Surface Analysis of the Negative Electrode in Lithium-Sulfur Secondary Batteries

Novel solutions for energy storage are considered for mobile applications, especially for the electromobility which is deemed to replace the combustion engine in the future. The electrochemical battery stores electricity by a chemical conversion or intercalation. The state-of-the-art technology is the Lithium-Ion Battery (LIB) which is utilized in mobile devices like notebooks, shavers, cellular phones, various electrical tools, and also electric vehicles. However, this technology utilizes costly transition metals and may not provide sufficient storage capacity for an acceptable range for a commercially available electric car. Therefore, novel battery systems with a higher specific energy density are investigated within the electrochemistry community and by the chemical as well as the automotive industry. Particularly, the battery system based on sulfur as positive electrode material is of interest because the material is abundant, inexpensive, and non-toxic. Companies like SION Power or OXIS Energy have been specializing in the development of lithium- sulfur batteries. Also, many chemical companies like the BASF or LG Chem which are focusing on the role as supplier for the necessary chemical components, the electrolyte compounds in particular, are also researching lithium-sulfur batteries. Indeed, the lithiumsulfur battery is not fully understood and many drawbacks have not been overcome by a complete control of the battery chemistry yet. In this thesis, the focus is set on the surface analysis of the negative electrode, the lithium metal electrode, in the mentioned lithium-sulfur battery system. Lithium is a reactive material that causes side reactions besides the cell reaction of lithium and sulfur to lithium sulfide. These side reactions render a crucial drawback of the lithium-sulfur cell and necessarily need to be kept in check. However, the formation of a surface film on the lithium electrode, the Solid Electrolyte Interphase, by the side reactions is likewise deemed to be beneficial since the film intrinsically prevent further reactions of the lithium metal with the electrolyte. Therefore, the side reactions with the electrolyte, generally applied in lithium-sulfur cells, are investigated with X-ray Photoelectron Spectroscopy and Secondary Ion Mass Spectrometry as surface analytical tools in this work