An AC impedance spectroscopic study of the Li x CoO 2 electrode in the temperature range of 0-30 °C is presented. The results are interpreted on the basis of an equivalent circuit that includes elements related to the electronic and ionic transport in addition to the charge transfer process. The evolution of the impedance spectra with the temperature shows that a thermally activated insulator to metal transition occurs at the beginning of the deintercalation process. At intermediate intercalation degrees, the effects of the finite electronic conductivity of the material and of the charge transfer process clearly appear as separate features in the spectra at low temperature.

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found to be able to exchange Li + and e -both by conversion and alloying processes. As a consequence Fe, LiZn, Li 2 O are formed upon lithiation, which are fi nely dispersed into a carbonaceous matrix, [ 7 ] according to a reversible reaction involving nine lithium ions per formula unit of ZFO and resulting in a capacity of ≈1000 mAh g −1 . [ 7 ] While the lithiation kinetics have already been probed by electrochemical impedance spectroscopy (EIS) and X-ray diffraction (XRD) analysis, [ 5,7 ] very little is known about the evolution of passivation layer properties on ZFO-C.The aim of this work is to study the evolution of the SEI in this innovative anode material at selected charging steps by exploiting the surface sensitivity [10][11][12] of the soft X-ray absorption spectroscopy (XAS). This technique requires synchrotron radiation and was never used before for such a purpose, although it appears to be very suitable for a detailed depth profi ling of the SEI of advanced electrodes. In fact, XAS experiments in the 50-1000 eV photon energy range can be typically performed using both total electron (TEY) and total fl uorescence (TFY) yield techniques for which effective probing depths are around 2-10 nm and 70-200 nm, respectively. In this study, ex situ TEY and TFY X-ray absorption experiments have been conceived and realized to study the modifi cation of the signals related to the various atomic species in ZFO-C electrodes selected at different states of charge during the fi rst Li insertion process. XAS measurements have been preceded and corroborated by a complete electrochemical characterization including galvanostatic intermittent titration technique (GITT) and EIS, with the aim of correlating each XAS experiment with half-cell open-circuit potential (OCV) and charge, and to crosscheck the SEI evolution with the polarization of the electrodes.The samples for the experiments were prepared using carbon-coated ZFO nanoparticles (ZFO-C), obtained [ 7 ] by dispersing 1 g of ZFO powder (<100 nm, Aldrich Chemistry) in 1.5 mL of an aqueous carbon precursor solution of sucrose (Acros Organics), followed by an annealing step under inert gas atmosphere. The weight ratio was 1:0.75 for ZFO:Suc. The obtained dispersion was homogenized by means of a planetary ball mill (Vario-Planetary Mill Pulverisette 4, FRITSCH, 2× 45 min at 400/−800 rpm with 10 min rest in between). Subsequently, the dispersion was dried at 70 °C under ambient atmosphere. After grinding, the resulting composite powder was annealed in a tubular furnace (R50/250/12, Nabertherm) at 450 °C for 4 h under a constant argon gas stream. The heating rate was set to 3 °C min −1 . The material was investigated by SEM (Scanning Electron Microscopy) and TEM (Transmission Electron Microscopy) revealing that it is formed by nanoparticles of average linear dimensions of about 50 nm, with formation of some ZnFe 2 O 4 Li-ion batteries (LIBs) represent a reliable, affordable, and safe energy storage technology for use in portable application. However, current LIB active ...

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We examine the formation of the solid electrolyte interface (SEI) on anodes made of carbon encapsulated zinc ferrite (ZnFe 2 O 4 ) nanoparticles (50 nm ZFO-C) as a standard metal oxide electrode prototype. The SEI formation and phase evolution are studied by two soft X-ray absorption techniques with different probing depths in the 10−100 nm range and by surface-sensitive X-ray photoemission spectroscopy at several specific capacities of the ZFO-C anodes. These techniques are shown to be able to provide information about the nature and extension of the individual chemical species within the SEI with a typical spatial resolution of 1−5 nm. A peculiar footprint of the interphase formations is obtained by comparing the chemical history of the reactive element sites in the anodes. The progressive development of the SEI in the first cycle and the variety of compositional transformations prior to stabilization are elucidated. Formation of a reversible alkyl carbonate layer, with maximum thickness of 7 nm, is detected at the SEI topmost region. On the basis of these results, we have obtained a map of suitable spatial resolution of the evolution of the different components of the interface layer.

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