Rechargeable magnesium batteries were first presented about seven years ago. [1][2][3] Their components included magnesium metal or a Mg alloy anode, Mg x Mo 6 S 8 (0 < x < 2) Chevrel phase cathodes, and electrolyte solutions that contained an ether solvent and a complex electrolyte, a product of the reaction between a MgBu 2 Lewis base and an AlCl 2 Et Lewis acid (Bu = butyl, Et = ethyl). These systems, while demonstrating impressive cycleability, suffered from several drawbacks:i) The micrometric size Mg 0-2 Mo 6 S 8 Chevrel phase cathode suffers from some kinetic limitation and the phenomenon of partial charge trapping (of Mg ions) at low temperatures. [4,5] ii) The electrochemical window of the first generation of electrolyte solutions, THF/Mg(AlCl 2 BuEt) 2 was around 2.2 V, which limited the possible use of cathode materials with a higher redox potential (and higher capacity) than Chevrel phases. iii) For practical use, the synthesis of the components of rechargeable Mg batteries needs simplification. Chevrel phases (CPs), M x Mo 6 T 8 (M = metal, T = S, Se), are of great interest owing to their remarkable electromagnetic, thermoelectric, and catalytic properties [6][7][8][9] . Exceptionally fast cation transport for multi-valent ions (compared to any other inorganic host material) made these materials unique cathodes in Mg batteries. [1][2][3] However, the kinetics of Mg diffusion in the CPs is strongly affected by their composition and temperature. At ambient temperature, the selenide shows excellent Mg mobility in the full intercalation range from 0 to 2 Mg 2+ ions per formula unit, [4] while Mg trapping occurs in the sulfide. During the first magnesiation of Mo 6 S 8 , 2 Mg ions are inserted (i.e., the full theoretical capacity is realized), upon further electrochemical deintercalation of Mg x Mo 6 S 8 , part of the Mg 2+ ions (20-25 %) are trapped and can be removed from the crystal structure, only at elevated temperatures (i.e., only 75-80 % of the theoretical capacity is involved in reversible cycling at low temperatures).[5]Detailed studies [10,11] of the crystal structure of the Mg-containing CPs made it clear that the trapping in the sulfide is caused by a unique ring arrangement of closely located cation sites with low potential energy. The triclinic distortion in the selenide changes the geometry of the cation sites, resulting in the degeneracy of the effect. It can be suggested that the presence of relatively small amounts of Se will be sufficient to improve the kinetics of the Mg 2+ cations in CPs. In fact, in addition to compounds with a single anion, the Chevrel family includes also solid solutions where sulfur and selenium atoms form a common anion framework. [12] Thus, in order to optimize the cathode composition in Mg batteries, it is of great importance to study the influence of the S-Se substitution in the host on the electrochemical behavior. Mg insertion into the binary hosts occurs in two stages: [1][2][3]

A combination of ab initio calculations and experimental methods (high-resolution neutron and powder X-ray diffractions) was used to solve the crystal structure of Mg x Mo6S8 (x = 1 and 2). It was shown that at room temperature, the latter are similar to the crystal structure of classic Chevrel phases (CPs) such as Cu x Mo6S8:  space group R3̄, a r = 6.494 Å, α = 93.43° for MgMo6S8 and a r = 6.615 Å, α = 95.16° for Mg2Mo6S8. For x = 1, one Mg2+ cation per formula unit is distributed statistically between inner sites. For x = 2, the second Mg2+ cation per formula unit is located in the outer sites. Peculiarities of the electrochemical behavior of the CPs as electrode materials for Mg batteries were understood on the basis of the analysis of the interatomic distances. It was shown that the circular motion of the Mg2+ ions between the inner sites in MgMo6S8 is more favorable than their progressive diffusion in the bulk of the material, resulting in relatively slow diffusion and Mg trapping in this phase. In contrast, in Mg2Mo6S8, the repulsion between the Mg2+ ions located in the inner and outer sites facilitates their transport through the material bulk.