The decrease of ionic conductivity of polymeric separators in high-power Li-ion cells, which were cycled or stored at elevated temperatures, was accompanied by dramatic changes in separator surface morphology. The source and nature of polymer separator degradation in high-power Li-ion batteries have been studied. We attributed the observed porosity loss to a deposit, which precipitated onto the separator surface from the electrolyte and clogged separator pores. This deposit resulted from a homogenous decomposition process of the LiPF6-ethylene carbonate-ethyl-methyl carbonate electrolyte, which was significantly accelerated at elevated temperatures. The electrolyte decomposition products consisted of lithium halophosphates and displayed very strong fluorescence. They contributed to the solid electrolyte interphase layers on both cathode and anode where they underwent further oxidation or reduction reactions, respectively. (C) 2004 The Electrochemical Society.