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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.

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The behavior of Li electrodes was studied in ethylene and diethyl carbonates (EC-DEC) solutions of LiAsF6, LiClO4, LiBF4, and LiPF~. The correlation of the surface chemistry to the interfacial properties, morphology, and Li cycling efficiency was investigated using surface sensitive Fourier transform infrared spectroscopy and impedance spectroscopy, scanning electron microscopy, x-ray energy dispersive microanalysis, and standard electrochemical techniques. The Li surface chemistry is initially dominated by EC reduction to an insoluble species, probably (CH2OCO2Li)2. Upon storage, several aging processes may take place, depending on the salt used. Their mechanisms are discussed. Although EC-DEC solutions were found to be adequate for Li ion rechargeable batteries, this work indicates that they are not suitable as electrolyte solutions for batteries with Li metal electrodes. This is mostly because Li electrodes cannot be considered stable in these systems and Li deposition is highly dendritic.

Failure mechanisms due to high charging rates of rechargeable lithium batteries comprised of Li metal anodes, Li 0.3 MnO 2 cathodes (tunneled structure), and electrolyte solutions based on the combination of 1,3-dioxolane (DN), LiAsF 6 , and tributylamine (antipolymerization stabilizer) were explored with the aid of postmortem analysis. It was found that at high charging rates, lithium deposition produces small grains, which are too reactive toward the electrolyte solution, in spite of the excellent passivation of lithium in this solution. In practical batteries such as AA cells with spirally wound configurations, the amount of solution is relatively small, and the solution is spread throughout the battery in a thin layer. Therefore, upon cycling, the Li-solution reactions deplete the amount of the solution below a critical value, so that only part of the active materials continues to function. This leads to a pronounced increase in the internal resistance of these batteries, which fail as a result of their high impedance and the decrease in the effective working electrodes area. Another failure mechanism relates to the extremely high charge-discharge current densities developed as the active electrode area decreases. These high currents, developed after prolonged cycling, lead to the formation of dendrites that short-circuit the battery, thus terminating its life.