Synthese des Olivinphosphats LiMnPO4_{4} und Untersuchung des elektrochemischen Mechanismus zum Ein- und Ausbau von Lithium

Das Olivinphosphat LiMnPO4 wurde mit drei verschiedenen Syntheseverfahren dargestellt: dem Polyol-Verfahren, dem Sol-Gel-Prozess und der Festkörperrektion. Die Produkte wurden mittels Röntgenbeugung (XRD) und Rasterelektronenmikroskopie (REM) charakterisiert. Für die Sol-Gel-Synthese wurde ein Reaktionsmechanismus zur Bildung von LiMnPO4 auf Grundlage einer Hochtemperatur-XRD-Messung hergeleitet. Die Produkte aus den Synthesen wurden elektrochemisch untersucht und eine geeignete Aufarbeitung ermittelt. Neben der spezifischen Kapazität wurde für das Kathodenmaterial aus der Sol-Gel-Synthese die Ratenfähigkeit, die Zyklenfestigkeit und die Zyklierbarkeit bei erhöhten Temperaturen bestimmt. Außerdem wurde der elektrochemische Mechanismus zum Ein- und Ausbau von Lithium untersucht. Dazu wurden in situ und ex situ XRD-Messungen durchgeführt. Das Kathodenmaterial wurde bezüglich einer partiellen Amorphisierung und einer Jahn-Teller Verzerrung untersucht. Die lokale Umgebung von Lithium und Phosphor in den ex situ Proben wurde mit Hilfe von magic angle spinning (MAS) Kernspinresonanzspektroskopie (NMR) analysiert. Die Ergebnisse wurden durch magnetische Messungen und Untersuchungen zur thermischen Stabilität von MnPO4 ergänzt

Electrochemical Lithium Extraction and Insertion Process of Sol-Gel Synthesized LiMnPO4LiMnPO_{4} via Two-Phase Mechanism

Olivine-type LiMnPO4 was synthesized via a citric-acid assisted sol-gel method. The structural evolution of the as-prepared material during calcination and the crystallization of $LiMnPO_{4}$ was investigated by XRD measurements. A reaction mechanism including two intermediate phases was derived for the synthesis. Furthermore, the electrochemical Li extraction and insertion processes were studied by in situ/ex situ XRD and MAS NMR measurements. The phase transition from $LiMnPO_{4}$ to $MnPO_{4}$ and vice versa proceeds via a two-phase mechanism. The 31P MAS NMR spectra of $MnPO_{4}$, which forms upon delithiation as confirmed by XRD patterns, shows a very broad signal. Possible explanations for the large width of this signal are given and evaluated

Thermally Induced Structural Reordering in Li- and Mn-Rich Layered Oxide Li Ion Cathode Materials

In recent years, a considerable amount of effort has been put into a better understanding of the correlation of structure and electrochemical properties in Li- and Mn-rich NCM layered oxide Li ion battery cathode materials, such as the intensely investigated Li$_{1.2}$Ni$_{0.15}$Co$_{0.1}$Mn$_{0.55}$O$_2$ composition. A gradual transformation from a trigonal R_{3̅m} layered structure toward a cubic Fd3̅m spinel structure during electrochemical cycling results in an unwanted decay of the mean charge and discharge voltages, called “voltage fade”. This transformation proceeds via an interim phase, which is characterized by the local deordering of cations and the introduction of various lattice defects after the formation of the initially well-ordered material. In this study, these structural changes are studied in detail on a long-range atomic scale by synchrotron radiation powder diffraction as well as on a very local atomic scale by 7Li nuclear magnetic resonance and X-ray absorption spectroscopy. A structural reordering was induced by a mild thermal treatment (150–300 °C) in lithiated (discharged to 2.0 V) as well as in delithiated (charged to 4.7 V) electrodes, which results either in a partial recovery of the initial well-ordered state or in an intensification of the structural degradation toward a spinel-type cation ordering, respectively. The structural reordering thus obtained was again studied on a long-range and local atomic scale and correlated with the electrochemical properties. To complement the experiment, an electrochemically highly fatigued electrode (300 cycles, discharged to 2.0 V) showing a pronounced voltage fade was thermally treated, which resulted again in a partial recovery of the initial well-ordered structure and its accompanying electrochemical properties. Finally, the results are summarized in a model explaining the influence of the local cation ordering, lattice defects, and the oxygen sublattice on the electrochemical properties, such as the oxygen redox activity and the voltage fade

High-Coordinate Gold(I) Complexes with Dithiocarboxylate Ligands

Ferrocene dithiocarboxylate has been introduced into the chemistry of gold­(I) and copper­(I). First, a modified synthesis of piperidinium ferrocene dithiocarboxylate (<b>1</b>) is reported. Reaction of this reagent with [Au­(tht)­Cl] in the presence of different phosphines resulted in monomeric, dimeric, and polymeric structures. Although gold­(I) is usually two coordinate, mainly three- and four-fold coordinated compounds were obtained by using ferrocene dithiocarboxylate as ligands. The isolated compounds are [(FcCSS)­Au­(PPh₃)₂] (<b>2</b>) (FcCSS = ferrocene dithiocarboxylate), [(FcCSS)­Au₂(dppm)₂] (<b>3</b>) (dppm = bis­(diphenylphosphino)­methane), and [(FcCSS)­Au­(dppf)]_<i>n</i> (<b>4</b>) (dppf = bis­(diphenylphosphino)­ferrocene) [{(FcCSS)­Au}₂(dppp)] (<b>5</b>) (dppp = bis­(diphenylphosphino)­propane). The FcCSS ligand shows a remarkable flexible coordination mode. It coordinates either in a monodentate, a chelating, or in a metal bridging mode. In the four gold­(I) complexes <b>2</b>–<b>5</b> four different coordination modes of the FcCSS ligand are seen. Attempts to extend this rich coordination chemistry to other coinage metals were only partly successful. [(FcCSS)­Cu­(PPh₃)₂] (<b>6</b>) was obtained from the reaction of piperidinium ferrocene dithiocarboxylate with [(Ph₃P)₃CuCl]. ⁵⁷Fe–Mössbauer spectroscopy was performed for compounds <b>2</b>–<b>4</b>. The spectra show isomer shifts and quadrupole splittings that are typical for diamagnetic ferrocenes