A method for manufacturing battery separators for use in a lithium-ion battery containing non-aqueous electrolytes, producing batteries being resistant to thermal runaway and explosion, includes the steps of preparing a dryblend comprising two UHMW Polyethylenes and Calcined Kaolin, feeding said dry-blend into an extruder, melt-kneading said dry blend in the extruder while feeding mineral oil, making a solution from a die into the form of a sheet, using casting rolls, thereby cooling the solution down, producing a thick gel sheet, stretching the gel sheet in both machine and transverse directions, producing a 20 micron thick gel sheet containing oil thereby, extracting the oil by use of a solvent and drying the film, heat-setting the film and producing a microporous membrane.