Oliver Clemens

Facile proton conduction in H+/Li+ ion-exchanged garnet-type fast Li-ion conducting Li5La3Nb2O12

Garnet-type Li5La3Nb2O12, among several other Li-stuffed garnet compounds, is known to be unstable under moisture and undergoes H+/Li+ ion-exchange at room temperature. While there have been reports on proton-exchange in water, the transport of the protons in the garnet structure is yet to be characterized by electrical methods. In this work, proton-exchange was performed on the pellet form of Li5La3Nb2O12 for 4, 7, and 10 days, while the pellet generally showed less proton-exchange due to less surface area exposed to water, longer treatment times did allow the reaction to proceed further. Neutron diffraction (ND) studies were utilized to locate the H-site in the garnet-type structure and to confirm which Li ion sites were exchanged. For H exchanged Li5La3Nb2O12, the neutron diffraction studies indicated that H exchanged preferentially for Li in the distorted octahedral sites of the Iad space group (no. 230). AC impedance measurements were done on as-prepared Li5La3Nb2O12 under 3% H2O + N2 and 3% D2O + N2, which suggested proton conduction under humid conditions at 23–300 °C. Li5La3Nb2O12 shows a proton conductivity of 10−5 S cm−1, of the same order of magnitude as Li ion conduction, at room temperature in humidity where protons seem to be incorporated from the solid and gas phase equilibrium reaction. Proton reversible Nafion electrode experiments indicate facile proton conduction in Li garnets under humidity.

Topochemical modifications of mixed metal oxide compounds by low-temperature fluorination routes

Abstract In this review, we discuss recent developments in the use of low-temperature fluorination routes for the topochemical modification of mixed metal oxide compounds. By applying such methods, material properties (such as magnetism, superconductivity, electrical conductivity, and ionic conductivity) can be tuned in a wide range. Furthermore, oxide fluoride compounds are interesting from a structural point of view, and while differentiating between oxide and fluoride ions has proved to be difficult using diffraction methods, strategies (e.g., bond valence sum calculations) to overcome this problem have been shown to be possible. In addition, this review concludes with an outlook on future prospects in the field of oxide fluoride compounds.

VOCl as a Cathode for Rechargeable Chloride Ion Batteries.

A novel room temperature rechargeable battery with VOCl cathode, lithium anode, and chloride ion transporting liquid electrolyte is described. The cell is based on the reversible transfer of chloride ions between the two electrodes. The VOCl cathode delivered an initial discharge capacity of 189 mAh g(-1) . A reversible capacity of 113 mAh g(-1) was retained even after 100 cycles when cycled at a high current density of 522 mA g(-1) . Such high cycling stability was achieved in chloride ion batteries for the first time, demonstrating the practicality of the system beyond a proof of concept model. The electrochemical reaction mechanism of the VOCl electrode in the chloride ion cell was investigated in detail by ex situ X-ray diffraction (XRD), infrared spectroscopy (FTIR), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The results confirm reversible deintercalation-intercalation of chloride ions in the VOCl electrode.

Introducing a large polar tetragonal distortion into Ba-doped BiFeO3 by low-temperature fluorination.

This article reports on the synthesis and crystallographic and magnetic structure of barium-doped BiFeO3 compounds with approximate composition Bi(1-x)Ba(x)FeO(3-x/2), as well as those of the fluorinated compounds Bi(1-x)Ba(x)FeO(3-x)F(x) (both with x = 0.2, 0.3), prepared by low-temperature fluorination of the oxide precursors using polyvinylidenedifluoride. Whereas the oxide compounds were obtained as cubic (x = 0.2) and slightly tetragonal (x = 0.3, c/a ≈ 1.003) distorted perovskite compounds, a large tetragonal polar distortion was observed for the oxyfluoride compounds (c/a ≈ 1.08 for x = 0.2 and ∼1.05 for x = 0.3), being isostructural to tetragonal PbTiO3. Although described differently in previous reports on Ba-doped BiFeO3, the observed remanent magnetization is found to agree well with the amount of BaFe12O19 only detectable by neutron diffraction and the well-known magnetic properties of BaFe12O19. The oxyfluoride compounds show G-type antiferromagnetic ordering with magnetic moments lying in the a/b plane.

Multicomponent equiatomic rare earth oxides

ABSTRACT Multicomponent rare earth oxide (REO) nanocrystalline powders containing up to seven equiatomic rare earth elements were successfully synthesized in a single-phase CaF2-type (Fm-3 m) structure. The addition of more than six elements resulted in the formation of a secondary phase. Annealing at 1000°C for 1 h led to the formation of a single-phase (Ia-3) even in the 7-component system. In the absence of cerium (Ce4+), secondary phases were observed irrespective of the number of cations or the extent of thermal treatment indicating that cerium cations played a crucial role in stabilizing the multicomponent REOs into a phase pure structure. GRAPHICAL ABSTRACT IMPACT STATEMENT Multicomponent equiatomic rare earth oxides pioneer a new group of materials that crystallize into a single-phase structure with the dominant role of a single element instead of entropy.

Synthesis, structural characterisation and proton conduction of two new hydrated phases of barium ferrite BaFeO2.5−x(OH)2x

Materials exhibiting mixed electronic and proton conductivity are of great interest for applications ranging from electrodes for proton conducting ceramic fuel cells to hydrogen separation membranes. In this work, we report a detailed investigation of the effect of water incorporation in BaFeO2.5 on the structure and conductivity. BaFeO2.5 is shown to be topochemically transformed to two different hydrated modifications, low-water (LW-) and high-water (HW-) BaFeO2.5. A combined analysis of neutron and X-ray diffraction data was used to determine the crystal structure of LW-BaFeO2.5 (BaFeO2.33(OH)0.33), which shows a unique ordering pattern of anion vacancies for perovskite type compounds, with structural relaxations around vacancies being similar to the chemically similar compound BaFeO2.33F0.33. Approximate proton positions were determined using the bond valence method. Conductivity studies of hydrated and pure BaFeO2.5 (with additional comparison to oxidized BaFeO2.5) show a significant enhancement of the conductivity on water incorporation, which can be attributed to proton conductivity. This is the first report of significant grain proton conduction (∼10−6 to 10−7 S cm−1) in an iron based perovskite. Water uptake is further shown to be completely reversible, with reformation of BaFeO2.5 when heating the compound to temperatures above ∼450 K under Ar.

La2CoO4: a new intercalation based cathode material for fluoride ion batteries with improved cycling stability

In this study, we report on the electrochemical cycling behavior of La2CoO4 (against a composite of Pb/PbF2 as the anode material) for use as an intercalation-based cathode material for fluoride ion batteries (FIBs). The material can intercalate approximately 1.2 fluoride ions per formula under the formation of La2CoO4F1.2 resulting in a strong increase of the cell volume, confirmed by means of ex situ X-ray diffraction studies at various charging capacities and additional complementary chemical fluorination experiments. Furthermore, by only regulating the cut-off capacity we were able to remarkably avoid unwanted side reactions which were previously assumed to be a major problem for achieving high cycling numbers of LaSrMnO4. In this respect, electrochemical impedance spectroscopy was used to determine an initial critical specific charge capacity of ∼65 mA h g−1. By carefully designing the charging process, a discharge capacity as high as 32 mA h g−1 could be obtained, which is the highest capacity for an intercalation based cathode material reported so far, with a capacity retention of ∼25% of the initial discharge capacity after 50 cycles. In addition, we discuss why avoiding the cross-reactivity of the carbon additive is more difficult for fluoride ion batteries than for lithium ion batteries, showing that this is an important challenge for the successful implementation of high voltage cathode materials.

Topochemical Fluorination of La2NiO4+d: Unprecedented Ordering of Oxide and Fluoride Ions in La2NiO3F2

The Ruddlesden-Popper (K2NiF4) type phase La2NiO3F2 was prepared via a polymer-based fluorination of La2NiO4+ d. The compound was found to crystallize in the orthorhombic space group Cccm ( a = 12.8350(4) Å, b = 5.7935(2) Å, c = 5.4864(2) Å). This structural distortion results from an ordered half occupation of the interstitial anion layers and has not been observed previously for K2NiF4-type oxyfluoride compounds. From a combination of neutron and X-ray powder diffraction and 19F magic-angle spinning NMR spectroscopy, it was found that the fluoride ions are only located on the apical anion sites, whereas the oxide ions are located on the interstitial sites. This ordering results in a weakening of the magnetic Ni-F-F-Ni superexchange interactions between the perovskite layers and a reduction of the antiferromagnetic ordering temperature to 49 K. Below 30 K, a small ferromagnetic component was found, which may be the result of a magnetic canting within the antiferromagnetic arrangement and will be the subject of a future low-temperature neutron diffraction study. Additionally, density functional theory-based calculations were performed to further investigate different anion ordering scenarios.

Proton Conduction in Grain-Boundary-Free Oxygen-Deficient BaFeO2.5+δ Thin Films

Reduction of the operating temperature to an intermediate temperature range between 350 °C and 600 °C is a necessity for Solid Oxide Fuel/Electrolysis Cells (SOFC/SOECs). In this respect the application of proton-conducting oxides has become a broad area of research. Materials that can conduct protons and electrons at the same time, to be used as electrode catalysts on the air electrode, are especially rare. In this article we report on the proton conduction in expitaxially grown BaFeO2.5+δ (BFO) thin films deposited by pulsed laser deposition on Nb:SrTiO3 substrates. By using Electrochemical Impedance Spectroscopy (EIS) measurements under different wet and dry atmospheres, the bulk proton conductivity of BFO (between 200 °C and 300 °C) could be estimated for the first time (3.6 × 10−6 S cm−1 at 300 °C). The influence of oxidizing measurement atmosphere and hydration revealed a strong dependence of the conductivity, most notably at temperatures above 300 °C, which is in good agreement with the hydration behavior of BaFeO2.5 reported previously.

Vanadium Oxyfluoride/Few-Layer Graphene Composite as a High-Performance Cathode Material for Lithium Batteries

Metal oxyfluoride compounds are gathering significant interest as cathode materials for lithium ion batteries at the moment because of their high theoretical capacity and resulting high energy density. In this regard, a new and direct approach is presented to synthesize phase-pure vanadium oxyfluoride (VO2F). The structure of VO2F was identified by Rietveld refinement of the powder X-ray diffraction (XRD) pattern. It crystallizes in a perovskite-type structure with disorder of the oxide and fluoride ions. The as-synthesized VO2F was tested as a cathode material for lithium ion batteries after being surface-coated with few-layer graphene. The VO2F delivered a first discharge capacity of 254 mA h g(-1) and a reversible capacity of 208 mA h g(-1) at a rate of C/20 for the first 20 cycles with an average discharge voltage of 2.84 V, yielding an energy density of 591 W h kg(-1). Improved rate capability that outperforms the previous report has been achieved, showing a discharge capacity of 150 mA h g(-1) for 1 C. The structural changes during lithium insertion and extraction were monitored by ex-situ XRD analysis of the electrodes discharged and charged to various stages. Lithium insertion results in an irreversible structural change of the anion lattice from (3)/4 cubic close packing to hexagonal close packing to accommodate the inserted lithium ions while keeping the overall space-group symmetry. For the first time we have revealed a structural change for the ReO3-type structure of as-prepared VO2F to the RhF3 structure after lithiation/delithiation, with structural changes that have not been observed in previous reports. Furthermore, the new synthetic approach described here would be a platform for the synthesis of new oxyfluoride compounds.