Invited: Critical Role of Si Nanoparticles Surface on Lithium Cell Electrochemical Performance
Silicon is always recovered by a native silicon oxide layer. An electrochemical reduction of the SiO2 surface layer occurs at the first cycle with the formation of lithium silicate, Li4SiO4, and Li2O as degradation products as proven by XPS . Moreover, the silicon oxide layer is composed of hydroxylated silanol (SiOH) groups which likely dramatically increases the reactivity compared to the dehydroxylated siloxane phase (Si-O-Si) . As a consequence various efforts have been made to reduce the thickness of SiO2 layer on silicon nanoparticles to decrease the first irreversible capacity and to enhance electrochemical performance.
Although the surface of Si particles is playing a major role in the electrochemical performance, it has rarely been characterized in depth. A combination of techniques is here used to finely describe the surface of Si nanoparticles: Diffuse Reflectance Infrared Fourier Transform (DRIFT) spectroscopy combined with TGA, Raman spectroscopy, solid-state high-resolution Nuclear Magnetic Resonance (MAS NMR), TEM coupled with Electron Energy Loss Spectroscopy (EELS), X-Rays Photoelectron Spectroscopy (XPS), and finally Broad Band Dielectric Spectroscopy (BDS).
With respect to previous works and common belief, we demonstrate, on the electrochemical performance, a favorable effect of a particular thin layer silicon oxide with a well-defined SiO2 composition at its extreme surface.
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Figure 1. Discharge capacity for SiA (commercial powder), SiU (home-made powder) and SiUd (home-made powder with designed durface) in LP30+FEC-VC electrolyte vs Li metal electrode in Swagelock cell. Composite electrode with CMC binder and prepared at pH=3.