Changes in the Lattice Constants of Thin-Film LiCoO2 Cathodes at the 4.2 V Charged State

Abstract

The lattice constants of thin-film Li12xCoO2 cathodes at the 4.2 V charged states were influenced by various deposition conditions. Li12xCoO2 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 Li12xCoO2 powders exhibiting ;3% c axis expansion at x > 0.5 ~from ;14.05 to ;14.45 Å!. The total energy of the constrained Li0.5CoO2 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 Li0.5CoO2 ~or ;80 meV/Li0.5CoO2), 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 6 0.05 Å as LiCoO2 before charging, and the other has a lattice parameter of 14.40 6 0.05 Å, which is similar to the Li0.5CoO2 powders. In contrast, the lattice constants c of the Li12xCoO2 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.