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

摘要

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 (ROCO_2Li), Li_2CO_3 and the salt reduction products Li_xAsF_y and Li_xPF_y. LiAsF_6 and LiPF_6 salts are reduced to Li_xAsF_y and Li_xPF_y 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 LiPF_6 compared with that of LiAsF_6 accounts for the poorer passivity of the graphite electrode in LiPF_6 + EC + DEC solution than that in LiAsF_6 + EC + DEC solution.