Since the earlier works in 2000 by Aurbach et al¹, 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 Mo₆S_8. The Chevrel phase, synthesized by R. Chevrel et al² 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 Mo₆S₈ clusters.

The electrochemistry highlighted 2 types of insertion sites able to host Mg²⁺ at different potentials.

Mg trapping into the Chevrel Phase was also noticed but curiously this phenomenom doesn’t occur in the analogous phase Mo₆Se₈⁴ thanks to the lower ionicity of Se compared to S.

The anionic framework plays a major role in the redox process during Mg insertion, this study is a comparative analysis of Mo₆S₈ and analogous Mo₆Se₈ using electrochemistry and X-ray Photoemission Spectroscopy to get a direct observation of the redox mecanism during charge/discharge of the material.

We performed ex situ XPS core peak analyses on Mo₆S₈ and Mo₆Se₈ analogous electrodes at different states of charge to probe the molybdenum, the sulfur and the selenium oxydation states and the environment (polarized or metallic) of inserted magnesium ions.

These results will be discussed based on a step-by-step XPS study before and after cycling, and will be correlated with the electrochemical performances.

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, 1974.

(3) R. Chevrel, M. Sergent Superconductivity in Ternary Compounds I Volume 32 of the series Topics in Current Physics pp 25-86

(4) M.D. Levi et al, Solid State Ionics Volume 176, Issues 19–22, June 2005, Pages 1695–1699