Evolved gas analysis (EGA) by mass spectroscopy (MS) was carried out for the pyrolysis of Li1−x Co1/3Ni1/3Mn1/3O2 (185mAhg−1 of charge capacity) and the results were compared with that of Li1−x CoO2 (140mAhg−1). Electrochemically prepared Li1−x Co1/3Ni1/3Mn1/3O2 clearly shows that O2 evolution begins at much higher temperature than Li1−x CoO2, suggesting that Li1−x Co1/3Ni1/3Mn1/3O2 is superior to LiCoO2 with respect to thermal stability. High-temperature XRD measurements of charged LiCo1/3Ni1/3Mn1/3O2-electrodes at 4.45V were also carried out and shown that the decomposition product by heating was identified as a cubic spinel consisting of cobalt, nickel, and manganese. This indicates that phase change from a layered to spinel-framework structure plays a crucial role in the suppression of oxygen evolution from the solid matrix. In order to show practicability of the new material, lithium-ion batteries with graphite-negative electrodes are fabricated and examined in the R18650-hardware. The new lithium-ion batteries show high rate discharge performances, excellent cycle life, and safety together with high-energy density.