Identification of Surface Films Formed on Lithium in Propylene Carbonate Solutions

FTIR, IR, and XPS have been used to study the films formed on lithium in propylene carbonate solutions of LiC104, LiAsF,, and LiSO~CF~. Over a range of conditions, the main components detected in the initial surface films were lithium alkyl carbonates (RCO~Li, R = alkyl). Another alkyl carbonate solvent, diethyl carbonate, was found to react with lithium to form lithium ethyl carbonate, CH3CH~CO3Li. In addition to solvent reduction, XPS measurements gave indication of salt reduction reactions. LiC104, LiAsFG, and LiSO.~CF. ~ were reduced by lithium to form halide ions, which were detected on the l ithium surface. Two possible mechanisms for the formation of alkyl carbonates are discussed. One is the nucleophilic reaction of propylene carbonate with basic species uch as OH-, while the other involves one-electron reduc-tion of propylene carbonate by lithium metal, followed by free radical termination reactions. When high concentrations of water were present, lithium carbonate was formed by further reaction of the alkyl carbonates with water. On lithium sur-faces without a mechanically stable surface film, such as those of lithium/mercury amalgams, the reduction reaction is be-lieved to proceed by an overall two-electron process, and the primary product is lithium carbonate. Lithium electrodes are observed to be highly stable in a variety of polar aprotic solvent systems with which l ithium, on a thermodynamic basis, should be reactiv