Direct observation of important morphology and composition changes at the surface of the CuO conversion material in lithium batteries

Morphology and composition changes occurring at the surface of CuO thin film electrode, used as conversion material for lithium-ion battery, were investigated at different stages of the electrochemical cycling by atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). Analyses were performed on samples obtained in LiPF6 containing carbonate-based electrolyte and prepared by linear sweep voltammetry. The formation/partial dissolution of a solid electrolyte interphase (SEI) was evidenced by XPS analyses. The SEI layer formed on lithiated CuO thin film during the first reduction between [3.5-0.8] V/Li+/Li is characterized by a LiF-rich inner layer and a Li2CO3-rich outer layer. Furthermore, reduction/oxidation cycles induced important surface morphology modifications. In particular, the growth and then the vanishing of large spherical nodules (∼400 nm in diameter) were observed during the lithium insertion and the subsequent deinsertion. During the twenty first cycles performed, even if a stable electrochemical behavior was shown, breathing of the thin film was also clearly evidenced by AFM images with the appearance and disappearance of numerous cracks as a function of Li removal and Li insertion processes.