Investigating Mechanisms Leading to Enhanced Performance of Carbon-Coated Si Anodes By Chemical Vapor Deposition Method

Abstract

Prior research on carbon-coated silicon (Si@C) for Si-based lithium-ion batteries have shown improved electrochemical performance upon coating. However, the underlying mechanisms responsible for the enhancement in performance have not been fully explored and are typically attributed to the protective influence of the carbon coating around the Si particles as well as the improved electronic conductivity of the electrode. In this work, we investigate the contribution of coating on the processing of the electrodes as well as on the formed solid electrolyte interphase (SEI). Si@C particles were prepared by chemical vapor deposition (CVD) technique to achieve a homogeneous carbon layer on the Si surface. The influence of coating time as well as processing temperature was investigated. X-ray photoelectron spectroscopy (XPS), neutron reflectivity (NR) measurements, and transmission electron microscopy (TEM) reveal the coating characteristics such as its extremely thin nature, ~2.5 nm, allowing us to rule out the protective function of carbon coating to prevent Si volume expansion as the governing mechanism for the improved electrochemical behavior. Instead, Raman mapping of the electrodes reveal a uniform distribution of Si and carbon conductive additive, indicating that the coating on the Si particles can facilitate slurry processing which yields to more homogeneous electrodes. Consequently, this leads to the commonly reported improved electronic connections within the electrodes as supported by electrochemical impedance spectroscopy (EIS) data. Furthermore, SEI studies using XPS reveal similar SEI characteristics of the Si electrodes after cycling in half and full cell format. In summary, we assign the improved performance of Si@C on the beneficial effect of carbon coating on the processing of Si electrodes, but not on the electrochemical passivation.