A comparison between intercalation of Li and Mg ions into the model Chevrel phase compound (M_xMo₆S₈): Impedance spectroscopic studies

Comparison between Li and Mg ions insertion into the Chevrel phase M _xMo₆S₈ (M = Li, Mg) shows interesting features related to the different interactions of the monovalent and the divalent ions with the Mo clusters and the anionic framework. It is clear that Mg ions insertion is much more complicated than Li ions insertion. An attempt was made to explain the peculiar behavior of the impedance spectra related to Mg ions insertion into the Mo₆S₈ electrode by a recent model for two non-equivalent lattice sites with two different energy barriers, which predicts the so-called "Gerischer-type impedance" at high frequencies and at the low-frequencies the appearance of additional semicircle, the latter being especially pronounced at the beginning of intercalation. We measure a charge-trapping of the Mg ions in the Chevrel phase at the initial stage of intercalation and at the final stage of deintercalation. An increase in the intercalation level (accompanied by a decrease in potential) correlates with a decrease in the trapping resistance, R_trap. We explain these features of the impedance response of the Mg ions insertion by the peculiar nature of the sites for small ions accommodation in the Chevrel phase structure. The divalent nature of the Mg-ions leads to their slower diffusion compared to the single charge Li ions. In general, the diffusion barrier of the trapped ions decreases with the increase in concentration due to the high electrostatic repulsion among the divalent cations.