Thermal evolution of the layered oxide Li2/3Co2/3Mn1/3O2, showing a T#2 stacking and prepared by a Na/Li ion exchange in P2-Na2/3Co2/3Mn1/3O2, was investigated by thermal analyses and X-ray diffraction. A thermal expansion of the T#2 orthorhombic unit cell is observed from 25 to 350 °C; from 350 °C the T#2 stacking is destabilized to the benefit of an O6-type stacking obtained from the former through slab gliding. The T#2 to O6 phase transformation is allowed to occur from a stacking with larger interlayer distances and the lithium ions in tetrahedral sites to a stacking with smaller interlayer distances and the lithium ions in octahedral sites. This phase transition from T#2 to O6 is reversible, even though its kinetic can be very slow: the thermal treatment of the T#2-type Li2/3Co2/3Mn1/3O2 phase at 450 °C with a quenching in air has shown to stabilize the O6HT-Li2/3Co2/3Mn1/3O2 phase. At temperatures higher than 450 °C, the layered oxide Li2/3Co2/3Mn1/3O2 is gradually decomposed into Li2MnO3 and Co3O4. First electrochemical tests performed in lithium batteries have revealed that O6HT-Li2/3Co2/3Mn1/3O2 delivers as positive electrode material a high reversible capacity of 230 mAh*g-1 over two voltage domains around 3 and 4 V vs Li+/Li.< Leer menos