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 LiClO₄, LiAsF₆, and LiSO₃CF₃. Over a range of conditions, the main components detected in the initial surface films were lithium alkyl carbonates (RCO₃Li, R = alkyl). Another alkyl carbonate solvent, diethyl carbonate, was found to react with lithium to form lithium ethyl carbonate, CH₃CH₂CO₃Li. In addition to solvent reduction, XPS measurements gave indication of salt reduction reactions. LiClO₄, LiAsF₆, and LiSO₃CF₃ were reduced by lithium to form halide ions, which were detected on the lithium surface. Two possible mechanisms for the formation of alkyl carbonates are discussed. One is the nucleophilic reaction of propylene carbonate with basic species such as OH^-, while the other involves one-electron reduction 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 surfaces without a mechanically stable surface film, such as those of lithium/mercury amalgams, the reduction reaction is believed to proceed by an overall two-electron process, and the primary product is lithium carbonate.