Time evolution of the impedance response of a passive film - A simple application to the Li/SOCl2 system

Time development of impedance spectra measured during passivation of lithiu m in 1.5 M LiAlCl4/SOCl2 is modeled quantitatively. The approach, which rel ates the changes in the high-frequency are with microstructural changes of the secondary film, is general enough to be applied to other passivating sy stems in which the impedance response varies considerably with time. Extens ive correlation between the suggested approach and the well-known model of a parallel-plate capacitor is given. The low-frequency impedance are is ass ociated with diffusion of Li vacancies and hopping electrons within the sol id part of the passive film. The contentrations and diffusion coefficients of both carriers are estimated to lie in the range of 5000 ppm and 5 X 10(- 11) m(2)/s, respectively. The apparent increase in "liquid electrolyte resi stance" during passivation is explained in terms of the suggested modeling. (C) 1999 The Electrochemical Society. S0013-4651(98)06-108-4. All rights r eserved.