Electrochemical and Spectroscopic Characterization of LiCoO2 Thin-Film as Model Electrode

Abstract

LiCoO2 thin-films deposited on various platinum substrates, i.e. single crystals of Pt(100), Pt(110) and Pt(111), and polycrystals (Pt-poly), are characterized by cyclic voltammetry (CV), X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) to discuss appropriateness of thin-film modeling of composite electrodes from the electrochemical and spectroscopic viewpoints. Structural characterization indicates (001) orientation of LiCoO2/Pt(100) and LiCoO2/Pt(111) thin-films, and (104) orientation of LiCoO2/Pt(110). LiCoO2/Pt-poly thin-film is polycrystalline with a broad distribution of tilt angle of LiCoO2 c-axis. The LiCoO2/Pt(100) and LiCoO2/Pt-poly thin-films exhibit reversible CV behavior, while the other two show irreversible behavior. The structural and electrochemical characterization suggests that the c-axis tilt angle is not the predominant factor for reversible electrochemical behavior of the LiCoO2 electrodes. Instead, polycrystalline structure with weak orientation seems to be preferable for high reversibility. Orientation dependence of XAS also indicates that the LiCoO2/Pt-poly thin-film shows similar spectrum to LiCoO2 powder. Thin-films having strong orientation lead to strong dependence of XAS spectra, and this can result in lack of spectroscopic information. From both the electrochemical and XAS viewpoints, the LiCoO2/Pt-poly thin-film having the polycrystalline structure with the broad distribution of the c-axis tilt angle is the most suitable for the thin-film modeling of the composite electrodes.