From Composites to Solid Solutions: Modeling of Ionic Conductivity in the CaF2-BaF2System

On the CaF2-BaF2 interface

Ionic redistribution at solid interfaces in ionic materials is the keystone of nanoionics. An experimental master piece has been provided by CaF2-BaF2 heterolayers. Meanwhile this system and the involved heterojunctions are extraordinarily well-understood. The present paper gives an account of this model system by reviewing not only transport experiments and defect-chemical modeling as a function of temperature and spacing of the individual layers, but also transition from semi-infinite to mesoscopic conditions, transition from Mott–Schottky to Gouy–Chapman behavior as well as the impact of ionic redistribution on the electronic minority carriers. Owing to the availability of bulk transport data, the analysis works well for in-plane and out-of-plane measurements with only the space charge potential as fit parameter. Space charge effects are able to provide an interpretation of the annealing behavior, too. The experiments are corroborated by molecular dynamics simulations. Extrapolating the ionic redistribution effects down to the atomic level may even explain homovalent doping effects in non-equilibrium mixtures of the two fluorides.

Rapid Photo-Sonotherapy for Clinical Treatment of Bacterial Infected Bone Implants by Creating Oxygen Deficiency Using Sulfur Doping

Periprosthetic infection is considered the main cause of implant failure, which is expected to be solved by fabricating an antibacterial coating on the surface of the implant. Nevertheless, systemic antibiotic treatment still represents the mainstream method for preventing infection, and few antibacterial coatings are applied clinically. This is because the externally introduced traditional antibacterial coatings suffer from the risk of invalidation and tissue toxicity induced by the consumption of antibacterial agents, degradation, and shedding. In this work, we proposed a rapid photo-sonotherapy by creating an oxygen deficiency on a titanium (Ti) implant through sulfur (S)-doping (Ti-S-TiO_2-x), which endowed the implants with great sonodynamic and photothermal ability. Without introducing an external antibacterial coating, it reached a high antibacterial efficiency of 99.995% against Staphylococcus aureus under 15 min near-infrared light and ultrasound treatments. Furthermore, bone infection was successfully treated after combination treatments, and improved osseointegration was observed. Importantly, the S-doped Ti implant immersed in water for 6 months showed an unchanged structure and properties, suggesting that the Ti implant with intrinsic modification showed stable antibacterial performance under exogenous stimuli with a high antibacterial performance in vivo. This photo-sonotherapy based on sulfur doping is also promising for cancer therapy with biosafety.

Mismatch in cation size causes rapid anion dynamics in solid electrolytes: the role of the Arrhenius pre-factor

Mixed (Ba,Ca)F₂ reveals highly correlated F anion diffusion in disordered potentials landscapes.

Challenges and Perspectives for NASICON-Type Electrode Materials for Advanced Sodium-Ion Batteries

Sodium-ion batteries (SIBs) have attracted increasing attention in the past decades, because of high overall abundance of precursors, their even geographical distribution, and low cost. Apart from inherent thermodynamic disadvantages, SIBs have to overcome multiple kinetic problems, such as fast capacity decay, low rate capacities and low Coulombic efficiencies. A special case is sodium super ion conductor (NASICON)-based electrode materials as they exhibit - besides pronounced structural stability - exceptionally high ion conductivity, rendering them most promising for sodium storage. Owing to the limiting, comparatively low electronic conductivity, nano-structuring is a prerequisite for achieving satisfactory rate-capability. In this review, we analyze advantages and disadvantages of NASICON-type electrode materials and highlight electrode structure design principles for obtaining the desired electrochemical performance. Moreover, we give an overview of recent approaches to enhance electrical conductivity and structural stability of cathode and anode materials based on NASICON structure. We believe that this review provides a pertinent insight into relevant design principles and inspires further research in this respect.

Longitudinal conductivity of LaF3/SrF2 multilayer heterostructures

LaF₃/SrF₂ multilayer heterostructures with thicknesses of individual layers in the range 5-100 nm have been grown on MgO(100) substrates using molecular beam epitaxy. The longitudinal conductivity of the films has been measured using impedance spectroscopy in the frequency range 10^-1-10⁶ Hz and a temperature range 300-570 K. The ionic DC conductivities have been determined from Nyquist impedance diagrams and activation energies from the Arrhenius-Frenkel equation. An increase of the DC conductivity has been observed to accompany decreased layer thickness for various thicknesses as small as 25 nm. The greatest conductivity has been shown for a multilayer heterostructure having thicknesses of 25 nm per layer. The structure has a conductivity two orders of magnitude greater than pure LaF₃ bulk material. The increasing conductivity can be understood as a redistribution of charge carriers through the interface due to differing chemical potentials of the materials, by strong lattice-constant mismatch, and/or by formation of a solid La_1-xSr_xF_3-x solution at the interface during the growth process.

Structure and ion dynamics of mechanosynthesized oxides and fluorides

In many cases, limitations in conventional synthesis routes hamper the accessibility to materials with properties that have been predicted by theory. For instance, metastable compounds with local non-equilibrium structures can hardly be accessed by solid-state preparation techniques often requiring high synthesis temperatures. Also other ways of preparation lead to the thermodynamically stable rather than metastable products. Fortunately, such hurdles can be overcome by mechanochemical synthesis. Mechanical treatment of two or three starting materials in high-energy ball mills enables the synthesis of not only new, metastable compounds but also of nanocrystalline materials with unusual or enhanced properties such as ion transport. In this short review we report about local structures and ion transport of oxides and fluorides mechanochemically prepared by high-energy ball-milling.

Self-Supported Nanotube Arrays of Sulfur-Doped TiO2 Enabling Ultrastable and Robust Sodium Storage

Self-supported nanotube arrays of sulfur-doped TiO2 on metal substrates are fabricated using electrochemical anodization and subsequent sulfidation. The nanotube arrays can serve as an efficient anode for sodium storage, enabling ultrastable cycling (retaining 91% of the 2nd capacity up to 4400 cycles) and robust rate capability (167 mA h g(-1) at 3350 mA g(-1)), remarkably outperforming any other reported TiO2 -based electrodes.

Control parameters for electrochemically relevant materials: the significance of size and complexity

This contribution is concerned with the control parameters for arriving at defined, electrochemically relevant materials. The treatment is precise as far as the equilibrium situation of simple crystals is concerned, but becomes more and more qualitative if the distance from equilibrium or the (structural or compositional) complexity increases. It proves useful to distinguish between in situ parameters and ex situ parameters, the number ratio of which decreases with increasing distance from equilibrium. A particularly complex situation is met if not only size, shape and phase distribution are important, but even morphological details are of relevance, as it is the case for modern battery electrodes (“electrochemical integrated circuits”). For such cases archetypical examples along with their advantages or disadvantages for electrochemical storage properties are discussed. In this context, special emphasis is placed upon the dimensionality and distribution topology of building elements.