Production of a separator, preferably in the form of a compartment, for insertion therein of a battery electrode, preferably by dip-coating a flexible porous substrate, e.g., potassium titanate paper or nylon, in a mixture preferably of a major portion of an inorganic or ceramic separator material, a minor portion of potassium titanate in short fiber form, and a minor portion of an organic polymer, e.g., polyphenylene oxide, dissolved in a suitable solvent such as chloroform. The flexible porous substrate is placed in the mixture, preferably maintained under vibration, and stirred; the substrate and applied coating are removed from the mixture, and the coating is dried to remove solvent and is cured at elevated temperature. The resulting porous separator, coated, filled or impregnated with the above mixture, has low resistivity, is smooth and uniform in thickness, and is flexible, and has high resistance to alkali and to elevated temperatures. Preferably the flexible porous substrate is first formed into a box or bag and then treated with the above mixture to form a separator. An electrode, such as a zinc electrode, is inserted in the bag either before or after coating and impregnation. Alternatively, a film or sheet of the substrate is treated with the above mixture to form a flexible separator, and the resulting flexible separator is then formed into a boxlike configuration for receiving an electrode, or is wrapped around an electrode or electrodes.