Kinetic and Thermodynamic Studies of Mg[sup 2+] and Li[sup +] Ion Insertion into the Mo[sub 6]S[sub 8] Chevrel Phase

Abstract

Slow-scan rate cyclic voltammetry (SSCV) and chronopotentiometry were used for a quantitative comparison of the thermodynamic and kinetic characteristics of Li+Li+ and Mg2+Mg2+ -ion insertion into the Mo6S8Mo6S8 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,Li1Mo6S8, Li3Mo6S8,Li3Mo6S8, and Li4Mo6S8,Li4Mo6S8, respectively. The kinetics of the intercalation is relatively fast. Mg-ion insertion was found to have the stoichiometry 2:2, i.e., Mg1Mo6S8Mg1Mo6S8 and Mg2Mo6S8Mg2Mo6S8 are formed. The initial magnesiation and the final demagnesiation of the chevrel phase (Mo6S8↔Mg1Mo6S8)(Mo6S8↔Mg1Mo6S8) 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 MgxMo6S8MgxMo6S8 phase. A moderate increase in temperature results in a drastic increase of ion mobility. In Mg(AlCl(4−n)Rn)2Mg(AlCl(4−n)Rn)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. © 2004 The Electrochemical Society. All rights reserved.