SEI formation and evolution in graphite and ZFO-C electrodes probed by X-ray absorption spectroscopy

Rechargeable lithium-ion cells battery are widely used today for commercial applications due to their high energy density, and good cyclability. Starting from the first full cycle of the cell a layer of decomposed electrolyte, called SEI (solid electrolyte interphase), forms on the electrodes surface; this leads to alterations of efficiency, durability and to an irreversible capacity loss. Understanding the complex surface chemistry of the electrodes is crucial for enhancing the long-term variability of those cells [1,2]. Graphite is widely used as commercial carbon insertion host due to its relatively high capacity (372 mAh/g), safety and low cost. More recently, zinc ferrite (ZnFe2O4) encapsulated in a carbonaceous matrix (ZFO-C) has been proposed as mixed alloying-conversion anode material showing an higher charge/discharge capacity and cycling efficiency mostly related to the formation of a stable and efficient SEI layer[3]. In this work, performed within an European FP7 project (SIRBATT) collaboration effort, we present a study of the SEI at selected anode capacities using the technique of X-ray absorption spectroscopy (XAS). Arsenic atoms, present in the electrolyte as Li salts (LiAsF6), are used as local probe for As K-edge XAS for monitoring the SEI evolution on the electrodes. Analysis of X-ray Absorption Near-Edge spectra has been performed to measure the quantity of As in its different valence states, then, Extended X-ray Absorption Fine Structure refinement[4] has been used for the detailed characterization of the local structure around As atoms within the SEI. Those experiments gave unique results about the SEI evolution[5]: as the estimated thickness, weight of different As oxidation states, average As − F distances and coordination numbers. References: [1] E. Peled; Journal of The Electrochemical Society 1979, 126(12), 2047–2051. [2] P. Verma, P. Maire and P. Novák; Electrochimica Acta 2010, 55(22), 6332–6341. [3] D. Bresser, E. Paillard, et al.; Advanced Energy Materials 2013, 3(4), 513–523. [4] A. Filipponi, A. Di Cicco and C. R. Natoli; Physical Review B 1995, 52 (21), 15122-15134.