Al- or Mg-substituted LiCo1-zMzO2 (M = Al or Mg) (z = 0, 0.05, 0.1) samples were synthesized by a coprecipitation-based method followed by heating with LiOH center dot H2O at 500 degrees C for 3 h and again at 900 C for 3 h. Electrochemical studies showed that Al or Mg substitution reduces the initial discharge capacity of LiCo1-zMzO2 cells but does not strongly impact their cycle life. The effects of Al or Mg substitution on the thermal stability of charged LiCo1-zMzO2 were studied by accelerating rate calorimetry and indicate that Al substitution is far more effective than Mg substitution in improving the thermal stability of LiCo1-zMzO2. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3223910] All rights reserved.