Layered Li[Li0.12Ni z Mg0.32−z Mn0.56]O2 oxide cathodes containing lithium atoms in the transition metal layers were synthesized and characterized using X-ray diffraction (XRD), galvanostatic cycling, and differential scanning calorimetry (DSC). The Li[Li0.12Ni z Mg0.32−z Mn0.56]O2 cathodes deliver a specific discharge capacity of about 190mAh/g at room temperature and 236mAh/g at 55°C when cycled between 2.7 and 4.6V versus Li/Li+. Excellent capacity retention and smooth potential profiles at room and elevated temperatures over extended cycles suggest that this material does not convert into a spinel structure.