LiNi0.8Co0.2O2 and Mg-doped LiNi0.8Co0.2O2 have been synthesized via the rheological phase reaction route. The particle size distribution is obtained using laser scattering measurements. The morphological features are observed by a scanning electron microscope. Electrochemical tests show that the Mg- doped LiNi0.8Co0.2O2 exhibits an improved capacity, cycling stability, and rate performances compared to undoped samples. The samples are also characterized by cyclic voltammetry and electrochemical impedance spectroscopy. Furthermore, possible mechanisms of Mg- doping are discussed based on the defect reaction. (c) 2008 The Electrochemical Society.