Mona Shirpour

Titanate Anodes for Sodium Ion Batteries

For reasons of cost and supply security issues, there is growing interest in the development of rechargeable sodium ion batteries, particularly for large-scale grid storage applications. Like the much better known and technologically important lithium ion analogs, the devices operate by shuttling alkali metal cations between two host materials, which undergo insertion processes at different electrochemical potentials. A particular challenge for the sodium systems is identification of a suitable anode material due to the fact that sodium does not intercalate into graphite. Although several alternatives, including disordered carbons and alloys are being investigated, the most promising options at present lie with titanates, not in the least because of attractive characteristics such as low toxicity, ease of synthesis, wide availability, and low cost. A large variety of sodium titanate compounds can be prepared, many of which have tunnel or layered structures that can readily undergo reversible reductive intercalation reactions. A brief overview of the physical, structural, and electrochemical characteristics of several of the most promising materials for sodium-ion battery applications is given in this paper, and a comparison is made between the sodium and the lithium insertion behaviors. For some of these compounds, insertion of sodium occurs at unusually low potentials, a feature that has important implications for the design of high-energy sodium-ion systems.

Evolution of solid/aqueous interface in aqueous sodium-ion batteries

The microstructural and compositional evolution at the solid/aqueous solution interfaces are investigated to monitor the electrical properties of superionic conducting phosphates and the electrochemical failure of aqueous sodium-ion batteries.

Grain boundary characterization of electroceramics : acceptor-doped BaZrO3, an intermediate temperature proton conductor

Acceptor doped BaZrO 3 exhibit a high bulk proton conductivity of 2 ·10 -2 S/cm to 5·10 S/cm in the temperature range of 500°C to 100°C. Neverth eless, highly resistive grain boundaries and notoriously small grain sizes seriously hamper th application of this material as an electrolyte in intermediate-temperature solid oxide fu l cells. Up to now, none of the usual methods (such as powder synthesis via chemical meth ods, reactive sintering, and sintering aids) could considerably improve the specific GB co nductivity of this material. Investigation on sintering properties showed that acceptor-doped BaZrO3 exhibits a limited grain growth resulting in grain sizes 10 times smaller than undo pe BaZrO3. Larger grains, by decreasing the GB number density, can improve the total conduc tivity of BaZrO3. Nevertheless, due to the limited range for the expected grain size incre ase, a modification of specific GB conductivity seems much more effective. This work is focused on characterization of grain b oundaries to get a better understanding of the blocking effect in this perovskite structure pr oton conductor. It was attempted to check the most frequently reported origins for the blocking c haracter of GBs in ion conductors: (i) Presence of continuous secondary/amorphous phas e at GB (ii) Distorted crystallographic structure of grain boundary region which can result in a low water solubility and/or lower proton mobility in GB region (iii) Inhomogeneous distribution of dopant cations i GB region (iv) Depletion of protonic charge carriers in space harge layers which form to compensate an excess charge in the GB core Based on TEM images showing clean GB, secondary pha ses t the GB could be ruled out as origin for the blocking character. The similar chan ge of bulk and GB conductivity upon

Mapping grain boundary potentials in ceramics by nonlinear inline electron holography and impedance spectroscopy

The electrostatic potential arising from charge bound at grain boundary cores in ceramics and the accumulation of space charge in their vicinity is in many cases made responsible for the ion blocking or conducting behavior of electroceramics. While interpretation of impedance spectra of nominally undoped and acceptor-doped SrTiO3 ceramics and bicrystals implies that grain boundaries are positively charged and accompanied by a fairly wide region of negative space charge on both sides, a critical analysis of off-axis and inline electron holography data available in the literature yields very narrow potential profiles of the opposite sign. Some of the advantages of inline holography over off-axis holography are: (a) very simple experimental setup (works in any FEG-TEM, and for small details even with a LaB6 source), (b) possibility to record holograms far away from the specimen edge, (c) large fields of view because there is no need to oversample, and (d) specimen drift may easily be compensated, even during exposure. We will report on the application of a recently developed flux-preserving inline holography reconstruction algorithm [1] which allows the reconstruction of focal series recorded over a large focal range. Fig. 1 shows the (amplified) image distortions of the 15 images within a focal series of a near Σ13 grain boundary in SrTiO3. These image distortions are a side effect of changing the objective lens current by a large amount (the defocus ranged from -16μm to +13μm) and have been determined and corrected for by the reconstruction algorithm. In addition to fitting and correcting relative image distortions the reconstruction algorithm also refines the relative defocus and translation of each image in the course of the reconstruction. The reconstructed phase maps are shown in Figure 2. A double-tilt rotation holder has been used during the experiment. The grain boundary could therefore be aligned with the axis of the specimen holder. Rotating the holder by 18° therefore allowed a tilted projection of the grain boundary projection to be obtained (Fig. 2b). Using the local specimen thickness extracted from Fig. 2b the phase shift (Fig 2a) could be converted into a map of the mean inner potential (Fig. 3). Inline holography results of various bicrystal geometries processed under varying oxygen partial pressures will be presented and will be correlated with grain boundary potential profiles fitted to impedance spectroscopy data as well as analytical TEM data.