TECHNOLOGY FOCUS - POLYMERS - Preferred Separator: The separator contains spherosilicates and/or silsesquioxanes as polyhedral oligomeric silicon-oxygen (Si-O) clusters, especially functionalized cluster units of structure 1, 2, 3 or 4:X1 = a substituent of type X or -O-SiX3;X2 = a substituent of type X, -O-SiX3, R or -O-SiR3The cluster has a molecular weight of not less than 400 g/mole. The polymer matrix contains at least one polypropylene or polyethylene. The porous film is 1-300 microns thick and has 30-80% porosity. The separator has 2 or 3 porous films, at least one of polyethylene, and has a tear strength of over 15 N/cm.Production: (claimed) The separator is made by (a) producing a polymer matrix containing clusters of formula (I), (b) producing a non-porous film and (c) providing the film with pores.Preferred Production: The polymer matrix is made simply by mixing the cluster with polymer(s), preferably dissolved or molten polymer(s). The clusters may be chemically bound in the matrix with the polymer chains, preferably by grafting. The non-porous film is made by extrusion. The film is provided with pores by (i) stretching; or (ii) dissolving a constituent of the film with a solvent, in which case the polymer matrix used in step (a) preferably contains 30-70 vol.% paraffin wax, which is dissolved with a suitable solvent in step (b). Several porous films may be laminated. EXAMPLE - An incompletely condensed silsesquioxane with 3 free hydroxyl (OH) groups was synthesized by adding 13 g (544 mmole) lithium hydroxide to a mixture of 84 ml methanol, 620 ml acetone and 16 ml water at 60degreesC, then dripping 140 ml (97% pure, 711 mmole) isobutyl trimethoxysilane into the mixture in 15 minutes, with rapid stirring. After stirring at 60degreesC for 2 days, the solution was treated with a mixture of 150 ml water and 50 ml 37% hydrochloric acid (0.6 mole) and the resultant precipitate was washed with methanol and dried, giving 64 g (78% yield) of (isobutyl)7Si7O9(OH)3 (II) as a white solid. 2.0 ml (13.1 mmole) vinyltrimethoxysilane were added to a solution of 10.0 g (12.6 mmole) (II) in 20 ml tetrahydrofuran at 20degreesC. After adding 0.5 ml tetraethylammonium hydroxide (35% aqueous solution, 1.2 mmole base, 18 mmole water), the mixture was stirred over night. The resultant turbid solution was treated with 200 ml methanol and filtered and the residue was washed with acetone. This gave 6.1 g (60% yield) of (isobutyl)7(vinyl)Si8O12 (IA) as a white powder. A mixture of 85 wt.% polypropylene and 15 wt.% polyhedral Si-O cluster (IIA) was extruded to a film, which was prestretched 20% at -196degreesC and stabilized in the stretched stage by tempering at 110degreesC. The film was then stretched by a factor of 3.2 at 130degreesC. This resulted in the formation of 1.2 microns pores. The porosity was 38%. This type of separator was preferred in a lithium-ion battery.