Multiscale testing of mechanical properties of lithium anodes in conventional electrolytes in comparison to ionic liquids.

The mechanical properties of a solid-electrolyte interphase (SEI), such as hardness and Young's modulus, influence the ion transport pathways through the SEI as well as affecting the dendrite formation processes. Moreover, the local composition of the SEI has a strong influence on mechanical variations at small scales, making a thorough understanding of such properties with high detail in depth and with high spatial resolution crucial. Within this contribution, in operando atomic force microscopy (AFM) and nanoindentation are used to investigate the mechanical properties of the SEI in conventional electrolytes in comparison to those in ionic liquids. While both methods have been shown to be powerful tools to investigate SEI formation, distinctive mechanical fingerprints are demonstrated for SEIs formed in carbonate electrolytes on lithium deposited on copper in AFM-based experiments. Such fingerprints change significantly when ionic liquids are used as electrolytes and depend critically on the exact composition. These results are correlated with nanoindentation, performed on freshly cleaved lithium surfaces in an inert atmosphere and carbonate solvents. Additionally, lithium surfaces treated with varying amounts of and exposure times to ionic liquids are studied. In both cases, AFM-based experiments and nanoindentation, higher stiffnesses and storage moduli are found, indicating a highly complex SEI structure.