A battery separator for a lithium-ion secondary battery is a microporbus membrane with an adjuvant. The microporous membrane is made of a thermoplastic, and has a thickness of 25 μm or less. The adjuvant, in an effective amount, is mixed into the membrane or coated thereon. The adjuvant is a material adapted to reduce or eliminate energy concentrations around the separator. The energy concentration is sufficient to initiate a reaction of the components of the lithium ion secondary battery.