Impedance spectroscopic study for the initiation of passive film on carbonelectrodes in lithium ion batteries

The formation of passive film at the interface between the mesocarbon micro beads (MCMB) electrode and the organic electrolyte in a lithium-ion battery during the initial period of intercalation was investigated by a.c.-impeda nce spectroscopy. An equivalent-circuit model consisting of five parallel R C-circuits in series combination was adopted for the curve-fitting analysis of the obtained impedance spectra. The results indicated that both the tot al interfacial resistance and the passive film thickness increased with dec reasing intercalation potential in the ethylene carbonate (EC) or dimethyl carbonate (DMC) single-solvent system, whereas an opposite trend was observ ed in the system containing diethyl carbonate (DEC) only. In addition, the total interfacial resistance was clearly affected by the porous structure o f the passive film in a single-solvent system. In binary solvent systems su ch as EC/DEC and EC/DMC, on the other hand, the effect of the porous struct ure on the total interfacial resistance was negligible. The total interfaci al resistance and the passive film thickness were also smaller in these sys tems than those in single-solvent systems. Finally, the variation of the to tal interfacial resistance and of the passive film thickness in the EC/DEC (or EC/DMC) system were also found to be similar to those in the parent DEC (or DMC) system during intercalation.