In-situ spectroelectrochemical analysis of the passivating surface film formed on a carbon film electrode as a function of the water content in 1 M LiPF6-EC/DEC solution

The in-situ spectroelectrochemical technique has been applied to investigate the role of water in the formation of a passivating surface film on a plasma enhanced chemical vapor deposited (PECVD) carbon film electrode in 1 M LiPF₆-ethylene carbonate (Li-EC) and diethyl carbonate (DEC) solution, combined with cyclic voltammetry. In-situ Fourier transform infra-red (FTIR) spectra of the surface film showed that all the peak intensities of the Li₂CO₃, ROCO₂Li, and LiPF₆ constituents significantly increase with increasing water content under application of the negative potentials with respect to open circuit potential (OCP). It is suggested that the reduction of Li-EC to ROCO₂Li runs via a one-electron transfer pathway with the help of the unrestricted supply of the electron transfer path as a result of diffusion of water through the surface film; then Li₂CO₃ formation proceeds concurrently by the chemical reaction of ROCO₂Li with water. Moreover, the compact sedimentation of ROCO₂Li in the presence of water in the electrolyte is subjected to severe interference of the salt reduction product, Li_xPF_y, than in the absence of water in the electrolyte. These FTIR results coincide well with those of cyclic voltammetry. From the combined results of in-situ FTIR spectroscopy and cyclic voltammetry, it is indicated that, unlike other salt and solvent reaction products, ROCO₂Li, Li₂CO₃ and Li_xPF_y simultaneously increase to constitute the outer layer of the surface film with equal amounts. Received: 24 February 1998 / Accepted: 6 June 1998