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Abstract: Ultra-thin Cr2O3 films (12.0, 17.3 and 29.6nm thick) were produced on Cr metal by thermal oxidation, and their electrochemical properties in 1M LiClO4 in propylene carbonate (PC) were investigated by cyclic voltammetry and chronopotentiometry. The reductive electrolyte decomposition and the conversion/deconversion process were observed and analyzed by X-ray photoelectron spectroscopy (XPS), polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). The initial irreversible capacity due to the reduction of electrolyte and the incomplete deconversion process during the first cycle is 70% of the first discharge capacity. A stable charge/discharge capacity of 460mAhg−1 was obtained in the 3rd to 10th cycles. XPS and PM-IRRAS evidenced the growth of a solid electrolyte interphase (SEI) layer that is constituted of Li2CO3 formed by reductive decomposition of the electrolyte. The SEI layer thickness and/or density is modified by the conversion/deconversion reaction. ToF-SIMS evidenced the volume expansion/shrink resulting from the conversion/deconversion reaction. ToF-SIMS also revealed an incomplete conversion process limited by mass transport, which partitions the oxide into a converted outer part assigned to Li2O containing Cr traces and an unconverted inner part ascribed to Cr2O3 or lower Cr oxide containing Li. It was found that the deconversion re-homogenizes the oxide film in a single layer but with lithium trapped in it. The present study provides a detailed understanding of the interfacial reaction on the oxide anode undergoing a conversion/deconversion reaction. [Copyright &y& Elsevier]

Abstract: The intercalation and deintercalation mechanisms of lithium into V2O5 thin films prepared by thermal oxidation of vanadium metal have been studied by X-ray photoelectron spectroscopy (XPS) using a direct anaerobic and anhydrous transfer from the glove box (O2 and H2O<1ppm), where the samples were electrochemically treated, to the XPS analysis chamber. Vanadium in the as-prepared oxide films is mostly (from 93 to 96% depending on samples) in a pentavalent state (V5+) with a stoichiometric O/V concentration ratio fitting that of V2O5. Four to seven percent of VO2 is also observed. After the 1st and the 2nd intercalation steps at E =3.3 and 2.8V versus Li/Li+, respectively, the V2p core level spectra evidence a partial reduction to V4+ states with a remaining concentration of 73 and 56% of V5+, in agreement with the intercalation of about 1/2mol of Li per V2O5 mol at each intercalation step. Intercalated lithium was observed at a binding energy of 56.1eV for Li1s. Changes of the electronic structure of the V2O5 thin film after intercalation are evidenced by the observation, at a binding energy of 1.3eV, of occupied V3d states (V4+) originally empty in the pristine film (V5+). The V2p and Li1s core level spectra show that the process of Li intercalation is partially irreversible. In the first cycle, 34 and 14% of the vanadium ions remain in the V4+ state after deintercalation at E =3.4 and 3.8V versus Li/Li+, respectively, indicating a partially irreversible process already after the 1st deintercalation. The analyses of C1s and O1s XP spectra show the formation of a solid–electrolyte interface (SEI). The analyzed surface layer includes lithium carbonate and Li-alkoxides. [Copyright &y& Elsevier]

Abstract: Ultra-thin (5–50nm) layers of aluminium and tantalum oxides deposited by atomic layer deposition (ALD) on a stainless steel substrate (316L) for corrosion protection have been investigated by electrochemical methods (linear scan voltammetry, LSV, and electrochemical impedance spectroscopy, EIS) and time-of-flight secondary ion mass spectrometry, ToF-SIMS. The effects of the deposition temperature (250°C and 160°C) and coating thickness were addressed. ToF-SIMS elemental depth profiling shows a marked effect of the organic and water precursors used for deposition and of the substrate surface contamination on the level of C and OH trace contamination in the coating, and a beneficial effect of increasing the deposition temperature. The polarization data show a decrease of the current density by up to four orders of magnitude with increasing coating thickness from 5 to 50nm. The 50nm films block the pitting corrosion in 0.8M NaCl. The uncoated surface fraction (quantified from the current density and allowing a ranking of the efficiency of the coating, also confirmed by the capacitance and resistance values extracted from the EIS data) was 0.03% with a 50nm thick Al2O3 film deposited at 250°C. The correlation between the porosity values of the coatings and the level of C and OH traces observed by ToF-SIMS points to a marked effect of the coating contaminants on the sealing performance of the coatings and on the corrosion resistance of the coated systems. [Copyright &y& Elsevier]

Abstract: Combined analysis by electrochemical impedance spectroscopy (EIS), time-of-flight secondary ion mass spectrometry (ToF-SIMS) and field emission scanning electron microscopy (FESEM) of the corrosion protection provided to carbon steel by thin (50nm) Al2O3 coatings grown by atomic layer deposition (ALD) and its failure mechanism is reported. In spite of excellent sealing properties, the results show an average dissolution rate of the alumina coating of ∼7nmh−1 in neutral 0.2M NaCl and increasing porosity of the remaining layers with increasing immersion time. Alumina dissolution is triggered by the penetration of the solution via cracks/pinholes through the coating to the substrate surface where oxygen reduction takes place, raising the pH. At defective substrate surface sites of high aspect ratio and concentrated residual mechanical stress (along scratches) presumably exposing a higher steel surface fraction, localized dissolution of the coating is promoted by a more facile access of the solution to the substrate surface enhancing oxygen reduction. De-adhesion of the coating is also promoted in these sites by the ingress of the anodic dissolution trenching the steel surface. Localized corrosion of the alloy (i.e. pitting) is triggered prior to complete dissolution of the alumina film on the elsewhere still coated surface matrix. [Copyright &y& Elsevier]