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

The basic structural chemistry of O3–Li_xCoO₂ (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_xCoO₂ 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_0.75CoO₂ and Li_0.25CoO₂ supercells crystallize with the space group R3̅m, a₄ = 5.6234 Å and 5.624 Å, and c₄ = 14.2863 Å and 14.26 Å, respectively, whereas the Li_0.5CoO₂ supercell crystallizes with the space group P2₁/m, a₇ = 4.865 Å, b₇ = 2.809 Å, c₇ = 9.728 Å, and β₇ = 99.59°. The Li_0.33CoO₂ supercell may crystallize in different unit cells (hexagonal or orthorhombic or monoclinic). For Li_0.75CoO₂, 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_0.5CoO₂ monoclinic structures (P2/m, a₅ = 4.865(3) Å, b₅ = 2.809(3) Å, c₅ = 5.063(3) Å, β₅ = 108.68(5)°) with the lithium and vacancies alternating the 1g and 1f atomic positions, in two successive layers, along the c direction. For Li_0.33CoO₂, in most cases, the Li and vacancy ordering are similar to Li and Mn ordering in the Li₂MnO₃ structure. The phase transition mechanisms from O3–LiCoO₂ to O3–Li_0.25CoO₂ and from O3–LiCoO₂ to spinel–Li_0.5CoO₂ have been determined, and the structural relationship between O3–LiCoO₂ and Li₂MnO₃ has been discussed in detail.