Due to its low atomic weight and reduction potential, the use of lithium metal as an anode material in secondary batteries has the potential to substantially increase the energy density of energy storage systems. However, the high reactivity of lithium with the electrolyte and its tendency to form dendritic structures upon deposition preclude its application in commercial secondary batteries. One possible approach to mitigate these undesired properties is to enable the formation of an electronically passivating, ionically conductive interphase layer (SEI) between the lithium anode and the electrolyte, which prevents a continuous reduction of the electrolyte while allowing lithium ions to travel through it. This interface layer should also be ionically conductive to allow the lithium ions to reach the electrode. While the reduction of electrolyte components is known to form such a layer on graphite-based anodes, the higher reactivity of lithium metal and the substantial volume changes inherent to the deposition mechanism impose more stringent requirements on the interphase layer formed on lithium.
Understanding the interphase formed on metallic lithium in liquid electrolytes requires, among other factors, an understanding of its mechanical properties. While extensive research has been conducted on the composition and physical/chemical properties of SEI layers, comparatively little attention has been paid to their mechanical properties. Furthermore, the studies that address this aspect almost exclusively focus on elastic properties. The scope of this research is to evaluate the use of nanoindentation to determine the elastic and plastic properties of interphase layers formed on lithium metal.
Initial measurements on electrochemically deposited lithium in LP57 electrolyte proved challenging due to multiple potential error sources, such as the highly heterogeneous surface morphology and electrolyte salt residues. To reduce the number of open variables, a method for cutting a lithium chip in liquid was developed, exposing an uncontaminated lithium surface to the electrolyte or electrolyte component in question. Samples with an even surface and low surface roughness were obtained, which is expected to significantly reduce measurement errors caused by surface morphology. Initial indentation measurements performed on lithium chips cut in pure EMC solvent identified a surface layer with higher elastic modulus and hardness than the underlying lithium. This method will be used to characterize SEI layers formed by various isolated solvents and additives, as well as full electrolyte systems. Ultimately, this research aims to understand how different electrolyte components affect the mechanical properties and thickness of passivation layers on metallic lithium.