LiMnO2 is of interest as positive electrode material in Li-ion batteries because of its high theoretical specific capacity, low material cost and toxicity. However, this material suffers from a cycling-induced transformation from a layered crystal structure to a spinel-like that induces a reduction in specific capacity during the first few cycles. Computational predictions suggested this drawback could be overcome by aluminum substitution for manganese. Using a hydrothermal synthesis method, extensive aluminum substitution in o-LiMnO2 was achieved. Even at such high levels, preservation of the orthorhombic over the monoclinic polytype was found. This strategy was unsuccessful in preventing the structural evolution of the layered to a spinel phase during electrochemical cycling, as evidenced by in situ synchrotron X-ray diffraction for a material with 25% aluminum. This sample showed the best cycling performance with a first charge capacity and coulombic efficiency of 160 mAh/g and similar to 100%, respectively. The structural transformation during cycling was correlated with a rapid decay of specific capacity over ten cycles, for a retention of 67%. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.048301jes] All rights reserved.