Phase transitions and diffusion kinetics during Mg²⁺- and Li⁺-ion insertions into the Mo₆S₈ chevrel phase compound studied by PITT

Careful potentiostatic intermittent titrations (PITT) were performed in order to study Li⁺- and Mg²⁺-ion insertions into the Mo₆S₈ chevrel phase compound. Potential dependences of equilibrium and dynamic characteristics of the insertion processes, such as the differential intercalation capacitance, C_dif, the chemical diffusion coefficient, D, and the specific ionic conductivity, κ, were measured and interpreted in terms of lattice-gas models with highly attractive interactions between the inserted ions. Li-ion conductivity of the slightly intercalated chevrel phase is much higher than that of Mg-ions, which can be attributed to a higher charge density of the latter ion and to its higher polarization effect on the S₈ anions' lattice of the host material. Some crystallographic arguments related to the low mobility of the Mg-ion on the one hand, and the extremely high mobility of most of the divalent d-metal cations in the Mo ₆S₈ chevrel phase on the other hand, are discussed.