The Surface Chemistry of Lithium Electrodes in Alkyl Carbonate Solutions

The chemical composition of the surface films formed on lithium in alkyl carbonate solutions was explored using surface sensitive Fourier transform infrared spectroscopy (external reflectance mode). The solvents included propylene carbonate, ethylene carbonate, and dimethyl carbonate. The salts included LiAsF₆, LiCIO₄, LiBF₄, and LiPF₆. The advantages of this work over previous studies are that highly H reflective Li surfaces were prepared fresh in solution and that the aging processes of the surface films initially formed could be rigorously 11 investigated. Furthermore, these three important solvents were investigated in a single study. This work further proves that the films initially formed on Li surfaces in these solvents consist of ROCO₂Li as the major constituents. Upon storage, the films initially formed react with trace water to form Li₂CO₃, which gradually also becomes a major surface species. It was found that these aging processes also depend on the salts used (for example ROCO₂Li or Li₂CO₃ films are not stable in LiPF₆ or LiBF₄ solutions).