

Therefore, the active area inside the solid electrolyte tube decreases, making it impossible to discharge at a constant current density, and when discharging at a constant current, the current density mA/cm' of the solid electrolyte tube increases, causing local There is a risk of overheating.

In addition, in order to increase the battery capacity, a sodium reservoir is installed at the upper end of the solid electrolyte tube to store sodium.
During discharge, the active area has been kept constant by not using the sodium in the solid electrolyte tube, but using sodium supplied from the sodium reservoir.

However, with this structure, the battery size increases by the height of the sodium reservoir, and since the sodium in the solid electrolyte tube is not used, the weight efficiency and volumetric efficiency of the battery are significantly reduced.

Therefore, by providing a hollow tube inside the solid electrolyte tube and interposing a conductive porous body between the inner wall of the solid electrolyte tube and the outer wall of the tube, the upper sodium reservoir is removed and the inside of the hollow tube inside the solid electrolyte tube is removed. There is a method in which sodium is constantly supplied to the inner surface of the solid electrolyte tube through a conductive porous material to keep the active area constant.

Furthermore, improvements have been made such as instead of providing a hollow tube inside the solid electrolyte tube, a tubular conductive porous body is closely attached to the inner wall of the solid electrolyte tube, and the pore diameter of the tubular conductive porous body is made to be 20 μm or more.

Although the above improvements have improved the sodium utilization rate, if the battery capacity is to be maintained at the same level as conventional batteries, the solid electrolyte tube must have a large inner diameter.

Solid electrolyte tubes are difficult to manufacture as their diameter increases, and their quality is inconsistent.

By the way, since a sodium-sulfur battery has a higher internal resistance than a lead-acid battery, it generates a large amount of heat during high-rate discharge, and the temperature may be about 100° C. higher than the normal operating temperature.

As a result, the pore diameter of the porous body increases due to thermal expansion due to the viscosity, wettability, and surface tension of sodium as well as temperature changes.
The mobility of sodium increases, the wick effect of the conductive porous body decreases, and if the solid electrolyte tube is damaged due to thermal shock, the amount of sodium that directly reacts with sulfur increases, which is extremely dangerous.

In addition, the battery assembly process has traditionally been extremely difficult from a technical standpoint, and the effect of the porous material on the sodium utilization rate differs between batteries, and the adhesion of the porous material deteriorates due to temperature changes, resulting in poor battery performance and, ultimately, There were problems with battery reliability and safety.

The present invention is achieved by filling the inside of a bottomed tubular solid electrolyte, which is a cathode chamber, with molten sodium-resistant metal fibers with a fiber diameter (fineness) of 19μ or less so that the porosity is 85 to 99.8%. By arranging molten sodium-resistant metal fibers in the lengthwise direction of the bottomed tubular solid electrolyte, it simplifies the battery assembly process, improves the utilization rate of sodium in the solid electrolyte tube, and is stable against temperature changes. We were able to.

An example battery of the present invention will be explained with reference to a vertical cross-sectional view in FIG.

1 is molten sodium, 2 is a bottomed solid electrolyte tube made of β-alumina, and 3 is an internal reservoir made of iron-nickel alloy.
Molten sodium 1 is completely filled into solid electrolyte tube 2 and inner reservoir 3.

4 is a molded body made of α-alumina for connecting the solid electrolyte tube 2 and the inner reservoir 3, which are bonded together with glass sealing solder.

5 is a molten sodium resistant metal fiber filled in the solid electrolyte tube 2 (for example, stainless steel, Fe Ni alloy, chromium, Fe-Cr alloy, Fe alloy, nickel alloy, cobalt alloy, tungsten, molybdenum, tantalum, F e -N i -Cr alloy, Ni-Cr alloy, etc.), 6 is an exhaust pipe (e.g. steel pipe, stainless steel coated pipe) inserted through the hole in the upper lid of the inner reservoir 3 and welded; 7
The cathode terminal (e.g., a copper rod or copper tube coated with stainless steel) is inserted into the metal fiber 5 through a hole in the upper lid of the inner reservoir 3, and the exhaust pipe 6 has a hole in the cathode terminal T. It can also be substituted by

8 is a metal outer reservoir (for example, made of stainless steel) for electrically insulating the anode chamber B and the inner reservoir 3, and 9 is a metal outer reservoir (for example, made of stainless steel) for electrically insulating the anode chamber B and the inner reservoir 3; 10 is molten sulfur; 11 is an auxiliary conductive material (e.g. carbon felt, graphite felt) wrapped around the solid electrolyte tube 2 to impart conductivity to the molten sulfur; 12 is an auxiliary material; The auxiliary current collector (for example, molybdenum) that presses and tightens the conductive material 11 from the outside and collects current, and the auxiliary current collector 12 may have a cylindrical shape, a ribbon shape, or the like.

13 is an auxiliary current collecting wire (e.g. molybdenum) for holding and tightening and fixing the auxiliary conductive material 11 and the auxiliary current collector 12; 14 is a battery case (e.g. stainless steel, molybdenum alloy) for accommodating them as a whole and collecting current; or stainless steel coated with molybdenum).

The upper end flange of the battery case 14, the upper end flange of the outer reservoir 8, and the auxiliary current collector 12 are integrally welded, and the anode chamber B is vacuum-sealed.

15 is an anode terminal having an L-shaped cross section (for example, made of stainless steel), which is welded at the same time when the battery case 14, the outer reservoir 8, and the auxiliary current collector 12 are welded together; 16 is a cathode fitted inside the anode terminal 15; α- with holes for terminal 1 and exhaust pipe 6
The member 16, which is made of alumina or the like, serves to completely electrically insulate the cathode and anode and to prevent the inner reservoir 3 from being eccentric.

EXAMPLES In order to clarify the effects of the present invention, examples will be described below.

Example 1 Stainless steel fibers with a fineness of 2 μm were attached to the solid electrolyte tube 2 with a porosity (
v-a/v%; V is the volume of the part of the cathode chamber filled with metal fibers, and a is the volume occupied by the metal fibers themselves, so a-W
/ρ; W is the weight of the filled metal fiber, ρ is the density of the metal constituting the fiber) is 75%, 80%, 85%, 95%, 9
Filled to 9.8% and 99.99%, Na/β-
About alumina/Na (within stainless steel fiber) cell 35
Constant current discharge test at a temperature of 0°C (current is applied in the direction that sodium in the cathode chamber decreases).

) was carried out. Figure 2 shows the battery voltage change at that time (corresponding to the internal resistance change).

) and sodium utilization rate (the ratio of the amount of sodium transferred to the counter electrode through β-alumina due to discharge to the amount of sodium filled in the cathode chamber).

).

When the porosity is less than 85%, the force that holds sodium within the fiber is strong and it is difficult to release it to the β-alumina interface due to the wick effect, so the amount of sodium released is smaller than the amount of sodium consumed electrochemically. becomes smaller.

That is, due to poor sodium mobility, sodium deficient spots occur on the β-alumina surface, which increases internal resistance, increases battery voltage, and as a result of energization being stopped, the sodium utilization rate deteriorates.

Therefore, it is not suitable for use where the porosity is less than 85%.

On the other hand, when the porosity exceeds 99.8%, the wick effect by the capillary tubes formed between the single metal fibers decreases, and the ability to retain sodium within the fibers and to suck up and supply it to the entire β-alumina surface decreases. However, due to the gravitational effect, alumina becomes β
- Accumulates in the lower part of the alumina tube, creating a sodium-deficient area in the upper part, increasing internal resistance, and reducing the sodium utilization rate as the current is stopped in a short period of time.

On the other hand, when the porosity is in the range of 85% to 99.8%, the rate of sodium release from the metal fiber to the β-alumina interface is stable, and the rate of sodium consumption due to discharge (the rate of electrochemically moving sodium) is stable. ), the sodium mobility is good, and the characteristics are stable without any sudden increase in internal resistance (increase in battery voltage) until the sodium utilization rate reaches approximately 100 cm.

Note that this tendency was the same when electricity was applied at a high current density, and no significant increase in internal resistance was observed when the porosity was 85% to 99.8%.

Example 2 Fineness lμ, 4μ, 8μ, 12μ, 1 in solid electrolyte tube 2
Stainless steel fibers of 6μ, 19μ, 22μ, and 30μ were arranged so that the porosity was 96.1%, and an alternating current test was conducted at 350 μm in a Na/β-alumina/Na (stainless fiber) cell.
When carried out at a temperature of °C, it was found that if the fineness was thicker than 19μ, the internal resistance increased and the sodium retention capacity decreased, making it unsuitable for high current density conduction, and the inside of the solid electrolyte tube 2. It was also difficult and inappropriate to fill the container.

On the other hand, when the fineness is 19μ or less, the sodium retention is good, and even in the high current density energization test, no increase in internal resistance is observed, and the sodium utilization rate in the solid electrolyte tube is 97.
% or more.

Example 3 A solid electrolyte tube was filled with copper fibers with a fineness of 4 μ and a porosity of 86%, and a Na/β-alumina/Na (inside the copper fiber) cell was
A constant current density alternating energization test was conducted at a temperature of 50°C.

Since copper fiber is used, internal resistance is also reduced, resulting in 98.
2% sodium could be used effectively.

Example 4 Iron and nickel alloy fibers with a fineness of 8μ were arranged in a solid electrolyte tube so that the porosity was 93.1%, and the Na/β-alumina/Na (intrafiber) cell was heated at a high current density at a temperature of 350°C. When an alternating energization test was conducted, the results shown in FIG. 3 were obtained, and the sodium utilization rate was 98.0%.

Example 5 A reservoir approximately half the height of the conventional one is connected to the open upper end of the solid electrolyte tube 2 with glass solder, and iron and chromium alloy fibers with a fineness of 10μ and a porosity of 93.1 are placed inside the reservoir and the solid electrolyte tube. % of Na/β-
When carried out in an alumina/Na (in iron-chromium alloy fiber) cell, the same electrical capacity as the conventional one was obtained.

As a result, the height of the conventional battery can be reduced to about 2/3.

Example 6 Nickel heat-treated (heat-treated around the sealing temperature of glass solder for sealing the solid electrolyte tube) in a zero atmosphere with an inert gas (for example, nitrogen gas, argon gas, etc.) in the solid electrolyte tube 2,
The porosity of chromium alloy fiber with a fineness of 10μ is 91.3.
%, and the sodium utilization rate was determined at a temperature of 350° C. in a Na/β-alumina/Na (nickel, chromium alloy fiber) cell and found to be approximately 98.9%.

By heat-treating the metal fibers, wetting with sodium was improved, the battery assembly process was simplified, and the characteristics of the batteries were also consistent.

Example 7 A cathode terminal was inserted into the metal fiber layer filled in the solid electrolyte tube (approximately 2 cm from the open end of the β-alumina tube).
m depth), the current collection effect was further enhanced, and the internal resistance was reduced by about 5% compared to the case without insertion.

Example 8 When metal fibers made of the same material as the filling metal fibers were welded to the insertion portion of the cathode terminal shown in Example 7, the current collection effect was even better than that shown in Example 7, and the internal resistance was the same as that of Example 7. It decreased by about 3% compared to the previous year.

Example 9 When the tip of the insertion portion of the cathode terminal shown in Example 7 was made into a needle shape, the efficiency of insertion of the cathode terminal into the metal fiber in Example 7 was further increased.

Example 10 When a notch and a protrusion were provided at the insertion portion of the needle-shaped cathode terminal shown in Example 9, the current collection effect was good, and the internal resistance was reduced by about 2% compared to Example 9.

Example 11 A solid electrolyte tube was filled with chromium fibers in the length direction of the solid electrolyte tube, and a constant current density alternating energization test was conducted at 350°C in a Na/β-alumina/Na (inside fiber) cell. The results shown in Examples 1 to 6 were obtained, and when a cathode terminal was further inserted into the fiber, Examples 7 to 1 were obtained.
The internal resistance was slightly decreased compared to the results shown in 0.

Example 12 It became clear from the test results regarding the sodium utilization rate using the Na/β-alumina/Na (within stainless steel fiber) cell that the utilization rate was greatly improved, so S/β-
Alumina/Na (in stainless steel fibers) cells were constructed and tested.

The prototype battery had the same weight as a conventional S/β-alumina/Na battery, and the capacity was increased by increasing the amount of S to account for the improved sodium utilization rate.

The results are shown in FIG. 4 for comparison between battery A of the present invention and conventional battery B.

From this result, the conventional energy density is 170WH/
kg, 190 WH/l, but in the battery according to the present invention, it is 200 WH/kg, 255 WH/kg.
As a result, battery performance has greatly improved.

From the results shown in the above examples, the sodium utilization rate is greatly improved, and compared to the structure in which the tube body is placed inside the solid electrolyte tube as shown in conventional improved batteries, the solid electrolyte tube with a small diameter can be used. The present invention has made it possible to create a battery in which adhesion and sodium permeability are not affected at all by temperature changes associated with high current density discharge.

Additionally, further advances have been made in the battery manufacturing process, battery performance, and safety.

As described above, the present invention has great industrial value. Note that the length of the single fiber of the molten sodium-resistant metal fiber is not particularly limited, and various lengths can be used depending on the battery dimensions and the solid electrolyte tube dimensions.

Furthermore, the material of the metal fiber is not limited as long as it is resistant to molten sodium.

[Brief explanation of the drawing]

Fig. 1 is a longitudinal cross-sectional view of a sodium-sulfur battery according to one embodiment of the present invention, Fig. 2 is a diagram of sodium utilization rate versus battery voltage characteristics at metal fiber density, Fig. 3 is a characteristic diagram of another embodiment, and Fig. 4 is a diagram of the present invention. FIG. 3 is a comparative characteristic curve diagram of an inventive battery and a conventional battery. DESCRIPTION OF SYMBOLS 1... Molten sodium, 2... Solid electrolyte tube, 5... Metal fiber, 7... Cathode terminal.

Claims (1)

[Claims] 1. In a sodium-sulfur battery using molten sodium as a cathode active material, molten sulfur as an anode active material, and a solid electrolyte tube as an electrolyte, the solid electrolyte tube has a fineness (fiber diameter) of 19
Melt resistant sodium metal fiber with a porosity of 85 to 9 μ or less
A sodium-sulfur battery characterized in that the sodium-sulfur content is 9.8%. 2. The sodium-sulfur battery according to claim 1, wherein the melt-resistant sodium metal fibers are also arranged within the sodium reservoir. 3. The sodium-sulfur battery according to claim 1 or 2, wherein the molten sodium-resistant metal fiber is a fiber heat-treated in an inert gas atmosphere. 4 In a sodium-sulfur battery that uses molten sodium as the cathode active material, molten sulfur as the anode active material, and a solid electrolyte tube as the electrolyte, the solid electrolyte tube has a fineness (fiber diameter) of 19
Molten resistant sodium metal fiber with a porosity of 85~
99.8%, and a cathode terminal is inserted into the metal fiber layer. 5. The sodium-sulfur battery according to claim 4, wherein metal fibers are welded to the surface of the cathode terminal. 6. The sodium-sulfur battery according to claim 4, wherein the tip of the cathode terminal is needle-shaped. 7. The sodium-sulfur battery according to claim 4, wherein the cathode terminal has a notch or a protrusion.