Separators of such cells must be resistant to corrosion and insensitive to cyclical changes of volume. Appropriate materials (e.g. BeO, ThO/sub 2/, Si/sub 3/N/sub 4/) are however not ion-conducting, and must therefore be used with an open structure. In well-known ceramic separators, the space between the electrodes has small separator material poured into it, and this causes variations in porosity and density; also the cells are loaded by the necessary shaking and vibration. The process of the invention consists of mixing the finely divided separator material and/or the electrodes with electrolyte powder and a plastic which is completely decomposed by heat. The mixture is rolled into a flat plate, and this is fitted in the cell individually or as a compound electrode/separator plate. After assembly, the whole cell is heated above the decomposition temperature of the plastic. A solution of polyisobutylene in cyclohexane is preferred, where the proportion is 2-5%. The separator material is used with two grain sizes, large particles of 2 mm diameter (40-80%) and fine particles in the range 5-60 ..mu..m (60-20%).