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]

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The correlation between the electrochemical properties of Li carbon intercalation electrodes and their surface chemistry in solutions was investigated. The carbons investigated were primarily graphite and petroleum coke, and the solvent systems included methyl formate (MF), propylene and ethylene carbonates, ethers and their mixtures. The surface chemistry of the electrodes was studied using mainly diffuse reflectance Fourier transform infrared spectroscopy. The following aspects were studied: (i) the effect of temperature on the buildup of the surface films; (it) the effect of additives (e.g., CO2, crown ethers), (iii) the behavior when the passive layer is built in one solution followed by cycling in another; and (iv) the

Magnesium can be reversibly deposited from ethereal solutions of Grignard salts of the RMgX type ( R = alkyl , aryl groups, and X = halides : Cl, Br), and complexes of the Mg ( AX 4 − n R n ′ R n ″ ′ ) 2 type ( A = Al , B; X = Cl , Br; R, R ′ = alkyl or aryl groups, and n ′ + n ″ = n ) . These complexes can be considered as interaction products between R 2 Mg bases and AX 3 − n R n Lewis acids. The use of such complexes in ether solvents enables us to obtain solutions of reasonable ionic conductivity and high anodic stability, which can be suitable for rechargeable Mg battery systems. In this paper we report on the study of variety of Mg ( AX 4 − n R n ) 2 complexes, where A = Al , B, Sb, P, As, Fe, and Ta; X = Cl , Br, and F; and R = butyl , ethyl, phenyl, and benzyl (Bu, Et, Ph, and Bz, respectively) in several solvents, including tetrahydrofuran (THF), 2Me-THF, 1-3 dioxolane, diethyl ether, and polyethers from the “glyme” family, including dimethoxyethane (glyme), ( CH 3 OCH 2 CH 2 ) 2 O ( diglyme ) , and CH 3 ( OCH 2 CH 2 ) 4 OCH 3 (tetraglyme), as electrolyte solutions for rechargeable magnesium batteries. It was found that Mg ( AlCl 4 − n R n ′ R n ″ ′ ) 2 complexes (R, R ′ = Et , Bu and n ′ + n ″ = n ) in THF or glymes constitute the best results in terms of the width of the electrochemical window ( > 2 V ) , from which magnesium can be deposited reversibly. These solutions were found to be suitable for use in rechargeable magnesium batteries. A variety of electrochemical and spectroscopic studies showed that these solutions have a complicated structure, which is discussed in this paper. It is also clear from this work that Mg deposition-dissolution processes in these solutions are far from being simple reactions of Mg / Mg + 2 redox couple. The conditions for optimized Mg deposition-dissolution processes are discussed herein. © 2001 The Electrochemical Society. All rights reserved.