The structures, electrochemical properties and thermal stability of Al-substituted lithium-excess oxides, Li1.2Ni0.16 Mn0.56Co0.08-yAlyO2 (y = 0, 0.024, 0.048, 0.08), are reported, and compared to the stoichiometric compounds, LiNizMnzCo1-2zO2. A solid solution was found up to at least y = 0.06. Aluminum substitution improves the poor thermal stability while preserving the high energy density of lithium-excess oxides. However, these high manganese compositions are inferior to the lithium stoichiometric materials, LiNizMnzCo1-2zO2 (z = 0.333, 0.4), in terms of both power and thermal stability. (C) 2011 The Electrochemical Society. [DOI: 10.1149/2.044202jes] All rights reserved.