A. Mitelman

Phase diagram of mg insertion into chevrel phases, MgxMo6T8 (T = S, Se). 2. The crystal structure of triclinic MgMo6Se8

Chevrel phases (CPs), M x Mo 6 T 8 (M = metal, T = S, Se), may be used as unique cathode materials for rechargeable Mg batteries because they ensure the high mobility of multivalent cations. However, the electrochemical behavior is strongly affected by the host composition. For the selenide, the intercalation process is completely reversible, while partial Mg trapping occurs upon its extraction from the sulfide at room temperature. A combination of powder X-ray and high-resolution neutron diffraction was used to study the crystal structure of triclinic MgMo 6 Se 8 , especially for determining the precise location of the Mg 2+ cations within the host lattice. It was shown that the crystal structure of the selenide is similar to that of triclinic Fe 2 Mo 6 S 8 : The Mg 2+ cations are distributed between two sites (per formula unit) with a square-pyramidal anion coordination. The environment analysis of all the cation sites based on the bond valence sum theory led us to propose the most favorable routes for Mg 2+ ion transport, as well as to explain the peculiarities of the electrochemical behavior of the CPs as intercalation materials for Mg batteries.

Phase Diagram of Mg Insertion into Chevrel Phases, MgxMo6T8 (T = S, Se). 3. The Crystal Structure of Triclinic Mg2Mo6Se8

This series of papers is devoted to unique cathode materials for Mg batteries, MgxMo6T8 (T = S, Se, x = 1 and 2) Chevrel phases (CPs). In this part, a combination of neutron and high-resolution synchrotron X-ray diffractions was used to study the crystal structure of Mg2Mo6Se8, which is triclinic at room temperature (space group P1, a = 6.868 A, b = 6.921 A, c = 6.880 A, alpha = 93.00 degrees , beta = 94.40 degrees , gamma = 96.22 degrees ). In contrast to other members of the MgxMo6T8 family, this compound does not follow the classic scheme of successive cation insertion into so-called inner and outer sites: Both the Mg(2+) ions per formula are located in the tetrahedral sites of the outer ring. This surprising cation location, predicted previously for Mg-containing CPs by ab initio calculations, provides the uniform distribution of the cation charge in the triclinic structure, which is similar to that of rhombohedral CPs. A mapping of the cation sites was widely used to demonstrate the variety of cation arrangement in CPs and the factors affecting this arrangement, as well as to clarify the origin of the exceptionally high mobility of the Mg(2+) ions in Mg2Mo6Se8.

New cathode materials for rechargeable Mg batteries: fast Mg ion transport and reversible copper extrusion in CuyMo6S8 compounds

We report on a discovery of fast cathode materials, ternary Chevrel phases (CPs), CuyMo6S8, for rechargeable magnesium batteries; the related electrochemical process displays a unique coupling between reversible Mg insertion, and Cu extrusion/ reinsertion; this coupling results in an entirely new intercalation mechanism which combines the total chemical reversibility of the electrochemical reaction of MgxCuyMo6S8 with irreversibility of its separate stages once Cu extrusion stage is reached (in MgxCuyMo6S8: 0.5x + y > 4).