Electrochemical Charge Transfer Reaction Kinetics at the Silicon-Liquid Electrolyte Interface

Since the gravimetric lithiation capacity of silicon is roughly ten times that of graphite, while their mass densities are comparable, for the same particle size the current density required to cycle a silicon electrode at a given C-rate is about ten times greater than that of graphite. Depending on the magnitude of the corresponding Butler-Volmer exchange current density, jo, such high current densities may cause the charge transfer kinetics at the silicon-electrolyte interface to become rate limiting. Previously reported values of jo for Si differ by about 10 orders of magnitude. Here we report jo measurements using electrochemical impedance spectroscopy (EIS) for single crystal electronically conductive silicon wafers with well-defined (100) and (111) orientations and active surface areas. The electrochemical cycling regime was designed to avoid artifacts due to stress-induced surface cracking of Si upon lithiation. The exchange current density of the silicon-electrolyte interface is found to be 0.1 ± 0.01 mA/cm² when using electrolyte consisting of 1 M LiPF₆ in EC/DMC (1/1 by wt) + FEC (10 wt%) + VC (2 wt%). These results are then used to illustrate the dependence of kinetic overpotential on particle size and C-rate for silicon compared to lower volumetric capacity compounds such as graphite