Model Studies on the Solid Electrolyte Interphase Formation on Graphite Electrodes in Ethylene Carbonate and Dimethyl Carbonate: Highly Oriented Pyrolytic Graphite

Abstract

Aiming at a deeper understanding of the solid electrolyte interphase (SEI) formation on carbon anodes in lithium-ion batteries, we performed a combined electrochemical and spectroscopic model study using structurally well-defined graphite model electrodes (highly oriented pyrolytic graphite, HOPG) and simplified model electrolytes (ethylene carbonate (EC) + 1 M LiPF 6 or dimethyl carbonate (DMC) + 1 M LiPF 6 ). In cyclic voltammetry measurements, we find initial activation of the reductive electrolyte decomposition at faster scan rates (1 or 10 mV s À 1 ), whereas this is not the case at a slower scan rate (0.1 mV s À 1 ). This activation effect, which is more pronounced for DMC, is explained by an increase in the HOPG surface area, presumably by electrode exfoliation; it is not observed on surface-defect-rich samples. XPS analysis shows that, regardless of the solvent and the scan rate, the SEI is mainly composed of LiF and only small amounts of solvent and other salt decomposition products.[a] I.

Model Studies on Solid Electrolyte Interphase Formation on Graphite Electrodes in Ethylene Carbonate and Dimethyl Carbonate II: Graphite Powder Electrodes

“…Finally, we will compare the present results with our previous findings obtained on HOPG model electrodes in single-solvent electrolytes. [33] Before presenting and discussing the results, we will briefly summarize previous studies and conclusions relevant for the understanding of our data. Numerous electrochemical studies employed HOPG as a model electrode with a low defect site density to investigate the formation and composition of the SEI under structurally well-defined conditions.…”

“…Nevertheless, a detailed understanding of the SEI formation mechanisms is still missing, mainly due to the complex situation in realistic LIBs which include electrode materials, electrolyte and reaction conditions. We thus started an extensive study of the SEI formation process, employing materials with reduced complexity such as structurally well-defined highly-oriented pyrolytic graphite (HOPG) as model systems [32,33] and simplified reaction conditions such as ultrahigh vacuum conditions [32] or electrochemical measurements in single-solvent model electrolytes. [33] In the present work, we extended this to a more realistic situation, studying the SEI formation at two different binder-free graphite powder film electrodes in 1 M LiPF 6 -containing single-solvent electrolytes, either ethylene carbonate (EC) or dimethyl carbonate (DMC), which are components of the commonly used battery electrolyte LP30 (1 M LiPF 6 + EC/ DMC, 1 : 1 v/v).…”

“…[22,40,41] The edge plane, on the other hand, was shown to be active both for preferential salt decomposition [22,40,41] and for Li + (de-)intercalation. [43,44] Several studies reported exfoliation of the HOPG electrode, most likely induced by solvent co-intercalation (especially in DMC [15,33] ). Individual, HOPG-supported graphite particles, which were studied by electrochemical atomic force microscopy, displayed an enhanced SEI formation activity and a better cell performance, which was attributed to the higher concentration of defect sites.…”

Surface Science and Electrochemical Model Studies on the Interaction of Graphite and Li‐Containing Ionic Liquids

“…Such an increase in reductive currents in Li + ‐containing electrolyte at potentials below 0.2 V is usually attributed to Li + intercalation into graphite . Although the intercalation of Li + into the basal plane of HOPG substrates is very slow, it still may take place, for example, via step and edge defects on the surface . In addition to Li + intercalation, the currents observed around 0.2 V have also been attributed both to [TFSI] − decomposition and to [BMP] + intercalation as competing processes .…”

“…The resulting XP spectra were evaluated by using a semi‐quantitative approach similar to the one described in detail in ref. . Briefly, all binding energy scales were calibrated by using the F 1s peak of [TFSI] − at 689.3 eV as reference.…”