A Modular Atom Probe Concept: Design, Operational Aspects, and Performance of an Integrated APT-FIB/SEM Solution.

Atomic probe tomography (APT) is able to generate three-dimensional chemical maps in atomic resolution. The required instruments for APT have evolved over the last 20 years from an experimental to an established method of materials analysis. Here, we describe the realization of a new modular instrument concept that allows the direct attachment of APT to a dual-beam SEM microscope with the main achievement of fast and direct sample transfer and high flexibility in chamber and component configuration. New operational modes are enabled regarding sample geometry, alignment of tips, and the microelectrode. The instrument is optimized to handle cryo-samples at all stages of preparation and storage. It comes with its own software for evaluation and reconstruction. The performance in terms of mass resolution, aperture angle, and detection efficiency is demonstrated with a few application examples.

Slow-Moving Phase Boundary in Li4/3+ x Ti5/3 O4.

Lithium titanate is one of the most promising anode materials for high-power demands but such applications desire a complete understanding of the kinetics of lithium transport. The poor diffusivity of lithium in the completely lithiated and delithiated (pseudo spinel) phases challenges to explain the high-rate performance. This study aims at clearing the kinetics of lithium transport using an innovative technique that employs optical microscopy in a constrained region of sputter-deposited thin-film samples. It enables the in situ observation of the transport of lithium through the electrode. Furthermore, with a thermostatically controlled cell, the Arrhenius-like temperature dependence is revealed. The quantitative findings demonstrate that indeed the end phases have poor diffusivity which is, however, accelerated at intermediate Li concentrations in the spinel structured Li4/3+ δ Ti5/3 O4 phase. Surprisingly, the slow migration of the phase boundary hinders the formation of the Li-rich (rock-salt) phase in the initial stages. Such kinetic control by the phase boundary stands in obvious contrast to a prior (theoretical) study postulating almost "liquid" behavior of the interface. Only after the Li diffusion into the Li-poor (spinel) phase has faded, when approaching the solubility limit, the further growth of the rock-salt phase becomes diffusion controlled.

Electrochromic Behavior and Phase Transformation in Li4+xTi5O12 upon Lithium-Ion Deintercalation/Intercalation.

The impact of phase transformation from spinel-structured Li4Ti5O12 to rocksalt-type Li7Ti5O12 on the electrochromic properties of the material is studied. Thin films of Li4Ti5O12 are deposited on platinum-coated substrates using radio-frequency-ion beam sputtering. In situ and ex situ optical spectroscopy (in reflectance geometry) is performed along with electrochemical characterization. In situ measurements demonstrate the reversible electrochromic behavior of the deposited thin films and the effect of the change of lithium content on the reflectance spectrum. Ex situ measurements quantify the optical constants of thin films for different charge states by modeling the reflectance spectrum with a Clausius-Mossotti relation. The model reveals the presence of one or two dominant resonant frequencies in the case of Li4Ti5O12 or Li7Ti5O12, respectively, in the UV/visible/NIR region of light. The single strong resonance in the case of Li4Ti5O12 is assigned to transition from O 2p to Ti t2g, that is, across the band gap, whereas for the Li7Ti5O12 phase, the two resonances correspond to the electronic transitions from O 2p to empty Ti t2g and from filled Ti t2g to empty Ti eg. The concentration dependence of the derived dielectric constants points out a fast lithium ion transport through the grain boundaries, thereby segregating a conductive lithium-rich phase at the grain boundaries. This increases the electronic conductivity of the thin films in the initial stages of intercalation and explains the debated mechanism of the fast discharge/charge capability of Li4Ti5O12 electrodes.

High capacity rock salt type Li2MnO3-δ thin film battery electrodes.

Recent investigations of layered, rock salt and spinel-type manganese oxides in composite powder electrodes revealed the mutual stabilization of the Li-Mn-O compounds during electrochemical cycling. A novel approach of depositing such complex compounds as an active cathode material in thin-film battery electrodes is demonstrated in this work. It shows the maximum capacity of 226 mA h g-1 which is superior in comparison to that of commercial LiMn2O4 powder as well as thin films. Reactive ion beam sputtering is used to deposit films of a Li2MnO3-δ composition. The method allows for tailoring of the active layer's crystal structure by controlling the oxygen partial pressure during deposition. Electron diffractometry reveals the presence of layered monoclinic and defect rock salt structures, the former transforms during cycling and results in thin films with extraordinary electrochemical properties. X-ray photoelectron spectroscopy shows that a large amount of disorder on the cation sub-lattices has been incorporated in the structure, which is beneficial for lithium migration and cycle stability.

Ionic conductivity of melt-frozen LiBH4 films.

The fast Li conductivity of LiBH4 envisages its use in all-solid-state batteries. Powders are commonly applied. But here, we study the formation of dense micrometer films by melting, spinning and subsequent solidifying. Characterized by electron microscopy, and spectroscopy (EDX/XPS/impedance), a reversible phase transformation is confirmed as well as a maximum conductivity of 103 S cm-1.