Layered LiMn1-xMxO2 (M=Ni, Al, Mg) has been synthesized over a wide range of composition by ion-exchange from their sodium precursors and characterized. Both the lattice parameters (a and c) of the hexagonal unit cell increase with x for Ni and Mg substitutions but decrease for Al substitution. As reflected by the broad powder X-ray diffraction peaks, scanning electron micrographs show agglomeration of submicrometer-size particles. The discharge capacity of these cathode materials varies with the nature and concentration of the substituents. LiMn0.95Ni0.05O2 exhibits a specific capacity of about 220 mAh g(-1) during the initial cycling, which decays gradually to about 150 mAh g(-1) after 30 charge-discharge cycles. LiMn0.95Al0.05O2 and LiMn0.95Mg0.05O2 yield capacity values of 180 and 150 mAh g(-1), respectively. Cyclic voltammetry, galvanostatic intermittent titration, and ac impedance data corroborate the observed discharge capacity behavior of these cathode materials. (c) 2005 The Electrochemical Society.