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

Abstract

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 (Cu1.83Mo3S4), 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 x Mo6T8 (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 Re. In contrast, the interfacial resistance Ri 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.