Influence of Film-Forming Additives on Protective Layer in Lithium Metal Batteries Identified

Despite their high energy density, lithium metal batteries still have to overcome challenges with regard to their safety and lifetime. A key factor to further develop the technology is the intensive analysis to elucidate the formation of the protective layer between the metal anode and the electrolyte, the solid electrolyte interphase (SEI). A team from MEET Battery Research Center at the University of Münster and the Massachusetts Institute of Technology (MIT) has now investigated the qualitative and quantitative composition of the primary SEI in lithium metal batteries. The researchers focused in particular on identifying the influence of the common film-forming electrolyte additives vinylene carbonate (VC) and fluoroethylene carbonate (FEC) on the formation of the SEI.

Newly Developed Method for Compound Accumulation Enables Detailed Analyses

Initially, the scientists developed an innovative process to accumulate a sufficient analyte amount. This is essential to allow analysis in the first place, as the layer is much thinner before cyclization than afterwards and the components are therefore only available in very small quantities. Subsequently, they examined the compounds with a wide range of analytical methods ranging from quantitative gas analysis, over qualitative electrolyte analysis using nuclear magnetic resonance and mass spectrometry to solid residue analysis using X-Ray diffraction and quantitative gas chromatography.

“We discovered that VC independently is more effective than FEC in forming a stable SEI,” says MEET researcher Dr Bastian von Holtum. “FEC can only achieve its full polymerization potential if the electrolyte solvent ethylene carbonate (EC) is present.” In addition, the team identified further differences between the additives: both the formation of inorganic SEI compounds and the formation of gaseous and electrolyte-soluble by-products varied. “In our study, we gained insights into the processes in lithium metal batteries that could not be analyzed previously. They are an important step towards further developing the technology,” explains von Holtum.

Detailed Results Online Available

The entire study has been published by the authors Dr Bastian von Holtum, Dr Christoph Peschel, Dr Uta Rodehorst, Dr Sascha Nowak and Dr Simon Wiemers-Meyer, MEET Battery Research Center, Daniel Wang and Prof. Dr Yang Shao-Horn, Massachusetts Institute of Technology, as well as Prof. Dr Martin Winter, MEET Battery Research Center and Helmholtz Institute Münster of Forschungszentrum Jülich, in the “Journal of the Electrochemical Society”.