Magnesium Insertion into the Chevrel Phase Mo6S8: XPS Analysis

Abstract

Since the earlier works in 2000 by Aurbach et al 1, magnesium-ion batteries emerged as an alternative to lithium-ion batteries thanks to many advantages: magnesium is the 5th most abundant metal in the earth crust, its divalent character and its ionic radius similar to lithium ionic radius gives it a volumetric capacity higher than lithium (3833 Ah/L and 2046 Ah/L respectively). Eventually the absence of dendritic growth on metallic magnesium is a plus in term of security. Several cathode materials were proposed for Mg-ions battery, nevertheless the reference positive electrode material in Mg-Ion technology remained the Chevrel Phase with the formula Mo6S8. The Chevrel phase, synthesized by R. Chevrel et al 2 in 1974, was described as an octahedral cluster of molybdenum located in a cube of sulfur that forms a Mo6S8 pattern. Structural analysis3 (X-Ray Diffraction) showed the existence of 12 insertions sites localized inside a cube of sulfur surrounded by Mo6S8clusters as illustrated in Fig. 1. The electrochemistry highlighted 2 types of insertion sites able to host Mg2+at different potentials. Being considered as the reference cathode material for Mg-ion systems, it has been extensively studied, especially on the structural and electrochemical point of view. However, the precise red-ox processes taking place during Mg insertion-deinsertion remain unclear. In this study X-ray Photoemission Spectroscopy (XPS) has been used to get a direct observation of the redox mecanism during charge/discharge of the material. We performed ex-situ XPS core peak analyses on Mo6S8electrodes at different states of charge to probe the molybdenum and sulfur oxydation state and the environment (polarized or metallic) of inserted magnesium ions. These results were discussed based on a step-by-step XRD and XPS study before and after cycling, and correlated with the electrochemical performance of Chevrel phase. Fig 2 illustrates Mo 3d and Mg 2p orbitals in Mg1.72Mo6S8. In one side we can see the evolution of molybdenum oxidation states during discharge and on the other side the two environments of inserted magnesium is underlined. References : (1) D. Aurbach, Z. Lu, A. Schechter, Y. Gofer, H. Gizbar, R. Turgeman, Y. Cohen, M. Moshkovich & E. Levi, Nature vol 407 (2000) 724 (2) R. Chevrel et al, Mat. Res. BuLl. Vol. 9, pp. 1487-1498, 1974M. D. Levi, E. Lancry, H. Gizbar, Z. Lu, E. (3) R. Chevrel, M. Sergent Superconductivity in Ternary Compounds I Volume 32 of the series Topics in Current Physics pp 25-86 Figure 1