Compositional and structural control in LLZO solid electrolytes.

Garnet-based solid-state electrolytes (SSEs) represent a promising class of materials for next-generation batteries with improved safety and performance. However, lack of control over the composition and crystal structure of the well-known Li7La3Zr2O12 (LLZO) garnet material has led to poor reproducibility with a wide range of ionic conductivities reported in the literature. In this study, the role of precursor homogeneity in controlling the compositional and structural evolution of Al-doped LLZO is explored. A novel solution-based synthesis approach is employed to demonstrate enhanced atomic-scale mixing of the starting materials in comparison to conventional solid-state preparation methods. Through this technique, it is shown that the stability and formation temperature of the highly conductive cubic phase is directly impacted by the spatial distribution of the doping element and reactant species in the precursor mixture. Precursor homogeneity was also an important factor in mitigating the formation of unwanted secondary impurities. These findings can be used to guide the synthesis of SSEs with reproducible material characteristics and enhanced electrolytic performance.

THz/Far infrared synchrotron observations of superlattice frequencies in MgB2.

THz/Far Infrared synchrotron absorption experiments on pure and doped MgB2 samples show that the absorption spectral weight at low wavenumber (i.e., <110 cm-1) evolves as the temperature is reduced to 10 K. Distinct spectral peak intensities increase as the temperature of MgB2 and doped MgB2 approaches, and then crosses, the superconducting transition temperature. These experimental data suggest a strong link to superconductivity induced by subtle shifts in structural symmetry. Significant increases in absorption are observed at frequencies that correspond to the superconducting gaps for doped and pure MgB2, and at fractions of these frequency (or energy) values. This low wavenumber spectral transition is consistent with the notion that superlattice frequencies contribute to the optic modes of the MgB2 phonon dispersion and are critical to the superconducting transition for this structure. Key integer ratios are identified in real and reciprocal spaces that link bonding character, Fermi vectors and Fermi surfaces as well as phonon properties with geometric parameters and specific superlattice symmetries for MgB2. Similarly consistent spectral data at low wavenumber are also obtained for carbon doped Mg11B2. Density Functional Theory calculations of superlattice phonon dispersions result in folded mode frequencies that match these observed low wavenumber experiments. These results show that symmetry reductions, largely electronic in character although coupled to vibrations, occur with change in temperature and imply strong links to superconductivity mechanisms.

Validating the Electronic Structure of Vanadium Phosphate Cathode Materials.

Investigation of the electronic structure of contending battery electrode materials is an essential step for developing a detailed mechanistic understanding of charge-discharge properties. Herein, we use synchrotron soft X-ray absorption spectroscopy (XAS) in combination with complementary experiments and density functional theory calculations to map the electronic structure, band positioning, and band gap of prototype vanadium(III) phosphate cathode materials, Na3V2(PO4)3, Li3V2(PO4)3, and K3V3(PO4)4·H2O, for alkali-ion rechargeable batteries. XAS fluorescence yield and electron yield measurements reveal substantial variation in surface-to-bulk atomic structure, vanadium oxidation states, and density of oxygen hole states across all samples. We attribute this variation to an intrinsic alkali metal surface depletion identified across these alkali metal vanadium(III) phosphates. We propose that an alkali-depleted surface provides a beneficial interface with the bulk structure(s) that raises the Fermi level and improves surface charge transfer kinetics. Furthermore, we discuss how this effect can play a significant role in reducing the electronic and ionic diffusion limitations of alkali vanadium phosphates in alkali-ion rechargeable batteries. These findings clarify the electronic structure and properties of alkali metal vanadium phosphates and offer guidance on future strategies to improve vanadium phosphate battery performance.

Effects of Al substitution by Si in Ti3AlC2 nanolaminate.

Recently, a series of high-purity Ti3(Al1-xSix)C2 solid solutions with new compositions (x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0) have been reported with interesting mechanical properties. Here, we have employed density functional theory for Ti3(Al1-xSix)C2 solid solutions to calculate a wider range of physical properties including structural, electronic, mechanical, thermal and optical. With the increase of x, a decrease of cell parameters is observed. All elastic constants and moduli increase with x. The Fermi level gradually increases, moving towards and past the upper bound of the pseudogap, when the value of x goes from zero to unity, indicating that the structural stability reduces gradually when the amount of Si increases within the Ti3(Al1-xSix)C2 system. In view of Cauchy pressure, Pugh's ratio and Poisson's ratio all compositions of Ti3(Al1-xSix)C2 are brittle in nature. Comparatively, low Debye temperature, lattice thermal conductivity and minimum thermal conductivity of Ti3AlC2 favor it to be a thermal barrier coating material. High melting temperatures implies that the solid solutions Ti3(Al1-xSix)C2 may have potential applications in harsh environments. In the visible region (1.8-3.1 eV), the minimum reflectivity of all compositions for both polarizations is above 45%, which makes them potential coating materials for solar heating reduction.

Synthesis and Characterization of a Novel Hydrated Layered Vanadium(III) Phosphate Phase K3V3(PO4)4·H2O: A Functional Cathode Material for Potassium-Ion Batteries.

Hydrated vanadium(III) phosphate, K3V3(PO4)4·H2O, has been synthesized by a facile aqueous hydrothermal reaction. The crystal structure of the compound is determined using X-ray diffraction (XRD) analysis aided by density functional theory (DFT) computational investigation. The structure contains layers of corner-sharing VO6 octahedra connected by corner and edge-sharing PO4 tetrahedra with a hydrated K+ ion interlayer. The unit cell is assigned to the orthorhombic system (space group Pnna) with a = 10.7161(4) Å, b = 20.8498(10) Å, and c = 6.5316(2) Å. Earlier studies of this material report a K3V2(PO4)3 stoichiometry with a NASICON structure (space group R3®c). Previously reported XRD and electrochemical data on K3V2(PO4)3 are critically evaluated and we suggest that they display mixed phase compositions of K3V3(PO4)4·H2O and known electrochemically active phases KVP2O7 and K3V(PO4)2. In the present study, the synthesis conditions, structural parameters, and electrochemical properties (vs K/K+) of K3V3(PO4)4·H2O are clarified along with further physical characterization by scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), X-ray fluorescence (XRF), Raman spectroscopy, Fourier transform infrared (FT-IR), and thermogravimetric analysis (TGA).

Re-evaluation of experimental measurements for the validation of electronic band structure calculations for LiFePO4 and FePO4.

Experimental measurements used to validate previous electronic band structure calculations for olivine LiFePO4 and its delithiated phase, FePO4, have been re-investigated in this study. Experimental band gaps of LiFePO4 and FePO4 have been determined to be 6.34 eV and 3.2 eV by electron energy loss spectroscopy (EELS) and UV-Vis-NIR diffusion reflectance spectroscopy, respectively. X-ray photoemission (XPS) and Raman spectroscopy show that the surfaces of very carefully synthesized LiFePO4 display Li-depletion, which affects optical reflectance determinations. Based on these experimental measurements, functionals for density functional theory (DFT) calculations of the electronic properties have been revisited. Overall, electronic structures of LiFePO4 and FePO4 calculated using sX-LDA show the best self-consistent match to combined experimentally determined parameters. Furthermore, the open-circuit voltages of the LiFePO4 half-cell have been interpreted in terms of both Fermi levels and Gibbs free energies, which provides additional support for the electronic band structures determined by this research.

Phonon anomalies predict superconducting T(c) for AlB2-type structures.

We show that the well-known Kohn anomaly predicts Tc for ordered AlB2-type structures. We use ab initio density functional theory to calculate phonon dispersions for Mg1-xAlxB2 compositions and identify a phonon anomaly with magnitude that predicts experimental values of Tc for all x. Key features of these anomalies correlate with the electronic structure of Mg1-xAlxB2. This approach predicts Tc for other known AlB2-type structures as well as new compositions. We predict that Mg0.5Ba0.5B2 will show Tc = 63.6 ± 6.6 K. Other forms of the Mg1-xBaxB2 series will also be superconductors when successfully synthesised. Our calculations predict that the end-member composition, BaB2, is likely to show a Tc significantly higher than currently achieved by other diborides although an applied pressure ∼16 GPa may be required to stabilise the structure.

Coherent phonon decay and the boron isotope effect for MgB2.

Ab initio DFT calculations for the phonon dispersion (PD) and the phonon density of states (PDOS) of the two isotopic forms ((10)B and (11)B) of MgB2 demonstrate that use of a reduced symmetry super-lattice provides an improved approximation to the dynamical, phonon-distorted P6/mmm crystal structure. Construction of phonon frequency plots using calculated values for these isotopic forms gives linear trends with integer multiples of a base frequency that change in slope in a manner consistent with the isotope effect (IE). Spectral parameters inferred from this method are similar to that determined experimentally for the pure isotopic forms of MgB2. Comparison with AlB2 demonstrates that a coherent phonon decay down to acoustic modes is not possible for this metal. Coherent acoustic phonon decay may be an important contributor to superconductivity for MgB2.

Phonon modes of MgB2: super-lattice structures and spectral response.

Micrometre-sized MgB2 crystals of varying quality, synthesized at low temperature and autogenous pressure, are compared using a combination of Raman and infra-red (IR) spectroscopy. These data, which include new peak positions in both spectroscopies for high quality MgB2, are interpreted using DFT calculations on phonon behaviour for symmetry-related structures. Raman and IR activity additional to that predicted by point group analyses of the P6/mmm symmetry are detected. These additional peaks, as well as the overall shapes of calculated phonon dispersion (PD) models are explained by assuming a double super-lattice, consistent with a lower symmetry structure for MgB2. A 2× super-lattice in the c-direction allows a simple correlation of the pair breaking energy and the superconducting gap by activation of corresponding acoustic frequencies. A consistent physical interpretation of these spectra is obtained when the position of a phonon anomaly defines a super-lattice modulation in the a-b plane.

Synthesis of MgB2 at Low Temperature and Autogenous Pressure.

High quality, micron-sized interpenetrating grains of MgB2, with high density, are produced at low temperatures (~420 °C < T < ~500 °C) under autogenous pressure by pre-mixing Mg powder and NaBH4 and heating in an Inconel 601 alloy reactor for 5-15 h. Optimum production of MgB2, with yields greater than 75%, occurs for autogenous pressure in the range 1.0 MPa to 2.0 MPa, with the reactor at ~500 °C. Autogenous pressure is induced by the decomposition of NaBH4 in the presence of Mg and/or other Mg-based compounds. The morphology, transition temperature and magnetic properties of MgB2 are dependent on the heating regime. Significant improvement in physical properties accrues when the reactor temperature is held at 250 °C for >20 min prior to a hold at 500 °C.