The shutdown function of a separator is an important factor in the safety of advanced lithium-ion batteries (ALB). When a separator without proper shutdown function is used, battery safety would depend on the thermal stability of electrode materials. Results show that thermal stability of a battery, contributed from both the anode and cathode, decreases noticeably after cycling. DSC shows that exothermicity from SEI decomposition and the reaction of the lithiated graphite and electrolyte around 140°C increases as cycle number increase; main reason is the gradual thickening of passivation film, observed through three-electrode ac impedance measurements. DSC also shows a similar trend of exothermicity for Li x CoO2 cathode. The lesser the amount of lithium (x-value) in Li x CoO2, the larger the exothermicity and the lower the decomposition temperature. Using a three-electrode system to observe the changes of open-circuit potential in Li x CoO2 cathode, thermal instability is a consequence of decreased lithium content as cycle increases.