Structure Evolution of Na2O2 from Room Temperature to 500 °C.

Na2O2 is one of the possible discharge products from sodium-air batteries. Here, we report the evolution of the structure of Na2O2 from room temperature to 500 °C using variable-temperature neutron and synchrotron X-ray powder diffraction. A phase transition from α-Na2O2 to ß-Na2O2 is observed in the neutron diffraction measurements above 400 °C, and the crystal structure of ß-Na2O2 is determined from neutron diffraction data at 500 °C. α-Na2O2 adapts a hexagonal P62m (no. 189) structure, and ß-Na2O2 adapts a tetragonal I41/acd (no. 142) structure. The thermal expansion coefficients of α-Na2O2 are a = 2.98(1) × 10-5 K-1, c = 2.89(1) × 10-5 K-1, and V = 8.96(1) × 10-5 K-1 up to 400 °C, and a ∼10% volume expansion occurs during the phase transition from α-Na2O2 to ß-Na2O2 due to the realignment/rotation of O22- groups. Both phases are electronic insulators according to DFT calculations with band gaps (both indirect) of 1.75 eV (α-Na2O2) and 2.56 eV (ß-Na2O2). An impedance analysis from room temperature to 400 °C revealed a significant enhancement of the conductivity at T ≥ 275 °C. α-Na2O2 shows a higher conductivity (∼10 times at T ≤ 275 °C and ∼3 times at T > 275 °C) in O2 compared to in Ar. We confirmed, by dielectric analysis, that this enhanced conductivity is dominated by ionic conduction.

Effects of Mixed Valency in an Fe-Based Framework: Coexistence of Slow Magnetic Relaxation, Semiconductivity, and Redox Activity.

A 2-D coordination framework, (NEt4)2[Fe2(fan)3] (1·5(acetone); H2fan = 3,6-difluoro-2,5-dihydroxy-1,4-benzoquinone), was synthesized and structurally characterized. The compound is structurally analogous to a formerly elucidated framework, (NEt4)2[Fe2(can)3] (H2can = 3,6-dichloro-2,5-dihydroxy-1,4-benzoquinone), and adopts a 2-D (6,3) topology with the symmetrical stacking of [Fe2(fan)3]2- sheets that are held in position by the NEt4+ cations between the sheets. The investigation of the dc and ac magnetic properties of 1·5(acetone) revealed ferromagnetic ordering behavior and slow magnetization relaxation, as evinced from ac susceptibility measurements. Furthermore, the exposure of 1·5(acetone) to air led to the formation of a heptahydrate 1·7H2O which displayed distinct magnetic properties. The study of the redox state and extent of delocalization in 1·5(acetone) was undertaken via crystallography, in combination with Mössbauer and vis-NIR spectroscopy, to reveal the mixed-valence and delocalized nature of the as-synthesized material. As a result, the conductivity studies conducted on a pressed pellet showed a relatively high conductivity of 1.8 × 10-2 S cm-1 (300 K). In order to compare structurally related anilate-based structures, a relationship among the redox state, spectroscopic properties, and electronic properties was elucidated in this work. A preliminary investigation of 1·5(acetone) as a candidate anode material in lithium ion batteries revealed a high reversible capacity of 676.6 mAh g-1 and high capacity retention.

Crystal and Magnetic Structures of Melilite-Type Ba2MnSi2O7.

Melilite-type Ba2MnSi2O7 was synthesized by a standard powder solid-state reaction route, and its magnetic properties were studied at low temperature. The magnetic structure was found to be C-type pointing along the c axis from neutron powder diffraction, which is different from the G-type ordering previously reported in all other 2-2-4-2 melilites with manganese as the B'-site transition metal. Ab initio (density functional theory) and magnetic dipole-dipole calculations were used to understand the magnetic structure by determining the spin supersuperexchange parameters as well as the relative influence of spin-orbit coupling and the magnetic dipole-dipole interactions.

Synthesis-Controlled Polymorphism and Magnetic and Electrochemical Properties of Li3Co2SbO6.

Li3Co2SbO6 is found to adopt two highly distinct structural forms: a pseudohexagonal (monoclinic C2/m) layered O3-LiCoO2 type phase with "honeycomb" 2:1 ordering of Co and Sb, and an orthorhombic Fddd phase, isostructural with Li3Co2TaO6 but with the addition of significant Li/Co ordering. Pure samples of both phases can be obtained by conventional solid-state synthesis via a precursor route using Li3SbO4 and CoO, by controlling particle size, initial lithium excess, and reaction time. Both phases show relatively poor performance as lithium-ion battery cathode materials in their as-made states, but complex and interesting low-temperature magnetic properties. The honeycomb phase is the first of its type to show A-type antiferromagnetic order (ferromagnetic planes, antiferromagnetically coupled) below TN = 14 K. Isothermal magnetization and in-field neutron diffraction below TN show clear evidence for a metamagnetic transition at H ≈ 0.7 T to three-dimensional ferromagnetic order. The orthorhombic phase orders antiferromagnetically below TN = 112 K and then undergoes two more transitions at 80 and 60 K. Neutron diffraction data show that the ground state is incommensurate.

Tunable Polymer Nanoreactors from RAFT Polymerization-Induced Self-Assembly: Fabrication of Nanostructured Carbon-Coated Anatase as Battery Anode Materials with Variable Morphology and Porosity.

We demonstrate a modular synthesis approach to yield mesoporous carbon-coated anatase (denoted as TiO2/C) nanostructures. Combining polymerization-induced self-assembly (PISA) and reversible addition-fragmentation chain-transfer (RAFT) dispersion polymerization enabled the fabrication of uniform core-shell polymeric nanoreactors with tunable morphologies. The nanoreactors comprised of a poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) shell and a poly(benzyl methacrylate) (PBzMA) core. We selected worm-like and vesicular morphologies to guide the nanostructuring of a TiO2 precursor, namely, titanium(IV) bis(ammonium lactato)dihydroxide (TALH). Subsequent carbonization yielded nanocrystalline anatase and simultaneously introduced a porous carbon framework, which also suppressed the crystal growth (∼5 nm crystallites). The as-prepared TiO2/C materials comprised of a porous structure, with large specific surface areas (>85 m2/g) and various carbon contents (20-30 wt %). As anode components in lithium-ion batteries, our TiO2/C nanomaterials improved the cycling stability, facilitated high overall capacities, and minimized the capacity loss compared to both their sans carbon and commercial anatase analogues.

3D Printing of Monoliths Using TIFSIX-Ni-Based Formulations for the Electric Swing Adsorption of CO2.

The hybrid ultraporous material TIFSIX-Ni ([Ni(pyrazine)2(TiF6)]n) was incorporated into a composite ink for the first time for the three-dimensional (3D) printing of monoliths. The large-scale synthesis of TIFSIX-Ni was completed using two different Ni(II) salts, with CO2 uptakes of 1.90 mmol g-1 achieved using mechanochemically assisted thermal synthesis. The monoliths were then tested for the capture and release of CO2 gas using electric swing adsorption (ESA) under dry and humid conditions. A working capacity of 1.7% was achieved (comparing dynamic data with isotherm data) when a current of 2.1 A was applied for 10 min. The monolith could be cycled repeatedly for 6 h without impacting the performance of the material or loss of capacity. Part of this work explored the improvement of mechanochemically assisted synthetic methods of TIFISX-Ni in reducing the costs associated with large-scale production, allowing for improvements in the overall scale-up and processability of the material for industrial applications.

Interfacial Reactions between Lithium and Grain Boundaries from Anatase TiO2-TUD-1 Electrodes in Lithium-Ion Batteries with Enhanced Capacity Retention.

The synergistic incorporation of anatase TiO2 domains into siliceous TUD-1 was optimized in this work and the resulting sample was implemented as the electrode in lithium-ion batteries. Triethanolamine was used as both the templating and complexing agent, the Si/Ti ratio was controlled, and the formation of Ti-O-Si bridges was optimized, as revealed through Fourier transform infrared spectroscopy, with the porous character of the materials being confirmed with N2 adsorption-desorption isotherms. The controlled formation of Ti-O-Si bridges resulted in attractive specific charge capacities, high rate capability, and a good retention of capacity. The electrochemical performance of the composite material clearly demonstrates a synergistic effect between pure TiO2 in its anatase form and the otherwise inactive siliceous TUD-1 matrix. Specific capacities of 300 mA h g-1 with a retention of 94% were obtained at a current density of 0.1 A g-1 over 100 cycles. This work showcases the use of bifunctional templating agents in the improvement of the performance and the long-term cyclability of composite electrodes, which can be potentially applied in future synthesis of energy materials.