Structural mechanism of the phase transitions in the Mg-Cu-Mo ₆S₈ system probed by ex situ synchrotron X-ray diffraction

Mo₆S₈ is a unique cathode material for rechargeable magnesium batteries, but its theoretical capacity cannot be realized at ambient temperature due to partial Mg trapping. This work shows that this trapping can be avoided by using Cu_∼1Mo₆S₈ instead of Mo₆S₈. The phase diagram of Mg insertion into Cu _∼1Mo₆S₈ was studied by a combination of cyclic voltammetry and ex situ synchrotron X-ray diffraction. Similarly to the previously studied Li-M-Mo₆S₈ (M = metal) systems, the insertion results in Cu extrusion from the intercalation compound, but contrary to the known cases, this process is fully reversible. The structural mechanism of the insertion reactions was established by Rietveld analysis performed for nine new Mg_xCu_yMo₆S₈ phases. It was found that the crystal structure of the quaternary intercalation compounds in the Mg-Cu-Mo₆S₈ system is similar to that of the ternary phases: both Mg²⁺ and Cu⁺ cations are located in the tetrahedral sites of the inner and outer rings, while the occupancy of the sites increases with intercalation level. However, the cation distribution is not disordered. It can be characterized by (i) Mg preference for the inner sites and (ii) cation segregation. The latter is typical for the separate cation groups in the same intercalation compound, but it appears also as cation segregation in different phases. As a result, similarly to the previously studied Li-Cu-Mo₆S₈ system, the phase diagram of Mg insertion into Cu_∼1Mo₆S₈ is rather complex. It includes three phase regions and the coexistence of Cu-rich and Cu-depleted compounds. The structural mechanism proposed in this work sheds light on the electrochemical behavior of the Cu_∼1Mo₆S₈ electrodes in Mg battery; in particular, it explains the absence of Mg trapping.