The effect of the anionic framework of Mo6X8 Chevrel phase (X=S, Se) on the thermodynamics and the kinetics of the electrochemical insertion of Mg2+ ions

The absence of Mg-ion trapping in selenium-based Chevrel phase (in contrast to charge trapping in MgxMo6S8) was carefully monitored using a combination of three electroanalytical tools: SSCV, PITT and EIS. We discuss the difference between MgxMo6S8 and MgxMo6Se8 (0<2) in terms of the higher polarizability of the host's Se2- anions framework, compared to that of S2-, in sulfur-based Chevrel phase, which presents lower diffusion barrier for the mobile Mg ions. In addition, it is shown that the formal equilibrium potential of Mg ions insertion is higher for Mo6S8 compared to that of Mo6Se8. This difference relates also to the higher stability of the selenium anions framework compared to the sulfur-based anionic system. (c) 2005 Elsevier B.V. All rights reserved.