Eli Lancry

Kinetic and Thermodynamic Studies of Mg2 + and Li + Ion Insertion into the Mo6 S 8 Chevrel Phase

Slow-scan rate cyclic voltammetry (SSCV) and chronopotentiometry were used for a quantitative comparison of the thermodynamic and kinetic characteristics of Li + and Mg 2+ -ion insertion into the Mo 6 S 8 chevrel phase compound. The Li-insertion process consists mainly of three stages with the relative stoichiometries 1:2:1, corresponding to the formation of Li 1 Mo 6 S 8 , Li 3 Mo 6 S 8 , and Li 4 Mo 6 S 8 , respectively. The kinetics of the intercalation is relatively fast. Mg-ion insertion was found to have the stoichiometry 2:2, i.e., Mg 1 Mo 6 S 8 and Mg 2 Mo 6 S 8 are formed. The initial magnesiation and the final demagnesiation of the chevrel phase (Mo 6 S 8 ↔ Mg 1 Mo 6 S 8 ) reveal intrinsically slow kinetics, accompanied by a substantial decrease in the intercalation level. This probably results from a low ionic conductivity of the electrode bulk caused by both small concentration and low mobility of the Mg ion in this potential region, related to the sites that the Mg intercalants occupy in the Mg x Mo 6 S 8 phase. A moderate increase in temperature results in a drastic increase of ion mobility. In Mg(AlCl (4-n) R n ) 2 solution, the difference of the two sequential insertions of Mg ion into the chevrel phase was found to be 0.26 V, i.e., by 0.08 V lower than that for the insertion of Li ion.

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.

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).