A separator for a battery that includes a first polymer to provide structural support and a second polymer mixed with the first polymer. The second polymer provides an open channel for ionic transport through the separator. The separator also includes a third polymer interspersed with the first polymer and the second polymer. The third polymer is an insulator when a potential in the battery is less than a switching voltage, and is a conductor when the potential in the battery is greater than the switching voltage.
 The subject matter described herein was supported in part by the Department of Energy under Grant No. DE-FG02-05ER84249 and FA9300-05-M-3104. The U.S. Government may have certain rights in the technology.