Passivation kinetics of surface films formed on a graphite electrode in organic lithium salt solution as a function of salt anion type

The passivation kinetics of a graphite electrode have been investigated as functions of applied potential and salt anion type by using cyclic voltammetry and current transient experiments supplemented with ac-impedance spectroscopy. The cyclic voltammogram and current transient were obtained after abrading the graphite bulk electrode subjected to various negative potentials with respect to the open circuit potential of about 3.0 V (Li\Li+) in 1 M LiAsF6, LiPF6 and LiN(SO2CF3)(2) + ethylene carbonate (EC) + diethyl carbonate (DEC) solutions. The current transients illustrated the passivation kinetics which can be split into two steps of prepassivation and passivation. The LiAsF6 salt gave a reduced reduction current in the second cycle of the voltammogram, the steeper slope of the current transient and the lowered ratios of R-porous to R-compact and of C-compact to C-porous obtained from the measured impedance spectra as compared to LiPF6 and LIN(SO2CF3)(2) salts. The three factors are characterized by the improved passivity of the surface film formed in LiAsF6 + EC + DEC solution. The passivity of the surface film on the graphite electrode has been discussed in terms of the reactivity of the salt anion towards reduction at the electrode. (C) 1997 Elsevier Science S.A.