Kinetic and Thermodynamic Studies of Mg[sup 2+] and Li[sup +] Ion Insertion into the Mo[sub 6]S[sub 8] 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 Mg 2 + -ion insertion into the Mo 6 S 8 chevrel phase compound. The Li-insertion process consists mainly of three stages with the relative stoichiometries 1:2:1, corresponding to the formation of Li 1 Mo 6 S 8 , Li 3 Mo 6 S 8 , and Li 4 Mo 6 S 8 , respectively. The kinetics of the intercalation is relatively fast. Mg-ion insertion was found to have the stoichiometry 2:2, i.e. , Mg 1 Mo 6 S 8 and Mg 2 Mo 6 S 8 are formed. The initial magnesiation and the final demagnesiation of the chevrel phase ( Mo 6 S 8 ↔ Mg 1 Mo 6 S 8 ) 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 x Mo 6 S 8 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.