XPS, XRD and SEM characterization of a thin ceria layer deposited onto graphite electrode for application in lithium-ion batteries

Thin ceria layer deposited by electro-precipitation onto graphite was synthesised and characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The electro-precipitated ceria has a cubic structure with nanocrystallites of about 6 nm. The SEM analyses shows that the ceria layer reflects the morphology of the graphite electrode, exhibits small cracks usually found on the electro-precipitated films but covers almost completely the surface of the graphite. The ceria layer is composed of 75% Ce(IV) and 25% Ce(III) oxides as indicated by the XPS analyses. Cyclic voltammetry and galvanostatic charge-discharge tests in ethylene carbonate/dimethyl carbonate (1/1) (wt/wt) in the presence of 1 M LiPF₆ show that reversible lithium insertion and deinsertion occurs in the graphite/ceria electrode and that the ceria layer on the graphite electrode prevents from the loss of capacity during the first four cycles. The reduction of the electrolyte occurs at about 0.7 V vs Li/Li⁺ on both electrodes but XPS and SEM analyses show that the SEI layer is thin and not as homogenous on the graphite as on the graphite/ceria electrode. The composition of the SEI layer on the graphite/ceria electrode, mainly composed of Li₂CO₃, ROCO₂Li, R-CH₂OLi and LiF, is different than those obtained on the graphite.