Implantable battery having thermal shutdown separator

An electrochemical cell comprising:

• an anode including an active anode material in contact with an anode current collector;
• a cathode including an active cathode material in contact with a cathode current collector;
• a separator disposed between the anode and the cathode;
• an electrolyte;
• and
• a housing containing the anode, cathode, separator, and electrolyte, the separator comprising a first sheet consisting essentially of a single layer material and a second sheet distinct from the first sheet,
• wherein the second sheet consists essentially of an inner microporous layer laminated between two outer layers, and
• wherein the first sheet is separated from the second sheet by the electrolyte.

A battery comprising:

• a plurality of anode layers;
• a plurality of separator layers;
• a plurality of cathode layers;
• an electrolyte;
• wherein the cathode and anode layers are configured as a stack having one of the plurality of separator layers disposed therebetween,
• wherein the one of the plurality of separator layers includes a first sheet and a second sheet, the first sheet consisting essentially of a unitary polymeric material having an electrical resistivity of less than about 500 ohms per square centimeter, wherein the first sheet is separated from the second sheet by the electrolyte.