X-ray Photoelectron Spectroscopy Studies of Lithium Surfaces Prepared in Several Important Electrolyte Solutions. A Comparison with Previous Studies by Fourier Transform Infrared Spectroscopy

Abstract

Li electrodes prepared in situ in solutions and then stored in them for different periods were studied by X-ray photoelectron spectroscopy (XPS) including depth profiling performed by argon sputtering followed by XPS. A set of solvents, propylene carbonate (PC), ethylene carbonate (EC)−dimethyl carbonate (DMC) mixtures, and 1,3-dioxolane, and a set of salts, LiAsF6, LiBF4, LiPF6, LiN(SO2CF3)2, and LiC(SO2CF3)3, were investigated with respect to the effect of storage time. The results of this study were compared with previous studies of Li electrodes in the same solutions by in situ and ex situ Fourier transform infrared spectroscopy. Basically, the results thus obtained are in line with the previous studies. The Li surface chemistry is dominated by solvent reactions. However, all the above salt anions are also reduced to form insoluble species which also contribute to the build-up of the surface films (e.g., the salt anions of the type MF y - (M = As, P, B) are reduced to LiF and species of the Li x MF z type). The surface reactions of these solvents and the salts on Li are discussed in detail. Depth profiling of the surface films formed on Li in solutions indicates that they have a multilayer structure. The concentration of the organic salts in the surface layers decreases as the layer is closer to the Li−film interface.

Reduction Reactions of Electrolyte Salts for Lithium Ion Batteries: LiPF6, LiBF4, LiDFOB, LiBOB, and LiTFSI

“…This mechanism is consistent with a combination of previously reported computational and experimental results. 14 …”

“…The SEI has been proposed to contain lithium alkyl carbonates, lithium carbonate, lithium oxalate, lithium alkoxides, and lithium oxide from the carbonate solvents and LiF, lithium fluorophosphates, lithium fluoroborates, and lithium oxalate from the reduction of electrolyte salts, depending upon the salt utilitzed. [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] Electrolyte additives have also been used to tailor the properties of the SEI through preferential reduction on anode. 1 Despite significant effort over the last two decades, the formation mechanism of the SEI is not well understood.…”

Evaluation of the SEI Using a Multilayer Spectroscopic Ellipsometry Model

“…2 Generally the SEI is viewed as consisting of multiple components with a primarily inorganic inner layer (adjacent to the electrode) with a more organic outer layer (adjacent to the electrolyte). [3][4][5][6][7][8] Spectroscopic ellipsometry (SE), by following changes in the polarization of light as it interacts with surface films, provides an in situ route to follow thickness changes in the SEI. [9][10][11][12][13][14] In typical experiments, the amplitude ratio ( ) and the phase difference ( ) are used to describe the orientation of the reflected light based on Fresnel amplitude reflection coefficients.…”

“…Keeping this tenant in mind, the present work looks to build on prior studies of SEI formation using SE by looking at an optical model for SEI growth which incorporates two layers to more closely align with non-SE studies of SEI composition and growth. [3][4][5][6][7][8] Experimental SE experiments used an M2000V spectroscopic ellipsometer (JA Woollam) and a custom designed cell ( Figure 1) which consisted of a fused silica tube (19 mm od) with one end enclosed and the other end capped using a Swagelok vacuum fitting. 12,15 No loss or change in electrolyte appearance was observed during evaporation studies of up to two weeks indicating the cell maintained an inert atmosphere.…”

Review—Post-Mortem Analysis of Aged Lithium-Ion Batteries: Disassembly Methodology and Physico-Chemical Analysis Techniques

“…Early FTIR investigations of materials in Li-ion cells were conducted in Aurbach's group 169,170 and by Yoshida et al 171 and focused on understanding the chemical characteristics of the SEI on Li and graphite-based anodes. These studies allowed identifying important reflections bands of the SEI as the asymmetric carbonyl stretching at 1650 cm FTIR studies on samples from Post-Mortem analyses have also been conducted aiming to tackle differences when using electrolyte additives.…”