In-situ spectroelectrochemical analysis of the passivating surface film formed on a graphite electrode during the electrochemical reduction of lithium salts and organic carbonate solvent

The in-situ spectroelectrochemical technique has been applied to qualitative analysis of the passivating surface film on the graphite powder electrode as a function of applied negative potential with respect to open circuit potential (ocp) of about 3.0 V (Li\Li+) in organic carbonate solutions. In parallel, the passivity of the surface film formed was assessed by using cyclic voltammetry with the help of abrading the graphite bulk electrode. The results of in-situ Fourier transform infra-red (FTIR) spectroscopy showed that the passivating surface film consists mainly of alkyl carbonate lithium (ROCO2Li), Li2CO3 and the salt reduction products LixAsFy and LixPFy, LiAsF6 and LiPF6 salts are reduced to LixAsFy and LixPFy in the potential ranges of 0.6 to 0.8 V (Li\Li+) and 0.4-0.6 V (Li/Li+), respectively, indicating the lower reactivity of the latter salt towards the electrochemical reduction as compared with that of the former salt. From the combined result of in-situ FTIR spectroscopy and cyclic voltammetry, it is suggested that the lower reduction reactivity of LiPF6 compared with that of LiAsF6 accounts for the poorer passivity of the graphite electrode in LiPF6 + EC + DEC solution than that in LiAsF6 + EC + DEC solution. (C) 1998 Elsevier Science S.A. All rights reserved.