Phase Transition Mechanisms in Li_xCoO₂ (0.25 ≤ x ≤ 1) Based on Group–Subgroup Transformations

The basic structural chemistry of O3–Li x CoO 2 (0.25 ≤ x ≤ 1) oxides is reviewed. Crystal chemical details of selected compositions and group–subgroup schemes are discussed with respect to phase transitions upon electrochemical or chemical deintercalation of the lithium atoms. Furthermore, the theoretical crystal structures of Li x CoO 2 supercells ( x = 0.75, 0.5, 0.33, and 0.25) are reported for the first time based on the combination of transmission electron microscopy (TEM) and X-ray (XRD) or neutron diffraction (ND) experiments. Li 0.75 CoO 2 and Li 0.25 CoO 2 supercells crystallize with the space group R 3̅ m, a 4 = 5.6234 Å and 5.624 Å, and c 4 = 14.2863 Å and 14.26 Å, respectively, whereas the Li 0.5 CoO 2 supercell crystallizes with the space group P 2 1 / m, a 7 = 4.865 Å, b 7 = 2.809 Å, c 7 = 9.728 Å, and β 7 = 99.59°. The Li 0.33 CoO 2 supercell may crystallize in different unit cells (hexagonal or orthorhombic or monoclinic). For Li 0.75 CoO 2, the TEM superstructure reflections are due to only one type of lithium and vacancy ordering within the lithium layers; however, for x = 0.5, the superstructure reflections are due to an intergrowth of two Li 0.5 CoO 2 monoclinic structures ( P 2/ m, a 5 = 4.865(3) Å, b 5 = 2.809(3) Å, c 5 = 5.063(3) Å, β 5 = 108.68(5)°) with the lithium and vacancies alternating the 1 g and 1 f atomic positions, in two successive layers, along the c direction. For Li 0.33 CoO 2, in most cases, the Li and vacancy ordering are similar to Li and Mn ordering in the Li 2 MnO 3 structure. The phase transition mechanisms from O3–LiCoO 2 to O3–Li 0.25 CoO 2 and from O3–LiCoO 2 to spinel–Li 0.5 CoO 2 have been determined, and the structural relationship between O3–LiCoO 2 and Li 2 MnO 3 has been discussed in detail.