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 140degreesC 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 LixCoO2 cathode. The lesser the amount of lithium (x-value) in LixCoO2, the larger the exothermicity and the lower the decomposition temperature. Using a three-electrode system to observe the changes of open-circuit potential in LixCoO2 cathode, thermal instability is a consequence of decreased lithium content as cycle increases. (C) 2003 Elsevier Ltd. All rights reserved.