Jamie L. Adcock

Controlled formation of mixed nanoscale domains of high capacity Fe2O3-FeF3 conversion compounds by direct fluorination.

We report a direct fluorination method under fluorine gas atmosphere using a fluidized bed reactor for converting nanophase iron oxide (n-Fe2O3) to an electrochemically stable and higher energy density iron oxyfluoride/fluoride phase. Interestingly, no noticeable bulk iron oxyfluoride phase (FeOF) phase was observed even at fluorination temperature close to 300 °C. Instead, at fluorination temperatures below 250 °C, scanning transmission electron microscopy coupled with electron energy loss spectroscopy (STEM-EELS) and X-ray photoelectron spectroscopy (XPS) analysis showed surface fluorination with nominal composition, Fe2O3-xF2x (x < 1). At fluorination temperatures of 275 °C, STEM-EELS results showed porous interconnected nanodomains of FeF3 and Fe2O3 coexisting within the same particle, and overall the particles become less dense after fluorination. We performed potentiometric intermittent titration and electrochemical impedance spectroscopy studies to understand the lithium diffusion (or apparent diffusion) in both the oxyfluoride and mixed phase FeF3 + Fe2O3 composition, and correlate the results to their electrochemical performance. Further, we analyze from a thermodynamical perspective, the observed formation of the majority fluoride phase (77% FeF3) and the absence of the expected oxyfluoride phase based on the relative formation energies of oxide, fluoride, and oxyfluorides.

A Topotactic Synthetic Methodology for Highly Fluorine‐Doped Mesoporous Metal Oxides

Ordered mesoporous materials have attracted much attention for years because of their excellent physical properties, such as very large specific surface area, nanometer-scale fine pores, and high stability, which are beneficial for their use as catalysts, separators, adsorbents, and so forth. The first ordered mesoporous materials were synthesized in around 1990 using soft templating pathways. Following the soft templating method, different kinds of mesoporous silica and nonsilica materials such as carbon, metal phosphates, gAl2O3, [9] Cr2O3, [10] lanthanide oxides, TiO2, ZrO2, Al2O3, Nb2O5, Ta2O5, and WO3 were synthesized. [12] Nanocasting is an alternative method for preparing mesoporous materials that are difficult to synthesize with the conventional soft templating method. This hard templating approach has been applied to prepare different kinds of metal oxides, such as CoO, Co3O4, [15] Mn3O4, [16] Fe2O3, [17] Fe3O4, [18] and NiO. Following the work by Lu et al., several ordered mesoporous materials, including MgO, boron nitride, g-Al2O3, [23] aluminosilicate, CuO, and ZnO, were nanocast by using ordered mesoporous carbon CMK-3 as a matrix. The control of interfacial properties through tunable surface functionalities is essential in the development of functional mesoporous materials. To the best of our knowledge, no versatile method for preparation of fluorine-doped mesoporous metal oxides has been reported previously. Mesoporous materials with high F atomic concentrations could be considered as a new class of mesoporous materials (mesoporous metal oxyfluorides). Owing to the high acidity of host frameworks, favorable fluoride ionic conduction and a spectral response in the visible region, these materials have properties of interest for numerous potentially valuable applications such as electric conductors, primary and secondary batteries, catalysis, and photocatalysis. There are only three reports on fluorine-containing mesoporous materials for mesoporous carbons prepared in a one-pot synthesis route using p-fluorophenol/phenol–formaldehyde as carbon precursors or through direct fluorination. By employing mesoporous metal oxides (TiO2 ZrO2, Al2O3, Nb2O5, and Ta2O5) as the starting materials and by using the most reactive fluorination agent, elemental fluorine (F2), we developed a novel, straightforward topotactic route towards F-doped mesoporous metal oxides with high fluorine content (up to 40%). The BET surface areas, pore sizes, pore volumes, and F atomic concentrations for these materials could be adjusted by varying the fluorination time and temperature (BET= Brunauer–Emmett–Teller). Our topotactic preparation method presents a general and facile approach to synthesis of F-doped mesoporous metal oxides and could be carried out on a large scale. Mesoporous metal oxides can be synthesized easily by a solvent-evaporation-induced self-assembly method by using metal alkoxides as the metal source and triblock copolymer F127 as the template. The overall synthetic procedure is illustrated in Scheme 1. We used five typical mesoporous metal oxides as precursors: TiO2, ZrO2, Al2O3, Nb2O5, and Ta2O5. The topotactic reaction proceeded by the chemical substitution of O atoms in the metal oxides by F atoms, resulting in F-doped mesoporous metal oxides. In other words, these mesoporous materials with crystalline (TiO2) or

Towards the selective modification of soft-templated mesoporous carbon materials by elemental fluorine for energy storage devices

Graphite fluoride is classified into (CF)n and (C2F)n types based on its structure and composition. The former (CF)n has been widely prepared and commercially utilized as electrodes in primary lithium ion batteries (LIBs). Porous electrodes, i.e. templated mesoporous carbons, can greatly improve first Coulombic efficiencies. For achieving the highest discharge potentials and rate capabilities, the extent of fluorination reactions and the retention of a conductive carbon backbone are required. Hence, the choice of the starting carbon nanomaterial and of the fluorination conditions is detrimental to the design of future energy storage and conversion devices, namely LIBs and pseudosupercapacitors.

Energetics of Defects on Graphene through Fluorination

Functionalized graphene sheets (FGSs) comprise a unique member of the carbon family, demonstrating excellent electrical conductivity and mechanical strength. However, the detailed chemical composition of this material is still unclear. Herein, we take advantage of the fluorination process to semiquantitatively probe the defects and functional groups on graphene surface. Functionalized graphene sheets are used as substrate for low-temperature (<150 °C) direct fluorination. The fluorine content has been modified to investigate the formation mechanism of different functional groups such as C-F, CF2, O-CF2 and (C=O)F during fluorination. The detailed structure and chemical bonds are simulated by density functional theory (DFT) and quantified experimentally by nuclear magnetic resonance (NMR). The electrochemical properties of fluorinated graphene are also discussed extending the use of graphene from fundamental research to practical applications.

Fluorination of “brick and mortar” soft-templated graphitic ordered mesoporous carbons for high power lithium-ion battery

Ordered mesoporous carbon–graphitic carbon composites prepared by the “brick and mortar” method were fluorinated using F2 and investigated as cathodes for primary lithium batteries. The resulting materials have a rich array of C–F species, as measured by XPS, which influence conduction and voltage profiles.

Role of precursor chemistry in the direct fluorination to form titanium based conversion anodes for lithium ion batteries

A new synthetic route for the formation of titanium oxydifluoride (TiOF2) through the process of direct fluorination via a fluidized bed reactor system and the associated electrochemical properties of the powders formed from this approach are reported. The flexibility of this synthetic route was demonstrated using precursor powders of titanium dioxide (TiO2) nanoparticles, as well as a reduced TiOxNy. An advantage of this synthetic method is the ability to directly control the extent of fluorination as a function of reaction temperature and time. The TiOF2 synthesized from TiO2 and TiOxNy showed reversible capacities of 300 mA h g−1 and 440 mA h g−1, respectively, over 100 cycles. The higher reversible capacity of the TiOF2 powders derived from TiOxNy likely relate to the partial reduction of the Ti in the fluorinated electrode material, highlighting a route to optimize the properties of conversion electrode materials.

Comparison of rheological properties of short-chain perfluoropolyethers through simulation and experiment

Four short-chain perfluoropolyethers (PFPEs) with varied architectural modifications have been studied rheologically using non-equilibrium molecular dynamics simulation and experiment. An explicit-atom potential, which treats all fluorine atoms equivalently, is not capable of reproducing the experimentally observed trends in viscosity among the four compounds. Rather, the parameters of the potential governing the interaction of fluorine must reflect the proximity of the fluorine to the oxygen in the ether linkage. Defining four different types of fluorine atoms, we are able to reproduce the experimentally observed trends in viscosity among the four compounds. We examine the effect of this potential change on the structural properties as well.