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

The crystal structures and electrochemical properties of Zn_xMo₆S₈ Chevrel phases (x = 1, 2) prepared via electrochemical Zn²⁺-ion intercalation into the Mo₆S₈ host material, in an aqueous electrolyte, were characterized. Mo₆S₈ [trigonal, R3, 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.