Role of Surface Oxides in the Formation of Solid–Electrolyte Interphases at Silicon Electrodes for Lithium-Ion Batteries

Abstract

Nonaqueous solvents in modern battery technologies undergo electroreduction at negative electrodes, leading to the formation of a solid-electrolyte interphase (SEI). The mechanisms and reactions leading to a stable SEI on silicon electrodes in lithium-ion batteries are still poorly understood. This lack of understanding inhibits the rational design of electrolyte additives, active material coatings, and the prediction of Li-ion battery life in general. We prepared SEI with a common nonaqueous solvent (LiPF6 in PC and in EC/DEC 1:1 by wt %) on silicon oxide and etched silicon (001) surfaces in various states of lithiation to understand the role of surface chemistry on the SEI formation mechanism and SEI structure. Anhydrous and anoxic techniques were used to prevent air and moisture contamination of prepared SEI films, allowing for more accurate characterization of SEI chemical stratification and composition by X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (TOF-SIMS) depth profiling. Additionally, multivariate statistical methods were used to better understand TOF-SIMS depth profiling studies. We conclude that the absence of native-oxide layer on silicon has a significant impact on the formation, composition, structure, and thickness of the SEI.

The Effect of Fluoroethylene Carbonate as an Additive on the Solid Electrolyte Interphase on Silicon Lithium-Ion Electrodes

“…Additionally, when discussing the ion signals it is important to remember that the overlapping intensities are the result of multiple factors, including knock-in, recoil, and interfacial roughness effects. 40 Comparing the lithiated electrodes (Figures 3a,b and 4a,b), we observe changes in the chemical composition and stratification consistent with the calculations done by Balbuena and co-workers. First, from the XPS, the SEI resulting from the EC/DEC/FEC electrolyte had increased concentration of inorganic species (Figure 3b), specifically LiF (reactions 1 and 2, Scheme 1).…”

“…The resulting sputtering rates were used to transform sputtering time into depth using a simple two-layer sputtering model. 40,46,47 A homemade script executed in the iPython nootbook interpreter environment using the numpy, scipy, and pandas libraries organized and transformed the data from the time domain to depth. 48−51 The Si − mass fragment signal was used to define the relative contributions of each sputtering rate in the transition between the SEI and the silicon active material.…”

“…There are, however, two structural differences: (1) SEI formed from EC/DEC had a thicker organic stratum on top of the inorganic species compared to the SEI formed from EC/ DEC/FEC; and (2) 40 This implies that there is less Li x SiO y on the electrode cycled in EC/DEC/FEC. The simplest explanation for this observation is that the FEC reduction leads to the formation of a radical organic anion and fluoride ion (similar to the reactions 1 and 2, Scheme 1) and then reacts with native oxide present on the surface, following reaction 3:…”

A hard X-ray photoelectron spectroscopy study on the solid electrolyte interphase of a lithium 4,5-dicyano-2-(trifluoromethyl)imidazolide based electrolyte for Si-electrodes

“…A lot of work has been done to understand the SEI on graphitic negative electrodes while the field of SEI formation on silicon is still a young area with only a rather small number of publications. [4,[7][8][9][10][11][12][13][14][15][16][17][18][19] Philippe et al have…”

Estimating the thickness of diffusive solid electrolyte interface

“…On the other hand, the SEI structure on the Si surface was similar to that on the graphite surface, which was not observed before. The disappearing of the SEI structure may be caused by the drastic change of electrode surface morphology [26,28]. The electrode cracked and a new electrolyte interface was formed during the intercalation of the Si electrode simultaneously.…”