ABS T R A C T Free-standing Ga-doped Li 7 La 3 Zr 2 O 12 (LLZO) films are fabricated by a scalable tape casting technique. The thin (25 -50 mu m) ceramics exhibit a relative density of 98 +/- 1 % and record-high ionic conductivity (1.41 +/- 0.05 mS cm -1 ) for tape cast LLZO. A solution-based approach is used to synthesize the green powders which enable sintering at relatively low temperatures (1050 degrees C, 30 min). The phase chemistry and microstructure are wellcontrolled during the sintering process. The tapes are further developed into multilayer porous/dense/porous structures to provide a fast Li + conducting ceramic framework for all-solid-state batteries (ASSBs). X-ray micro computed tomography (microCT) analysis reveals a fully interconnected pore network in the outer porous layers. In addition, the multilayer structures exhibit a critical current density (CCD) of 2.5 mA cm -2 at room temperature and are stable over 200 cycles at a current density of 0.5 mA cm -2 in symmetric Li cells. Overall, this work offers guidance for the scalable production of thin LLZO ceramics with enhanced physical and electrochemical properties.

Free-standing Ga-doped Li7La3Zr2O12 (LLZO) films are fabricated by a scalable tape casting technique. The thin (25–50 μm) ceramics exhibit a relative density of 98 ± 1 % and record-high ionic conductivity (1.41 ± 0.05 mS cm−1) for tape cast LLZO. A solution-based approach is used to synthesize the green powders which enable sintering at relatively low temperatures (1050 °C, 30 min). The phase chemistry and microstructure are well-controlled during the sintering process. The tapes are further developed into multilayer porous/dense/porous structures to provide a fast Li+ conducting ceramic framework for all-solid-state batteries (ASSBs). X-ray micro computed tomography (microCT) analysis reveals a fully interconnected pore network in the outer porous layers. In addition, the multilayer structures exhibit a critical current density (CCD) of 2.5 mA cm−2 at room temperature and are stable over 200 cycles at a current density of 0.5 mA cm−2 in symmetric Li cells. Overall, this work offers guidance for the scalable production of thin LLZO ceramics with enhanced physical and electrochemical properties.

The densification of Ga-doped Li7La3Zr2O12 (LLZO) ceramics was optimized by controlling the phase chemistry and morphology of green powders after calcination. A solution-based approach was used to synthesize the green powders which were calcined at temperatures between 600 degrees C and 950 degrees C and free-sintered into dense pellets. The ionic conductivity and relative density were optimized at a calcination temperature of 700 degrees C to be 1.28x10- 3 S/cm and 96.2%, respectively. At this condition, the green powder consisted primarily of La2Zr2O7 pyrochlore nanoparticles (80-100 nm) and LLZO particles (0.5-1 mu m), which promoted reaction-driven densi-fication and enabled a favorable bimodal size distribution for improved packing density. Further increasing the calcination temperature was correlated with a reduction in density due to the detrimental effects of particle coarsening on sintering activity. These findings show that controlling the calcination conditions is an effective method for tailoring green body properties for improved densification of LLZO.

The Li + conductivity of Li7La3Zr2O12 (LLZO) solid electrolytes was optimized by controlling the addition of the dopant (Al) into the crystal structure. A solution-based synthesis method was used to minimize Al segregation and ensure a homogeneous substitution into the garnet framework. Neutron and x-ray diffraction were used to monitor the structural transition from tetragonal to cubic LLZO. It was found that the critical dopant concentration (xc) required to stabilize the cubic phase was 0.18 mol pfu. The maximum ionic conductivity of 5.54 x 10-4 S/cm was obtained at the critical composition. The enhanced conductivity was achieved by accurately controlling the vacancy density at the active Li octahedral sites. Neutron structure refinements were used to validate the changes in Li vacancy distribution at different dopant concentrations. Impedance measurements reveal a strong dependence of Li+ transport properties on Al concentration, specifically around the critical dopant concentration xc = 0.18 mol pfu. These findings demonstrate the importance of fine-tuning composition and provide guidance for engineering improvements in ionic conductivity.

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.

Structural analyses and phase quantification of solution-synthesised and commercially available boron carbide powders are conducted via Rietveld refinement of X-ray diffraction data. A fixed incidence parallel beam geometry is compared against the more commonly used Bragg-Brentano geometry. Fixed incidence parallel beam is found to have a significant advantage in phase quantification through the minimisation of peak aberrations due to the high X-ray transparency of boron carbide. The enhanced resolution and accuracy afforded by this technique allows for fundamental new insights into the phase content of boron carbide powders. Specifically, commercially obtained boron carbide powders are found to be multi-phasic, containing varying contents of both B~4C and B~6.5C phases. Solution-synthesised boron carbide produced in this study is found to comprise of B4.05C only. These findings have significant importance to the development of boron carbide for high-performance applications and present a potential avenue for the synthesis of high purity boron carbide powders with controlled stoichiometry.

The surface chemistries of carefully synthesized, phase pure lithium transition-metal phosphates LiTMPO4 (TM = Fe, Mn, Co, Ni) have been determined using a combination of surface spectroscopic techniques. Results obtained with X-ray photoelectron spectroscopy, Raman spectroscopy, and soft X-ray absorption spectroscopy indicate the presence of surface Li-depletion and mixed chemical states of transition-metal (TM) ions across the family. The extent of surface TM mixed chemical states has been found responsible for changes in color and optical absorption edge positions, which are often used to validate electronic band structure calculations. These characteristics are common in many complex metal oxide ceramics and may not be unique to lithium metal phosphate battery materials.

LiFePO4 (LFP) is one of the important commercial battery materials, as such, many efforts have been made to understand its electrical and ionic conductivities and electrochemical properties. In this study, we have investigated electrochemical, electrical and magnetic properties of carbon coated LFP down to cryogenic temperatures. The fact that the practical material really consists of a core-shell structure with a shell of delithiated material and carbon coating determines the measured properties, which are often mistakenly attributed to pure LFP core behaviour. An electronic resistivity drop (11 +/- 0.5% based on the resistivity at room temperature), preceded by a gradual increase feature between 100 and 30 K, was observed when the temperature was below the Neel temperature at low applied currents, indicating a likely interaction between the magnetic configuration of the core LFP and electronic transport mechanisms. Metallic Fe3P was precipitated on the samples surfaces after annealing at high temperature in Argon. The existence of Fe3P was found to significantly improve the electronic conductivity but it took a toll on the electrochemical performance.

The optical absorption properties of lithium transition-metal phosphates (LiTMPO4, TM = Mn, Fe, Co, Ni) with nanoscale particle sizes (300-500 nm in diameter) have been measured. The measured edges have been compared with band gaps determined from their corresponding electronic band structure calculations. Various functionals for density functional theory calculations have been compared and validated against experimental results. Gradual increases and intermediate peaks in optical absorption spectra before the main absorption edges have been attributed to intervalence charge transfer between TM2+ and TM3+ due to surface Li depletion. The functional, screened-exchange local density approximation (sX-LDA), which includes screened Hartree-Fock exchange, shows the highest overall accuracy for electronic band structure prediction with acceptable computational cost. The voltage plateaus determined from calculated enthalpies also display the best match with calculations using sX-LDA.

This study investigates a series of cation dopants for LiFePO4 that are reported to be beneficial for rate performance. A solid-state synthesis has been used to be consistent with a common processing method used in prior doping investigations and in commercial processes for this compound. Increased ratios of Fe3+/Fe2+ oxidation on the particle surfaces have been determined using X-ray photoelectron spectroscopy (XPS) and soft X-ray absorption spectroscopy (sXAS) for the doped LiFePO4, for all chosen dopants and valences (<, = or >2+), while the cores remain much Valence s.a.et closer to the characteristics of the pristine (undoped) material. These results indicate that the dopants are predominantly pushed to the particle surfaces during phase formation, even when the dopants are added at the initial fine mixing of precursors. The differential distribution of dopants between the cores and surface layers of the particles results in conductivity improvements and reduction of polarizations in electrochemical impedance spectroscopy measurements of the doped LiFePO4.

In this study, the rate performance of a LiFePO4 (LFP) electrode has been enhanced by optimization of the particle size distribution of the LFP particles. Two LFP samples with different particle sizes (∼50 and ∼350 nm) are mixed with various ratios and the electrochemical performance has been evaluated. Reduction of the contact resistance and increase of the Li diffusion coefficient have been achieved. The electrode with a mixing ratio of 50:50 shows an improved initial capacity at C/10 and superior rate capability compared with the two pristine materials.

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.

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.

Electron density differences resulting from atom displacement patterns aligned with phonon modes in MgB2 have been calculated using density functional theory (DFT). The extent of phonon anomalies, identified as indicators of the superconducting transition temperature, Tc, under a range of conditions in AlB2-type structures, reduce as boron atoms are displaced from their equilibrium positions along E2g mode directions. The Fermi energy for displacements along the directions of the E2g phonon mode accounts for changes in the covalent B-B bond electronic charge density. We applied differential atom displacements to show that the shifted σ band structure associated with the light effective mass became tangential to the Fermi level and that the Fermi surface undergoes a topological transition at a critical relative displacement of ~0.6% of the boron atoms from equilibrium. The difference in Fermi energies at this critical displacement and at the equilibrium position correspond to the superconducting energy gap. The net volume between tubular σ surfaces in reciprocal space correlated with the depth of the phonon anomaly and, by inference, it is a key to an understanding of superconductivity. This ab initioapproach offers a phenomenological understanding of the factors that determine Tc based on knowledge of the crystal structure.

Raman spectroscopy of many metal hexaborides is well understood for vibrations of Pm3 over bar m symmetry that can be ascribed to the breathing and deforming modes of boron octahedral clusters at the corners of the cubic unit cell. However, the significance of the lower energy modes has been subject to debate wherein interpretation is dependent on "rattling" of the metal ion within the boron cage or on alternative mechanisms such as the presence of defects or of isotope clusters. Furthermore, converged calculations of phonon dispersions (PDs) using Density Functional Theory (DFT) for YB6 with Pm3 over bar m symmetry in the published literature are limited and equivocal. Converged PDs across all reciprocal lattice orientations are an important indicator of thermodynamic stability for a phase. We use this criterion to evaluate symmetry conditions for hexaboride structures and compare DFT models of PDs with Raman spectra for the hexaborides BaB6, LaB6, and YB6. We demonstrate that models with lower symmetry using a P4/mmm superlattice along one primary axial direction allow the lower energy modes to be assigned to a point symmetry. This approach, which models displacements of the metal position along the superlattice direction, results in a convergent PD for YB6, an outcome difficult to achieve with Pm3 over bar m symmetry.

We have calculated the extent of the E-2g phonon anomaly for Mg1-xMxB2 where M = Sc, Ti, Cd and Ba for 0 < x < 1 using ab initio DFT models with the LDA and GGA functionals. Using superlattice models along the c axis to represent metal substitution in MgB2, we show that phonbn dispersion (PD) plots vary significantly with x, in particular, the nature and extent of the phonon anomaly around the origin, G, of the reciprocal lattice. Measurement of this phonon anomaly along the G-K and G-M directions provides an estimate of the thermal energy, T-delta, of the anomaly, which approximates experimentally determined T-c, within standard error for Sc and Ti substitution. We demonstrate that substitutions of Cd and Ba in MgB2 show a higher calculated T-delta than MgB2 by more than 20 K. Syntheses of these Cd and Ba compositions are not extant and may not be possible given the limited solubility of metals in MgB2. Nevertheless, ab initio DFT models of phonon behaviour in AIB(2)-type structures provide an effective tool for prediction of physical properties and for design of new materials. (C) 2017 The Author(s). Published by Elsevier B.V.

We evaluate, via the Local Density and the Generalised Gradient Approximations to the Density Functional Theory (DFT), the change in form and extent of the E-2g phonon anomaly of MgB2 with increase in applied pressure up to 20 GPa. Ab initio DFT calculations on the phonon dispersion (PD) for MgB2 show a phonon anomaly symmetrically displaced around Gamma, the reciprocal lattice origin. This anomaly is related to nesting between diametrically opposite sides of tubular elements of Fermi surfaces, which correspond to sigma bonding and run approximately parallel to the Gamma-A reciprocal space direction. The anomaly is parallel to Gamma-A and along Gamma-M and Gamma-K. The extent of the E-2g phonon anomaly, delta, along Gamma-M and Gamma-K is a measure of the thermal energy, T-delta, that matches within error the experimental onset superconducting transition temperature, T-c. Ab initio DFT calculations with pressure for -5 GPa < P < 20 GPa show a linear reduction in T-delta that closely matches experimental T-c values for MgB2. For phonon-mediated superconductors with AlB2-type structures, the thermal energy of the phonon anomaly, T-delta, is a reliable predictor of T-c. (C) 2017 Elsevier B.V. All rights reserved.

A predictive tool for the design of new, higher-temperature superconductors requires a simple, first-principles technique based on well-established axioms as embodied in density functional theory (DFT), without post-calculation corrections or increased levels of complexity. We show that anomalies in the calculated phonon dispersions of compounds with AlB2-type structures are good descriptors of their superconducting transition temperatures. This ab initio methodology has proven robust for descriptions of the superconductivity of MgB2 and other related AlB2-type structures, the different isotopic forms of MgB2, a series of substituted MgB2 compositions with Al and with transition metals, the pressure dependence of the superconductivity of MgB2, and various temperature effects. Calculated values are well correlated with experimentally determined values within experimental errors, when sufficiently fine k-grids are used to resolve Fermi surface details. This methodology provides invaluable insight on the mechanisms of superconductivity, can be extended to other crystal systems, and has been used to predict superconducting T (c) for new compounds.

The production of boron carbide powder with uniform particle size in high yield is demonstrated via precise control of precursor processing and handling conditions. A gel is formed by complete dissolution of boric acid (H3BO3) and sucrose in water which is dried, pre-treated at 550 °C and then calcined at 1400 °C to form the boron carbide product. Optimised synthesis conditions are obtained by managing exposure of precursor powders to atmospheric conditions. Exposure of precursor powders to humid environments causes significant changes in morphology due to water adsorption by dehydrated H3BO3 variants. This hydration drives a loss of contact surfaces between boron and carbon components, decreasing boron carbide yield and increasing residual carbon impurity. Reactant dispersion in precursor powders is also shown to have a direct effect on formed boron carbide morphology.