Visualization of Degradation Mechanisms of Negative Electrodes Based on Silicon Nanoparticles in Lithium-Ion Batteries via Quasi In Situ Scanning Electron Microscopy and Energy-Dispersive X-ray Spectroscopy

Understanding the degradation mechanisms of negative electrodes based on nanoparticles like silicon is key for developing solutions against active material cracking, pulverization, or extensive growth of the solid electrolyte interphase (SEI). Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) are common ex situ methods for post mortem analysis of lithium (ion) battery materials. Beyond that, in situ/operando methods provide more accurate insights but require specially designed cells and electrolytes with a high evaporation temperature. This study combines aspects of in situ and ex situ SEM/EDX measurements to enable detailed insights on the degradation mechanisms of silicon nanoparticle-based electrodes during charge/discharge cycling. Therein, a protective quasi in situ environment was ensured by handling electrodes of repeatedly disassembled cells under an inert atmosphere and performing a same-spot analysis by SEM and EDX in the pristine state and after formation and subsequent cycles. This so-called quasi in situ analysis allowed accurate tracking of the degradation mechanisms such as irreversible expansion, cracking, and pulverization of active material particles and evolution of the SEI. Thus, extensive irreversible volume expansion of silicon nanoparticles was detected during charge/discharge cycling at low C rates, while higher C rates facilitated pulverization. Furthermore, EDX analysis revealed pronounced SEI growth at conductive additive-rich areas.