Microscopy Study of Structural Evolution in Epitaxial LiCoO₂ Positive Electrode Films during Electrochemical Cycling

The evolution of interface between the epitaxial thin film LiCoO2 (LCO) electrode and liquid electrolyte and inside the LCO film during electrochemical cycling has been analyzed by high resolution scanning transmission electron microscopy. Relaxation of sharp translational domain boundaries with mismatched layers of CoO2 octahedra occurs during cycling and results in formation of continuous CoO2 layers across the boundaries. The original trigonal layered structure of LiCoO2 tends to change into a spinel structure at the electrode/electrolyte interface after significant extraction of Li from LCO. This change is more pronounced at 4.2 V peak of CV, indicating lower stability of the layered LCO structure near its surface after Li is extracted above 60%. The transformed structure is identified to be close to Co3O4, with Co both on tetrahedral and octahedral sites, rather than to LiCo2O4 as it was suggested in earlier publications. Electron energy-loss spectroscopy measurements also show that Co ions oxidation state is reduced to mixed valence state Co(2+)/Co(3+) during the structure changes to spinel rather than oxidized.