Leaching chemistry and the performance of the Mo6S8 cathodes in rechargeable Mg batteries

The influence of leaching chemistry on the performance of Mo6S8 cathodes for rechargeable Mg batteries was studied by XRD, XPS, and electrochemical methods. It is shown that Cu-2-Mo6S8, chemically leached in I-2/AN (acetonitrile) or in HCl/H2O, is able to accommodate the theoretical amount of Mg2+ ions upon the first discharge of the electrode (two Mg atoms per formula unit). In contrast, the process of Mg extraction upon charging the electrode cannot be completed at room temperature, resulting in capacity loss between the first and subsequent cycles. Electrochemical experiments at elevated temperatures reveal that the capacity loss is not related to the leaching process, but results from an intrinsic kinetic problem of the Mg extraction from the MgMo6S8 phase. Despite the initial capacity loss, the materials leached in I-2/AN or in HCl/H2O show excellent stability upon long-term cycling, with a specific capacity of 90-100 mA.h/g.