Insights into the SEI Formation Mechanism By in-Situ Scanning Tunneling Microscopy and Electrochemical Quartz Crystal Microbalance Studies

The strongly increasing worldwide energy demand threatens to lead to a critical shortage of fossil fuel resources as well as a strong climate change in the next decades. The changeover to renewable energy sources like sun and wind and a complete transition in the transport sector to electromobility are potential solutions for the existing problems and thus under intensive discussion nowadays [1]. For this reason, strong efforts have been undertaken for the further development and improvement of electric energy storage devices such are batteries [2,3]. Li⁺-ion batteries as the front-runners especially in the area of electromobility undergo intensive research for improving efficiency, lifetime and safety [4]. These properties are crucially affected by the properties of the formed SEI (Solid Electrolyte Interphase) on the battery electrodes [5]. In order to optimize the battery performance, a complete understanding of the formation mechanism, morphology and chemistry of this interphase is inevitable. Therefore, the formation of the SEI has been in the focus of recent research [6, 7].

In this study two powerful tools, namely In-Situ Scanning Tunneling Microscopy and the Electrochemical Quartz Crystal Microbalance (EQCM) Technique, have been applied to gain further insights into the SEI formation and its morphology. The STM studies were building on earlier work by Inaba et al. [8]. Highly oriented pyrolytic graphite (HOPG) has been chosen as the model electrode for intercalation of Li⁺-ions from commercially available and alternative electrolytes. Both in-situ and ex-situ imaging was carried out. It could be demonstrated that the initially formed SEI is reversible, i.e. it dissolves when increasing the potential, while at lower potentials it becomes irreversible. Also the insertion of solvated ions could be visualized. The STM studies were complemented by EQCM measurements. EQCM with bare Au electrode coated quartz resonators have been shown to provide information on SEI formation [7]. In our studies the electrodes were modified in order to be more representative of real battery anodes and to represent more closely the experimental conditions applied in STM. The conclusions from the results of these measurements for the SEI formation in different electrolytes are discussed in this contribution. .

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