Phase Transition
Mechanisms in Li<sub><i>x</i></sub>CoO<sub>2</sub> (0.25
≤ <i>x</i> ≤
1) Based on Group–Subgroup Transformations
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Abstract
The basic structural chemistry of
O3–Li<sub><i>x</i></sub>CoO<sub>2</sub> (0.25 ≤ <i>x</i> ≤
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<sub><i>x</i></sub>CoO<sub>2</sub> supercells (<i>x</i> = 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<sub>0.75</sub>CoO<sub>2</sub> and Li<sub>0.25</sub>CoO<sub>2</sub> supercells crystallize
with the space group <i>R</i>3̅<i>m</i>, <i>a</i><sub>4</sub> = 5.6234 Å and 5.624 Å, and <i>c</i><sub>4</sub> = 14.2863 Å and 14.26 Å, respectively,
whereas the Li<sub>0.5</sub>CoO<sub>2</sub> supercell crystallizes
with the space group <i>P</i>2<sub>1</sub>/<i>m</i>, <i>a</i><sub>7</sub> = 4.865 Å, <i>b</i><sub>7</sub> = 2.809 Å, <i>c</i><sub>7</sub> = 9.728
Å, and β<sub>7</sub> = 99.59°. The Li<sub>0.33</sub>CoO<sub>2</sub> supercell may crystallize in different unit cells
(hexagonal or orthorhombic or monoclinic). For Li<sub>0.75</sub>CoO<sub>2</sub>, the TEM superstructure reflections are due to only one type
of lithium and vacancy ordering within the lithium layers; however,
for <i>x</i> = 0.5, the superstructure reflections are due
to an intergrowth of two Li<sub>0.5</sub>CoO<sub>2</sub> monoclinic
structures (<i>P</i>2/<i>m</i>, <i>a</i><sub>5</sub> = 4.865(3) Å, <i>b</i><sub>5</sub> =
2.809(3) Å, <i>c</i><sub>5</sub> = 5.063(3) Å,
β<sub>5</sub> = 108.68(5)°) with the lithium and vacancies
alternating the 1<i>g</i> and 1<i>f</i> atomic
positions, in two successive layers, along the <i>c</i> direction.
For Li<sub>0.33</sub>CoO<sub>2</sub>, in most cases, the Li and vacancy
ordering are similar to Li and Mn ordering in the Li<sub>2</sub>MnO<sub>3</sub> structure. The phase transition mechanisms from O3–LiCoO<sub>2</sub> to O3–Li<sub>0.25</sub>CoO<sub>2</sub> and from O3–LiCoO<sub>2</sub> to spinel–Li<sub>0.5</sub>CoO<sub>2</sub> have been
determined, and the structural relationship between O3–LiCoO<sub>2</sub> and Li<sub>2</sub>MnO<sub>3</sub> has been discussed in detail
