, , , Ichiro Takeuchi,
The evolution of interface between the epitaxial LiCoO2 cathode and liquid electrolyte and the boundaries inside the cathode during electrochemical cycles has been analyzed by high resolution scanning transmission electron microscope. Relaxation of sharp translational domain boundaries with shifted layers of CoO2 octahedra occurs during cycling in order to align and make continuous the CoO2 layers across the boundaries. The original trigonal/monoclinic 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, indicating lower stability of the layered LCO structure near its surface after Li is extracted above 50%. The transformed structure is identified to be close to Co3O4 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 Co2+/Co3+ during the structure changes to spinel rather than oxidized.
ACS Applied Materials and Interfaces