Electrolyte solutions for rechargeable Mg batteries were developed, based on reaction products of phenyl magnesium chloride ͑PhMgCl͒ Lewis base and AlCl 3 Lewis acid in ethers. The transmetallation of these ligands forms solutions with Mg x Cl y + and AlCl 4−n Ph n − ions as the major ionic species, as analyzed by multinuclei nuclear magnetic resonance spectroscopy. Tetrahydrofuran ͑THF͒ solutions of ͑PhMgCl͒ 2 -AlCl 3 exhibit optimal properties: highly reversible Mg deposition ͑100% cycling efficiency͒ with low overvoltage: Ͻ0.2 V and electrochemical windows wider than 3 V. A specific conductivity of 2-5 ϫ 10 −3 ⍀ −1 cm −1 could be measured between −10 and 30°C for these solutions, similar to that of standard electrolyte solutions for Li batteries. Mg ions intercalate reversibly with Chevrel phase ͑Mg x Mo 6 S 8 ͒ cathodes in these solutions. These systems exhibit high thermal stability. The solutions may enable the use of high voltage, high-capacity Mg insertion materials as cathodes and hence open the door for research and development of high-energy density, rechargeable Mg batteries.

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]

Electrodes F 3000 Progress in Rechargeable Magnesium Battery Technology. -Mo6S8-nSen (n = 0, 1, 2) Chevrel phases are synthesized from Cu 2.5 Mo 6 S 8-n Se n nanomaterials from which Cu is leached in an oxygen containing acid aqueous solution. The precursors are prepared by heating MoS2, MoSe2, Mo, and Cu in stainless-steel cells at 900°C for 16 h. A partial substitution of sulfur by selenium in Mo6S8-nSen Chevrel phases enables to elaborate fast cathodes for rechargeable magnesium batteries in which phenomena such as charge (Mg 2+ ) trapping are avoided. The practical capacity of Mo 6 S 6 Se 2 cathodes is higher than that of Mo6S8 cathodes. In addition, new solutions prepared from phenyl-MgCl and AlCl3 etheral solutions allow highly reversible behavior of Mg electrodes and high conductivity. -(AURBACH*, D.; SURESH, G. S.; LEVI, E.; MITELMAN, A.; MIZRAHI, O.; CHUSID, O.; BRUNELLI, M.; Adv.