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XPS Comparative Analysis of the Chevrel Phase Mo6S8 and the Analogous Mo6Se8 during Mg Insertion

Sunday, 28 May 2017: 15:00
Grand Salon D - Section 24 (Hilton New Orleans Riverside)

ABSTRACT WITHDRAWN

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 Mo6S8. 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