BATTERY SEPARATOR, BATTERY INCLUDING THE SEPARATOR, AND METHOD AND SYSTEM FOR FORMING SAME

摘要

Microporous sheet product suitable for use as a battery separator and method of forming the same. According to one embodiment, the method involves forming an extrusion mixture of one or more thermoplastic polymers and a fluid having a high vapor pressure. Next, the mixture is extruded through a die head, cooled and shaped in a first vapor zone under an above-UEL condition to form a solid sheet material. Next, the sheet material is subjected to a two-step process in a second vapor zone under a below-LEL condition, the first step involving a first stretching/fluid vaporization at a higher temperature, the second step involving a second stretching/fluid vaporization at a lower temperature. The resultant sheet is then annealed, and the remainder of fluid is removed to form a sheet product having a thickness characterized by a structure of smaller and larger pore strata across its thickness.