Al was substituted for Co in LiNi1/3Mn1/3Co1/3O2 by two different routes: during coprecipitation of the hydroxide precursor or afterward as AI(OH)3 introduced during solid-state sintering. The latter method avoids the formation of layered double hydroxides during the coprecipitation. but may not ensure uniform distribution of the Al atoms in the mixed cation layers. The electrochemical performance and thermal stability of Al-substituted LiNi1/3Mn1/3Co(1/3-z)AlzO2 was studied by coin cell testing and accelerating rate calorimetry tests, respectively. High resolution compositional analysis showed that the Al distribution was not uniform in material prepared by solid-state synthesis at 900 degrees C, unlike materials prepared by coprecipitation at 900 or 1000 degrees C or by solid-state synthesis at 1000 degrees C. Nonuniform Al distribution does not affect the electrochemical properties strongly, but has a strong negative impact on the thermal stability of the charged electrode materials. The solid-state sintering substitution of Al for Co can be successful if the sintering temperature is high enough to ensure a uniform Al distribution. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3184606] All rights reserved.