Oxyfluoride Materials Research Articles

Nb-based AxNbOFy:Mn4+ (A=Na, Cs; x=2, 3; y=5, 6) red phosphors with excellent water resistance and enhanced luminescence by W6+/Mo6+ charge compensation

Mn4+ activated fluoride red phosphors are used in warm white light emitting diodes (WLEDs). However, their inherently low moisture resistance is a major barrier to their use in high humidity conditions. In this work, the Nb-based oxyfluoride phosphors AxNbOFy:Mn4+ (A = Na, Cs; x = 2, 3; y = 5, 6) were synthesized by the co-precipitation method. The crystal structure, morphology, and luminescence properties are investigated with composition variation. The non-equivalent replacement of Nd5+ ions by Mn4+ results in bright red emission under blue light excitation. These phosphors showed very good moisture resistant luminescence. The best performance was achieved by the Na3NbOF6:0.1%Mn4+, which retained 98 % of the initial fluorescence intensity after immersion in water for 60 min. In addition, the W6+ and Mo6+ are used to compensate for the charge difference between Mn4+ and Nd5+ in Na3NbOF6:Mn4+. The compensated phosphors have shown large enhancements of 169 % (W6+) and 168 % (Mo6+) in luminescence intensity. This strategy of non-equivalent doping approach of Mn4+ into the Nb-based oxyfluoride materials and the charge compensation by W6+/Mo6+ to enhance the luminescence intensity provides a new way to realize high performance red phosphors for WLEDs.

Exploring the potential of lanthanide-doped oxyfluoride materials for bright green upconversion and their promising applications towards temperature sensing and drug delivery.

The most efficient upconversion (UC) materials reported to date are based on fluoride hosts with low phonon energies, which reduce the amount of nonradiative transitions. In particular, NaYF4 doped with Yb3+ and Er3+ at appropriate ratios is known as one of the most efficient UC phosphors. However, its low thermal stability limits its use for certain applications. On the other hand, oxide hosts exhibit better thermal stability, yet they have higher phonon energies and are thus prone to lower UC efficiencies. As a result, developing host nanomaterials that combine the robustness of oxides with the high upconversion efficiencies of fluorides remains an intriguing prospect. Herein, we demonstrate the formation of ytrrium doped oxyfluoride (YOF:Yb3+,Er3+) particles, which are prepared by growing a NaYF4:Yb3+,Er3+ layer around SiO2 spherical particles and consecutively applying a high-temperature annealing step followed by the removal of SiO2 template. Our interest lies in employing these materials as Boltzmann type physiological range luminescence thermometers, but their weak green emission is a drawback. To overcome this issue, and engineer materials suitable for Boltzmann type thermometry, we have studied the effect of introducing different metal ion co-dopants (Gd3+, Li+ or Mn2+) into the YOF:Yb3+,Er3+ particles, focusing on the overall emission intensity, as well as the green to red ratio, upon 975 nm laser excitation. These materials are explored for their use as ratiometric thermometers, and further also as drug carriers, including their simultaneous use for these two applications. The investigation also includes examining their level of toxicity towards specific human cells - normal human dermal fibroblasts (NHDFs) - to evaluate their potential use for biological applications.

Evaluate the Limits of F Covalent Bonding with Transition Metals at High Valent States in Li2MO2f ( M = Mn, Co, Ni ) Oxyfluoride

The capacity of transition metal oxides as Li-ion battery cathodes are limited by instabilities that arise when high states of charge are achieved1. Oxyfluorides with a disordered rock-salt structure have emerged as attractive alternatives2, but the role of F in their electrochemical function, particularly when metals reach high formal oxidation states, remains to be ascertained so far. In our recent study3, using X-ray Absorption Spectroscopy (XAS) measurements of Mn, O and F, we revealed the existence of Mn-F covalent interactions in Li2MnO2F. The results challenged the assumption of F as largely a spectator ion, providing instead a nuanced picture of redox compensation in oxyfluorides. They suggested the existence of unique knobs of design of battery cathodes in these chemical spaces, by manipulating the covalent interactions between transition metals and two different anions. To further expand the understanding of F participation in the covalent bonding and its electrochemical effect on oxyfluoride materials, we synthesized Li2CoO2F and Li2NiO2F4. Solid fluorides of ions like Co(IV) and Ni(IV) are known to be aggressive oxidizers5. Analysis of the transition metal fluoride literature reveals that oxidation states of IV and higher could lead to unstable phases for late transition metals, particularly Co or Ni5. Unlike oxides, rather than reductively releasing F2, they have a strong propensity to act as highly oxidizing F- donors6,7, being able to oxidize even other halogen cations to their VII state. Yet the oxidation of Ni(II) features prominently in oxyfluoride cathodes that were recently discovered. This puzzle and the exact role of F in modulating the formal redox chemistry of a late metal like Ni and Co in the presence of O remains to be elucidated. To address this question and determine a periodic trend on the role of a mixture of anions in improving the energy density in such cathode materials, we conducted a deep dive into Li2CoO2F and Li2NiO2F. We interrogated the covalent interaction between the oxygen 2p states, fluorine 2p states, and the transition metal 3d orbitals, and their respective contribution to the charge compensation mechanism using XAS. These two oxyfluorides data were compared with the previous Li2MnO2F data to understand the effect of varying the transition metal and how that affects the overall electrochemistry of these materials. We also have estimated the M-O and M-F hybridization and provided a periodic trend. This study allowed us to define a rule to manipulate each element in a oxyfluoride that determines the electrochemical properties of these cathode materials.

Rapid green preparation of Lu7O6F9:RE (RE = Eu3+/Gd3+, Yb3+/Er3+) phosphors using ionic liquids

Rare earth oxyfluoride materials are a fascinating class of host matrixes for various applications. However, rapid green preparation of multifunctional rare earth oxyfluoride phosphors remains a huge challenge. In this work, we report the fast synthesis of Lu7O6F9 using ionic liquids (ILs), which enables the precursors to be obtained at 10 min. The effects of calcination temperature, reaction temperature, reaction time and fluorine source on the phase and morphology of precursors and products are elaborated and the growth mechanism is put forward. Eu3+ alone and Yb3+, Er3+ co-doped samples were prepared to investigate the down-conversion (DC) and up-conversion (UC) properties, respectively. Besides, Gd3+ ions were introduced to the Eu3+ doped products, endowing the phosphors more intense luminescence and paramagnetic properties. The proposed facile synthesis route, excellent luminescence, thermal stability and paramagnetic properties open up the way to obtain rare earth fluoride and oxyfluoride for multifunctional applications.

Bulk and Surface Stabilization Process of Metastable Li-Rich Disordered Rocksalt Oxyfluorides as Efficient Cathode Materials

Manganese based disordered rocksalt systems have attracted attention as Co-free and high capacity cathode materials for Li-ion batteries. However, for a practical application these materials are considered as metastable and exhibit too limited cyclability. In order to improve the structural stability of the disordered rocksalt Li1+xMn2/3Ti1/3O2Fx (0 ≤ x ≤ 1) system during cycling, we have introduced a mild temperature heat treatment process under reducing atmosphere, which is intended to overcome the structural anomalies formed during the mechanochemical synthesis. The heat-treated samples presented better electrochemical properties, which are ascribed to a structural defect mitigation process both at the surface and in the bulk, resulting in improved crystal structure stability. In addition, the optimized particle size and the smaller BET surface area induced by the recrystallization contributes to the observed enhanced performance. Among the studied compositions, the heat treated Li2Mn2/3Ti1/3O2F sample displayed better electrochemical performance with a discharge capacity of 165 mAh g−1 after 100 cycles at 0.1 C (∼80% of the initial capacity), when combined with further conditioning of the cells. The results point explicitly towards a guided stabilization approach, which could have a beneficial effect regarding the application of DRS oxyfluoride materials for sustainable LIBs.

Lanthanum Oxyfluoride modifications boost the electrochemical performance of Nickel-rich cathode

Lanthanum oxyfluoride (LaOF) materials have appreciable higher conductivity, favorable thermal and chemical stability, herein the LaOF coating layer was synthesized on the cathode LiNi0.8Co0.1Mn0.1O2 through one-step solid-phase method, to study the effect of LaOF modification of the electrochemical performance. The modified sample 0.4LaOF-NCM exhibits the initial discharge capacity of 182.8 mAh·g−1 and the retention of 87.8% after 300 cycles at 1C and 2.8–4.3 V (half-cell, room temperature); meanwhile, its full cell has the capacity of 179.8 mAh·g−1 and the retention of 83.5% after 300 cycles, superior to the pristine NCM (175.8 mAh·g−1 and 79%, full cell). Through analyses of XRD, SEM, HRTEM and XPS test results, it indicates that the LaOF amorphous coating layer on the surface of the NCM material can inhibit the side reactions between the cathode and the electrolyte during the cycling, and consequently improve the cycling stability. After 100th cycles, the Li+ diffusion rate during the delithiation and lithiation process is 1.63 × 10−10 and 1.02 × 10−10 cm2 s−1 for the pristine NCM, whereas for the 0.4LaOF-NCM the Li+ diffusion rate is respectively 2.08 × 10−10 and 1.45 × 10−10 cm2 s−1, which are more than 1.3 times faster than those of the pristine NCM. It is suggested that such LaOF modification would have promising applications to improve the performance of LIB.

Calcium-free double-layered cuprate superconductors with critical temperature above 100\u2009K

Calcium is a vital constituent in multilayered cuprate superconductors with critical temperatures (Tc) above 100 K, because it plays a key role in separating CuO2 planes. Here, we demonstrate the synthesis of calcium-free double-layered cuprates: Sr2SrCu2O4(X,O)2(X = F, Cl, and Br) and M(Sr,Ba)2SrCu2Oy(M = Hg/Re, Tl, and B/C), where strontium exists between the CuO2 planes. Oxyfluoride and mercury-based materials show a Tc of 107 K and 110 K, respectively, which are high compared to existing calcium-free cuprates. These findings indicate Tc greater than 100 K can be realized by replacing both barium and calcium, which have been indispensable in conventional multilayered cuprates, with strontium. Furthermore, the non-toxicity of Sr2SrCu2O4F2 and (B,C)Sr2SrCu2Oy simplifies the synthesis process and ensures their safety in potential applications. We also perform a comparison of the characteristic structural parameters between the calcium-free and calcium-containing cuprates considering the number of CuO2 planes.

(Charles W. Tobias Young Investigator Award Address) Understanding Reactivity at Electrode-Electrolyte Interfaces in Li-O2 and Li-ion Batteries

When expanding the limits of voltage stability, capacity retention, and rate capability in batteries, parasitic processes at electrode-electrolyte interfaces-- such as electrolyte degradation, oxide surface densification, and Li metal plating on graphite electrodes-- play a critical role in defining battery performance. Our laboratory’s objective is to combine quantitative analytical techniques with spectroscopy to understand, and ultimately suppress, these process’s role in observed system lifetime, capacity, and efficiency. This presentation will provide a perspective on our understanding of interfacial reactivity in two systems: the positive electrode in the nonaqueous metal-O2 batteries, and interfaces in Li-ion batteries operating at either high voltage or high rates. First, the impact of electrolyte composition on product formation mechanisms in metal-O2 batteries will be discussed, as will strategies to suppress reactive oxygen-induced degradation. Second, drawing a comparison to Li-O2 batteries, the influence of anionic oxygen redox in interfacial degradation processes will be discussed in emerging high-capacity cathode materials, such as cation disordered rocksalt oxide and oxyfluoride materials and Ni-rich NMC layered oxide materials. Finally, to highlight the versatility of gas evolution-based quantitative techniques developed to study oxygen activity in metal-O2 and Li-ion oxide cathodes, the accurate detection of Li plating onset during fast charging of graphite electrodes will be shown by monitoring hydrogen evolution upon exposure of nominally delithiated graphite electrodes to acid.

Stabilization of Li-Rich Disordered Rocksalt Oxyfluoride Cathodes by Particle Surface Modification.

Promising theoretical capacities and high voltages are offered by Li-rich disordered rocksalt oxyfluoride materials as cathodes in lithium-ion batteries. However, as has been discovered for many other Li-rich materials, the oxyfluorides suffer from extensive surface degradation, leading to severe capacity fading. In the case of Li2VO2F, we have previously determined this to be a result of detrimental reactions between an unstable surface layer and the organic electrolyte. Herein, we present the protection of Li2VO2F particles with AlF3 surface modification, resulting in a much-enhanced capacity retention over 50 cycles. While the specific capacity for the untreated material drops below 100 mA h g–1 after only 50 cycles, the treated materials retain almost 200 mA h g–1. Photoelectron spectroscopy depth profiling confirms the stabilization of the active material surface by the surface modification and reveals its suppression of electrolyte decomposition.

Ceramic synthesis of disordered lithium rich oxyfluoride materials

Disordered lithium-rich transition metal oxyfluorides with a general formula Li1+xMO2Fx (M being a transition metal) are gaining more attention due to their high specific capacity which can be delivered from the face-centered cubic (fcc) structure. The most common synthesis procedure involves use of mechanosynthesis. In this work, ceramic synthesis of lithium rich iron oxyfluoride and lithium rich titanium oxyfluoride are reported. Two ceramic synthesis routes are developed each leading to the different level of doping with Li and F and different levels of cationic disorder in the structure. Three different Li1+xMO2Fx samples (x = 0.25, 0.3 and 1) are compared with a sample prepared by mechanochemical synthesis and non-doped LiFeO2 with fcc structure. The obtained lithium rich iron oxyfluoride are characterized by use of Mössbauer spectroscopy, X-ray absorption spectroscopy, NMR and TEM. Successful incorporation of Li and F have been confirmed and specific capacity that can be obtained from the samples is in the correlation with the level of disorder introduced with doping, nevertheless oxidation state of iron in all samples is very similar. Conclusions obtained from lithium rich iron oxyfluoride are validated by lithium rich titanium oxyfluoride.

Disordered Rocksalt Type Li-Rich Manganese Oxyfluoride As High-Capacity Cathode Material for Rechargeable Lithium Ion Batteries

For the coming full-fledged xEV era, rechargeable lithium ion batteries with higher capacities are strongly desired. However, the current commercial cathode materials, such as layered-rocksalt oxide, are reaching the limit of their theoretical capacities. Li-rich oxide cathode materials, which can access not only to transition metal redox reactions but also to redox reactions during the Li insertion/de-insertion, are expected as candidates for the next generation high capacity cathode materials as they can store much more Li ions than the current commercial cathode materials. However, one of the main drawback of these materials are voltage-fading with structural transition upon electrochemical cycling. It is considered due to oxygen loss at surface and transition metal rearrangement [1][2]. To suppress the oxygen loss without decreasing capacity, we focused on the oxyfluoride cathode material. Especially, we confirmed that disordered Rocksalt type-Li2MnO2F synthesized by mechanochemical process exhibit a discharge capacity of 330mAh/g with both transition metal and oxygen redox reaction, which is much higher than conventional cathode materials (200mAh/g) [3]. Recently, Bruce et al, disclosed that the Li1.9Mn0.95O2.05F0.95 does not exhibit oxygen loss from the lattice. This indicates that the Li-rich oxyfluoride is one of the noteworthy material for the high capacity cathode materials. In this study, we will discuss the possibility of the oxyfluoride materials for the next-generation high capacity cathode material from the view point of structures and compositions by comparing with some other oxide cathode materials without fluorine doping. [1] N. Yabuuchi et al, J. Am. Chem. Soc. 133 (2011) 4404 [2] Y. S. Meng et al, Energy Environ. Sci., 4 (2011) 2223 [3] R. Natsui, K Nakura, POSITIVE ELECTRODE ACTIVE MATERIAL AND BATTERY, WO2017/013848 [4] P. G. Bruce et al, Energy Environ. Sci., (2018) Advanced Article

Color-tunable self-activated oxyfluoride phosphors induced by anion-defect with cation-disorder

Cation-disordered oxyfluoride materials, Sr3-p-qBapNaqAl1-zSnzO4F (p = 0–0.7 and q, z = 0, 0.05) and Sr2.975-pBapNa0.05AlO4F (p = 0–0.7), were prepared via solid-state reactions and subsequently induced with defects under appropriate reduction conditions. The anion-deficient self-activating phosphors can be identified using broadband-shaped emissions ranging from 300 to 700 nm when excited with ultraviolet (UV) light. The luminescence properties of the defect-induced self-activating phosphors were examined using cation-disordered [(Sr,Ba) (Al,Sn)O4]3- and [(Sr,Na)2F]3+ clusters in the oxyfluoride lattice. The luminescent emissions of Sr2.45Ba0.5Na0.05Al0.95Sn0.05O4-αF1-δ phosphors were significantly enhanced by the combination of the cation disorder with anion defect. Color-tunable emissions in the defect-induced self-activating Sr2.45Ba0.5Na0.05Al0.95Sn0.05O4-αF1-δ phosphor were obtained. The desired Commission Internationale de I'Eclairage (CIE) values corresponding to the spectral emission from blue to white and yellow regions of the spectra were attained by exciting the defect-induced self-activating Sr2.45Ba0.5Na0.05Al0.95Sn0.05O4-αF1-δ phosphors with UV light.

Effect of the initial reagents concentration on final crystals size and luminescence properties of PbF2:Eu3+ phosphors

The silica oxyfluoride materials doped with Eu3+ ions have been obtained by a facile sol-gel method. The structure of xerogels and powders as well as derivative heat-treated at 350 °C phosphors was examined using scanning transmission electron microscopy (STEM), X-ray diffraction (XRD) measurements as well as FT-IR and Raman spectroscopy. Obtained results clearly indicate the formation of β-PbF2 micro- and nanocrystalline phase dispersed in amorphous silica matrices during ceramization process. Importantly, it was observed that shape and size modifications of β-PbF2 crystals are closely related to the quantitative composition of studied samples. Furthermore, the luminescence properties of Eu3+ ions have been characterized based on excitation and emission spectra as well as luminescence decay measurements from the 5D0 excited state. The characteristic visible emission in reddish-orange spectral scope corresponding to the 4f6-4f6 intra-configurational 5D0 → 7FJ (0-4) transitions has been observed before and after annealing process. The luminescence intensity R/O-ratios were also calculated. Obtained luminescence spectra and double-exponential character of decay curves measured for powders and glass-ceramic samples after heat-treatment indicated the significant changes in local crystal field around Eu3+ ions. In result, conducted luminescence studies and structural characterization indicated the successfully migration of optically active Eu3+ ions from amorphous silica framework to low-phonon energy β-PbF2 micro-/nanocrystal phase after heat-treatment.

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).

Synthesis and characterisation of the Sr xBa 1− xFeO 3− y-system and the fluorinated phases Sr xBa 1− xFeO 2F

Compounds in the system Sr x Ba 1− x FeO 3− y have been prepared under different partial pressures of oxygen. In this system, different perovskite-type structures are found depending highly on the values of x and y. Fluorination using polyvinylidenedifluoride (PVDF) gives oxyfluoride materials of composition Sr x Ba 1− x FeO 2F, which normally crystallize in the cubic perovskite structure. Rietveld refinement results provide information about the packing density for oxide and oxyfluoride samples and allow a general comparison between these two different types of materials. Furthermore, the determination of the average iron oxidation state also showed that the oxygen deficiency, y, depends significantly on the value of x and the structure determined by the Sr/Ba ratio.

Chemical processing for inorganic fluoride and oxyfluoride materials having optical functions

Chemical processing such as a sol–gel method can offer interesting and useful routes for designing and synthesizing inorganic metal fluoride and oxyfluoride materials for applications in optics and photonics. In our series of studies during the last decade, a variety of fluoride materials including alkaline earth fluorides (MgF 2, CaF 2, SrF 2 and BaF 2), rare-earth fluorides (LaF 3, NdF 3, GdF 3, etc.), rare-earth oxyfluorides (LaOF, EuOF, GdOF, Sm 4O 3F 6, Er 4O 3F 6, etc.) and complex fluorides (SrAlF 5, BaMgF 4, BaLiF 3, LiGdF 4, etc.) have been prepared, using trifluoroacetic acid as a fluorine source, in the form of nanoparticles, thin films and oxide/fluoride nanocomposites. They can be utilized as anti-reflective coatings, luminescent materials, VUV materials, IR materials, and so forth. This article summarizes fundamentals and possible applications of optically useful inorganic fluoride and oxyfluoride materials, with emphasis on porous single-layer anti-reflective coatings and visible photoluminescence of doped Eu 3+ or Eu 2+ ions. Furthermore, our recent results on LaF 3:Ce 3+ and LaOF:Ce 3+ are originally reported here.

Iron Oxyfluorides as High Capacity Cathode Materials for Lithium Batteries

Nanostructure iron oxyfluoride compounds of tunable oxygen content were fabricated by a solution process utilizing iron metal and fluorosilicic acid solutions as precursors. Simple adjustment of the synthesis atmosphere, temperature, and time enabled the fabrication of iron oxyfluoride materials with compositions spanning over the entire range from pure to FeOF. Nanocomposites were fabricated from iron oxyfluorides of various oxygen content and activated carbon in order, for the first time, to evaluate the impact of oxygen on the iron fluoride electrochemical performance. The introduction of oxygen proved beneficial to cycling stability but at the expense of reversible capacity. A combination of electrochemical and structural characterization analyses was performed to identify the iron oxyfluoride reaction mechanism.

NMR evidence of LiF coating rather than fluorine substitution in Li(Ni 0.425Mn 0.425Co 0.15)O 2

A series of “Li 1+ z /2(Ni 0.425Mn 0.425Co 0.15) 1− z /2O 2− z F z ” materials was prepared by a coprecipitation route and their structure was characterized using X-ray diffraction (XRD), as well as 7Li and 19F Magic Angle Spinning (MAS) NMR spectroscopy. Two hypotheses were considered: (i) formation of layered oxyfluoride materials and (ii) formation of a mixture between the layered material and LiF. Structural parameters were refined by the Rietveld method, using XRD diffraction data. The refinement results did not allow us to choose between these two hypotheses: no significant change in crystallinity and structural parameters was observed irrespective of the fluorine ratio. 7Li and 19F MAS NMR analyses showed signals with isotropic positions characteristic of LiF, but envelopes characteristic of very strong dipolar interactions with the electron spins of the material, demonstrating that LiF was not incorporated into the layered oxide structure but was instead present as a coating.

A Solid-State NMR, X-ray Diffraction, and Ab Initio Investigation into the Structures of Novel Tantalum Oxyfluoride Clusters

A series of tantalum oxyfluoride materials containing the [Ta4F16O4]4− and [Ta8F24O12]8− anion clusters have been synthesized and characterized using X-ray diffraction (XRD) and solid-state nuclear magnetic resonance (SSNMR) spectroscopy. The structure of both tantalum oxyfluoride materials display octahedrally bonded tantalum atoms with bridging oxygen and terminal fluoride atoms. The [Ta4F16O4]4− cluster is an eight-membered ring, whereas the [Ta8F24O12]8- cluster forms a cagelike structure. Solid-state dynamics of these clusters were explored by monitoring the impact of temperature on the one-dimensional (1D) 19F magic angle spinning (MAS) NMR, 13C cross-polarization (CP) MAS NMR, and two-dimensional (2D) double quantum (DQ) 19F MAS NMR spectra. The DQ 19F NMR correlation experiments allowed the through space connectivity between the different resolved fluorine environments to be determined, thus aiding in the spectral assignment and structural refinement of these materials. Ab initio 19F NMR chemical shift calculations were used to assist in the interpretation of the 19F NMR spectra. The influence of scalar relativistic and Ta−F spin–orbit coupling on the 19F NMR shielding calculation arising from bonding to tantalum atoms is also addressed.