SEI Formation and Evolution in Graphite and ZFO-C Electrodes Probed by X-ray Absorption Spectroscopy
Authors: Rezvani S. J. , Ciambezi M. , Gunnella R. , Minicucci M. , Nobili Francesco , Pasqualini M. , Munoz-Marquez M. A. , Trapananti A. , Witkowska A. , Di Cicco A. 
Published: February 2020

Li-ion batteries (LIBs) are considered to be one of the most reliable, and hazard free energy storage devices known by today’s conventional technology. However, they face several limitation in energy storage due to their dependence on the solid state electrolyte-electrode reactions. The efficiency and durability of cells are also significantly altered by SEI (Solid Electrolyte Interphase) medium that is built within electrode-electrolyte admix with various celerities during the first cell full cycle. The study and hence control of the SEI formation process remains one of the challenges in cell design. Recent efforts have been dedicated to investigate the SEI phase evolution mechanism and explore new active materials to enhance the durability and energy storage capability of the cells [1-3].

Natural graphite occupies a considerable share of the market in the anode materials in lithium-ion batteries by virtue of its low and flat potential, low cost, relatively high reversible capacity of 370 mAh/g and high columbic efficiency in proper electrolytes. On the other hand Zinc iron oxide (ZnFe2O4, ZFO) encapsulated by a carbonaceous matrix (ZFOC) is recently explored to be a high capacity anode replacement [2]. In these systems the exchange of Li+ and e− via the alloy generation and conversion. Thus, metaphases such as Li, Zn, Li2O, LiC6 are formed upon lithiation, which are dispersed in the host matrix. While the lithiation kinetics have already been probed by electrochemical impedance spectroscopy (EIS) and x-ray diffraction (XRD) analysis, clear understanding of the evolution of passivation layer evolution and properties on ZFO-C and graphite is still lacking.

In this work, performed within a European FP7 project (SIRBATT) collaboration effort, we present a study of the evolution of the SEI in these innovative anode material at selected charging steps by using the x-ray absorption fine structure (XAFS) technique. The local probe chosen for the SEI evolution are the As atoms embedded in the solid interphase as an effect of the reaction of the electrolyte (LiAsF6) at the electrode surface. The SEI evolution is thus monitored collecting As K-edge XAFS spectra thus the experiments require synchrotron radiation, used for the first time for such a purpose. The aim is to provide detailed and precise fine structural analysis of the SEI of these innovative electrodes.

 As K-edge XAFS experiments on ZFO-C and graphite based anodes at different Li charge uptakes were thus carefully performed at the LISA BM08 beamline in fluorescence mode. XAFS spectra were used to analyze the sub monolayer structure of the electrolyte related phase evolution within the thin (typically 2-20 nm) SEI layer. Graphite electrodes showed a much faster SEI formation procedure. Typical reactions leading to changes in As valence and coordination are of the type:

AsF6 +2xLi+2xe-  → LixAsF3-x +xLiF

Combined data-analysis of near-edge and extended XAFS spectra has shown the occurrence of transformations from As5+ valence state (typical of the pure electrolyte) to a combination of As3+ and As2+ valences with increasing As2+ ratio by the charging steps. Quantitative XAFS analysis [4] also confirms the transitions between the polyhedral structures of As-F formed on the SEI and a corresponding reduction of the first-neighbor coordination number of As atoms.

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