Mg Desolvation and Intercalation Mechanism at the Mo6S8 Chevrel Phase Surface

In this work, we examine the Mg-ion desolvation and intercalation process at the Chevrel phase Mo6S8 cathode surface from first principles. It is reported that in electrolytes based on chlorides in tetrahydrofuran (THF), Mg2+ is strongly coordinated by the counterion Cl- and can form singly charged MgCl+ and Mg2Cl3+ species in solution. During cell discharge, intercalation of Mg into the Chevrel phase requires breaking the strong, ionic Mg-Cl bond. Our simulation results indicate that the stripping of Cl- is facilitated by the existence of another cationic species, Mo on the Chevrel phase surface. Once Mg is intercalated, it leaves the counterion, Cl-, on the surface, bound to Mo. It is found that the chlorinated surface presents higher activation barriers to further intercalate Mg. Instead, the chlorinated surface continues to interact with incoming MgCl+ species and form various MgCly surface adsorbates. With certain energy costs, the neutral MgCl2 unit may be released from these surface adsorbates to reopen Mo sites on the surface and permit continuous Mg intercalation. Presuming compatibility of chloride electrolytes with the Mg metal anode, our work implies that finding a compatible cathode material will depend critically on its ability to catalyze Mg-Cl bond breaking. This may explain the success of the Chevrel phase, with its open Mo sites, permitting intercalation of Mg from the halide solutions, whereas higher-voltage transition metal oxides, which typically lack open metal sites, require more weakly coordinating anions in their electrolytes.