Molybdenum Oxyfluoride Research Articles

Nucleation and growth of molybdenum disulfide grown by thermal atomic layer deposition on metal oxides

To enable greater control over thermal atomic layer deposition (ALD) of molybdenum disulfide (MoS2), here we report studies of the reactions of molybdenum hexafluoride (MoF6) and hydrogen sulfide (H2S) with metal oxide substrates from nucleation to few-layer films. In situ quartz crystal microbalance experiments performed at 150, 200, and 250 °C revealed temperature-dependent nucleation behavior of the MoF6 precursor, which is attributed to variations in surface hydroxyl concentration with temperature. In situ Fourier transform infrared spectroscopy coupled with ex situ x-ray photoelectron spectroscopy (XPS) indicated the presence of molybdenum oxide and molybdenum oxyfluoride species during nucleation. Density functional theory calculations additionally support the formation of these species as well as predicted metal oxide to fluoride conversion. Residual gas analysis revealed reaction by-products, and the combined experimental and computational results provided insights into proposed nucleation surface reactions. With additional ALD cycles, Fourier transform infrared spectroscopy indicated steady film growth after ∼13 cycles at 200 °C. XPS revealed that higher deposition temperatures resulted in a higher fraction of MoS2 within the films. Deposition temperature was found to play an important role in film morphology with amorphous films obtained at 200 °C and below, while layered films with vertical platelets were observed at 250 °C. These results provide an improved understanding of MoS2 nucleation, which can guide surface preparation for the deposition of few-layer films and advance MoS2 toward integration into device manufacturing.

Order and Disorder: Toward the Thermodynamically Stable α-BaMoO2F4 from the Metastable Polymorph

A fully ordered noncentrosymmetric barium molybdenum oxyfluoride, α-BaMoO2F4, has been synthesized by a hydrothermal reaction at 200 °C in a concentrated hydrofluoric acid solution. A centrosymmetr...

Ba(MoO2F)2(QO3)2 (Q = Se, Te): Partial Fluorination of MoO6 Octahedra Enabling Two Polar Solids with Strong and Phase Matchable SHG Response

The first polar molybdenum oxyfluoride in selenite and tellurite systems, namely, Ba(MoO2F)2(SeO3)2 and Ba(MoO2F)2(TeO3)2, was achieved successfully by partial fluorination of MoO6 octahedra under ...

Synthesizing High-Capacity Oxyfluoride Conversion Anodes by Direct Fluorination of Molybdenum Dioxide (MoO2 ).

High-capacity metal oxide conversion anodes for lithium-ion batteries (LIBs) are primarily limited by their poor reversibility and cycling stability. In this study, a promising approach has been developed to improve the electrochemical performance of a MoO2 anode by direct fluorination of the prelithiated MoO2 . The fluorinated anode contains a mixture of crystalline MoO2 and amorphous molybdenum oxyfluoride phases, as determined from a suite of characterization methods including X-ray diffraction, Raman spectroscopy, and X-ray photoelectron spectroscopy, and scanning transmission electron microscopy. Electrochemical measurements indicate that fluorination facilitates the conversion reaction kinetics, which leads to increased capacity, higher coulombic efficiency, and better cycling stability as compared to the nonfluorinated samples. These results suggest that fluorination after prelithiation not only favors formation of the oxyfluoride phase but also improves the lithium-ion diffusivity and reversibility of the conversion reaction, making it an attractive approach to address the problems of conversion electrodes. These findings provide a new route to design high-capacity negative electrodes for LIBs.

Improved Electrode Performance of Lithium-Excess Molybdenum Oxyfluoride: Titanium Substitution with Concentrated Electrolyte

A titanium-substituted lithium-excess molybdenum oxyfluoride, Li2.1–yTi0.2Mo0.7O2F, is synthesized by mechanical milling and tested as a positive electrode material in a conventional carbonate-based electrolyte or concentrated electrolyte. Reversibility as the electrode material is significantly improved by suppression of dissolution of the molybdenum oxyfluoride on electrochemical cycles. Li2.1–yTi0.2Mo0.7O2F delivers a reversible capacity of 265 mA h g–1 after 30 cycles at a rate of 50 mA g–1 with concentrated electrolyte. This finding contributes to the development of high-energy and long-cycle-life rechargeable lithium batteries with lithium-excess metal oxyfluorides in the future.

Lithium Molybdenum Oxyfluorides As High-Capacity Positive Electrodes for Rechargeable Lithium Batteries

The demand for the increase in energy density of rechargeable lithium batteries are still growing. Recently, the use of two-electron redox of vanadium ions coupled with a high electronegativity anion in the crystal lattice is proposed as an effective strategy to enhance the energy density of positive electrode materials.[1] Theoretical capacities of lithium insertion materials depend on both total extractable lithium ions and electron numbers in the structure of positive electrodes. Therefore, in addition to the lithium-enrichment in the structure, the use of multi-electron redox processes for transition metals is expected to be an important strategy to further increase energy density of positive electrode materials. We have recently proposed a new lithium-excess oxide, Li9/7Nb2/7Mo3/7O2, which delivers a large reversible capacity of ca. 290 mAh g-1 based on highly reversible three-electron redox of Mo3+/Mo6+.[2] Three-electron redox of molybdenum ions is expected to be a promising system to further increase energy density of positive electrode materials with less transition metal ions. Nevertheless, redox potential of Mo3+/Mo6+in the oxide framework structure is relatively low for positive electrodes. In this study, to increase in the redox potential of three-electron redox reaction of Mo3+/Mo6+, a mixed anion system, oxyfluorides, is targeted. Crystal structures and electrode performance of a new series of xLiF–LiMoO2 binary system, Li1+x MoO2F x , as oxyfluoride positive electrodes are systematically examined. The highest theoretical capacity based on Mo3+/Mo6+ is expected for x = 2 (Li3MoO2F2) in this binary system. Li3MoO2F2 was prepared by mechanical milling from LiMoO2 and LiF. A mixture of LiMoO2 and LiF was mechanically milled with a ZrO2 container and balls. Li3MoO2F2 is found to crystallize into an anion-/cation-disordered rocksalt structure with low crystallinity. Electrochemical properties of Li3MoO2F2 before and after mechanical milling are compared in Figure 1. The sample after mechanical milling delivers a large reversible capacity of ca. 280 mAh g-1, which nearly corresponds to that of theoretical capacity based on the two-electron redox reaction of Mo3+/Mo5+. However, higher average voltage for the molybdenum oxyfluoride is evidenced compared with those of oxides.[2] From these results together with structural and electrochemical data of Li1+x MoO2F x (x= 0.5, 1, and 1.5), we will discuss the feasibility of molybdenum oxyfluoride materials with multi-electron redox reactions as positive electrode materials for rechargeable lithium batteries.

Revealing the electronic structure and optical properties of K6[Mo4O8F10] novel molybdenum oxyfluoride materials

The electronic structure and optical properties of the molybdenum oxyfluoride K6[Mo4O8F10] single crystal were investigated. Based on X-ray diffraction data, the crystal structure was optimised so as to minimise the forces acting on each atom. The calculation reveal that K6[Mo4O8F10] is an indirect band gap semiconductor of about 1.51 eV, using the local density approximation, 1.72 eV, using gradient approximation (PBE-GGA), 1.81 eV using Engel–Vosko generalized gradient approximation and 2.01 eV, from the recently modified Becke–Johnson potential (TB-GGA-mBJ). Partial density of states reveals the orbitals’ contribution and the degree of the hybridisations between the orbitals. The contours of the valence electronic charge density of each atom in K6[Mo4O8F10] reveal that there are some electrons from Mo, K, F and O orbitals that are transferred to the valence bands and participate in the interactions between the Mo, K, F and O atoms to form covalent bonding. The strength of the covalent bond depends on the degree of the hybridisation and the electro-negativity differences between Mo, K, F and O atoms. The calculated bond lengths exhibit good agreement with the experimental data. The optical properties help to get deep insight into the electronic structure and reveal the types of the orbitals that participate in the optical transitions. It has been found that the investigated crystal possesses negative uniaxial anisotropy (δε = −0.102) and positive birefringence ( = 0.2765).

Fluorine substituted molybdenum oxide as cathode material for Li-ion battery

Molybdenum oxyfluoride MoO2.8F0.2 has been synthesized by solid-phase method. Its morphological and structural features were studied by scanning electron microscopy and X-ray powder diffraction. It was established that the substitution of the O atoms for F atoms in MoO3 structure leads to charge redistribution in the crystal lattice and conductivity increasing from 4.4·10−9Scm−1 to 1.8·10−6Scm−1. It was shown that molybdenum oxyfluoride characterized by higher discharge–charge stability in comparison with the oxide itself: 10-fold cycling in the range from 3.5 to 1.5V yields 160mAhg−1 and 120mAhg−1 for MoO2.8F0.2 and MoO3, respectively.

Mechanochemical synthesis of metallic Ag 2F and application to Ag 3MoO 3F 3 based nanocomposites for lithium batteries

Metallic Ag 2F phase has been successfully synthesized through the mechanochemical reaction of Ag and AgF. Its unique metallic character leads to a good electronic conductivity of 7.89 × 10 −2 S cm −1. The efficacy of silver molybdenum oxyfluoride (SMOF) composites consisting of various conducting matrices (carbon black, Ag 2F and Ag) for lithium battery was investigated through discharge rate studies. Results indicate that Ag 2F phase can be utilized as an alternative conductive additive with a high density.

Electrochemistry of a Novel Silver Molybdenum Oxyfluoride Perovskite as Cathode Material for Lithium Batteries

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Electrochemically active silver molybdenum oxyfluoride perovskite: Synthesis and in situ electrochemical characterization

An electrochemical characterization of a silver molybdenum oxyfluoride perovskite positive electrode for Li batteries was investigated as a function of synthesis condition, stoichiometry and effect of Mo and Ag derived second phases. A detailed in situ electrochemical study by XAS, Raman, and XRD was performed, revealing a 3 electron silver displacement or conversion reaction at >3 V and a 2 electron reduction of Mo 6+ to Mo 4+ in the region <3 V.

ChemInform Abstract: A Novel Silver Molybdenum Oxyfluoride Perovskite as a Cathode Material for Lithium Batteries.

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A Novel Silver Molybdenum Oxyfluoride Perovskite as a Cathode Material for Lithium Batteries

Novel electroactive nanostructured silver molybdenum oxyfluoride (SMOF) perovskites Ag1+3Mo6+(O3F3) have been successfully synthesized by a mechanochemical reaction. The formation of this perovskite was investigated throughout the Ag−Mo composition range with the use of either Ag1+ or Ag2+ in the form of AgF and AgF2 as the reactant, respectively. The compositional study combined with X-ray diffraction and extensive Raman investigation was utilized to determine structure and cation distribution and infer oxidation state. Finally, preliminary electrochemical characterization of the perovskites vs lithium was investigated.

Synthesis and Electrochemical Properties of a Novel Silver Molybdenum Oxyfluoride Perovskite

A novel electroactive nanostructured silver molybdenum oxyfluoride (SMOF) has been successfully synthesized through the mechanochemical reaction of AgF/AgF2 and MoO3. A combination of XRD and Raman studies revealed the structure to be perovskite. In-situ XRD revealed that half the specific capacity was realized through a silver displacement reaction. The new electrode material was found to exhibit good electrochemical activity with high volumetric energy density.

Structure and Electrochemistry of Novel Silver Metal Oxyfluoride Perovskite: Cathode Materials for High Volumetric Energy Storage

ABSTRACTA novel electroactive nanostructured silver molybdenum oxyfluoride (SMOF) perovskite was successfully synthesized via mechanochemistry through the use of AgFx and MoO3. The structure of the SMOF perovskite has been examined utilizing X-ray powder diffraction (XRD) combined with X-ray absorption spectroscopy (XAS) and elemental analysis. The SMOF perovskite exhibited a very good electrochemical activity. A detailed in-situ electrochemical study by XRD and XAS was carried out. Finally, preliminary work on fabrication of a metallic Ag2F phase and optimization of the rate capability of SMOF perovskite is presented.

Synthesis and Electrochemical Properties of a Novel Silver Molybdenum Oxyfluoride Perovskite

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