Why is the Solid-Electrolyte-Interphase Selective? Through-Film Ferrocenium Reduction on Highly Oriented Pyrolytic Graphite

To determine if the solid-electrolyte-interphase (SEI) selectively permits reduction of redox shuttles because of specific surface interactions, through-film ferrocenium reduction was studied on the edge and basal planes of highly oriented pyrolytic graphite (HOPG). Monitoring ferrocene kinetics in-situ during SEI formation demonstrated that the SEI formed in LiPF 6 -based electrolyte is unstable on both graphite orientations, and that introducing just 1.6 mM of PF − 6 to the electrolyte chemically dissolves the SEI formed in LiClO 4 -based electrolyte. When PF − 6 was avoided completely, a 2 V overpotential was sufficient to reduce ferrocenium through the SEI on the basal plane, suggesting through-film tunneling. On the edge plane, more complicated behavior was observed. On both orientations, the shuttle cation can function as an oxidizing agent that corrodes the SEI.