Antoine Seyeux

Effect of Lithiation Potential and Cycling on Chemical and Morphological Evolution of Si Thin Film Electrode Studied by ToF-SIMS

Si thin films obtained by plasma enhanced chemical vapor deposition (PECVD) were used to investigate chemical and morphological modifications induced by lithiation potential and cycling. These modifications were thoughtfully analyzed by time-of-flight secondary ion mass spectrometry (ToF-SIMS) depth profiling, which allows to distinguish the surface and bulk processes related to the formation of the solid electrolyte interphase (SEI) layer, and Li-Si alloying, respectively. The main results are a volume expansion/shrinkage and a dynamic behavior of the SEI layer during the single lithiation/delithiation process and multicycling. Trapping of lithium and other ions corresponding to products of electrolyte decomposition are the major reasons of electrode modifications. It is shown that the SEI layer contributes to 60% of the total volume variation of Si electrodes (100 nm). The apparent diffusion coefficient of lithium (DLi) calculated from the Ficks second law directly from Li-ion ToF-SIMS profiles is of the order of ∼5.9 × 10(-15) cm(2).s(-1). This quite low value can be explained by Li trapping in the bulk of electrode material, at the interfaces, continuous growth of the SEI layer and increase of SiO2 quantity. These modifications can result in limitation the ionic transport of Li.

Anodic TiO2 Layer Conversion: Fluoride-Induced Rutile Formation at Room Temperature

The present work demonstrates that an amorphous anodic oxide on titanium can be converted to a micrometer thick nanoporous rutile layer upon extended polarization in a fluoride-containing electrolyte, whereas extended polarization in a fluoride-free electrolyte leads to a conversion to anatase. Titanium was preanodized in fluoride-free 1 M H 3 PO 4 for 10 min, then anodization was either continued for 5 h in the presence or absence of fluoride ions and the resulting layers characterized. Scanning electron microscopy, X-ray diffraction, and Raman and photocurrent spectroscopy confirm ion-selective structure conversion and extended layer growth at room temperature.

Aging-induced chemical and morphological modifications of thin film iron oxide electrodes for lithium-ion batteries.

Spectroscopic (XPS, ToF-SIMS) and microscopic (SEM, AFM) analytical methods have been applied to iron oxide (∼Fe2O3) using a thin film approach to bring new insight into the aging mechanisms of conversion-type anode materials for lithium-ion batteries. The results show that repeated lithiation/delithiation causes both chemical and morphological modifications affecting the electrochemical performance. The SEI layer formed by reductive decomposition of the electrolyte remains stable in composition (mostly Li2CO3) but irreversibly thickens upon multicycling. Irreversible swelling of the material accompanied by penetration of the SEI layer and accumulation of non-deconverted material in the bulk of the oxide thin film occurs upon repeated conversion/deconversion. After initial pulverization of the thin film microstructure, grain growth and aggregation are promoted by multicycling. This leads to capacity increase in the first few cycles, but upon further cycling volume expansion and accumulation of non-deconverted material lead to deterioration of the electrode performances.

Modeling of Growth and Dissolution of Nanotubular Titania in Fluoride-Containing Electrolytes

In this paper, model calculations of diffusion processes and pH profiles inside TiO 2 nanotubes are performed in order to explore key factors in the growth mechanism of this system in aqueous electrolytes. An electrochemical steady state featured by an equivalent rate between oxide growth and dissolution is reached for a given current efficiency. Electrochemical oxide growth is found to be exclusively located at the pore bottom, whereas chemical oxide dissolution is uniformly distributed over the whole nanotube. It can be deduced from the results that electrolyte resistance or diffusion processes in the electrolyte inside the tubes are not limiting.

Local Degradation Mechanisms by Tarnishing of Protected Silver Mirror Layers Studied by Combined Surface Analysis

In this work, we addressed the local degradation mechanisms limiting the prelaunch environmental durability of thin-layered silver stacks for demanding space mirror applications. Local initiation and propagation of tarnishing were studied by combined surface and interface analysis on model stack samples consisting of thin silver layers supported on lightweight SiC substrates and protected by thin SiO2 overcoats, deposited by cathodic magnetron sputtering and submitted to accelerated aging in gaseous H2S. The results show that tarnishing is locally initiated by the formation of Ag2S columns erupting above the stack surface. Ag2S growth is promoted at high aspect ratio defects (surface pores) of the SiC substrate as a result of an imperfect protection by the SiO2 overcoat. Channels most likely connect the silver layer to its environment through the protection layer, which enables local H2S entry and Ag2S growth. The silver sulfide columns grow in number and size eventually leading to coalescence with increasing H2S exposure. In more advanced stages, tarnishing slows down owing to saturation of all pre-existing imperfectly protected sites of preferential sulfidation. However, it progresses radially at the basis of the Ag2S columns underneath the protection layer, consuming the metallic silver layer and deteriorating the protecting overcoat.