Direct Imaging of the Structural and Morphological Evolution of Epitaxial LiCoO₂ Films During Charge and Overcharge

Capacity decay of layered cathodes in high voltage applications underscores the need to utilize accurate and precise techniques to understand the underlying mechanisms. Here, we use well-defined epitaxial LiCoO₂ (LCO) films on SrRuO₃/SrTiO₃ (SRO/STO) with controlled orientations and defect structures along with <i>in situ</i> electrochemical atomic force microscopy to probe the structural and morphological evolutions during the charge and overcharge processes. We quantitatively show the morphological changes in both reversible delithiation regime and irreversible over-delithiation regime, and correlate the overall electrochemical behaviors to atomic scale defect evolutions in the films. We also observe a significantly lower charging capacity for LCO/SRO/STO(111) compared to that of LCO/SRO/STO(001) films of the same thickness, which is ascribed to the different types of atomic scale defects formed during the film growth process. Our high-resolution scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) studies reveal that the antiphase boundaries in LCO/SRO/STO(001) act as viable channels for Li migration but are more susceptible to irreversible phase transitions, which then blocks subsequent Li diffusion. The use of LiCoO₂ thin films also allows us to design epitaxial interfaces and integrate multilayers which can serve as model materails systems for subsequent studies.