E. I. Voit

Nanostructured zirconia-doped titania as the anode material for lithium-ion battery

Nanostructured TiO2 and TiO2-ZrO2 (4 wt % ZrO2) in the anatase crystal modification have been synthesized through a template sol-gel route. Morphological and structural features of these compounds have been studied by scanning electron microscopy and Raman spectroscopy. The synthesized materials are porous nanostructured microtubes 10–300 μm in length and 3–5 μm in diameter. The size of nanoparticles forming tubes is 15–25 nm. Doping with zirconia (≤4 wt %) does not change the anatase crystal structure; at the same time, this is accompanied by an increase in the unit cell parameters and in the number of defects in the lattice. The availability of nanostructured TiO2-ZrO2 as an anode material for a Li-ion battery has been evaluated by the galvanostatic discharge-charge method. After 20 cycles in the range 3-1 V, the reversible capacity of TiO2-ZrO2 was 140 mA h/g, while the capacity of the undoped TiO2 was 65 mA h/g. The developed method for modification of titania is efficient from the standpoint of producing a promising anode material for Li-ion battery.

Nanostructured microtubes based on TiO2 doped by Zr and Hf oxides with the anatase structure

The nanostructured microtubes based on TiO2 have been prepared on the carbon fiber template using the sol-gel method. The microtubes consist of nanoparticles of metal oxides: TiO2/ZrO2 and TiO2/HfO2. The dependence of microtubes morphology and nanoparticles structure on the synthesis conditions has been studied using the methods of SEM, SAXS, and Raman spectroscopy. It has been demonstrated that at the stoichiometric ratio of up to 0.04 for Zr/Ti and up to 0.06 for Hf/Ti microtubes consist of uniform nanoparticles with the anatase structure. Along with further increase of the dopants content in the microtubes composition, nanoparticles acquire the core-shell structure. It has been suggested that nanoparticles have a core composed of the solid solutions Ti1-xZrxO2 or Ti1-xHfxO2 and a shell consisting of zirconium or hafnium titanate. The fabricated Zr- and Hf-doped TiO2 materials were investigated in view of their possible use as anode materials for Li-ion batteries. Charge- discharge measurements showed that the doped samples manifested significantly higher reversibility in comparison with the undoped TiO2. The method opens new prospects in synthesis of nanostructured materials for Li-ion batteries application.

Synthesis and studies of magnesium hexafluorozirconates MgZrF6 · nH2O (n = 5, 2, 0)

Crystalline magnesium hexafluorozirconate MgZrF6 · 5H2O isostructural to MnZrF6 · 5H2O, and having a chain-like structure, was synthesized and studied. According to thermogravimetry, the compound undergoes stepwise dehydration in the temperature range of 50–420°C to give the stable phase MgZrF6 · 2H2O and the final product MgZrF6 isostructural to the cubic modification of MZrF6 (M = Cu, Fe). The vibrational spectra of the initial compound and the dehydration products are analyzed and the structures of the compounds are considered.

Quantum chemical study of model fluorozirconate clusters

A number of fluorozirconate clusters having the general formula {fx41-01} were investigated by the DV-Xα quantum chemical method. The results of calculations are used to explain the existence of polyhedm with coordination numbers of the central atom 6, 7, and 8 in crystalline fluomzirconates. In the title series of clusters, the [ZrF6]2- octahedron and the dimer of edge-sharing seven-vertex polyhedm [Zr2F12]4-s are highly stable.

Quantum-Chemical Justification of the Structure of Fluorozirconate Glasses1

A number of fluorozirconate clusters of the general formula [ZrmFn](4m– n)and barium fluorozirconate clusters [BaZr2Fn](10 – n)are studied by the quantum-chemical discrete variational Xαmethod. It is shown that the mean energies of fluorine ion addition for all structural units observed in real fluorozirconate crystals lies in a narrow range. The vibrational spectra are analyzed within the energy approach. It is demonstrated that the coordination number of zirconium in a number of barium fluorozirconate glasses is equal to seven or eight. The possible mechanism of structural transformations in binary fluorozirconate glass with a variation in its composition is proposed. The rigidity of different bonds in fluorozirconate clusters is investigated qualitatively. It is established that the rigidities of zirconium bonds with bridging and terminal fluoride ions level off in polymers, which is a prerequisite for the glass formation in fluorozirconate systems.

Ab Initio study of the structure and vibrational spectra of ZrF n 4-n systems

The optimal geometry and vibrational spectra of a series of [ZrFn]4-n clusters (n = 4–8) are calculated in a nonrelativistic approximation of the Hartree-Fock-Roothaan method. The calculated frequencies of the totally symmetric stretching vibrations vs are in satisfactory agreement with the experimental data. The values of vs are very sensitive to minor changes in cluster geometry and to the outerspheric environment. The high stability of the [ZrF6]2− ion was confirmed in an MP2 calculation.

Structure of ZnZrF6 · nH2O (n = 6–2) and ZnZrF6 crystallohydrates according to vibrational spectroscopy data

Fluoridezirconate crystallohydrates ZnZrF6 · nH2O (n = 6–2) and anhydrous ZnZrF6 are investigated by vibrational spectroscopy and thermography. The influence of the hydrate number on the structure of the cationic and anionic sublattices of the crystallohydrates is studied. The changes in the strength of HOH···F and HOH···O hydrogen bonds of coordinated and outer-sphere water molecules occurring with variations in the hydrate number are determined by changes in the IR spectra. The IR spectra of ZnZrF6 · nH2O (n =6, 4) compounds, which have isolated complex anions [ZrF6]2– in their structure, revealed a band with two peaks in the range of 3470–3430 cm–1, which corresponds to stretching vibrations of coordinated water molecules. The spectra of ZnZrF6 · nH2O (n = 5, 3, 2, 1) crystallohydrates with a polymeric structure show a high-frequency shift of this band, which corresponds to weakening of hydrogen bonds. The vibrations of crystallization water molecules involved in the network of strong O–H···F and O–H···O hydrogen bonds manifest themselves in the spectra of ZnZrF6 · nH2O (n =5, 3) crystallohydrates by broad structureless bands in the region of stretching, bending, and libration vibrations.

Relationship between the energy characteristics of formation of fluorozirconates

A correlation is established between the mean energy of fluorine addition in a particular coordination polyhedron and the fluorine content in fluorozirconates by quantum chemical model cluster calculations and analysis of structural data for crystalline fluorozirconates. A new energy approach to studies of complex fluorides is suggested. It is shown that the mean energy of fluorine addition varies within a narrow range for any structural unit met in real fluorozirconate crystals. Based on the energy approach to analysis of vibrational spectra it is found that the coordination number of zirconium in a series of barium fluorozirconate glasses is 7 or 8.

Ionic mobility, structure, and conductivity of 45ZrF4-35BiF3-20CsF glass according to 19F NMR, IR, Raman, and impedance spectroscopy

Abstract19F NMR, IR, Raman and impedance spectroscopy are used to study the ionic mobility, structure, and conductivity of 45ZrF4-35BiF3-20CsF bismuth fluorozirconate glass. With the increase in temperature from 150 K to 500 K the fluorine-containing groups pass from the rigid lattice to local movements (reorientations), and then to diffusion. According to the results of IR and Raman spectroscopy, the lattice of this glass consists of ZrF8 polyhedra linked by their vertices into chains. The glass has high ionic conductivity: σ ≈ 1.8×10−4 S/cm in a temperature range of 480–485 K.

Effect of Hf-doping on electrochemical performance of anatase TiO2 as an anode material for lithium storage

Hafnium-doped titania (Hf/Ti = 0.01; 0.03; 0.05) had been facilely synthesized via a template sol–gel method on carbon fibre. Physico-chemical properties of the as-synthesized materials were characterized by X-ray diffraction, Raman spectroscopy, scanning electron microscopy, energy-dispersive X-ray analysis, scanning transmission electron microscopy, X-ray photoelectron spectroscopy, thermogravimetry analysis and Brunauer–Emmett–Teller measurements. It was confirmed that Hf4+ substitute in the Ti4+ sites, forming Ti1–xHfxO2 (x = 0.01; 0.03; 0.05) solid solutions with an anatase crystal structure. The Ti1–xHfxO2 materials are hollow microtubes (length of 10–100 µm, outer diameter of 1–5 µm) composed of nanoparticles (average size of 15–20 nm) with a surface area of 80–90 m2 g–1 and pore volume of 0.294–0.372 cm3 g–1. The effect of Hf ion incorporation on the electrochemical behaviour of anatase TiO2 in the Li-ion battery anode was investigated by galvanostatic charge/discharge and electrochemical impedance spectroscopy. It was established that Ti0.95Hf0.05O2 shows significantly higher reversibility (154.2 mAh g–1) after 35-fold cycling at a C/10 rate in comparison with undoped titania (55.9 mAh g–1). The better performance offered by Hf4+ substitution of the Ti4+ into anatase TiO2 mainly results from a more open crystal structure, which has been achieved via the difference in ionic radius values of Ti4+ (0.604 Å) and Hf4+ (0.71 Å). The obtained results are in good accord with those for anatase TiO2 doped with Zr4+ (0.72 Å), published earlier. Furthermore, improved electrical conductivity of Hf-doped anatase TiO2 materials owing to charge redistribution in the lattice and enhanced interfacial lithium storage owing to increased surface area directly depending on the Hf/Ti atomic ratio have a beneficial effect on electrochemical properties.