STM observation of the electro-adsorption of lithium ions onto graphite and of the ensuing solid electrolyte interphase formation

The basal planes of highly oriented pyrolytic graphite (HOPG) have been observed by near field microscopy in the course of their polarization in LiClO4/propylene carbonate (PC) solutions. At +2.4 V to +2.1 V vs. Li+/Li, i.e. at moderately negative potentials vs. the potential of zero charge of graphite (assumed at 3.0 V vs. Li+/Li), a near square two dimensional (2D) configuration with the symmetry of the 2D space group P2 was observed by electrochemical scanning tunneling microscopy. It was interpreted as the image of the Helmholtz electrical double layer, evidencing Li+ ions in-plane solvated by two PC dipoles in a “frozen” state. This state may originate from the choice of Li+ ions to settle above the centers of the graphite hexagons, hereafter favoring epitaxy. At lower potentials, formation of a non-homogeneous solid electrolyte interphase film was observed by atomic force microscopy. This film was characterized by FTIR spectroscopy as predominantly composed of lithium alkyl carbonates while Li2CO3 generally found appeared here in minority part.