Optical Modulation and Phase Distribution in LiCoO2 upon Li-Ion De/Intercalation

Modulation of reflectance resulting from the change in optical constants in Li x CoO2 during lithium de/intercalation is studied and quantified by in-operando and ex situ optical spectroscopy. To this aim, the LiCoO2 (LCO) thin films are sputter deposited using radio-frequency ion-beam sputtering. The films are structurally characterized by X-ray diffraction and transmission electron microscopy. The reversible electrochemical and electrochromic performance is determined by in-operando optical reflectance. Ex-situ reflectance, at particular charge states, is used to determine the optical constants by modeling the optical spectrum using the Clausius-Mossotti relation. The model reveals a dominant resonant wavelength at 646 nm for the fully intercalated state of LCO. For the delithiated state or Li0.5CoO2, a much broader and significantly larger absorption peak is obtained by the model description. This significantly broad and intense absorption peak can be associated with the conducting nature of the films upon lithium removal. Furthermore, the observed complex refractive index (CRI), evolving with the lithium content, is justified by the prior reported density of states calculations. With the CRI, the corresponding variation of the real and imaginary part of the dielectric function reveals that the intercalation of lithium and the consequent phase propagation follows a layer-like reaction.