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.

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.

New Spin on Organic Radical Batteries-An Isoindoline Nitroxide-Based High-Voltage Cathode Material.

Organic electrode materials are a highly promising and environmentally benign class of battery materials with radical polymers being at the forefront of this research. Herein, we report the first example of the 1,1,3,3-tetramethylisoindolin-2-yloxyl class of nitroxides as an organic electrode material and the synthesis and application of a novel styrenic nitroxide polymer, poly(5-vinyl-1,1,3,3-tetramethylisoindolin-2-yloxyl) (PVTMIO). The polymer was synthesized from the precursor monomer, 2-methoxy-5-vinyl-1,1,3,3-tetramethylisoindoline, and subsequent oxidative deprotection yielded the electroactive radical species. Cyclic voltammetry revealed a high oxidation potential of 3.7 V versus Li, placing it among the top of the nitroxide class of electrode materials. The suitability of PVTMIO for utilization in a high-voltage organic radical battery was confirmed with a discharge capacity of 104.7 mAh g-1, high rate performance, and stability under cycling conditions (90% capacity retention after 100 cycles), making it one of the highest reported organic p-dopable cathode materials.

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.