1. Field of the Invention
The invention relates to an electrochemical storage cell of the sodium and sulfur type with an anode and a cathode space which are separated from each other by a solid electrolyte and are bounded, at least in some regions, by a metallic housing. The cathode contains a sulfur electrode which is made of a sulfur-impregnated graphite or carbon fiber material.
2. Description of the Prior Art
Such electrochemical storage cells are highly suited as energy sources. They find application increasingly in the construction of storage batteries which are provided for the power supply of electric vehicles.
A specific example for these storage cells are those of the sodium and sulfur type, which are rechargeable and comprise a solid electrolyte of beta-aluminum oxide which separates the anode space from the cathode space. It is an advantage of these storage cells that no electrochemical secondary reactions take place during the charging. The reason for this is that only sodium ions can get through the solid electrolyte. The current yield of such sodium/sulfur storage cells is therefore near 100%. In these electrochemical storage cells the ratio of energy content to the total weight of the storage cell is very high as compared with lead storage cells since the reactants are light and much energy is released in the electrochemical reactions.
Besides the anode space which is filled with sodium, such a storage cell comprises a cathode space which is arranged in a normal storage cell between the metallic housing delineating the storage cell from the outside and the solid electrolyte. In all known storage cells, this cathode space is filled with a long-fiber graphite felt which is impregnated with sulfur. In the manufacture of the storage cells, two half shell-shaped elements are preferably formed, impregnated with sulfur and then inserted into the cathode space. The storage cells are manufactured at room temperature. For operation, the storage cells must be heated to a temperature of 350.degree. C. If a storage cell is exposed to such a temperature influence, the graphite felt expands as do, in particular, the two half-shells which are arranged in the cathode space. They expand until their end faces abut each other flush and the fibers of the one half extend beyond those of the other half, so that no space is left any more in the boundary region of the half-shells. During the discharge of storage cells, the sodium ions contained in the anode space migrate through the solid electrolyte into the cathode space and with the sulfur present form sodium polysulfide there. Due to the fact that the two half-shells formed of graphite felt now are close and touch each other, the sodium polysulfide can be distributed uniformly in the entire cathode space, especially also over the boundary surfaces of the two half-shells. If such a storage cell which contains appreciable amounts of sodium polysulfide in the cathode space, is cooled down, the sodium polysulfide solidifies to form a closed ring which firmly surrounds the solid electrolyte. The sodium polysulfide has a thermal coefficient of expansion higher than that of the solid electrolyte which is made of beta-aluminum oxide. This means that the ring formed of sodium polysulfide is shrunk onto the solid electrolyte when it gets cold. Thereby, it adheres very firmly to the outside surface of the solid electrolyte and exerts shear forces on the same in the event of temperature changes, which is caused by the different thermal coefficients of expansion of the two materials. These shear forces lead eventually to the formation of cracks in the solid electrolyte and thus to the destruction of the entire storage cell.