51 research outputs found
Electronic structure and optical properties of lightweight metal hydrides
We study the electronic structures and dielectric functions of the simple
hydrides LiH, NaH, MgH2 and AlH3, and the complex hydrides Li3AlH6, Na3AlH6,
LiAlH4, NaAlH4 and Mg(AlH4)2, using first principles density functional theory
and GW calculations. All these compounds are large gap insulators with GW
single particle band gaps varying from 3.5 eV in AlH3 to 6.5 eV in the MAlH4
compounds. The valence bands are dominated by the hydrogen atoms, whereas the
conduction bands have mixed contributions from the hydrogens and the metal
cations. The electronic structure of the aluminium compounds is determined
mainly by aluminium hydride complexes and their mutual interactions. Despite
considerable differences between the band structures and the band gaps of the
various compounds, their optical responses are qualitatively similar. In most
of the spectra the optical absorption rises sharply above 6 eV and has a strong
peak around 8 eV. The quantitative differences in the optical spectra are
interpreted in terms of the structure and the electronic structure of the
compounds.Comment: 13 pages, 10 figure
Electronic Structure of the Complex Hydride NaAlH4
Density functional calculations of the electronic structure of the complex
hydride NaAlH4 and the reference systems NaH and AlH3 are reported. We find a
substantially ionic electronic structure for NaAlH4, which emphasizes the
importance of solid state effects in this material. The relaxed hydrogen
positions in NaAlH4 are in good agreement with recent experiment. The
electronic structure of AlH3 is also ionic. Implications for the binding of
complex hydrides are discussed.Comment: 4 pages, 5 figure
Recent Advances in First Principles Computational Research of Cathode Materials for Lithium-Ion Batteries
Is it possible to prepare olivine-type LiFeSiO4?. A joint computational and experimental investigation
Silicates LiMSiO4 are potential positive electrode materials for lithium ion batteries. In this work we analyse from first principles calculations the relative stability of possible LiFeSiO4-polymorphs within four structural types. Olivine-LiFeSiO4 is predicted to be more stable than the LiFeSiO4 prepared by delithiation of Li2FeSiO4; the latter being the only LiFeSiO4 compound reported so far. Attempts to prepare olivine-LiFeSiO4 from a mixture of reactants at ambient pressure (600-1100 °C) resulted in a mixture of quartz-SiO2, Li2SiO3, LiFe5O8 and LiFeSi2O6 phases. Conducting the reaction under HP conditions (40 kbar) leads to the formation of LiFeSi2O6 as a majority phase, regardless the nature of the reactants/precursors. First principles calculations indicate that the preparation of the olivine-LiFeSiO4 is thermodynamically hindered due to the competition with the more stable LiFeSi2O6 pyroxene, in the range of pressure/temperature investigated. © 2008 Elsevier B.V. All rights reserved
Low-Potential Sodium Insertion in a NASICON-Type Structure through the Ti(III)/Ti(II) Redox Couple
We report the direct
synthesis of powder Na<sub>3</sub>Ti<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub> together with its low-potential electrochemical
performance and crystal structure elucidation for the reduced and
oxidized phases. First-principles calculations at the density functional
theory level have been performed to gain further insight into the
electrochemistry of Ti(IV)/Ti(III) and Ti(III)/Ti(II) redox couples
in these sodium superionic conductor (NASICON) compounds. Finally,
we have validated the concept of full-titanium-based sodium ion cells
through the assembly of symmetric cells involving Na<sub>3</sub>Ti<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub> as both positive and negative
electrode materials operating at an average potential of 1.7 V
Impedance Simulation of a Li-Ion Battery with Porous Electrodes and Spherical Li+ Intercalation Particles
We present a semimathematical model for the simulation of the impedance spectra of a rechargeable lithium batteries consisting of porous electrodes with spherical Li+ intercalation particles. The particles are considered to have two distinct homogeneous phases as a result of the intercalation and deintercalation of Li+ during charge and discharge. The diffusion of Li+ ions in the two phases and the charge transfer at the solid electrolyte interface (SEI) are described with a mathematical model. The SEI and the electrolyte are modeled using passive electronic elements. First, this model is derived for a single intercalation particle consisting of two different solid phases. This model is then transformed to a continuous model and applied to a single porous electrode, where the sizes of the particles are assumed to have on average two grain sizes where the radii are Gaussian distributions. Finally, this model is further developed to simulate the impedance of a rechargeable lithium-ion battery.ChemE/Chemical EngineeringApplied Science
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