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

Abstract

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.

Metal Carbide Additives in Graphite‐Silicon Composites for Lithium‐Ion Batteries

“…The presence of Si improves the capacity for all materials, i. e., almost double, during the first cycle. Besides, the SEI formation occurs in the discharge process between 0.5–0.7 V, Figure 2; [37] in subsequent cycles, this slope change disappears (Figure S1). The first discharge involves in the range of potential below 0.2 V the lithiation process of Li + ion intercalation in the structure of the active material: xLi + +6 C→Li x C 6 (0<x≤1) and the Li‐alloy formation: yLi+Si→Li y Si (1.71<y≤4.4) [38, 39] .…”

Superior compatibility of silicon nanowire anodes in ionic liquid electrolytes

“…The different cycling performances can be due to the different densities and porosity of the SEI on the Si surface. The SEI layer onto the Si anodes cycled in LiFSI-EMIFSI and organic electrolytes may be rich in stable LiF and -Si-F compounds which, thanks to their high bonding energy, do not decompose during cycling and promote stable interface with the electrolyte [81,82] . Although, in the other case, the surface layer can contain less stable compounds, such as metastable and less dense, linear alkyl carbonates [-Si-OCH 2 CH 2 OCO 2 Li, -Si-CH 2 CH 2 OCO 2 Li, R(OCO 2 Li) 2 ] and, due to their low bonding energy, can continue to decompose upon cycling, thus feeding irreversible redox reactions that promote silicon dendritization and pulverization, as has been shown by Bongiorno et al The low density of the SEI layer allows the transition of the electrolyte during the Li + insertion, which causes the breakup of the -Si-Si-network bond, giving rise to porous structures and cracking [83] .…”

Imaging the Surface/Interface Morphologies Evolution of Silicon Anodes Using in Situ/Operando Electron Microscopy

“…The unstable SEI keeps consuming and capturing lithium ions and electrolyte during electrochemical cycling as a result of the constant volume expansion. − A variety of rear coatings, different binders, conductive additives, and modifications to the conventional carbonate-based electrolyte were all used to try and lessen the impacts of SEI. − For instance, scanning transmission electron microscopy and electron energy loss spectroscopy provide a thorough understanding of FEC influence on SEI progression during electrochemical cycling in relation to composition and form . For example, in situ HRTEM experiments on a single Si/C nanowire also demonstrate the lithiation processes (Figure i,j).…”