Base films for impregnation, improved impregnated products, and related methods

A dry process polymer membrane adapted for use as or in a separator for a lithium battery, including at least one mesoporous, microporous, or nanoporous impregnated membrane film, with at least one area of surfaces of pores in the polymer membrane having a first thin layer of at least a first impregnate material, comprising a polymer resin (B) as a major component therein wherein the polymer resin (B) is polyvinylidene difluoride (PVdF), polycarbonate (PC) or a combination thereof, at least an about 10% impregnation depth into the thickness of the polymer membrane, and wherein the membrane film has a polymer resin (A) as a major component;

• and,
• and, wherein the dry process polymer membrane has a composite splittiness index of greater than 140 measured where composite splittiness index is defined by the equation below where composite splittiness index is a function of the first load peak, the second load peak, TD tensile strength, MD tensile strength, and TD elongation measured during puncture strength testing:
• CSI=(A−|B−A|1.8)×C×(D×E)/106
• where:
• A=First Load Peak/(Thickness×(1−% Porosity));
• B=Second Load Peak/Thickness;
• C=TD Elongation;
• D=MD Tensile Strength;
• E=TD Tensile Strength;
• and
• where First and Second Load Peak are in units of gram-force, thickness values are in microns, MD and TD tensile strength are in gram-force, and TD elongation is expressed as percentage;
• and
• wherein the dry process polymer membrane comprises groups of pores between adjacent lamellae with the lamellae oriented substantially along a transverse direction and fibrils or bridging structures between the adjacent lamellae oriented substantially along a machine direction and an outer surface of at least some of the lamellae being substantially flattened or planar.