XPS-Surface Analysis of SEI Layers on Li-Ion Cathodes: Part I. Investigation of Initial Surface Chemistry

Abstract

In this contribution, we investigate the initial surface chemistry on fully lithiated LiCoO 2 thin film model electrodes in the electrolyte solvent diethyl carbonate (DEC) and the LiPF 6 -electrolyte by means of soaking experiments. The interfacial layer composition is analyzed by X-ray photoelectron spectroscopy (XPS), and possible layer morphologies and spontaneous formation mechanisms are discussed in detail. Upon decomposition of DEC a layered system of surface-bound semi-organic components (inner layer) and cross-linked organic moieties (outer layer) is formed, while a change of the Co 3+ oxidation state and thus a surface corrosion of LiCoO 2 was not observed. In contrast, the solid electrolyte interface (SEI) film of the LiPF 6 -electrolyte soaked electrode showed an inner layer, containing predominantly corroded LiCoO 2 , i.e. Co(II,III) x O y (OH) z and LiF as well as aliphatic fluoroorganic species. The outer SEI layer consists mainly of a poly-organic network and randomly distributed Li x PO y F z domains. The thickness of the deposit on the electrolyte soaked electrode surface was only half as thick due to the significantly lower amount of organic and semi-organic compounds. Our investigation indicates that the solvent decomposition is related to the catalytically active LiCoO 2 surface, which is passivated by reaction products such as LiF originating from HF induced processes.

Exploring high-voltage fluorinated carbonate electrolytes for LiNi0.5Mn1.5O4 cathode in Li-ion batteries

“…After losing an electron, the acyl-oxygen bond in DEC molecule is broken to form the Intermediate A and free radical EtO • , then the Intermediate A further decomposed into CO 2 and ethyl carbocation. According to the result of XPS, the free radical EtO • will turn into EtOLi, CO 2 can react to form Li 2 CO 3 , ethyl carbocation will combine with F − in the electrolyte to form CH 3 CH 2 F. All above is the reaction mechanism of DEC on cathode surface, and something similar is presented by Schulz and coworkers in their report [38] .…”

Solid-electrolyte interphase nucleation and growth on carbonaceous negative electrodes for Li-ion batteries visualized with in situ atomic force microscopy

“…Not only anodic SEI but also cathodic electrolyte interface (CEI) is under ongoing investigation especially relevant for emerging high voltage materials, where 4.7 V vs. Li + /Li oxidation potential of common organic electrolytes may be surpassed. Different mechanisms underlying the first cycle irreversibility in layered oxides have been discussed, including formation of CEI [27][28][29][30][31][32][33][34] , side reactions 35 , and structural transformations [36][37][38] . The experimental results are still sparse due to absence of model samples such as HOPG and due to difficulty of in situ AFM measurements of powder samples.…”

Structural and Electrochemical Characterization of Radio Frequency Magnetron‐Sputtered LiCoO2 Thin Films

“…[ 24 ] The Li 1s signal of the LiCoO 2 surface in Figure 5c occurs at a binding energy of 55 eV, whereas the Li 1s photoelectron of LiCoO 2 is expected to arise at binding energies of ≈54 eV. [ 25,26 ] As already described for the O 1s signal, species Li 2 CO 3 , LiOH, and Li 2 O can be obtained within the Li 1s signal in the surface region of the thin film, shifting the signal to higher binding energies. For the C 1s spectrum of the analyzed film, three different types of carbon‐containing species can be observed.…”