On the Redox Activity of the Solid Electrolyte Interphase in the Reduction/Oxidation of Silicon Nanoparticles in Secondary Lithium Batteries

Chemical and structural modifications occurring in homogeneous crystalline Si nanoparticles (NPs) used as anode material in Li cells are investigated. State-of-the-art high-resolution scanning transmission electron microscopy coupled with electron energy loss spectroscopy resolved at the nanoscale is exploited. It is directly highlighted by electron spectromicroscopy that, above 0.1 V versus Li, the electrochemical activity of Si electrodes involves a complex interplay between Li incorporation, electrolyte degradation, and Si reduction/oxidation. These redox processes occur upon cycling through partially reversible reactions mediated by the solid electrolyte interphase. Overall, a SiO2 amorphous layer forms in the oxidized electrodes at the Si NPs interface with the electrolyte: this oxide shell partially dissolves upon reduction to give Li2CO3 and amorphous Si. Si NPs cores are therefore eroded upon cycling as their outer layers are directly involved in a reversible oxygen shifting mechanism at the interface, whereas unreacted SiO2 accumulates cycle-by-cycle. These findings extend the comprehension of the Si pulverization mechanism in Li batteries.