Kinetic and thermodynamic studies of Mg²⁺ and Li⁺ ion insertion into the Mo₆S₈ chevrel phase

Slow-scan rate cyclic voltammetry (SSCV) and chronopotentiometry were used for a quantitative comparison of the thermodynamic and kinetic characteristics of Li⁺ and Mg²⁺-ion insertion into the Mo ₆S₈ chevrel phase compound. The Li-insertion process consists mainly of three stages with the relative stoichiometries 1:2:1, corresponding to the formation of Li₁Mo₆S₈, Li₃Mo₆S₈, and Li₄Mo ₆S₈, respectively. The kinetics of the intercalation is relatively fast. Mg-ion insertion was found to have the stoichiometry 2:2, i.e., Mg₁Mo₆S₈ and Mg₂Mo ₆S₈ are formed. The initial magnesiation and the final demagnesiation of the chevrel phase (Mo₆S₈ ↔ Mg ₁Mo₆S₈) reveal intrinsically slow kinetics, accompanied by a substantial decrease in the intercalation level. This probably results from a low ionic conductivity of the electrode bulk caused by both small concentration and low mobility of the Mg ion in this potential region, related to the sites that the Mg intercalants occupy in the Mg_xMo ₆S₈ phase. A moderate increase in temperature results in a drastic increase of ion mobility. In Mg(AlCl_(4-n)R_n) ₂ solution, the difference of the two sequential insertions of Mg ion into the chevrel phase was found to be 0.26 V, i.e., by 0.08 V lower than that for the insertion of Li ion.