Failure mechanisms in silicon and silicon-graphite electrodes cycled in ionic liquid electrolyte

Combining silicon and graphite in electrode materials leverages the high energy density of silicon while using graphite to mitigate the significant volumetric expansion of silicon based electrodes during cycling. Such combination will impact the formation and evolution of the solid electrolyte interphase (SEI). In this study, we compare silicon (Si) and silicon/graphite blend Si/Gr (50:50) electrodes and investigated their electrochemical performance in a highly concentrated triethyl(methyl)phosphonium bis(fluorosulfonyl)imide (P1222FSI) ionic liquid electrolyte in both half-cells and full-cells. Through a combination of transmission electron microscopy (STEM-EDX), X-ray photoelectron spectroscopy (XPS), and magic angle spinning nuclear magnetic resonance (MAS-NMR), we monitored the evolution of the SEI and silicon particles morphology. Although displaying similar capacity fading profile, failure mechanisms for Si and Si/Gr electrodes cycled in full-cell configuration are completely different. For Si/Gr blend full-cells, nano-porosification of silicon particles occurs, leading to SEI accumulation and the capacity fade is largely attributed to the loss of cyclable lithium. Contrarily, the absence of nano-porosification and an extremely stable and blocking SEI characterize Si electrodes in full-cell. This study provides new insights into how SEI evolution and silicon morphology impact the electrochemical performance of Si and Si/Gr blend electrodes across different cell configurations in non-carbonated electrolytes. • SEI evolution for Si and Si/Gr electrodes in ionic liquid electrolyte. • Different failure mechanisms in half- and full-cells. • Stable SEI for Si electrode in highly concentrated ionic liquid electrolyte.