Phase formation in the V<Subscript>2</Subscript>O<Subscript>5</Subscript>-Nb<Subscript>2</Subscript>O<Subscript>5</Subscript>-MoO<Subscript>3</Subscript> system

The solid-phase interaction in the V₂O₅-Nb₂O₅-MoO₃ system has been investigated, and the formation of a solid solution bounded by the compositions MoNb₂V₄O_{18 ? δ}, Mo₂NbV₅O_{21 ? δ}, Mo₂Nb₃V₃O_{21 ? δ}, and Mo₄Nb₉V₉O_{57 ? δ} has been found (δ is nonstoichiometry). In the V₂O₅?Nb₂O₅ system, the formation of three compounds is verified, namely, VNbO₅ (tetragonal structure), VNb₉O₂₅, and V₂Nb₂₃O_62.5. The first two compounds are isostructural and form a continuous solid solution with tetragonal symmetry. A new compound of the composition Mo₃NbVO_{14 ? δ} has been synthesized. This compound is isostructural to the Mo₃Nb₂O₁₄ compound described in the literature and forms a tetragonal solid solution with it. The phase equilibria in the V₂O₅-Nb₂O₅-MoO₃ system in the subsolidus region have been determined.     

Modeling the active centers of V<Subscript>2</Subscript>O<Subscript>5</Subscript>/SiO<Subscript>2</Subscript> and V<Subscript>2</Subscript>O<Subscript>5</Subscript>/TiO<Subscript>2</Subscript> supported catalysts. DFT theoretical analysis of optical properties

Within the framework of the density functional theory (DFT), the electronic structure of monooxodioxovanadium functional groups in tetrahedral coordination, which model the active centers (ACs) of fine supported catalysts V₂O₅/SiO₂ and V₂O₅/TiO₂, has been analyzed. The optimal structures of three ACs as possible models of monomeric and polymeric oxovanadium forms on the carriers with low vanadium content were determined. The modified DFT method involving the time dependence of Kohn-Sham equation (TDDFT) was used for the adopted AC models to calculate the energies of the excited states, and optical spectra of the absorption in 25000–60000 cm^?1 region were reconstructed on their base. The spectrum in this region is due to O → V charge transfer. The features of electronic spectra with the charge transfer for V₂O₅/SiO₂ and V₂O₅/TiO₂ catalysts and the vibrational spectra of three AC models corresponding to the monomeric and dimeric oxovanadium forms of the supported catalysts V₂O₅/SiO₂ and V₂O₅/TiO₂ were defined. The detailed interpretation of normal vibration frequencies is given. The frequencies typical of the monomeric and dimeric oxovanadium forms on the carrier surface were identified.     

Phase relations in the Zn<Subscript>2</Subscript>V<Subscript>2</Subscript>O<Subscript>7</Subscript>-Cu<Subscript>2</Subscript>V<Subscript>2</Subscript>O<Subscript>7</Subscript> system from room temperature to melting

Phase relations in the Zn₂V₂O₇-Cu₂V₂O₇ system were studied by high-temperature X-ray diffraction and differential thermal analysis. The major phase constituents of the system are solid solutions based on Zn₂V₂O₇ and Cu₂V₂O₇ polymorphs and their coexistence regions. The generation of α-Zn_{2 − 2x} Cu_2x V₂O₇ solid solution, where 0 ≤ x ≤ 0.30, leaves almost unchanged the stabilization temperature of the high-temperature zinc pyrovanadate phase. The α-Cu_{2 − 2x} Zn_2x V₂O₇ homogeneity range is 5 mol % Zn₂V₂O₇. In the range 0.050 ≤ x ≤ 0.09 from 20 to ∼ 620°C, there is the two-phase field of α-Cu₂V₂O₇ and β-Cu₂V₂O₇ base solid solutions. At still higher temperatures, β-Zn_{2 − 2x} Cu_2x V₂O₇ and α-Cu_{2 − 2x} Zn_2x V₂O₇ coexist in the mixed-phase region. β-Zn_{2 − 2x} Cu_2x V₂O₇ solid solution, where 0 ≤ x ≤ 0.30, exists above 610 ± 5°C. The extent of the β′-Cu₂V₂O₇-base solid solution is 9 to 65 mol % Zn₂V₂O₇ at 615 ± 5°C, expanding to 0 mol % Zn₂V₂O₇ with rising temperature. Original Russian Text ￠ T.I. Krasnenko, M.V. Rotermel’, S.A. Petrova, R.G. Zakharov, O.V. Sivtsova, A.N. Chvanova, 2008, published in Zhurnal Neorganicheskoi Khimii, 2008, Vol. 53, No. 10, pp. 1755–1762.     

Ion state of atoms and the properties of perovskite-like compound CaCu<Subscript>3</Subscript>V<Subscript>4</Subscript>O<Subscript>12</Subscript>

The binding energies and valence state of atoms in the perovskite-like compound CaCu₃V₄O₁₂ have been determined using XPS spectroscopy. The stoichiometry of this phase is formulated as Ca²⁺Cu²⁺Cu ₂ ⁺ (V ₂ ⁵⁺ V ₂ ⁴⁺ O₁₂). Under an air atmosphere, the phase interacts with water vapor and oxygen. As a result, Ca(OH)₂ is formed on its surface, the Cu⁺ and V⁴⁺ ion concentrations decrease, and the Cu²⁺ and V⁵⁺ concentrations increase in association. Raman spectra show shortened cation-anion bond lengths and cation-anion-cation bond angles in CaCu₃V₄O₁₂ compared to perovskite CuVO₃; the two structures are alike. The electrical conductivity, magnetic susceptibility, thermal and sensor properties of CaCu₃V₄O₁₂ in aqueous salt solutions have been studied.     

Synthesis,structure and thermal decomposition of [Ni(NH<Subscript>3</Subscript>)<Subscript>6</Subscript>][VO(O<Subscript>2</Subscript>)<Subscript>2</Subscript>(NH<Subscript>3</Subscript>)]<Subscript>2</Subscript>

The compound [Ni(NH₃)₆][VO(O₂)₂(NH₃)]₂ was prepared and characterized by elemental analysis and vibrational spectra. The single crystal X-ray study revealed that the structure consists of [Ni(NH₃)₆]²⁺ and [VO(O₂)₂(NH₃)]⁻ ions. As a result of weak interionic interactions V′···O_p (O_p-peroxo oxygen), ([VO(O₂)₂(NH₃)]⁻)₂ dimers are formed in the solid-state. The thermal decomposition of [Ni(NH₃)₆][VO(O₂)₂(NH₃)]₂ is a multi-step process with overlapped individual steps; no defined intermediates were obtained. The final solid products of thermal decomposition up to 600°C were Ni₂V₂O₇ and V₂O₅.     

Nanofibers of V<Subscript>2</Subscript>O<Subscript>5</Subscript>/C@MWCNTs as the cathode material for lithium-ion batteries

Vanadium pentoxide (V₂O₅) nanofibers (NFs) with a thin carbon layer of 3–5 nm, which wrapped on V₂O₅ nanoparticles, and integrated multiwalled carbon nanotubes (MWCNTs) have been fabricated via simple electrospinning followed by carbonization process and post-sintering treatment. The obtained composite displays a NF structure with V₂O₅ nanoparticles connected to each other, and good electrochemical performance: delivering initial capacity of 320 mAh g^?1 (between 2.0 and 4.0 V vs. Li/Li⁺), good cycling stability (223 mAh g^?1 after 50 cycles), and good rate performance (~?150 mAh g^?1 at 2 A g^?1). This can attribute to the carbon wrapped on the V₂O₅ nanoparticles which can not only enhance the electric conductivity to decrease the impendence of the cathode materials but also maintain the structural stability to protect the nanostructure from the corruption of electrolyte and the strain stress due to the Li-ion intercalation/deintercalation during the charge/discharge process. And, the added MWCNTs play the role of framework of the unique V₂O₅ coated by carbon layer and composited with MWCNT NFs (V₂O₅/C@MWCNT NFs) to ensure the material is more stable.     

Electrospun Li<Subscript>4</Subscript>Ti<Subscript>5</Subscript>O<Subscript>12</Subscript>/Li<Subscript>2</Subscript>TiO<Subscript>3</Subscript> composite nanofibers for enhanced high-rate lithium ion batteries

Li₄Ti₅O₁₂/Li₂TiO₃ composite nanofibers with the mean diameter of ca. 60 nm have been synthesized via facile electrospinning. When the molar ratio of Li to Ti is 4.8:5, the Li₄Ti₅O₁₂/Li₂TiO₃ composite nanofibers exhibit initial discharge capacity of 216.07 mAh g^?1 at 0.1 C, rate capability of 151 mAh g^?1 after being cycled at 20 C, and cycling stability of 122.93 mAh g^?1 after 1000 cycles at 20 C. Compared with pure Li₄Ti₅O₁₂ nanofibers and Li₂TiO₃ nanofibers, Li₄Ti₅O₁₂/Li₂TiO₃ composite nanofibers show better performance when used as anode materials for lithium ion batteries. The enhanced electrochemical performances are explained by the incorporation of appropriate Li₂TiO₃ which could strengthen the structure stability of the hosted materials and has fast Li⁺-conductor characteristics, and the nanostructure of nanofibers which could offer high specific area between the active materials and electrolyte and shorten diffusion paths for ionic transport and electronic conduction. Our new findings provide an effective synthetic way to produce high-performance Li₄Ti₅O₁₂ anodes for lithium rechargeable batteries.     

Solid Solutions of the System V<Subscript>2</Subscript>O<Subscript>5</Subscript>-V<Subscript>2</Subscript>O<Subscript>4</Subscript>-TiO<Subscript>2</Subscript> in Air

Gravimetry in combination with X-ray phase analysis, X-ray crystallography, and X-ray densitometry were used to determine the contents of V⁵⁺, V⁴⁺, and Ti⁴⁺ ions and vacancies in solid solutions formed by the reaction of V₂O₅ with TiO₂ in air at atmospheric pressure.     

A New Molybdophosphate Constructed From {Mo<Stack><Subscript>2</Subscript><Superscript>V</Superscript></Stack>O<Subscript>4</Subscript>(H<Subscript>2</Subscript>O)<Subscript>6</Subscript>}<Superscript>2+</Superscript> and 1-Hydroxyethylidenediphosphonate

A new molybdophosphate (NH₄)₈{Mo₂^VO₄[(Mo₂^VIO₆)CH₃C(O)(PO₃)₂]₂}·14H₂O (1), has been synthesized by the reaction of {Mo₂^VO₄(H₂O)₆}²⁺ fragments with 1-hydroxyethylidenediphosphonate (hedp HOC(CH₃)(PO₃H₂)₂), and it is characterized by ³¹P NMR, IR, UV, element analysis, TG and single-crystal X-ray analysis. The structure analysis reveals that the polyoxoanion can be described as two {(Mo₂^VIO₆)(CH₃C(O)(PO₃)₂} units connected by a {Mo₂^VO₄}²⁺ moiety. In the structure, the six Mo atoms are arranged into a new “W-shaped” structure, which represents a new kind of molybdophosphate.     

Preparation and electrochemical properties of V<Subscript>2</Subscript>O<Subscript>5</Subscript> submicron-belts synthesized by a sol–gel H<Subscript>2</Subscript>O<Subscript>2</Subscript> route

One-dimensional (1D) submicron-belts of V₂O₅ have been prepared by a sol–gel route using V₂O_5, H₂O₂ and aniline as starting materials. Thermogravimetric and differential thermal analysis, X-ray diffraction, Fourier transform infrared spectroscopy and scanning electron microscopy were employed to characterize the samples. Electrochemical behaviors as cathode material in rechargeable lithium-ion batteries were investigated by galvanostatic charge–discharge measurement and cyclic voltammeter. The results showed that the synthesized V₂O₅ appeared to be submicron-belts and orthorhombic structure. The V₂O₅ submicron-belts exhibited a high initial discharge capacity of 346 mAh/g and stayed 240 mAh/g after 20 cycles at 0.1 C discharge rate in the potential region 1.8–4.0 V.     

Composition and structure of solid solutions in the Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>-SiO<Subscript>2</Subscript>-V<Stack><Subscript>2</Subscript><Superscript>5+</Superscript></Stack>O<Subscript>5</Subscript> system with the structure sillenite

In the Bi₂O₃-SiO₂-V₂⁵⁺+O₅ system, single crystal solid solutions of the sillenite family of the general composition Bi₂₄(Bi,Si,V)₂O₄₀ are obtained by a hydrothermal method and for the first time characterized by neutron and X-ray diffraction analysis. The tetrahedral position is found to contain vanadium ions with different formal charges (V⁴⁺ and V⁵⁺) responsible for green and orange colors, respectively, of the samples. For the first time, for some sillenites of this system dissymmetrization of the structure (a transition from the I23 space group into P23) is revealed, which is caused by the presence of several atoms in one crystallographic position and also by crystal growth conditions.     

Phase formation in the V<Subscript>2</Subscript>O<Subscript>5</Subscript>-Ta<Subscript>2</Subscript>O<Subscript>5</Subscript>-MoO<Subscript>3</Subscript> system

Solid-phase interactions in the V₂O₅-Ta₂O₅-MoO₃ system were studied. The formation of com- pounds TaVO₅ and VTa₉O₂₅ in the V₂O₅-Ta₂O₅ binary system was verified. Tetragonal VTa₉O₂₅-base solid solutions of the general formula Ta_{5 + 4x} V_{5 − 4x} O₂₅ (x = 0.25–1) and TaVO₅-base solid solutions of the general formula Ta _x Mo_{1 − x} V_{2 − x} O_{8 − 3x} (x = 0.625–1) were found to form. Subsolidus phase equilibria in the V₂O₅-Ta₂O₅-MoO₃ were determined.     

The use of thermal analysis in the study of Ca<Subscript>3</Subscript>Al<Subscript>2</Subscript>O<Subscript>6</Subscript> formation by the polymeric precursor method

Single-phase Ca₃Al₂O₆ was prepared via polymeric precursor method. The influence of the reactants nature in the Ca₃Al₂O₆ synthesis was investigated. For this purpose, citric acid and soluble salts of calcium (nitrate, chloride, carbonate) and aluminium (nitrate, chloride, acetate) were used as starting materials, in the presence and, respectively, in the absence of ethylene glycol. Ca₃Al₂O₆ resulted as single-phase after annealing at 1050 °C for 1 h only starting from calcium nitrate or carbonate and aluminium nitrate or acetate as salts precursor for Ca²⁺ and Al³⁺ cations. The formation of Ca₃Al₂O₆ is not conditioned by the ethylene glycol presence in these mixtures. Using calcium and aluminium chlorides, the phases present at 1050 °C are Ca₁₂Al₁₄O₃₃ and unreacted CaO.     

High-temperature heat capacity and thermodynamic properties of Pb<Subscript>4</Subscript>V<Subscript>2</Subscript>O<Subscript>9</Subscript> and Pb<Subscript>8</Subscript>V<Subscript>2</Subscript>O<Subscript>13</Subscript>

The molar heat capacity of Pb₄V₂O₉ and Pb₈V₂O₁₃ in the temperature range 350–1000 K was measured by differential scanning calorimetry. It was determined that the plot C_p = f(T) for Pb₈V₂O₁₃ has an extremum within the range 416–516 K, which is due to a phase transition. A correlation was found between the heat capacity and composition of oxides in the PbO–V₂O₅ system. The data obtained allowed one to predict the specific heat capacity value for Pb(VO₃)₂.     

Electrochemical performance of novel Li<Subscript>3</Subscript>V<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript> glass-ceramic nanocomposites as electrodes for energy storage devices

The novel Li₃V₂(PO₄)₃ glass-ceramic nanocomposites were synthesized and investigated as electrodes for energy storage devices. They were fabricated by heat treatment (HT) of 37.5Li₂O–25V₂O₅–37.5P₂O₅?mol% glass at 450 °C for different times in the air. XRD, SEM, and electrochemical methods were used to study the effect of HT time on the nanostructure and electrochemical performance for Li₃V₂(PO₄)₃ glass-ceramic nanocomposites electrodes. XRD patterns showed forming Li₃V₂(PO₄)₃ NASICON type with monoclinic structure. The crystalline sizes were found to be in the range of 32–56 nm. SEM morphologies exhibited non-uniform grains and changed with variation of HT time. The electrochemical performance of Li₃V₂(PO₄)₃ glass-ceramic nanocomposites was investigated by using galvanostatic charge/discharge methods, cyclic voltammetry, and electrochemical impedance spectroscopy in 1 M H₂SO₄ aqueous electrolyte. The glass-ceramic nanocomposites annealed for 4 h, which had a lower crystalline size, exhibited the best electrochemical performance with a specific capacity of 116.4 F g^?1 at 0.5 A g^?1. Small crystalline size supported the lithium ion mobility in the electrode by decreasing the ion diffusion pathway. Therefore, the Li₃V₂(PO₄)₃ glass-ceramic nanocomposites can be promising candidates for large-scale industrial applications in high-performance energy storage devices.     

Synthesis and characterization of macroporous Li<Subscript>3</Subscript>V<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript>/C composites as cathode materials for Li-ion batteries

The macroporous Li₃V₂(PO₄)₃/C composite was synthesized by oxalic acid-assisted carbon thermal reaction, and the common Li₃V₂(PO₄)₃/C composite was also prepared for comparison. These samples were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), and electrochemical performance tests. Based on XRD and SEM results, the sample has monoclinic structure and macroporous morphology when oxalic acid is introduced. Electrochemical tests show that the macroporous Li₃V₂(PO₄)₃/C sample has a high initial discharge capacity (130 mAh g⁻¹ at 0.1 C) and a reversible discharge capacity of 124.9 mAh g⁻¹ over 20 cycles. Moreover, the discharge capacity of the sample is still 91.5 mAh g⁻¹, even at a high rate of 2 C, which is better than that of the sample with common morphology. The improvement in electrochemical performance should be attributed to its improved lithium ion diffusion coefficient for the macroporous morphology, which was verfied by cyclic voltammetry and electrochemical impedance spectroscopy.     

Formation and properties of the Fe<Subscript>8</Subscript>V<Subscript>10</Subscript>W<Subscript>16–x</Subscript>Mo<Subscript>x</Subscript>O<Subscript>85</Subscript> type solid solution

Solid solution phases of a formula Fe₈V₁₀W_16–xMo_xO₈₅ where 0≤x≤4, have been obtained, possessing a structure of the compound Fe₈V₁₀W₁₆O₈₅. It was found on the base of XRD and DTA investigations that these solution phases melted incongruently, with increasing the value of x, in the temperature range from 1108 (x=0) to 1083 K (x=4) depositing Fe₂WO₆ and WO₃. The increase of the Mo⁶⁺ ions content in the crystal lattice of Fe₈V₁₀W₁₆O₈₅ causes the lattice parameters a=b contraction with cbeing almost constant. IR spectra of the Fe₈V₁₀W_16–xMo_xO₈₅ solid solution phases have been recorded.     

Physicochemical Properties of Li<Subscript>6</Subscript>V<Subscript>5</Subscript>O<Subscript>15</Subscript> as the Cathode Material for Lithium-Ion Batteries

Lithium-vanadium oxide with the formal composition Li₆V₅O₁₅, uniform microsctructure, and the particle size of 100 nm is synthesized by a solution method. The synthesized compound is characterized by the methods of X-ray diffraction analysis, Raman spectroscopy, and synchronous thermal analysis. The total electric conductivity is measured by the method of impedance spectroscopy and its electronic component is estimated by dc method. In the temperature range of 200–400°C, Li₆V₅O₁₅ represents a mixed electronic- ionic conductor with predomination of the ionic component and is thermally stable up to 550°С. Preliminary tests of a laboratory model of electrochemical cell Li|LiPF₆|Li₆V₅O₁₅ are carried out.     

Studies of porphyrin zinc compounds intercalated into V<Subscript>2</Subscript>O<Subscript>5</Subscript> xerogel

The intercalation of meso-tetrakis(4-pyridyl)porphyrin zinc, the cationic salts of meso-tetrakis(N-methylpyridinium-4-yl)porphyrin zinc, and zwitterionic meso-tetrakis-[N-(3-sulfonatopropyl)pyridinium-4-yl]porphyrin zinc from aqueous solutions as well as of meso-tetrakis(N-methylpyriclinium-4-yl)porphyrin zinc from pyridine solutions into V₂O₅ xerogel was studied. The intercalation complexes obtained were characterized by X-ray diffraction analysis, TG analysis, IR, and UV reflectance spectroscopy.     

Rational design of double-confined Mn<Subscript>2</Subscript>O<Subscript>3</Subscript>/S@Al<Subscript>2</Subscript>O<Subscript>3</Subscript> nanocube cathodes for lithium-sulfur batteries

Due to the high specific capacities and environmental benignity, lithium-sulfur (Li-S) batteries have shown fascinating potential to replace the currently dominant Li-ion batteries to power portable electronics and electric vehicles. However, the shuttling effect caused by the dissolution of polysulfides seriously degrades their electrochemical performance. In this paper, Mn₂O₃ microcubes are fabricated to serve as the sulfur host, on top of which Al₂O₃ layers of 2 nm in thickness are deposited via atomic layer deposition (ALD) to form Mn₂O₃/S (MOS) @Al₂O₃ composite electrodes. The MOS@Al₂O₃ electrode delivers an excellent initial capacity of 1012.1 mAh g^?1 and a capacity retention of 78.6% after 200 cycles at 0.5 C, and its coulombic efficiency reaches nearly 99%, giving rise to much better performance than the neat MOS electrode. These findings demonstrate the double confinement effect of the composite electrode in that both the porous Mn₂O₃ structure and the atomic Al₂O₃ layer serve as the spacious host and the protection layer of sulfur active materials, respectively, for significantly improved electrochemical performance of the Li-S battery.