Diana Nihtianova

Structure and reversible lithium intercalation in a new P′3-phase: Na2/3Mn1−yFeyO2 (y = 0, 1/3, 2/3)

In this contribution, new data on the reversible Li+ intercalation in iron substituted sodium manganates are provided. Novel Na2/3Mn1−yFeyO2 (y = 0, 1/3 and 2/3) compounds with a P′3-type structure are prepared from freeze-dried citrate precursors at 500 °C. A new structural element is the development of three-layer oxygen stacking contrary to the well-known P2-type Na2/3MnO2 with a two-layer sequence. The effect of Fe additives on the structure of Na2/3MnO2 was examined by XRD powder diffraction and TEM analysis. The oxidation state and the distribution of transition metal ions in Na2/3Mn1−yFeyO2 were analysed using electron paramagnetic resonance spectroscopy. The lithium intercalation in Na2/3Mn1−yFeyO2 was investigated in two-electrode lithium cells of the type Li|LiPF6 (EC:DMC)|Na2/3Mn1−yFeyO2. The stability of the layered phases during lithium intercalation was studied by ex situ Raman spectroscopy. It was found that the intermediate Na2/3Mn2/3Fe1/3O2 composition is able to intercalate Li+ reversibly in high amounts. Details of the structure and its stability during the Li+ intercalation are discussed.

Sodium deficient nickel–manganese oxides as intercalation electrodes in lithium ion batteries

Sodium deficient nickel–manganese oxides NaxNi0.5Mn0.5O2 with a layered structure are of interest since they are capable of participating in reactions of intercalation of Li+ and exchange of Na+ with Li+. Taking into account the intercalation properties of these oxides, we provide new data on the direct use of NaxNi0.5Mn0.5O2 as low-cost electrode materials in lithium ion batteries instead of lithium analogues. Sodium deficient nickel–manganese oxides NaxNi0.5Mn0.5O2 are prepared at 700 °C from freeze-dried acetate precursors. The structure of NaxNi0.5Mn0.5O2 is analyzed by means of powder X-ray diffraction, SAED and HRTEM. The oxidation states of nickel and manganese ions are determined by X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance spectroscopy (EPR). Model lithium cells are used to monitor the lithium intercalation into NaxNi0.5Mn0.5O2. The surface and composition stability of NaxNi0.5Mn0.5O2 during the electrochemical reaction is monitored by using ex situ XPS and LA-ICPMS. Layered oxides NaxNi0.5Mn0.5O2 exhibit a P3-type of structure, in which the solubility of sodium is limited between 0.5 and 0.75. At 700 °C, NaxNi0.5Mn0.5O2 consists of thin well-crystallized nanoparticles; some of the particles have sizes higher than 100 nm, displaying a trigonal superstructure. For all oxides, manganese ions occur in the oxidation state of +4, while the oxidation state of nickel ions is higher than +2 and depends on the sodium content. The electrochemical reaction occurs within two potential ranges at 3.1 and 3.8 V due to the redox manganese and nickel couples, respectively. During the first discharge, Li+ intercalation and Li+/Na+ exchange reactions take place, while the consecutive charge process includes mainly Li+ and Na+ deintercalation. As a result, all oxides manifest a reversible capacity of about 120–130 mA h g−1, corresponding to 0.5–0.6 moles of Li+. The formation of surface layers in the course of the electrochemical reaction is also discussed.

Catalytic reduction of NO with decomposed methanol on alumina-supported Mn-Ce catalysts.

A series of manganese-ceria supported on alumina catalysts with various Mn/Ce ratios are investigated in both methanol decomposition to CO and hydrogen and SCR of NO(x) with CO. The study is aimed at the potential application of both reactions in integrated devices, where NO(x) is reduced with the products of the decomposed methanol. The samples are characterized by nitrogen physisorption, XRD, TEM, XPS, UV-Vis, and TPR. It was established that manganese-ceria supported on alumina catalysts are perspective in both methanol decomposition and NO reduction at temperatures above 723 K, which are typical for exhausted gases from the vehicles and some stationary stations. The best catalytic activity and selectivity to the desired products under these conditions was found for the samples with Mn/Mn+Ce ratio of 0.5 and 0.7. This superior catalytic performance is related to the formation of mixed valence Mn(3+)/Ce(4+) and Mn(4+)/Ce(3+) active sites.

Influence of Ce addition on the catalytic behavior of alumina-supported Cu-Co catalysts in NO reduction with CO.

The effect of Ce addition to alumina-supported copper, cobalt, and copper-cobalt oxides with low loadings on the catalysts efficiency in NO reduction with CO was studied. The attention was focused on varying the impregnation procedure in the ternary-supported catalysts in order to determine the best catalyst as well as the reasons for the enhanced catalytic activity. Ternary Co-Cu-Ce and binary Co-Ce, Cu-Ce, and Cu-Co-supported alumina were prepared and characterized by ICP, XRD, TEM, adsorption studies, XPS, H(2)-TPR, and catalytic investigations. The high activity of the ternary and the binary catalysts was determined by the favorable influence of the added cerium on the dispersion of the copper and cobalt active phases. The presence of ceria contributes to the formation of appropriate active phases, resulting in catalytic sites on the surface of the samples that promote the reduction of NO with CO.

Competitive lithium and sodium intercalation into sodium manganese phospho-olivine NaMnPO4 covered with carbon black

In this contribution we provide novel data on the reversible lithium and sodium ion intercalation into a sodium-manganese phospho-olivine NaMnPO4, when it is used as a cathode in model lithium-ion cells. The ion-exchange reaction involving the participation of KMnPO4·H2O dittmarite as precursor was chosen for the preparation of NaMnPO4. The NaMnPO4 particles were covered with carbonaceous materials to improve the electrical conductivity and electrolyte wetting. The procedure includes ball-milling of NaMnPO4 with conductive carbon black additives Super C/65, followed by thermal treatment. The mechanically treated samples consist of well crystallized phospho-olivine phase NaMnPO4 free of any anti-site defects and disordered carbon species with graphite like medium-range order. The composite NaMnPO4/C material manifests a reversible capacity between 80–85 mA h g−1 in model lithium cells versus lithium anode. Prior to the electrochemical test, the chemical inertness of NaMnPO4 in the lithium electrolyte is studied by soaking phospho-olivines in the solution of LiPF6 in EC:DMC. The mechanism of the reversible intercalation/deintercalation cycling is investigated using ex situ X-ray powder diffraction, TEM and high-angle annular dark field STEM analysis, infrared spectroscopy and electron paramagnetic resonance spectroscopy (EPR). The study demonstrates, for the first time, that NaMnPO4 is able to intercalate reversibly both Na+ and Li+ ions following the chemical reaction LixNa1−xMnPO4 ↔ Li0.0Na0.5MnPO4 (0.25 ≤ x ≤ 0.45).

Precursor-based methods for low-temperature synthesis of defectless NaMnPO4 with an olivine- and maricite-type structure

We report precursor-based methods for low-temperature synthesis of two structure modifications of NaMnPO4. The maricite phase is thermodynamically more stable, while the olivine phase is of great interest as a positive-electrode material for lithium and sodium ion batteries. The advantage of synthetic procedures is the formation of defectless NaMnPO4 in the temperature range of 200–400 °C. The structure and morphology characterizations of two modifications are performed by powder XRD, SEM and TEM analyses. The oxidation state of the Mn ions in NaMnPO4 is determined by X-ray photoelectron spectroscopy. The local environment of Na in both structure modifications is assessed by 23Na MAS NMR spectroscopy. The synthesis methods are based on the formation of appropriate precursors that are easily transformed to target NaMnPO4. Thermal decomposition of freeze-dried phosphate–formate precursor yields NaMnPO4 with a maricite structure at 400 °C. KMnPO4·H2O with a dittmarite-type structure acts as a structure-template precursor for the preparation of NaMnPO4 with an olivine structure by an ion exchange reaction. Both olivine and maricite NaMnPO4 do not accommodate any anti-site mixing and Na,Mn deficiency. The morphology of NaMnPO4 consists of nano-sized particles (less than 50 nm) that are closely bound together into aggregates, the shape of the aggregates being dependent on the synthesis procedure used.

Layered P3-NaxCo1/3Ni1/3Mn1/3O2 versus Spinel Li4Ti5O12 as a Positive and a Negative Electrode in a Full Sodium–Lithium Cell

The development of lithium and sodium ion batteries without using lithium and sodium metal as anodes gives the impetus for elaboration of low-cost and environmentally friendly energy storage devices. In this contribution we demonstrate the design and construction of a new type of hybrid sodium-lithium ion cell by using unique electrode combination (Li4Ti5O12 spinel as a negative electrode and layered Na3/4Co1/3Ni1/3Mn1/3O2 as a positive electrode) and conventional lithium electrolyte (LiPF6 salt dissolved in EC/DMC). The cell operates at an average potential of 2.35 V by delivering a reversible capacity of about 100 mAh/g. The mechanism of the electrochemical reaction in the full sodium-lithium ion cell is studied by means of postmortem analysis, as well as ex situ X-ray diffraction analysis, HR-TEM, and electron paramagnetic resonance spectroscopy (EPR). The changes in the surface composition of electrodes are examined by ex situ X-ray photoelectron spectroscopy (XPS).

Dittmarite precursors for structure and morphology directed synthesis of lithium manganese phospho-olivine nanostructures†

We have demonstrated the capability of dittmarite-type precursors to act as structure and morphology directed agents for a facile synthesis of electrochemically active lithium manganese phospho-olivine nanostructures which are of interest as positive electrode materials for lithium ion batteries. Two types of compounds from the dittmarite series are considered: potassium and ammonium manganese phosphate monohydrates, M′MnPO4·H2O (M′ = K, NH4). Both KMnPO4·H2O and NH4MnPO4·H2O precursors interact with the eutectic LiCl–LiNO3 composition before its melting, leading to the formation of the target LiMnPO4. The reaction of KMnPO4·H2O with LiCl–LiNO3 takes place by a topotactic mechanism including a fast ion exchange of K+ for Li+ and H2O release, as a result of which nanostructured LiMnPO4 is formed. The NH4MnPO4·H2O precursor reacts with the lithium salts by massive evolution of H2O, NH3, NO and N2O gasses, resulting in fragmentation of the pristine plate-like aggregates into well-shaped and dispersed particles with nanosizes of about 20–50 nm.

Structural insights into M2O-Al2O3-WO3 (M = Na, K) system by electron diffraction tomography.

The M2O-Al2O3-WO3 (M = alkaline metals) system has attracted the attention of the scientific community because some of its members showed potential applications as single crystalline media for tunable solid-state lasers. These materials behave as promising laser host materials due to their high and continuous transparency in the wide range of the near-IR region. A systematic investigation of these phases is nonetheless hampered because it is impossible to produce large crystals and only in a few cases a pure synthetic product can be achieved. Despite substantial advances in X-ray powder diffraction methods, structure investigation on nanoscale is still challenging, especially when the sample is polycrystalline and the structures are affected by pseudo-symmetry. Electron diffraction has the advantage of collecting data from single nanoscopic crystals, but it is frequently limited by incompleteness and dynamical effects. Automated diffraction tomography (ADT) recently emerged as an alternative approach able to collect more complete three-dimensional electron diffraction data and at the same time to significantly reduce dynamical scattering. ADT data have been shown to be suitable for ab initio structure solution of phases with large cell parameters, and for detecting pseudo-symmetry that was undetected in X-ray powder data. In this work we present the structure investigation of two hitherto undetermined compounds, K5Al(W3O11)2 and NaAl(WO4)2, by a combination of electron diffraction tomography and precession electron diffraction. We also stress how electron diffraction tomography can be used to obtain direct information about symmetry and pseudo-symmetry for nanocrystalline phases, even when available only in polyphasic mixtures.

Fluoride etching of mordenite and its influence on catalytic activity

Due to its structure and high Si/Al ratio, zeolite mordenite has high thermal and acidic stability. Mordenite-type of zeolites have been used as catalysts in many industrially important reactions such as hydrocracking, hydroisomerization, alkylation, acid-catalyzed isomerization of alkanes and aromatics, reforming. In order to overcome the problem of the limited access to the active sites, OSDA-free synthesized mordenite undergoes fluoride etching as a post-synthetic treatment. The post-synthetic treatment is performed with hydrofluoric acid in combination with ammonium fluoride. Thus, the porosity is enhanced additionally without changing considerably the Si/Al ratio of the zeolite framework. All samples have been characterized by X-ray diffraction analysis, nitrogen adsorption, scanning electron microscopy, high-resolution transmission electron microscopy and solid-state nuclear magnetic resonance spectroscopy. The catalytic activity of the samples obtained has been investigated in the reaction of m-xylene transformation. All mordenite samples having undergone post-synthetic treatment exhibit catalytic activity higher than that of the parent sample.