Changes in the lattice constants of thin-film LiCoO₂ cathodes at the 4.2 V charged state

The lattice constants of thin-film Li_1-xCoO₂ cathodes at the 4.2 V charged states were influenced by various deposition conditions. Li_1-xCoO₂ thin films [yielding a strong (003) texture] on a Pt or Au current collector, which were unheated during sputtering deposition and ex situ annealed, showed negligible lattice expansion at 4.2 V during the first charge. This is in contrast to the Li_1-xCoO₂ powders exhibiting ∼3% c axis expansion at x ≅ 0.5 (from ∼ 14.05 to ∼ 14.45 Å). The total energy of the constrained Li_0.5CoO ₂ lattice (0% c axis expansion) obtained by a pseudopotential total-energy calculation was slightly higher than that of the relaxed one by ∼ 1.0 eV per 12 Li_0.5CoO₂ (or & sim;80 meV/Li _0.5CoO₂), indicating no difficulty of limited lattice expansion during the first cycle. However, splitting of the (009) diffraction peak was observed at 4.2 V as cycling proceeded: one has a lattice constant c of 14.01 ± 0.05 Å as LiCoO₂ before charging, and the other has a lattice parameter of 14.40 ± 0.05 Å, which is similar to the Li_0.5CoO₂ powders. In contrast, the lattice constants c of the Li_1-xCoO₂ thin films deposited at different conditions [yielding a weak (003) texture] expanded when first charged to 4.2 V, which is similar to that observed in the powder geometry.