Chemical, Electrochemical and Physical Properties of the Solid Electrolyte Interphase on HOPG Surface: Mechanisms of Formation and Transition Metal Deposition

Both the reaction pathways that lead to the formation of the solid electrolyte interphase (SEI) on the negative electrode of a lithium ion battery and the chemical, electrochemical and physical properties of the SEI are not well understood. Consequently, a full control of the SEI composition and properties through the selection of appropriate formation conditions is far from being achieved. In this contribution we present the influence of the formation potential on the chemical and electrochemical properties of SEIs formed on model carbon electrodes. Our results show that, on HOPG, SEI is formed via a two-step process, where the step occurs at respectively ~700 and ~400 mV vs. Li/Li⁺. The SEIs that are formed at potentials corresponding to the first or the second potential step exhibit drastically different characteristics in terms of chemical composition, thickness and transport properties. Our results are consistent with electrochemical investigations that are performed on commercial composite carbon electrodes. The SEI formed at higher potentials appears to be disadvantageous for Li⁺ ion intercalation. A further reduction of the SEI at lower potential is required to improve the intercalation.

The still debated mechanism of transition metal deposition on the anode of Li-ion batteries was also investigated on model HOPG electrode surfaces exposed to contaminated electrolyte after completion of the SEI formation. Our results indicate, that the deposition of transition metals is strongly affected by the nature of the SEI, in particular by its thickness. By introducing transition metal cations in our model system we demonstrate, that the respective deposition takes place at the SEI compact layer surface via an electron tunneling mechanism.