The present teachings include an electrochemical cell including an anode, a cathode, an electrolyte, a separator disposed between the cathode and anode, and a housing containing the anode, cathode, electrolyte, and separator. The separator can include a first sheet consisting essentially of a single layer material and a second sheet distinct from the first sheet. The second sheet can include an inner microporous layer laminated between two more outer layers. In some cells, the inner layer can have a transition temperature between a porous configuration and a substantially non-porous configuration that is between about 80 degrees C. and 150 degrees C., and in which the two more outer layers maintain their structural integrity to at least about 10 degrees C. greater than the first layer transition temperature.