Ex-Situ Electron Microscopy Study of Solid Electrolyte Interphase Formed by Charge-Discharge Reaction of Silicon Negative Electrode in Lithium-Ion Secondary Battery

A SEI (Solid Electrolyte Interphase) is formed on the surface layer of the negative electrode active material of a lithium ion secondary battery (LIB) during the initial charging process, and its morphology and structure significantly affect performance and safety. in this study, by conducting ex situ experiments, SEM, TEM and STEM-EELS observations were performed on Si negative electrodes under charge state within an actual battery and Si negative electrodes directly charged on a TEM thin film. revealed morphology and structure of the SEI. All of the processes from specimen preparation for electron microscopy observation to specimen transport were performed under non-atmospheric exposure conditions. The SEI on the surface of the Si negative electrode grew thicker as the charge depth increased. On the other hands, LixSi amorphous phase due to the lithiation by solid-state reaction was confirmed inside the Si negative electrode. It was found that Li2O was formed on the most surface of the Si negative electrode at the initial stage of charging. and the SEI was mainly composed of Li2O. The SEI of about 1 mu m was observed on the Si negative electrode after 40% charge, and the thickness of the SEI decreased to less than 1/5 after discharge.