Study of surface film formation on noble-metal electrodes in alkyl carbonates/Li salt solutions, using simultaneous in situ AFM, EQCM, FTIR, and EIS

In this study, surface film formation on nonactive-metal electrodes was analyzed using several in situ spectroelectrochemical techniques. These techniques included in situ Fourier transform infrared spectroscopy in both internal and external reflectance modes, impedance spectroscopy, electrochemical quartz crystal microbalance, and atomic force microscopy. The solutions studied included ethylene carbonate-dimethyl carbonate (EC-DMC) and EC-tetrahydrofurane (EC-THF) mixtures with Li salts, such as LiAsF₆, LiPF₆, LiClO₄, and LiBr, part of which are essential for practical Li ion batteries. This work aimed to determine the onset of surface film formation, the impact of the solvent and salt used, and to compare the stability of the surface films formed in the various systems. The onset of surface film formation in these systems usually approximated 1.5 V (Li/Li⁺). EC reduction products, probably (CH₂OCO₂Li)₂, are dominant constituents in the surface films. However, in LiPF₆ solutions, Li salt reduction products become dominant in the surface films. The surface film formation is accompanied by an injection of charge which does not form stable surface species and by dissolution processes of surface species (until the solution becomes saturated with them). The coherence of the data obtained from the four techniques mentioned above and the relative stability of the surface films formed in the various solutions are discussed.