1. Field of the Invention
The invention relates to an electrochemical storage cell of the alkali metal and sulfur type with an anode space and a cathode space which are separated from each other by an alkali ion-conducting solid electrolyte and are bounded at least in some regions by a metallic housing, a safety insert in the anode space for receiving the alkali metal from which the alkali metal can emerge via at least one discharge opening into a safety gap.
2. Description of the Prior Art
Such rechargeable electrochemical storage cells are used in high-temperature storage batteries which find application, for instance, as energy sources in electric vehicles.
Rechargeable electrochemical storage cells with solid electrolytes are highly suitable for the construction of storage batteries with high energy and power density. In these solid electrolytes used in alkali/chalcogen storage cells which are made, for example, of beta-aluminum oxide, partial conductivity of the mobile ion is very high and the partial conductivity of the electrons is smaller by many powers of ten. By using such solid electrolytes for the construction of electrochemical storage cells practically no secondary discharge takes place since the electron conductivity is negligible and the reaction substances cannot travel through the solid electrolyte as neutral particles.
A specific example of this are rechargeable electrochemical storage cells of the sodium and sulfur type which have a solid electrolyte of beta-aluminum oxide. An advantage of these electrochemical storage cells is that no electrochemical secondary reactions occur during charging. The reason for this is that only sodium ions can go through the solid electrolyte. The current yield of such a sodium/sulfur storage cell is therefore approximately 100%. In these electrochemical storage cells, the ratio of energy content to the total weight of such a storage cell is very high as compared to lead storage cells, since the reaction substances are light and much energy is liberated in the electrochemical reaction. Electrochemical storage cells of the sodium and sulfur type therefore have considerable advantages over conventional storage batteries such as lead storage batteries.
The solid electrolyte can break if a voltage applied to storage cells is too great. The same can also occur if the solid electrolyte is over age or mechanically damaged. In such a situation, confluence and direct reaction of increased amounts of sodium and sulfur results from the break in the electrolyte. To avoid this, U.S. Pat. No. 4,247,605 provides a safety insert in the anode space which can contain the alkali metal. This safety insert has at its lower end a small discharge opening via which the sodium can enter a safety gap between the safety insert and the solid electrolyte.
A disadvantage of this arrangement is that while no sudden surge of a large amount of sodium can flow into the cathode space in the event of a break of the solid electrolyte, smaller amounts of sodium can flow always and continuously.