Characterization of the SEI on a Carbon Film Electrode by Combined EQCM and Spectroscopic Ellipsometry

Abstract

The electrochemical quartz crystal microbalance ͑EQCM͒ and cyclic voltammetry have been applied simultaneously to characterize electron-beam deposited carbon film electrodes in LiClO 4 -containing mixed electrolytes of ethylene carbonate ͑EC͒ and dimethyl carbonate ͑DMC͒. The structure of the carbon electrode was found to be amorphous/disordered using Raman spectroscopy. Cyclic voltammetry in LiClO 4 /EC ϩ DMC demonstrated features typical of Li intercalation/deintercalation into/from the disordered carbon electrode, and EQCM showed a corresponding mass increase/decrease. Combined EQCM and spectroscopic ellipsometry data were compared, and the solid electrolyte interphase density after the first cycle in LiClO 4 /EC ϩ DMC was estimated to be 1. Carbon electrodes, in both graphitic and amorphous forms, are used as negative electrodes in secondary Li-ion batteries. The limiting capacity of LiC 6 is 372 mAh/g for graphite, but higher capacities have been observed with certain disordered carbons. When a carbon electrode in a nonaqueous electrolyte is charged for the first time from its open-circuit voltage ͑OCV͒ down to 0 V vs. Li/Li ϩ , Li ions are intercalated into the carbon structure. The potential region over which Li ions are intercalated depends on the structure of the carbon and the type of electrolyte. Along with Li intercalation, other electrochemical/chemical reactions that occur during the first few cycles are electrolyte decomposition on the carbon surface and the formation of an electrode surface layer, often referred to as the solid electrolyte interphase ͑SEI͒. The reactions involved in SEI formation are associated with so-called irreversible capacity loss. The SEI properties and chemical composition have been subjects of intense scientific interest because the SEI plays a critical role in the safety and cycle life of secondary Li-ion batteries.1-5 The current understanding of the role of the SEI is that solvated Li ions in the liquid electrolyte lose their solvation shells while penetrating the SEI and are incorporated into the carbon structure in a solvent-free form. In addition, the SEI not only dramatically slows the kinetics of electrolyte decomposition but also reduces active lithium consumption by forming a physical barrier between the lithiated carbon electrode and the electrolyte.Spectroscopic ellipsometry ͑SE͒ is a surface-sensitive tool that is capable of detecting changes in the surface composition/structure at the molecular level. Even a small change in species adsorption on a surface or the formation of nanometer-scale surface film can lead to measurable changes in the ellipsometry parameters ⌬ and ⌿. However, to obtain a direct and unambiguous correlation between the detailed processes occurring on the electrode surface with the changes in the ellipsometric parameters requires precise information on the optical properties of the surface and all candidate film components.The electrochemical quartz crystal microbalance ͑EQCM͒ technique is based on the piezoelectric effect of a quartz cry...

Evaluation of the SEI Using a Multilayer Spectroscopic Ellipsometry Model

“…3,4,7,8 While previous work has been performed using SE to evaluate the SEI in LIBs these works have focused almost solely on describing the SEI as a single component system. 9,10,12 The present work builds upon these prior studies by developing an optical model for SE analysis of the SEI which more closely mirrors the multilayer structure observed by other methods.As an in situ method, SE analysis escapes experimental difficulties associated with ex situ analysis including SEI exposure to air and …”

“…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.…”

“…[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 .…”

“…[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. As the optical properties and thicknesses of surface films change, so do and .…”

“…The loss in thickness can be attributed to the dissolution of semisoluble SEI components and corresponds, with reported mass loss as determined using a quartz crystal microbalance (EQCM) for similar CV-based experiments on a carbon electrode. 10 While the SE data suggest the multi-layer model adequately describes the system, secondary evaluation of the as formed SEI layers was performed using FTIR and SEM analysis. To aid in identifying how the layers are formed samples were cycled to either 0.1 or 1.25 V using the same voltammetric regime described above.…”