JPH0240866A - Sodium-sulfur battery - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a sodium-sulfur battery, and particularly to a negative electrode structure with high safety performance and high efficiency.

[Conventional technology]

A conventional cathode structure for a sodium-sulfur battery is described in Japanese Patent Application Laid-Open No. 59-51482.

This battery has a double bag tube structure, and since sodium is supplied to the surface of the solid electrolyte tube through a hole in the bottom of the metal bag tube that also serves as a current collector, the area between the solid electrolyte tube and the metal bag tube is in a vacuum state. has been done. The inner metal bag tube has a sulfur electrode (
This is provided to keep the amount of sodium that reacts with sulfur that enters the sodium electrode (cathode) from the anode (anode) or with high sulfide sodium with a high sulfur number to a low level, and the battery container does not melt due to the heat generated corresponding to the amount of reacting sodium. Because of this purpose, it is called a safety tube.

During charging and discharging of a battery, sodium ions move through the solid electrolyte, and it is known that the solid electrolyte gradually changes in quality and eventually forms many hair cracks inside. Damage to the battery occurs when arc cracks are connected to these, and the sulfur electrode side and the sodium electrode side are communicated through the crack. In a typical sodium-sulfur battery, the pressure on the sulfur side is higher than the sodium side unless special pressure adjustment is performed, and sulfur is injected into the sodium electrode through cracks generated by this pressure difference.

When sodium and sulfur come into contact, heat is generated, but the reaction product is low-sulfide sodium, which has a very high melting point.

For example, the melting point of NazS is 1170°C, and when the amount of reaction is small, after the direct reaction of Na and S, the reaction product contacts a substance with a large heat capacity at a low temperature and solidifies. In actual battery structures, NazS or NazSz is formed to fill cracks that occur in the solid electrolyte, inhibiting further flow of sulfur or sodium, and the reaction upon failure often ends. However, depending on the size of cracks that occur when the solid electrolyte is damaged, there may be cases where a reaction amount of sulfur is supplied that will raise the local temperature to continue the reaction while leaving the reaction product in a liquid or gaseous state. The exothermic reaction continues until all reactants in the vicinity of the damaged area have reacted. During this time, the temperature of the battery continues to rise, the internal pressure increases, and in the worst case, the battery container melts and the active material evaporates into the surroundings.

Keeping the distance between the solid electrolyte and the metal bag tube inside the solid electrolyte tube narrow as in the prior art reduces the absolute amount of active material that contributes to the direct reaction that occurs at the time of breakage, and allows the space where the direct reaction occurs to occur more quickly. This structure is favorable for safety because it has the effect of filling with low sulfide, which is a reaction product.

However, this structure alone does not have sufficient resistance to high-temperature corrosion of metal by sodium polysulfide, and poses a safety problem, particularly as a cathode structure for large batteries. The corrosive effect of sodium polysulfide becomes particularly pronounced at high temperatures. At high temperatures of 600° C. or higher, most metals are corroded, and when the temperature exceeds 800° C., the rate of corrosion increases, and even corrosion-resistant materials such as stainless steel develop corrosion holes in a short period of time. On the other hand, as the size of the battery increases, the anode section that separates the solid electrolyte, which generates heat during breakage, and the positive electrode container wall, which acts as a heat dissipation section, becomes thicker, and as a result, the temperature of the reaction section further increases for the same amount of reaction. For this reason, the effects of high-temperature corrosion become greater in larger batteries.

[Problem that the invention seeks to solve]

In the conventional technology, no consideration was given to ensuring stability in the event that a hole occurs in the safety pipe due to moisture corrosion resistance, and when a large battery breaks, the safety pipe melts and sulfur and high-sulfide sodium are released inside the safety pipe. There was a safety issue in that batteries could be destroyed by entering the environment, and corrosive active materials would be scattered around.

The purpose of the present invention is to improve the corrosion resistance of the safety tube, and to create a battery structure that prevents rapid exothermic reactions even if a melt hole is formed in the stability tube due to high-temperature corrosion, without deteriorating battery performance. It is about realization.

[Means for solving problems]

The present invention provides a cylindrical bag tubular solid electrolyte with a safety tube having pores at the bottom and holding the molten metal sodium therein, and a gap between the safety tube and the solid electrolyte and an outer side inside the safety tube. It is characterized by being filled with a flow resistance member and an inner flow resistance member.

[Effect]

The flow rate of sulfur flowing into the sodium electrode through the crack is lowered by the effect of the viscous flow by the outer flow resistance member, which has the effect of reducing the reaction rate and heat generation rate, which has the effect of keeping the temperature of the damaged part low. Furthermore, even if the safety tube melts due to high-temperature corrosion, the inner flow resistance member inside the safety tube reduces the mixing rate of the sodium inside the safety tube and the sodium sulfide outside the safety tube, thereby suppressing a sudden reaction. In particular, a large amount of sulfur has already been consumed by the corrosion reaction between the outside of the safety tube and the safety tube, so the sulfur molar ratio of the inflowing active material is small at the time it enters the safety tube, and the sodium content inside the safety tube is gradual. Assimilation of the reactants occurs due to the reaction, and the direct reaction converges inside the battery as well.

With the above functions, the inner and outer flow resistance members arranged inside and outside the safety pipe have the effect of literally acting as a flow resistance, but the outer flow resistance member on the outside of the safety pipe has a porous structure, so the surface tension It can also be used for the purpose of holding. Since the surface tension of sodium is constant, the pressure difference corresponding to the difference in sodium height inside and outside the safety tube is proportional to the difference in local curvature of the sodium surface at the sodium liquid level. This local curvature of sodium is almost inversely proportional to the average pore diameter of the porous structure, so if the average pore size of the porous structure outside the safety tube is made smaller than the average pore diameter inside the safety tube, the same effect as capillarity can be achieved. Since sodium is smoothly supplied from inside the safety tube to the surface of the solid electrolyte, a highly efficient battery with low internal resistance can be manufactured without using means such as vacuum suction. In addition, when the outer flow resistance member outside the safety tube has high wettability with respect to sodium, the entire surface of the solid electrolyte is uniformly contacted with sodium. Therefore, the current density of sodium ions throughout the solid electrolyte becomes uniform, and the accompanying deterioration of the solid electrolyte is uniform, so there is no concentration of internal stress due to material changes, and hair cracks occur in the electrolyte. At the final stage of deterioration, cracks are prevented from expanding due to stress, which reduces the size of initial cracks at the time of failure.

〔Example〕

An embodiment of the present invention will be described below with reference to FIG. Molten metal sodium 1 containing a cathode active material is sealed in a space formed by a cathode container 2 and a solid electrolyte tube 3. The space formed by the anode container 6 and the solid electrolyte tube 3 is impregnated with graphite felt material 5 in which sulfur 4, which is an anode active material, is impregnated with a graphite felt material 5 for the distribution of exchanged electrons generated or absorbed during battery reactions. is enclosed in. The cathode container 2 and the anode container 6 are thermo-pressure bonded to α-alumina 7, which has insulating properties against both sodium ion conduction and electron conduction, via an aluminum insert material 8. As the material for the solid electrolyte tube 3, β"-alumina is used because of its good sodium ion conductivity.
-Alumina is joined by glass solder.

Inside the solid electrolyte tube 3, a bag-shaped safety tube 9 made of stainless steel 316 material, which is electron conductive, is provided, and inside the safety tube 9, an inner flow resistance member 1o made of a felt material made of alumina fiber is provided. In the space between the safety tube 9 and the solid electrolyte tube 3, an outer flow resistance member 11 made of a felt material made of stainless steel or other wire material pressurized at about 900° C. and having a pore density of about 95% is disposed. ing.

The sodium 1 stored inside the safety tube 9 is
It flows to the outside of the safety tube 9 through the pore 12 opened at the bottom. In the space between the safety tube 9 and the solid electrolyte tube 3, sodium 1 is transferred to the solid electrolyte tube 3 due to capillary phenomenon or vacuum suction.
It is now being supplied throughout the area. It is particularly important here that no air bubbles are present within the outer flow-resisting member 11. This is due to the presence of air bubbles in the solid electrolyte tube 3.
This is because when a portion of the surface that is not wetted by sodium 1 occurs, the current density in that portion becomes non-uniform, causing local electrolyte deterioration and shortening the life of the battery. The generation of such bubbles can be prevented by evacuating the inside of the cathode container 2 when injecting the sodium 1 into the cathode container 2 to remove the gas held in the inner and outer flow resistance members 10.11. At this time, the flow resistance member 10.1 such as moisture or oil
If the substance adsorbed on 1 is not removed by raising the temperature and drawing a sufficient vacuum, it will cause bubbles to occur when the battery operating temperature is raised to 330° C. or higher, so care must be taken. The optimum temperature for deaeration is about 400°C, but if deaeration is performed with a large exhaust capacity and the adsorbed oil has been sufficiently decomposed by baking, it may be about 150°C.

If both the flow resistance members 10 and 11 inside and outside the safety tube 9 have a porous structure, they both absorb sodium by capillary action. However, due to the surface tension of sodium, the ability to absorb sodium through the porous structure is
It is equal to the surface tension of sodium plus the curvature of the sodium surface in minute areas at the sodium liquid level. In addition, since the liquid level curvature is approximately equal to the pore diameter of the porous structure, if the average pore diameter of the porous structure outside the safety tube 9 is made smaller than the average pore diameter of the porous structure inside the safety tube 9, it is possible to The liquid level becomes higher than the liquid level inside the safety pipe 9. For example, when wires with a wire diameter of 8 μm are used inside and outside the safety tube 9, and the pore density on the outside of the safety tube 9 is 95% and the pore density on the inside is 99%, the sodium level on the outside is the highest, and the sodium level on the inside is approximately 60a11 higher.

Next, the effect will be explained. Sodium supplied to the surface of the solid electrolyte tube 3 is ionized by the surrounding electric field and migrates in the solid electrolyte in the form of sodium ions. When a load is connected to a circuit outside the battery, electrons generated in the cathode due to ionization of sodium 1 are quickly supplied through the external circuit from the cathode container 6 into the anode through the graphite felt 5. Since the electrons generated during ionization are released to the outside, the sodium ions approach the sulfur atoms in the anode portion that have passed through the solid electrolyte tube 3 without causing interfacial polarization. At this time, the sulfur atom absorbs two electrons supplied from the graphite felt 5, becomes a divalent negative ion, and reacts in a manner that neutralizes the two sodium ions. The charge exchange reaction at this time occurs with an electromotive force of about 2V.

The above series of reactions occur continuously, and the discharge reaction of the battery progresses.

During charging, electrons are liberated from sodium sulfide due to an externally applied electric field, and sodium ions return to the cathode through sodium polysulfide and the solid electrolyte.
It recombines with electrons supplied from the anode through an external circuit. Thereafter, the sodium that has no longer entered the area between the solid electrolyte tube 3 and the safety tube 9 is returned into the safety tube 9 through the pore 12 at the bottom of the safety tube 9.

As mentioned above, the charging and discharging reactions of batteries involve the movement of sodium ions within the solid electrolyte. In this process, sodium ions in the solid electrolyte crystal jump from one crystal to the next, but as electricity is applied, a crystal structure different from the original state is formed with a certain probability. At this time, the solid electrolyte undergoes alteration, such as a change in the lattice constant of the crystal, and cracks occur within the solid electrolyte due to the generation of stress. When this crack grows and connects the sulfur bridge (anode) and the sodium electrode (cathode), sodium 1 and sulfur 4 react directly. At this time, all the energy generated by the reaction is consumed as thermal energy, so the temperature around the crack increases.

Normally, the movement of active material through the crack occurs in the form of sulfur 4 entering the sodium electrode. This means that even when both the anode and cathode are sealed in vacuum, the saturated vapor pressure of sulfur 4 is 300.
This is because the temperature is about 0.1 atmosphere or more at temperatures above ℃. Also,
Even when replacing argon with atmospheric pressure, sodium 1 moves to the sulfur bridge due to discharge, which causes the pressure at the sodium electrode to drop. Furthermore, at the time of initial damage,
Even if sodium 1 initially reacts with sulfur 4, the increased pressure of sulfur 4 generated by the temperature rise on the sulfur electrode side causes sulfur 4 to flow back through the crack, so sulfur 4 still flows into the solid electrolyte.

If the cracks generated in the solid electrolyte are small, the above direct reaction will be blocked by the low sodium sulfide produced by the reaction, and no further direct reaction will occur. When the crack becomes larger than a certain level, the amount of active material contributing to the reaction increases, and the reaction continues due to the mixing of the sodium 1 between the solid electrolyte and the safety tube 9 and the sulfur 4 supplied from the outside.

At this time, if there is no outer flow resistance member 11 in the area between the safety tube 9 and the solid electrolyte tube 3, mixing will occur quickly,
A large amount of heat is generated in a short period of time, leading to damage.

The outer flow resistance member 11 in the space between the solid electrolyte tube 3 and the safety tube 9 acts as a flow resistance at the time of breakage as described above. Therefore, this flow resistance slows down the progress of the reaction and increases the corrosion resistance of the safety tube. Furthermore, during normal battery operation, the solid electrolyte surface must be uniformly wetted with sodium 1, so the material for the outer flow resistance member 11 must be wettable with sodium 1. Therefore, as the material used for this part, a thin metal wire such as stainless steel is used in consideration of its corrosivity against sodium 1. Especially when using high-purity materials, the solid electrolyte absorbs oxygen generated as it deteriorates.
This is more preferable in that the active material sodium 1 is kept in a clean state and the life of the battery is improved.

When using sodium-sulfur batteries for power storage, increasing the capacity per cell is particularly important from an economic standpoint. However, when increasing the size of the battery structure, the distance from the solid electrolyte tube 3 to the anode container 6, where breakage occurs, increases, and the heat generated at the breakage part cannot be released to the outside quickly. As a result, the temperature of the damaged part increases, and the safety tube 9 melts due to high-temperature corrosion, causing the sodium 1 inside the safety tube 9 to melt.
and external low sulfides may be mixed. At this time, if the mixing speed is high, a rapid reaction may occur and the battery may be damaged. In particular, when a hole occurs above the sodium level inside the safety tube 9, liquid sulfur 4 falls by gravity into the sodium 1, which can generate large temperatures and pressures. In order to prevent the occurrence of such a situation, an inner flow resistance member 10 is disposed inside the safety tube 9 to increase the flow resistance inside the safety tube 9 and to suppress the mixing speed of the active material to a low level. There is.

It is desirable that the material of the inner flow resistance member 1o disposed within the safety pipe 9 has corrosion resistance against sodium polysulfide. This is because it is necessary to predict in advance the high temperature corrosion of the material at the ambient temperature that will cause the safety tube 9 to melt. There are ceramic materials such as alumina and zirconia that exhibit corrosion resistance against high-temperature sodium polysulfide, and by using short fibers made of these materials, the inner flow resistance member 10 having a porous structure can be formed. .

〔Effect of the invention〕

According to the present invention, since the flow resistance of sulfur is increased by the outer flow resistance member, rapid mixing and reaction of sodium and sulfur can be suppressed, and thereby heat generation can be suppressed to a minimum, thereby improving the corrosion resistance of the safety pipe. can be improved. Furthermore, even if a hole is made in the safety tube, the sulfur that has flowed into the safety tube will mix with sodium gradually rather than quickly due to the internal flow resistance, resulting in rapid heat generation inside the safety tube. Reactions can be prevented and safety can be improved. By using the present invention, it is possible to manufacture a sodium-sulfur battery in which the axial length of the reaction region is 60a11 or more.

[Brief explanation of the drawing]

FIG. 1 is a longitudinal sectional view showing one embodiment of the present invention. 1... Molten metal sodium, 2... Cathode container, 3...
...Solid electrolyte tube, 4...Sulfur, 5...Graphite felt, 6...Anode container, 7...α-alumina, 8...
・Aluminum insert material, 9...Safety pipe, 1o...Inner flow resistance member, 11...Outer flow resistance member, 12.
··pore.

Claims (1)

[Claims] 1. A sodium-sulfur battery using molten metallic sodium as a cathode active material, molten sulfur as an anode active material, and a sodium ion-permeable solid electrolyte as an electrolyte disposed at the boundary between the two active materials. A safety tube having a pore at the bottom and holding the molten metal sodium therein is provided in the solid electrolyte in the form of a cylindrical bag tube, and an outer flow is provided in the gap between the safety tube and the solid electrolyte and in the stabilization tube. A sodium-sulfur battery characterized by being filled with a resistance member and an inner flow resistance member. 2. In claim 1, the outer flow resistance member is formed of a material that is wettable with molten metal sodium, and the average pore diameter of the gaps in the outer flow resistance member is the average pore diameter of the gaps in the inner flow resistance member. Smaller sodium-
sulfur battery. 3. A sodium-sulfur battery according to claim 1 or 2, wherein the inner flow resistance member is made of a material having corrosion resistance against sodium polysulfide.