Probing Mechanical Properties of Solid-Electrolyte Interphases on Li Nuclei by In Situ AFM

Mechanical properties of solid-electrolyte interphases (SEIs) play pivotal roles in maintaining reversible cycling of Li metal anode. However less attentions have been paid to the integration of kinds of SEIs on Li nuclei, and precise characterization of mechanical properties of SEIs also appear challenging. Herein, we employ combined in-situ atomic force microscope (AFM) based nanoindentation and peak force quantitative nanomechanics (QNM) methods to explore structures, thickness and Young's moduli of three kinds of SEIs which may appear in "anode-free" Li metal batteries, and correlate mechanical properties with chemical and/or electrochemical origins. Results show that SEIs formed by electrochemical reduction bear conventional double layer structures and are much thicker with smaller Young's moduli, compared with the inorganic-organic hybrid SEIs formed with involvement of chemical reactions. In-situ AFM monitoring of morphology evolution shows that coexistence of different kinds of SEIs on individual Li nuclei, even with apparently minor differences in thickness and Young's moduli, could result in breakages of SEI shells upon dissolution of Li nuclei. Our work reveals the importance of integration of kinds of SEIs on Li nuclei and demonstrates the advantage of combined use of nanoindentation and QNM methods in understanding the cause mechanical point of view.