PNNL

Abstract

Previous studies have demonstrated that chemical vapor deposition carbon coating on silicon (Si@C) can enhance the electrochemical performance of Si-based batteries. However, the underlying mechanisms contributing to this improvement have not been fully explored. We show through Raman mapping of the electrodes that carbon coating by reducing acetylene (C2H2) on ball-milled Si particles benefits a homogeneous Si and carbon distribution during the slurry casting process, thereby promoting the Si utilization during galvanostatic cycling. X-ray photoelectron spectroscopy (XPS) confirms the carbon coating on the Si surface while transmission electron microscopy (TEM) and neutron reflectivity experiments indicate an extremely thin carbon layer of less than 5 nm. Electrochemical impedance spectroscopy (EIS) measurements upon cell cycling indicate carbon coating also reduces the overall resistance as benchmarked against bare Si. Galvanostatic cycling in half-cell studies revealed higher first Coulombic efficiency and specific capacities with increasing carbon coating time. However, solid electrolyte interphase (SEI) investigations using XPS showed very similar characteristics of the coated and uncoated samples, suggesting that the SEI may only have a negligible contribution to the improved performance of Si@C. Full-cell evaluation of the Si electrodes demonstrated consistency with half-cell results relating to performance and SEI properties, further supporting the conclusion that electronic and ionic percolation, enabled by effective electrode manufacturing, is the dominant factor contributing to the favorable performance of Si@C.