Jonas Sottmann

Perovskite to postperovskite transition in NaFeF3.

The GdFeO3-type perovskite NaFeF3 transforms to CaIrO3-type postperovskite at pressures as low as 9 GPa at room temperature. The details of such a transition were investigated by in situ synchrotron powder diffraction in a multianvil press. Fit of the p-V data showed that the perovskite phase is more compressible than related chemistries with a strongly anisotropic response of the lattice metrics to increasing pressure. The reduction in volume is accommodated by a rapid increase of the octahedral tilting angle, which reaches a critical value of 26° at the transition boundary. The postperovskite form, which is fully recoverable at ambient conditions, shows a regular geometry of the edge-sharing octahedra and its structural properties are comparable to those found in CaIrO3-type MgSiO3 at high pressure and temperature. Theoretical studies using density functional theory at the GGA + U level were also performed and describe a scenario where both perovskite and postperovskite phases can be considered Mott-Hubbard insulators with collinear magnetic G- and C-type antiferromagnetic structures, respectively. Magnetic measurements are in line with the theoretical predictions with both forms showing the typical behavior of canted antiferromagnets.

Versatile electrochemical cell for Li/Na‐ion batteries and high‐throughput setup for combined operando X‐ray diffraction and absorption spectroscopy

A fundamental understanding of de/intercalation processes (single phase versus multi-phase), structural stability and voltage–composition profiles is pivotal for optimization of electrode materials for rechargeable non-aqueous batteries. A fully operational setup (electrochemical cells, sample changer and interfacing software) that enables combined quasi-simultaneous operando X-ray diffraction (XRD) and absorption (XANES and EXAFS) measurements coupled with electrochemical characterization is presented. Combined XRD, XANES and EXAFS analysis provides a deep insight into the working mechanisms of electrode materials, as shown for the high-voltage Li insertion cathode material LiMn1.5Ni0.5O4 and the high-capacity sodium conversion anode material Bi2S3. It is also demonstrated that the cell design can be used for in-house XRD characterization. Long-term cycling experiments on both Li and Na electrode materials prove the hermeticity and chemical stability of the design as a versatile operando electrochemical cell.

Chemical Structures of Specific Sodium Ion Battery Components Determined by Operando Pair Distribution Function and X-ray Diffraction Computed Tomography

Phosphorus is one of the most promising anodes for sodium ion batteries (SIBs). Little is known about the structural mechanism of Na/P cycling due to the fact that only one of the structures involved (Na3P) is crystalline. Using operando X-ray diffraction computed tomography (XRD-CT) analysed by Rietveld and pair distribution (PDF) methods combined with density functional theory (DFT) calculations we show that the sodiation and desodiation mechanisms of phosphorus are very different. Sodiation follows the thermodynamic path of lowest energy from P via NaP to Na3P while desodiation follows a kinetically controlled deintercalation mechanism in which the layered Na3-xP type structure is maintained until P nanoclusters form. Using XRD-CT allows analysis of the 3D structure of the anode, but most importantly, removes the contributions from the sample container and other battery components, particularly important for PDF analysis.To improve lithium and sodium ion battery technology, it is imperative to understand how the properties of the different components are controlled by their chemical structures. Operando structural studies give us some of the most useful information for understanding how batteries work, but it remains difficult to separate out the contributions of the various components of a battery stack (e.g., electrodes, current collectors, electrolyte, and binders) and examine specific materials. We have used operando X-ray diffraction computed tomography (XRD-CT) to study specific components of an essentially unmodified working cell and extract detailed, space-resolved structural information on both crystalline and amorphous phases that are present during cycling by Rietveld and pair distribution function (PDF) methods. We illustrate this method with the first detailed structural examination of the cycling of sodium in a phosphorus anode, revealing surprisingly different mechanisms for sodiation and desodiation in this promising, high-capacity anode system.

A new approach towards the study of thermal decomposition and formation processes of nanoalloys: the double complex salt [Pd(NH3)4][PtCl6]

A new approach based on a combination of synchrotron radiation techniques, such as X-ray absorption fine structure (XAFS), X-ray photoelectron spectroscopy (XPS), hard X-ray photoelectron spectroscopy (HAXPES), and powder X-ray diffraction (PXRD), has been applied to in situ study the processes of thermal decomposition of inorganic compounds and the formation of bimetallic nanoalloys. As an example, a double complex salt, [Pd(NH3)4][PtCl6], was selected because of (i) its sufficiently complicated structure and previous studies conducted via thermal analysis, ex situ PXRD and XPS, and (ii) its use as a prospective single-source precursor for the preparation of bimetallic (PdPt) nanoparticles or nanoalloys. The differences between the mechanisms based on ex situ and in situ data have been discussed for the first time. It has been found that the first step of thermal decomposition is related to the formation of crystalline [Pd(NH3)2Cl2][Pt(NH3)2Cl4]. Further decomposition results in the formation of {PdCl2}, {PtCl2}, and (NH4)2[PtCl6] in the second step. In the final step, the intermediates are completely reduced, and a bimetallic nanoalloy is formed. The different means of Pd and Pt reduction on the surface and in the bulk result in the formation of a disordered nanoalloy with possible monometallic inclusions. Further heating orders the nanoalloy that is accompanied by a decrease in the positive charge on Pt.