1. Field of the Invention
The invention relates to a method for the manufacture of an electrochemical storage cell of the sodium and sulfur type, with at least one anode space and one cathode space as well as an alkali ion conducting solid electrolyte separating the two spaces, and with an least one cathodic current collector and one electron-conducting matrix material arranged in the cathode space, as well as to a storage cell produced by this method.
2. Description of the Prior Art
Such rechargeable electrochemical storage cells with solid electrolytes are well suited for building storage batteries of high energy and power density. The solid electrolytes used in alkali/sulfur storage cells, made for instance of .beta.-aluminum oxide, are characterized by the fact that the partial conductivity of the mobile ion is very high and the partial conductivity of the electrons is smaller by multiple powers of ten. By using such solid electrolytes for the construction of electrochemical storage cells, practically no self-discharge is obtained, since the electron conductivity is negligible and the reaction substances cannot travel through the solid electrolytes as neutral particles. It is an advantage of these electrochemical storage cells of the sodium and sulfur type that no electrochemical secondary reactions occur during the charging. The reason therefor is again that only one kind of ion can travel 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 a lead storage cell, since the reaction substances are light and considerable energy is released 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. To ensure proper operation of such storage cells, the cathode is made of an electron-conducting matrix material, in the pore structure of which the melted active matter, sulfur and sodium polysulfide, can be absorbed. The resistivity of the matrix material should not exceed 10 Ohm.cm, and should be, if possible, about 1 Ohm.cm. Because of the extraordinary aggressivity of the cathodic melt, only felt-like matrix materials of the carbon type have found acceptance to date. A common feature of these felts is the structure of the fiber layers, the fibers of which are connnected to each other by needling. The principal fiber directions are parallel to the felt plane in these felts. As a result, the electric conductivity parallel to the felt plane is greater than the electric conductivity perpendicular thereto. In the manufacture of sulfur electrodes for electrochemical storage cells care must therefore be taken that in the situation where the felt is built into the cathode space with its surface parallel to the surface of the solid electrolyte, a relatively large amount of felt is used in order to obtain the desired conductivity transversely to the principal fiber direction. As a result, the felt must be heavily compressed before it is built into the storage cell, particularly into the cathode space. This leads to breakage of a large number of fibers. This, in turn, leads to a loss of elasticity of the felt. The possibility exists of prefabricating the sulfur electrode and then building it into the cathode space, or else preparing the sulfur electrode directly within the cathode space. In prefabricated sulfur electrodes, contact with the cathodic current collector is usually established by the elastic expansion of the felt used as the matrix material, after the sulfur is melted. If a rigid matrix material is used for obtaining the sulfur electrode, the making of contact with one side of the matrix material and the housing and contact of the other side of the matrix with the solid electrolyte presents a problem that has not yet been solved satisfactorily.
U.S. Pat. No. 4,169,120, disclosed an electrochemical storage cell of the sodium and sulfur type. The cathode space of this storage cell is filled with graphite fibers which are cut very stort and are mixed with a resin. The matter formed of the graphite fibers and the resin is compressed and filled into the cathode space with a predetermined density. In addition, the entire cathode charge is coked and saturated with sulfur. The matrix material used here does not have the required elasticity, so that the desired contacts between the matrix material and the solid electrolyte, as well as with the cathode current collector, are not provided. Due to this disadvantage, the storage cell does not have the required conductivity which is necessary for proper functioning of the storage cell.