A rapid solid-state synthesis of electrochemically active Chevrel phases (Mo₆T₈; T = S, Se) for rechargeable magnesium batteries

High energy mechanical milling (HEMM) of a stoichiometric mixture of molybdenum and metal chalcogenides (CuT and MoT₂; T = S, Se) followed by heat treatment at elevated temperatures was successfully applied to synthesize Chevrel phases (Cu₂Mo₆T₈; T = S, Se) as positive electrodes for rechargeable magnesium batteries. Differential scanning calorimetry (DSC), thermogravimetric analyses (TGA), X-ray diffraction (XRD), and scanning electron microscopy (SEM) were used to understand the phase formation following milling and heat treatment. CuS and Mo were observed to react at 714–800 K and formed an intermediate ternary Chevrel phase (Cu_1.83Mo₃S₄), which further reacted with residual Mo and MoS₂ to form the desired Cu₂Mo₆S₈. Quantitative XRD analysis shows the formation of a ~96%–98% Chevrel phase at 30 min following the milling and heat treatment. The electrochemical performance of de-cuprated Mo₆S₈ and Mo₆Se₈ phases were evaluated by cyclic voltammetry (CV), galvanostatic cycling, and electrochemical impedance spectroscopy (EIS). The results of the CV and galvanostatic cycling data showed the expected anodic/cathodic behavior and a stable capacity after the first cycle with the formation of Mg_xMo₆T₈ (T = S, Se; 1 ≤ x ≤ 2). EIS at ~0.1 V intervals for the Mo₆S₈ electrode during the first and second cycle shows that partial Mg-ion trapping resulted in an increase in charge transfer resistance R_e. In contrast, the interfacial resistance R_i remained constant, and no significant trapping was evident during the galvanostatic cycling of the Mo₆Se₈ electrode. Importantly, the ease of preparation, stable capacity, high Coulombic efficiency, and excellent rate capabilities render HEMM a viable route to laboratory-scale production of Chevrel phases for use as positive electrodes for rechargeable magnesium batteries.