Evaluation of the SEI Using a Multilayer Spectroscopic Ellipsometry Model

Abstract

A multilayer spectroscopic ellipsometry (SE) model has been developed to characterize SEI formation. The model, which consists of two Cauchy layers, is constructed with an inner layer meant to model primarily inorganic compounds adjacent to an electrode and an outer layer which mirrors polymeric, organic constituents on the exterior of the SEI. Comparison of 1:1 EC:EMC and 1:4 EC:EMC with 1.0 M LiPF 6 shows distinct differences in the two modeled layers. The data suggest that the thickness of both layers change over a wide potential range. These changes have been linked with other reports on the growth of the SEI. In Li-ion batteries (LIBs) the electrolyte has a unique role, not only for transport of Li, but also for the formation of the solid electrolyte interphase (SEI) which provides cell stability and practical existence. 1As a film formed from electrolyte components, SEI growth is related to the reduction of electrolyte components at the anode interface. 2Generally 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).3-8 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. As the optical properties and thicknesses of surface films change, so do and .To appropriately analyze experimental data, optical models for SE analysis are constructed. A model which uses realistic refraction index (n), extinction coefficient (k) and other measures of physicality such as the number of films present is important when interpreting SE data. When applied to monitoring SEI growth, prior SE works have largely relied on describing the SEI as a single layer. 10,12,14 This is partially due to the desire to obtain a fit with the fewest variables possible. 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. 3-8Experimental 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. Two Teflon blocks were used to maintain parallel electrode placement (5.5 mm spacing). This spacing allows acquisition of SE data at angles near the Brewster angle for many solid-liquid interfaces (70-75• ) while minimizing electrolyte volume (nominally 1 mL...

Use of phosphoranimines to reduce organic carbonate content in Li-ion battery electrolytes

“…Blend 3 was chosen as it has the highest PA-5 content while still falling within the generally acceptable range for electrolyte viscosity and conductivity while blend 1 provides information on just PA-5. In Figure 5A it is seen that relatively high cathodic currents are observed on the initial sweep for blend 3 from open circuit on the Ni electrode with the reductive processes initiating near 1.7 V.A secondary, more clearly defined peak is observed near 1.2 V. These initial reductive processes correspond well with previous reports of EC-containing electrolytes on noble, Cu and Ni electrodes[29][30][31]. Upon change in sweep polarity there is no appreciable anodic current density (j) observed.…”

Galvanostatic Li Electrodeposition in LiTFSI-PC Electrolyte: Part I. Effects of Current Density in Initial Stage

“…3b for the current density of 8 to 60 mA cm −2 where ΔV clearly appears. Here, the starting point of Li deposition was defined as the time of Q L in Table I and the thickness of SEI was assumed to be δ = 30 nm measured by ellipsometer 20 and cryo-transmission electron microscopy 21 in previous reports.…”