Divalent cation incorporated Li1+xNiMgxO2(1+x) materials, corresponding to LiNi1-yMgyO2 [y = x/(1 + x)], represent a class of novel materials with unique electrochemical characteristics. Excellent cyclability (reversibility) when cycled up to high potentials (e.g., 4.4 V with respect to Li), as well as higher thermal stability in comparison to pure LiNiO2, are the major benefits of these divalent cation incorporated materials. Ln the present work, a series of material compositions (x = 0, 0.05, 0.10, 0.15, and 0.20) have been successfully synthesized and electrochemically characterized. The introduction of Mg results in the elimination of phase transformations and causes smaller unit cell volume changes during cycling. More importantly, it results in a significant decrease in the capacity fade rate. Results of these studies reveal that the suppression of decomposition reactions during cycling is the prime reason for this improvement in capacity fade rate rather than the elimination of phase transformations and the smaller unit cell volume change. Suppression of the decomposition reactions is thought to be due to the prevention of overcharging of the material as well as the stabilization of NiO2 slabs caused by the presence of Mg cations.