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Quasi-In-Situ Scanning Electron Microscopy and Energy Dispersive X-ray Spectroscopy during Cyclic and Calendaric Aging of Silicon Nanoparticle Anodes

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Obtaining a detailed understanding of cyclic and calendaric aging is essential for confronting the challenges of silicon anodes. Recent developments in in situ and in operando scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques showed insights into the volume expansion and associated crack formation during de-/lithiation of silicon. Due to the high vacuum in the SEM chamber in situ batteries require complex cell designs and uncommon electrolytes such as ionic liquids, polymer electrolytes or cooling of the cell to prevent electrolyte evaporation. This leads to a complex and restricted cell set-up. Furthermore, the in situ SEM set-up limits the resolution of the images to the micrometer scale due to the presence of the electrolyte. Furthermore, the measurement is often limited to only a few charge-/discharge-cycles due to beam damage. In addition, in situ energy dispersive X-ray spectroscopy (EDX) analysis of the electrode surface composition is challenging due to the presence of the electrolyte. On the contrary, ex situ SEM and EDX measurements compare different electrodes leading to possible misinterpretation due to artefacts and minor differences in the electrode structure. To overcome the challenges of in situ and ex situ SEM measurements the aspects of both techniques were combined in this study allowing comparable images and standard operation conditions regarding cell design, temperature and pressure with a carbonate-based electrolyte. SEM images and EDX spectra of the same spot of silicon nanoparticles were obtained after the initial cycle (formation procedure) and after 10 subsequent cycles or after 4 weeks respectively (calendaric aging) at the same state of charge. A quasi in situ (QUIS) environment was ensured by disassembling standard coin cells under argon atmosphere and transferring the anodes under vacuum in and out of the SEM. QUIS-SEM/EDX contributes to a more detailed understanding of degradation mechanisms of silicon nanoparticle-based anodes, because minor structural changes, that usually go unnoted in ex situ SEM/EDX, can be detected. Comparing anodes that were cycled at different C-rates after 10 cycles via QUIS-SEM/EDX showed that the irreversible volume expansion is dominant at lower C-rates whereas the fractured structure is dominant at higher C-rates. In consistency, calendaric aging showed a pronounced irreversible volume expansion at low potential. Furthermore, EDX spectra showed a pronounced SEI growth in the carbon black rich area, suggesting that the conductive additive must be considered for the formation of a homogeneous SEI.

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