PRODUCTION OF FLEXIBLE ENVELOPE FORMED OF FLEXIBLE SHEETS HEAT SEALED TOGETHER ALONG EDGE PORTIONS THEREOF, SAID FLEXIBLE ENVELOPE HAVING HIGH RESISTANCE TO ALKALI ATTACK AT ELEVATED TEMPERATURE OVER AN EXTENDED PERIOD, THE HEAT SEALED BOND BEING RESISTANT TO DELAMINATION OF THE BONDED SHEETS AND PROVIDING A STRONG BOND, BY HEAT SEALING SUCH FLEXIBLE SHEETS, E.G. FORMED OF A CHRYSOTILE ASBESTOS (PURE FUEL CELL GRADE), EMPLOYING A SEALANT IN THE FORM PREFERABLY OF A MIXTURE OF NEOPRENE AND POLYSULFONE. THE RESULTING FLEXIBLE ENVELOPE CAN BE COATED WITH A FLEXIBLE SUBSTANTIALLY INORGANIC FILM BY DIP-COATING 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 INORGANIC POLYMER, E.G. POLYPHENYLENE OXIDE, DISSOLVED IN A SUITABLE SOLVENT SUCH AS CHLOROFORM. THE RESULTING POROUS CHRYSOTILE ASBESTOS SEPARATOR ENVELOPE HEAT SEALED AS DESCRIBED ABOVE AND HAVING A FLEXIBLE COATING THEREON AS DESCRIBED ABOVE, CAN SERVE AS AN ELECTRODE COMPARTMENT FOR A BATTERY ELECTRODE, E.G. A ZINC ELECTRODE INSERTED THEREIN, AND SUCH SEPARATOR-ELECTRODE UNIT INCORPORATED IN A BATTERY IN SIDE-BY-SIDE RELATION WITHOTHER ELECTRODES, SUCH AS SILVER ELECTODES, THE FLEXIBLE ELECTRODE COMPARTMENT BEING RESISTANT TO SHORTING BY PENETRATION OF ELECTRODE MATERIAL INTO THE SEALED EDGE PORTIONS OF THE SEPARATOR ENVELOPE.