Time-Resolved X-ray Operando Observations of Lithiation Gradients across the Cathode Matrix and Individual Oxide Particles during Fast Cycling of a Li-Ion Cell

Lithiated transition metal oxides serve as active materials in the positive electrode (cathode) of lithium-ion cells. During electrochemical cycling, lithium ions intercalate and deintercalate into these oxide particles. This behavior causes two types of lithiation gradients to emerge: (i) a bulk gradient across the depth of the cathode matrix (averaged over individual oxide particles) and (ii) a microscopic gradient across the particles themselves, which also depends on their location in the electrode. Here we show how both gradients can be studied using operando X-ray diffraction during 4C charge and 4C discharge. The oxide (de)lithiation is estimated from the unit cell parameters by indexing the X-ray diffraction spectra. By fitting the lithiation profiles with orthogonal polynomials, the bulk gradients across the electrode thickness are quantified. These gradients develop as the current flows through the cell and dissipate during open-circuit and potentiostatic-hold periods. Further details of lithiation dynamics can be obtained through shape analysis of the Bragg peaks. In particular, from electrochemical model simulations, we show that the width and skewness of the (003) peak track (de)lithiation fronts moving across the individual oxide particles.

CHARACTERISATION OF OXIDES BY ADVANCED TECHNIQUES

For the safe development of GenIV nuclear reactors, it is necessary to study the compatibility of structural materials with new coolants. The current work describes the behavior of the ferritic-martensitic steel T91 in a Heavy Liquid Metal environment. Specimens were pre-stressed up to yield strength and subsequently exposed to lead-bismuth eutectic (LBE) in static conditions for 2000 hours. The aim was to identify the susceptibility to crack initiation in the selected experimental conditions. In a reference position of the sample the examination of the metal-LBE interface was carried out by means of SEM equipped with EDX. On the interface, the formation of oxide scales was observed without trace of crack initiation. The oxide was characterized by a two layers structure. From the sample, a TEM lamella was produced by FIB and subsequently the lamella was analyzed in HRTEM. The individual oxide layers were identified and characterized by SAED, EELS and EDS techniques. For a visualization of the interface between matrix and oxide STEM-HAADF and EFTEM techniques were used.

Photoelectrochemical solar cells made from SnO2/ZnO films sensitized with an indoline dye

A dye-sensitized photoelectrochemical (DS-PEC) cell consisting of SnO2 and ZnO nanoparticles was found to yield higher solar energy conversion efficiency than similar cells made of the individual oxide semiconductors when they were sensitized with an indoline dye. The SnO2/ZnO composite solar cell gave an overall energy conversion efficiency of 3.8% while the SnO2 and ZnO individual cells yielded efficiencies of 2.8% and 1.2%, respectively, under standard AM 1.5 irradiation (100 mW cm−2). The broadening of the absorption spectra and a large red shift of the absorption peak were observed by the adsorbing dyes on the composite film suggesting the formation of various kinds of J-aggregates. It is suggested that the formation of the J-aggregates and the presence of the ZnO barrier were responsible for the higher efficiency of the composite cell.

New Approach for Surface Microstructuring

A new approach for microstructuring of aluminum and alumina surfaces based on localized anodization of aluminum is described. It is based on the self-organized equilibrium at the metal/oxide interface and dynamic interactions between individual oxide growth sites. Hexagonal and square closed-packed arrays of hemispherical and pyramidal features from alumina and aluminum on the micrometer scale were prepared. Potential applications of such microstructured surfaces are discussed.