A bilayer conducting polymer composite separator provides overcharge protection for high voltage rechargeable lithium batteries. The polymer with the higher oxidation potential is placed in contact with the cathode and sets the overcharge protection limit, while the lower voltage polymer completes the reversible shunt and protects the high voltage polymer from degradation due to instability at the anode potential. The passage of an oxidation front through such a polymer bilayer was visualized using a transparent thin- film cell, and the approach was successfully used for internal protection of LiFePO4/ Li cells and external protection of LiNi0.8Co0.15Al0.05O2/ Li cells. (c) 2005 The Electrochemical Society.