Local Structure and Stability of SEI in Graphite and ZFO Electrodes Probed by As K-Edge Absorption Spectroscopy

The evolution of the solid electrolyte interphase (SEI) during the first Li uptake in advanced Li-ion electrodes is studied by X-ray absorption spectroscopy (XAS). The As atoms present in the electrolyte solution were used as a local probe for monitoring the SEI growth on different electrodes. High-quality As K-edge spectra were collected in fluorescence mode for a set of graphite and carbon-coated ZnFe2O4 electrodes. XAS measurements have been preceded and corroborated by electrochemical characterizations. SEI phase evolution was analyzed by distinct As valence states in the subsequent stages of SEI formation, while X-ray fluorescence (XRF) was used to estimate the As content. Detailed structural results are presented for different Li contents in different electrodes including the estimated thickness of the SEI layer, contribution of different As oxidation states, and As local structure. The formation of AsFN complexes with different local coordination N is clearly observed and measured at various SEI evolution stages. Evidence of a partially reversed redox process, taking place within the SEI by charge-discharge cycling, was also obtained.