Effect of Graphite Orientation and Lithium Salt on Electronic Passivation of Highly Oriented Pyrolytic Graphite

This work studies the effect of edge-to-basal plane ratio on the macroscopic formation kinetics and electrochemical properties of the solid-electrolyte-interphase (SEI). The relative fraction of edge and basal planes was calculated by measuring the double-layer capacitance of highly oriented pyrolytic graphite (HOPG) in 1.0 M KCl. The formation kinetics was studied using chronoamperometry (CA) and cyclic voltammetry (CV). The electrochemical properties of the SEI were studied by CV and electrochemical impedance spectroscopy (EIS) of ferrocene. Results show that, as expected, current due to both lithium intercalation and SEI formation increases with the fraction of edge planes. After SEI formation in LiClO4-based electrolyte, the edge plane permits slightly more electron transfer to ferrocene. Attempts to form the SEI incompletely by running CV scans to progressively lower voltages show that oxygen contamination produces a more passivating SEI. Ferrocene CV shows that the SEI formation causes mass-transport limitations by either formation of a porous layer or blocking the active area of the electrode, but the kinetics of the ferrocene reaction remains fast even in the presence of the SEI. Comparison of formation CVs for LiPF6- and LiClO4-based electrolytes shows that HOPG passivates much more rapidly with LiClO4