Impedance spectra of Celgard(R) 2300 membranes that were removed from high-power Li-ion cells showed a significant rise in membrane ionic resistivity for cells that were cycled or stored at elevated temperatures. Atomic force microscopy images revealed dramatic changes in membrane surface morphology. Swelling of the membrane polypropylene fibers and the presence of particles of electrode active material in the membrane pores effectively reduced the membrane porosity, and thereby account for the membrane impedance rise and part of the cell power loss. These results not only indicate that membrane degradation can contribute significantly to Li-ion cell power loss at elevated temperatures, but also reveal mechanisms of membrane degradation. (C) 2002 The Electrochemical Society.