Fluorinated Materials as Positive Electrodes for Li- and Na-Ion Batteries.

Fluorine is known to be a key element for various components of batteries since current electrolytes rely on Li-ion salts having fluorinated ions and electrode binders are mainly based on fluorinated polymers. Metal fluorides or mixed anion metal fluorides (mainly oxyfluorides) have also gained a substantial interest as active materials for the electrode redox reactions. In this review, metal fluorides for cathodes are considered; they are listed according to the dimensionality of the metal fluoride subnetwork. The synthesis conditions and the crystal structures are described; the electrochemical properties are briefly indicated, and the nature of the electron transport agent is noted. We stress the crucial importance of the elaboration processes to induce the presence of cation disorders, of anion substitutions (mainly F-/O2- or F-/OH-) or vacancies. Finally, we show that an accurate structural characterization is a key step to enable enhanced material performances to overcome several lasting roadblocks, namely the large irreversible capacity and poor energy efficiency that are frequently encountered.

TIPS-Diazoacetone Aldol Addition: Mechanistic Aspects and Contribution to the Synthesis.

Aldol addition of α-triisopropylsilyl-α-diazoacetone (TIPS-diazoacetone), promoted by excess lithium diisopropylamide (LDA), was developed and applied to the synthesis of original C-TIPS diazoaldols, C-TIPS diazoketols, and a related Mannich-type product. An unprecedented mechanistic pathway has been proposed, involving a lithiated triazene intermediate resulting from the nucleophilic addition of LDA on the diazo moiety, supported by experimental results and DFT calculations.

The Effects of Various Parameters of the Microwave-Assisted Solvothermal Synthesis on the Specific Surface Area and Catalytic Performance of MgF2 Nanoparticles.

High-specific-surface-area MgF2 was prepared by microwave-assisted solvothermal synthesis. The influences of the solvent and the magnesium precursors, and the calcination atmospheres, on the nanoparticle sizes and specific surface areas, estimated by X-Ray Powder Diffraction, N2 sorption and TEM analyses, were investigated. Nanocrystallized (~7 nm) magnesium partially hydroxylated fluorides (MgF2-x(OH)x) with significant specific surface areas between 290 and 330 m2∙g-1 were obtained. After activation under gaseous HF, MgF2-x(OH)x catalysts underwent a large decrease of both their surface area and their hydroxide, rates as shown by their 19F and 1H solid-state NMR spectra. Expect for MgF2 prepared from the acetate precursor, an activity of 30-32 mmol/h∙g was obtained which was about 40% higher compared with that of MgF2 prepared using Trifluoroacetate method (21.6 mmol/h∙g).

Stabilization of a mixed iron vanadium based hexagonal tungsten bronze hydroxyfluoride HTB-(Fe0.55V0.45)F2.67(OH)0.33 as a positive electrode for lithium-ion batteries.

In our search for novel insertion compounds for Li-based batteries, we have identified a new mixed iron vanadium based Hexagonal Tungsten Bronze (HTB) type phase. Its synthesis involves two steps which consist first of preparing mixed metal hydrated fluoride Fe1.64V1.36F8(H2O)2 by a microwave assisted thermal process, followed by thermal treatment under air to obtain metastable HTB-(Fe0.55V0.45)F2.67(OH)0.33 hydroxyfluoride. 57Fe Mössbauer spectrometry demonstrates the presence of oxidation states Fe2+ and Fe3+ in Fe1.64V1.36F8(H2O)2 as opposed to only Fe3+ in HTB-(Fe0.55V0.47)F2.67(OH)0.33. Moreover, the Mössbauer spectra recorded at 77 K reveal that none of the compounds shows magnetic ordering owing to the presence of V3+ distributed over the crystallographic sites of Fe3+. Complementary X-ray spectroscopy and Rietveld refinement further confirm the successful synthesis of HTB-(Fe0.55V0.45)F2.67(OH)0.33. Electrochemically, the new HTB-(Fe0.55V0.45)F2.67(OH)0.33 shows a first discharge capacity of 181 mA h g-1 with 67% of this capacity remaining upon cycling. Unlike HTB-FeF2.66(OH)0.34, the structure remains stable after the first discharge confirming the positive effect of vanadium in the HTB network.

Investigation of mixed-metal (oxy)fluorides as a new class of water oxidation electrocatalysts.

The development of electrocatalysts for the oxygen evolution reaction (OER) is one of the principal challenges in the area of renewable energy research. Within this context, mixed-metal oxides have recently emerged as the highest performing OER catalysts. Their structural and compositional modification to further boost their activity is crucial to the wide-spread use of electrolysis technologies. In this work, we investigated a series of mixed-metal F-containing materials as OER catalysts to probe possible benefits of the high electronegativity of fluoride ions. We found that crystalline hydrated fluorides, CoFe2F8(H2O)2 and NiFe2F8(H2O)2, and amorphous oxyfluorides, NiFe2F4.4O1.8 and CoFe2F6.6O0.7, feature excellent activity (overpotential for 10 mA cm-2 as low as 270 mV) and stability (extended performance for >250 hours with ∼40 mV activity loss) for the OER in alkaline electrolyte. Subsequent electroanalytical and spectroscopic characterization hinted that the electronic structure modulation conferred by the fluoride ions aided their reactivity. Finally, the best catalyst of the set, NiFe2F4.4O1.8, was applied as anode in an electrolyzer comprised solely of earth-abundant materials, which carried out overall water splitting at 1.65 V at 10 mA cm-2.

New iron tetrazolate frameworks: synthesis, temperature effect, thermal behaviour, Mössbauer and magnetic studies.

The exploration of the FeF3/FeF2-Hamtetraz-HF system in dimethylformamide by solvothermal synthesis evidences two isostructural 3D hybrid fluoroferrates. They are prepared from the same starting mixture at two different synthesis temperatures: 120 °C for [Hdma]·(Fe4(II)Fe(III)F8(H2O)2(amtetraz)4) () and 140 °C for [Hdma]1.5·(Fe4.5(II)Fe0.5(III)F7(H2O)(HCOO)(amtetraz)4) (). Both compounds are characterized by single crystal X-ray diffraction, X-ray thermodiffraction, TGA analysis, Mössbauer spectrometry and SQUID magnetometry. They crystallize in the monoclinic system and are built from two distinct chains connected by aminotetrazolate anions. The first chain ∞(Fe(II)FN4) is common to and and can be found in numerous fluorides. In the second chain ∞(Fe3X12) (X = F, N, O), iron cations adopt both valence states Fe(ii)/Fe(iii). The hydrolysis of DMF implies the formation of a [Hdma](+) cation and a (HCOO)(-) anion. The presence of Fe(3+) in both phases is evidenced by (57)Fe Mössbauer spectrometry. The magnetic properties are studied and two transitions from a paramagnetic regime to a long range ordered state below 30 K and 5 K are identified.

(H3O)2NaAl3F12, isostructural with A2NaAl3F12 (A = K(+), Rb(+), Cs(+)) fluorides having HTB-type sheets.

The title compound, (H3O)2NaAl3F12 [dihydronium sodium trialuminum(III) dodecafluoride], was obtained by solvothermal synthesis from the reaction of aluminium hydroxide, sodium hydroxide, 1,2,4-triazole and aqueous HF in ethanol at 463 K for 48 h. The structure consists of AlF6 octahedra organized in [AlF4(-)]n HTB-type sheets (HTB is hexagonal tungsten bronze) separated by H3O(+) and Na(+) cations.

New series of hybrid fluoroferrates synthesized with triazoles: various dimensionalities and Mössbauer studies.

The solvothermal reactions of an equimolar mixture of FeF2 and FeF3 with Htaz (1,2,4-triazole), aqueous HF and DMF (dimethylformamide) at 120 °C yielded a series of new hybrid fluoroferrates (1-5). Their structures were characterized by either single crystal or powder X-ray diffraction data analysis. Both classes of hybrid networks were observed according to the Fe(n+)/Htaz/HF starting ratio: class I for 1 and 2 and class II for 3, 4 and 5. Four compounds, [Hdma]·(Fe2(H2O)4F6) (1), [Hdma]·(Fe2(H2O)4F6)·0.5H2O (2), Fe2F5(Htaz) (3) and [Hdma]·(Fe2F5(H2O)(Htaz)(taz)) (4), exhibit both Fe(II) and Fe(III) oxidation states while [Hdma]·(Fe2F5(taz)2) (5) contains only Fe(III) cations. [Hdma]·(Fe2(H2O)4F6) (1) and [Hdma]·(Fe2(H2O)4F6)·0.5H2O (2) contain anionic inorganic chains of alternating corner-sharing Fe(II) and Fe(III) octahedra; they are weakly hydrogen bonded to dimethylammonium cations [Hdma](+) which are formed by the in situ hydrolysis of DMF. The structure of Fe2F5(Htaz) (3) exhibits a three dimensional inorganic network resulting from the association of HTB planes of corner sharing Fe(II)F4N2 and Fe(III)F6 octahedra. [Hdma]·(Fe2F5(H2O)(Htaz)(taz)) (4) and [Hdma]·(Fe2F5(taz)2) (5) reveal two original two-dimensional sheets. In 4, the deprotonated and neutral amines connect trinuclear Fe3F10N6 units of corner-sharing octahedra and mononuclear FeN4(H2O)2 octahedra. Infinite Fe2F5(taz)2 layers in 5 are built up from dinuclear species connected by deprotonated amines along two perpendicular directions. The thermal behavior and Mössbauer spectrometry results are detailed for the first tridimensional mixed valence hybrid fluoroferrate (3).