A multi-functional battery separator comprises two or more active separator layers deposited from different polymer solutions to form a multilayered unitary structure comprising a free-standing film, a multiplex film on one side of a porous substrate, or separate films or multiplex films on opposite sides of a porous substrate. In a preferred embodiment, the cascade coating method is used to simultaneously deposit the active separator layers wet so that the physical, electrical and morphological changes associated with the polymer drying out process are avoided or minimized. The multi-functional separator is inexpensive to fabricate, exhibits enhanced ionic conductivity and ionic barrier properties, and eliminates gaps between individual layers in a separator stack that can contribute to battery failure.