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

Abstract

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, LixPFy, 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 LixPFy simultaneously increase to constitute the outer layer of the surface film with equal amounts.