JP2646087B2 - Sodium-sulfur secondary battery - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a sodium-sulfur secondary battery, and more particularly, to a structure of a cathode portion thereof.

[Conventional technology]

A sodium-sulfur battery is connected (usually in a tube) through a solid electrolyte bag tube that allows only sodium ions to pass through.
This is a high-temperature type secondary battery in which a molten sodium (cathode compartment) as a cathode active material and a sulfur or sodium polysulfide (anode compartment) as an anode active material are disposed on the other side and operated at 300 to 400 ° C. The electrochemical reactions involved in charging and discharging in this sodium-sulfur battery are as follows.

2Na + xSNa ₂ Sx In other words, during discharge, sodium in the cathode chamber emits electrons to become sodium ions, enters the anode chamber through the solid electrolyte, and reacts with sulfur in the anode chamber to produce sodium polysulfide (Na ₂ Sx) . The electrons pass through the external circuit of the battery and are given to sulfur from the graphite wire in the anode.

As described above, in the sodium-sulfur battery, sodium in the cathode chamber gradually decreases due to discharge. Therefore, there is a conventional example in which molten sodium-resistant metal fibers are arranged in a sodium container (Japanese Patent Publication No. 58-62765). In this conventional example, it is intended to supply sodium to the entire surface of the solid electrolytic chamber by using metal fibers. That is, the sodium is raised from the lower part of the battery by the capillary action by the metal fiber, and the portion where there is no sodium which may be generated due to the battery reaction is supplied with sodium. According to such a conventional example, the battery density can be increased and the sodium utilization can be improved.

When metal fibers are placed in a sodium container, if the porosity is large, the amount of sodium retained is large, but sodium may not rise due to capillary action. Further, when the solid electrolyte tube is broken, sodium and sulfur may react rapidly. Therefore, when placing metal fibers in a sodium container, it is necessary to reduce the porosity, that is, to increase the filling rate of the metal fibers.

[Problems to be solved by the invention]

However, in the conventional example in which the metal fiber is simply filled in the sodium container, it is impossible to uniformly fill the metal fiber, and the filling itself is difficult. Further, when an attempt is made to increase the momentum filling rate, a large pressure is applied to the solid electrolyte tube, and the residual stress is applied to the solid electrolyte even after the battery is assembled. Therefore, there is a possibility that the solid electrolyte may be destroyed, and when the solid electrolyte is destroyed, the cracks may be expanded.

An object of the present invention is to provide a sodium-sulfur secondary battery in which metal fibers are filled in a sodium container without applying pressure to a solid electrolyte tube in order to solve the above problems.

[Means for solving the problem]

The metal fibers to be filled in the sodium container need to be filled with a constant porosity or with an appropriate value of the volume ratio of the fibers and the holes (here, the fiber / hole volume ratio). . That is, if the fiber / pore volume ratio is too small, there is a problem that molten sodium does not rise up the metal fiber tube due to capillary action. At this time, the amount of retained sodium is large. On the other hand, if the fiber / pore volume ratio is large, it is possible to ensure that sodium rises due to the capillary phenomenon, and to hold the sodium in the metal fiber tube when the solid electrolyte tube cracks, preventing a rapid reaction of sodium and sulfur. On the other hand, there is a problem that the amount of retained sodium decreases.

A good fiber / void volume ratio is between 0.85 and 0.98. If metal fibers are packed in a sodium container in order to secure such a volume ratio, the above-described problem of breakage of the solid electrolyte occurs. It is also possible to make the metal fibers into a plate shape and arrange them in a sodium container, but there is a possibility that a gap may occur between the solid electrolyte tube and the plate made of metal fibers, which is not sufficient for the solid electrolyte. The wettability of sodium cannot be ensured.

Therefore, in the present invention, a cathode portion made of a cathode active material containing sodium as an essential component and an anode portion made of an anode active material containing sulfur or sodium polysulfide as an essential component have a solid state having conductivity for sodium ions. In a sodium-sulfur secondary battery in which a mat made of metal fibers is loaded in the cathode portion, the contacts between the fibers of the mat are diffusion-bonded, and the volume of the pores in the mat is determined. A sodium-sulfur secondary battery characterized in that a fiber / pore volume ratio representing a fiber volume ratio is 0.85 to 0.98.

[Action]

According to the configuration of the present invention, only the contact points of the metal fibers are arranged, for example, in a sodium container by a mat bonded by diffusion bonding. Therefore, the metal fiber and the solid electrolyte are formed by the springback force having the mat. The contact with the tube can be sufficiently maintained. At the same time, the metal fiber is mat-shaped as compared with the case where the metal fiber is forcibly filled, so that a predetermined fiber / hole volume ratio can be secured without applying internal pressure to the solid electrolyte tube, and the solid electrolyte tube may be cracked. There is no.
In addition, a predetermined fiber / pore volume ratio can be designed when the mat is formed, so that the supply of sodium and a predetermined holding amount can be ensured.

As a result, it is possible to improve the utilization rate of sodium, prevent abrupt reaction when the solid electrolyte is damaged, and prevent the solid electrolyte from being damaged.

The metal fibers used in the present invention need to have sodium resistance. As such, for example, iron or iron alloy (stainless steel / Fe-Ni alloy,
Fe-Cr alloy) can be used. These fibers are pressurized to a predetermined fiber / hole volume ratio, and then heated in a vacuum or an inert gas to diffuse and bond the fiber joints, that is, the contact points between the fibers, to obtain an elastic stress. Can be formed. By adjusting the degree of heating, pressure, and fiber diameter, the fiber / hole volume ratio of the mat can be arbitrarily designed.

〔Example〕

Next, an embodiment of a sodium-sulfur secondary battery according to the present invention will be described with reference to the drawings.

FIG. 1 is a sectional view showing an embodiment of the present invention. First
In the figure, sodium 1 is held in a cathode container 7. On the other hand, the sulfur 3 impregnated in the carbon mat
It is held in the anode container 8. Sodium and sulfur are opposed to each other via a solid electrolyte tube 2 composed of a β ″ -aluminum bag tube. A sodium injection tube 9 is provided at the upper end of the battery, and sodium is supplied into the cathode container 7. The cathode container 7 is made of iron.

The solid electrolyte tube 2 is glass soldered to α-alumina 6 for electrical insulation, and the anode container 8 and the cathode container 7 are respectively joined to α-alumina 6 by thermal pressure welding. The cathode and anode are vacuum-sealed, and a cathode terminal 10 is welded to the upper end of the battery, and an anode terminal 11 is welded to the lower part.

A protective tube (for example, made of stainless steel) 5 having a small hole is inserted and arranged in the solid electrolyte tube 2 which is a bag tube.

There is no gap between the protective tube 5 and the solid electrolyte tube 2,
A mat 4 of metal fiber having sodium resistance is arranged.

As described above, in this mat, only the joints of the fibers are crushed at a constant pressure, and the joints have a joint surface due to intermetallic diffusion limited to the joints.
It is a 0.98 mat.

This mat is in contact with the solid electrolyte tube 2 by the force of the springback, and can ensure the wettability of β ″ -alumina surface with sodium. At the same time, a predetermined amount of pores is obtained by the mat. Therefore, no internal pressure is applied to the solid electrolyte tube 2.
Therefore, it is possible to prevent the solid electrolyte tube from being damaged and from spreading the damage.

The method of manufacturing the diffusion bonding mat 4 is described in
A flat mat shape formed by pressing and heating a set of stainless steel thin wires having a square cross section shown in -42703 described in an unoxidized atmosphere or vacuum can be used. However, it can also be manufactured by other methods.

The diffusion bonding mat configured as described above is wound so as to cover the protective tube 5 when the sodium-sulfur battery of this embodiment is manufactured, and the mat is inserted into the solid electrolyte tube 2. Therefore, since the battery assembly process can be facilitated,
The economic effect is great.

In addition to the method of inserting a mat between the protective tube and the solid electrolyte tube 2 in this manner, for example, a solid electrolyte bag tube-shaped ceramic (for example, α-Al ₂ O ₃ ) is used, and a fine wire is packed in the inside thereof. By pressing and heating, a mat that does not require rework for insertion can be formed. Further, a thin wire may be wound around the protective tube 5 and diffusion bonding may be performed by heating the protective tube together with the protective tube so that the mat is integrally formed with the protective tube.

The diffusion bonding mat has a fiber / pore volume in order to maintain a required amount of sodium and to prevent a rapid reaction between sodium and sulfur when the solid electrolyte tube is cracked, and to secure the movement of sodium by capillary action. Ratio 0.85-0.98
It is desirable that Such a mat has a fiber diameter of 2 to 5 μm and a thickness of 2 to several mm so that the above fiber / hole volume ratio can be achieved. This mat is desirably inserted over the entire sodium electrode. In other words, if the solid electrolyte tube is broken and the mat is inserted all over, regardless of the position of the broken part,
This is because a rapid reaction between sodium and sulfur can be prevented. In addition, since a predetermined fiber / pore volume ratio can be obtained by such a mat, a portion for protecting Na + spill can be provided.
The function of the Na + retention part can be divided.

As described above, the fiber / void volume ratio of the mat 4 is 0.85
The above is desirable. If the volume ratio is small, it is difficult to supply sodium. However, in order to prevent a rapid direct reaction between sodium and sulfur when the solid electrolyte is damaged, it is necessary to reduce the outflow rate of sodium. The minimum supply amount of sodium may be an amount sufficient for the sodium ions flowing through the solid electrolyte, but this would result in a large internal resistance. Although it depends on the diameter and thickness of the fiber constituting the mat, the outflow rate of sodium can be expected to be at least 10 times the minimum amount when the fiber diameter is 5 μm, the thickness of the mat is about 2 mm, and the fiber / pore volume ratio is 0.90. However, when a wool-like substance with a fiber diameter of 3 to 4 μm (SUS316) is packed, the fiber / hole volume ratio is increased at a pressing pressure of 1 kg / cm ^2.
0.98. If an attempt is made to increase the volume ratio more than this, a large pressure is required for the solid electrolyte tube 2, which makes assembly extremely difficult. On the other hand, if the above-mentioned mat is used, the mat can be arranged between the protective tube 5 and the solid electrolyte tube 2 by a simple operation.

The diffusion bonding mat is usually made of iron or iron alloy fiber. However, if metal fibers having resistance to molten sodium and capable of diffusion bonding having a specific gravity of about 2 are used, the battery becomes lighter in weight and energy density is improved as compared with the case where iron or iron alloy fibers are used. Furthermore, if the surface of the carbon fiber is treated, the wettability with molten sodium is improved, and furthermore, if this fiber aggregate can be formed into a mat having a fiber / void volume ratio of 0.98 or less by the same method as diffusion bonding, Energy density can be improved as compared with the case where iron or iron alloy fibers are used. As a heating method for forming the mat, in addition to heating in the furnace, electric heating and a combination of heating in the furnace and electric heating can also be used.

The thinner the diffusion bonding mat 4 is, the more effective it depends on the amount of sodium retained. If the thickness is thin, the weight of the battery is reduced and the energy density is improved.

 Next, specific examples will be described.

(Example 1) A stainless steel fiber (SUS316) having a wire diameter of 3 to 4 µm was diffused and bonded to a solid electrolyte tube (inner diameter 16 mm) using a mat.
Fillers having a pore volume ratio of 0.80, 0.85, 0.95, 0.98, 0.99 were filled. The fiber / void volume ratio can be determined as follows.

V ₀ is the volume when the mat is filled W is the weight of the filled metal fiber, Δ is the density of the fiber material. Next, when the utilization rate of Na was evaluated in a Na‖Na cell,
At a fiber / pore volume ratio of 0.80, the sodium wicking effect was not sufficient, and the sodium utilization was only 82%. On the other hand, when a mat having a fiber / void volume ratio of 0.99 is used, the flowability of sodium is extremely poor, and the internal resistance is 1.25 times that of a fiber / void volume ratio of 0.95. Usage rate was only 85%. When a cycle test was performed at a current density of 1 A / cm ² using a mat having a fiber / hole volume ratio of 0.85, 0.95, or 0.98, it was operated stably up to 1000 Ah / cm ² .

(Example 2) A fiber / hole volume ratio of 0.96 was obtained using a mat in which stainless steel fibers having a wire diameter of 3 to 4 μm were diffused and bonded in a solid electrolyte (inner diameter 16 mm).
Was inserted and a charge / discharge test was performed in a Na / β ″ -Al ₂ O ₃ / S cell. When an excessive current was applied to damage the β ″ -Al ₂ O ₃ , the cell temperature increased. Did not cause major damage below 160 ° C.

Example 3 Using the same β ″ -Al ₂ O ₃ as in Example 2, a fiber diameter of 3
A mat having a fiber / hole volume ratio of 0.958 obtained by diffusion bonding of SUS316 of ４4 μm in a vacuum furnace was inserted. Na in the Na に て Na cell
When the utilization rate was measured, it was 98% or more. Current density
In the 1 A / cm ² cycle test, β ″ -Al up to 1200 Ah / cm ²
_No breakage of ₂ O ₃ was observed.

(Example 4) A stainless steel wool having a fiber diameter of 4 to 5 µm was wound around the outside of a SUS304 safety tube having an outer diameter of 30 mm, and the outside was held down with an α-alumina ceramic tube (inner diameter of 54 mm).
Heated at ° C. At this time, the fiber / void volume ratio of the fiber layer was 0.
94. After cooling and replacing with argon gas, the safety tube and mat are integrated and the fiber / hole volume ratio of the mat part is
0.948. This was inserted into a β ″ -Al ₂ O ₃ tube with an inner diameter of 54 mm, and a Na‖Na cell was assembled to measure the utilization rate of Na.
98.2%. Further, the cell was extremely easy to assemble, and the operation of degreasing and cleaning of the mat was completed in one step.

(Example 5) Fiber / pore volume ratio produced in the same manner as in Example 4
A Na / β ″ -Al ₂ O ₃ / S cell was assembled using a protective tube with a mat of 0.92. A charge / discharge test was performed at 350 ° C. and a current density of 0.1 A / cm ^{2, and} it was stable up to 1000 Ah / cm ² Performance.

 Next, a comparative example will be described.

(Comparative Example 1) A wool β ″ -Al ₂ O ₃ tube made of SUS316 having a fiber diameter of 3 to 4 μm was packed so as to have a fiber / void volume ratio of 0.92, and Na / β ″ -A
The l ₂ O ₃ / S cell was assembled. 350 ° C., was subjected to charge and discharge test at a current density of 0.1 A / cm ^2, the solid electrolyte tube is damaged at 200 Ah / cm ^2.

(Comparative Example 2) Wool made of SUS316 having a fiber diameter of 3 to 4 µm was packed in a β ″ -Al ₂ O ₃ tube so as to have a fiber / hole volume ratio of 0.98, and Na / β ″.
-An Al ₂ O ₃ / S cell was assembled. When overcharging was performed and the cell was damaged artificially, the cell temperature rose to 2100 ° C. This is due to the rapid reaction between sodium and sulfur, and no suppression of the reaction between sodium and sulfur could be recognized.

〔The invention's effect〕

As described above, according to the present invention, since the joints of the fibers of metal fibers are joined in the sodium electrode, and the mat having a fiber / void volume ratio of 0.85 to 0.98 is inserted, the solid electrolyte The metal fiber can be inserted and arranged in the sodium electrode without applying internal pressure to the tube. Therefore, it is possible to prevent the solid electrolyte tube from being damaged and from spreading the damage. Furthermore, since the contact between the solid electrolyte tube and the metal fibers can be kept good by the springback force of the mat, the life of the battery can be greatly improved while the utilization rate of sodium can be increased.

[Brief description of the drawings]

FIG. 1 is a sectional view showing one embodiment of a sodium-sulfur battery according to the present invention. 1 ... molten sodium, 2 ... solid electrolyte, 3 ... sulfur, 4 ... diffusion bonding mat, 5 ... protective tube, 6 ... α-
Alumina, 7: cathode container, 8: anode container, 9: sodium injection tube, 10: cathode terminal, 11: anode terminal.

 ──────────────────────────────────────────────────の Continued on the front page (72) Inventor Hiroshi Tobita 4026 Kuji-cho, Hitachi City Inside Hitachi, Ltd.Hitachi Research Laboratory, Ltd. (72) Inventor Kazuki Fujita 4026 Kuji-cho, Hitachi City Hitachi, Ltd.Hitachi Research, Hitachi Ltd. In-house (72) Inventor Sadao Mori 2-4-1 Nishi-Atsujigaoka, Chofu City Tokyo Electric Power Co., Inc.Technical Research Laboratory (72) Inventor Masao Ogino 2-4-1 Nishi-Atsujigaoka, Chofu City Tokyo Electric Power Company Technical Research Institute ( 56) References JP-A-60-12680 (JP, A) JP-B-54-42703 (JP, B2)

Claims (3)

(57) [Claims] 1. A solid material comprising a cathode portion made of a cathode active material containing sodium as an essential component and an anode portion made of an anode active material containing sulfur or sodium polysulfide as an essential component have conductivity for sodium ions. In a sodium-sulfur secondary battery provided with an electrolyte as a boundary and a mat made of metal fibers loaded in the cathode portion, contacts of each fiber of the mat are diffusion bonded, and pores in the mat are provided. A sodium-sulfur secondary battery, wherein a fiber / hole volume ratio representing a ratio of a fiber volume to a volume of the fiber is 0.85 to 0.98. 2. The sodium-sulfur secondary battery according to claim 1, wherein said mat is formed by diffusion bonding of fibers of iron or an iron alloy in a vacuum or an inert gas. 3. The sodium-sulfur secondary battery according to claim 1, wherein said mat is made of 2 to 5 μm fibers.