Electrochemical Zinc-Ion Intercalation Properties and Crystal Structures of ZnMo6S8 and Zn2Mo6S8 Chevrel Phases in Aqueous Electrolytes

Abstract

The crystal structures and electrochemical properties of ZnxMo6S8 Chevrel phases (x = 1, 2) prepared via electrochemical Zn(2+)-ion intercalation into the Mo6S8 host material, in an aqueous electrolyte, were characterized. Mo6S8 [trigonal, R3̅, a = 9.1910(6) Å, c = 10.8785(10) Å, Z = 3] was first prepared via the chemical extraction of Cu ions from Cu2Mo6S8, which was synthesized via a solid-state reaction for 24 h at 1000 °C. The electrochemical zinc-ion insertion into Mo6S8 occurred stepwise, and two separate potential regions were depicted in the cyclic voltammogram (CV) and galvanostatic profile. ZnMo6S8 first formed from Mo6S8 in the higher-voltage region around 0.45-0.50 V in the CV, through a pseudo two-phase reaction. The inserted zinc ions occupied the interstitial sites in cavities surrounded by sulfur atoms (Zn1 sites). A significant number of the inserted zinc ions were trapped in these Zn1 sites, giving rise to the first-cycle irreversible capacity of ∼46 mAh g(-1) out of the discharge capacity of 134 mAh g(-1) at a rate of 0.05 C. In the lower-voltage region, further insertion occurred to form Zn2Mo6S8 at around 0.35 V in the CV, also involving a two-phase reaction. The electrochemical insertion and extraction into the Zn2 sites appeared to be relatively reversible and fast. The crystal structures of Mo6S8, ZnMo6S8, and Zn2Mo6S8 were refined using X-ray Rietveld refinement techniques, while the new structure of Zn2Mo6S8 was determined for the first time in this study using the technique of structure determination from powder X-ray diffraction data. With the zinc ions inserted into Mo6S8 forming Zn2Mo6S8, the cell volume and a parameter increased by 5.3% and 5.9%, respectively, but the c parameter decreased by 6.0%. The average Mo-Mo distance in the Mo6 cluster decreased from 2.81 to 2.62 Å.