A microporous separator film for electrochemical cells and a method of making such films is disclosed. The microporous separator film includes an intimate mixture of an electrically insulating matrix phase and a self-switching voltage activated conductive phase, wherein the voltage activated conductive phase provides a plurality of conductive paths from a first face of the microporous separator film to a second face of the microporous separator film. The method for making the composite microporous separator film includes the steps of forming an intimate mixture of at least an insulating matrix phase and a self-switching voltage activated phase, forming a film from the mixture, and generating pores within the film.