A systematic approach to understanding the safety fundamentals of Li-ion batteries was undertaken. Firstly, we present thermal characterization experiments of charged prismatic polymer lithium-ion batteries (PLBs). These cells, at different state of charge (SOC), were tested inside an accelerated rate calorimeter (ARC) to determine the onset-of-thermal runaway (OTR) temperatures. In addition, the thermally activated components of these cells were followed by monitoring both the impedance (at 1 kHz) and the open circuit voltage (OCV) as a function of temperature. An increase in the impedance was observed at around 133 degreesC corresponding to the polyethylene separator shutdown. Secondly, an original in situ ac impedance measurement was performed over a wide range of frequencies instead of I kHz when the battery was heated from ambient to 130 degreesC. Resulting impedance spectra were modeled using an appropriate equivalent circuit. It is concluded that the high frequency and the low frequency semicircles observed in the impedance spectra are due to processes occurring the anode/electrolyte and cathode/electrolyte interfaces, respectively. The activation energy E(a) was found in the ranges of 0.4-0.6 and 0.364.53 eV, for cathode and anode processes, respectively. In addition, it is assumed that change in the electrolyte composition is the main factor responsible for the rise in the cell impedance at high temperatures.