Quantenchemische Untersuchungen der Lithiumionendiffusion in Übergangsmetalldichalkogeniden

Das Ziel dieser Arbeit ist es, eine genaue theoretische Beschreibung der Schichtstruktur von Übergangsmetalldichalkogeniden (MX2), insbesondere von TiX2, zu finden, und ihre Eignung als Interkalationsmaterial in Lithiumionenbatterien zu untersuchen. Die theoretische Beschreibung der Schichtstruktur von MX2 stellt eine Herausforderung für die lokale Dichte- sowie die generalisierte Gradientennäherungen (GGA) in der Dichtefunktionaltheorie (DFT) dar. Die Schichten sind nur durch schwache van-der-Waals-Wechselwirkungen verbunden, resultierend aus langreichweitiger Elektronenkorrelation, die durch lokale und semilokale Dichtefunktionale nicht beschrieben wird. Zudem beschreiben GGA-Funktionale die elektronische und magnetische Struktur von TiX2 aufgrund des bekannten Selbstwechselwirkungsfehlers nicht korrekt. Qualitativ höherwertigere Hybridfunktionale würden bessere Ergebnisse erzielen, dabei würde sich aber die Rechenzeit um eine Größenordnung erhöhen. In der vorliegenden Arbeit wird dieses Problem unter Beibehaltung der Rechenzeit durch die Verwendung des GGA-Funktionals von Perdew, Burke und Ernzerhof (PBE) in Kombination mit Dispersions- und Hubbardkorrekturen, im folgenden als PBE+U-D3 bezeichnet, im Vienna Ab initio Simulation Package (VASP) angegangen. Bei Verwendung dieser Korrekturen liegt der Fehler des Gitterparameters c der TiX2- und LiTiX2-Strukturen im Vergleich zu den experimentellen Daten bei ±3 %. Mit dem reinen PBE-Funktional wird ein Fehler von bis zu 15 % erhalten. Neben der strukturellen Beschreibung verbessert sich auch die Beschreibung der Bandstruktur und Infrarot-Banden von TiS2, sowie die Quadrupolkopplungskonstante und die chemische Verschiebung von LiTiS2. Die Beschreibung der lithiierten und delithiierten TaX2 und VX2-Schichtstrukturen mit PBE+U-D3 resultieren in einem maximalen Fehlern von ±3 % für den Gitterparameter c im Vergleich zu den experimentellen Daten. Allerdings ist die Beschreibung der Spannungskurven gegen den Lithiumanteil x im Bereich 0 < x <1 für LixTaX2 und LixVX2 mit PBE+U-D3 problematisch. Für LixTiS2 und LixTiSe2 hingegen weisen die erhaltenen Spannungskurven eine gute Übereinstimmung mit den experimentellen Daten auf. Zusätzlich konnten die experimentellen Aktivierungsbarrieren für Li1.0TiS2, Li0.7TiS2 und Li0.7TiSe2 mit der gewählten Methode reproduziert werden. Der Ursprung der beiden, mit verschiedenen Kernspinresonanzmethoden gemessenen Aktivierungsbarrieren für Li1.0TiS2 konnte geklärt werden. Des Weiteren war es in dieser Arbeit möglich, die konzentrationsabhängige Aktivierungsbarriere für LixTiX2 mit 0 < x < 1 vorherzusagen, was keiner theoretischen Arbeit bisher gelungen ist.The aim of this thesis is to find an accurate theoretical description of layered transition metal dichalcogenides (MX2), especially TiX2, and investigate their properties regarding the suitability as lithium intercalation material utilized in lithium-ion batteries. The theoretical description of layered MX2 represents a challenge for local density and generalized gradient approximation (GGA) in density functional theory (DFT), since it does not take into account long-range electron correlation effects (London dispersion) which is responsible for the inter-layer interaction. In addition, GGA DFT does not reproduce the electronic and magnetic properties of TiX2, due to the well-known self-interaction error. Using a higher quality hybrid functional is expected to result in a better description but would increase computational costs by one order of magnitude. Keeping the computational costs on the same level as for pure GGA DFT calculations, these challenges are approached in this thesis using a GGA functional developed by Perdew, Burke and Ernzerhof (PBE) together with dispersion and Hubbard correction terms denoted as PBE+U-D3 in the Vienna Ab initio Simulation Package (VASP). With these corrections, the c lattice parameter of TiX2 and LiTiX2 is described within an error of ±3 % while pure PBE leads to an error of up to ±15 %. The description of the TiS2 band structure and vibrational frequency as well as the LiTiS2 quadrupole coupling constant and chemical shift are also improved. PBE+U-D3 calculations of lithiated and delithiated TaX2 and VX2 result in a maximum error of ±3 % for the c lattice parameter. However, applying this method to calculate the voltage vs composition curve for a lithium portion 0 < x <1 result is problematic for tantalum and vanadium dichalcogenides. For LixTiS2 and LixTiSe2 the voltage vs composition curves are in good agreement with the experimental data. The experimentally reported activation barriers for Li1.0TiS2, Li0.7TiS2 and Li0.7TiSe2 are reproduced with the corrected PBE functional. In addition the migration pathways observed for Li1.0TiS2 using various nuclear magnetic resonance techniques are identified. It was also possible to predict concentration-dependent activation barriers in the region of 0 < x < 1 for LixTiX2, which was not yet achieved by other theoretical investigations

Density Functional Theory Evaluated for Structural and Electronic Properties of 1T-LixTiS2 and Lithium Ion Migration in 1T-Li0.94TiS2

In many applications it has been found that the standard generalized gradient approximation (GGA) does not accurately describe weak chemical bond and electronic properties of solids containing transition metals. In this work, we have considered the intercalation material 1T-LixTiS2 (0≤x≤1) as a model system for the evaluation of the accuracy of GGA and corrected GGA with reference to the availabile experimental data. The influence of two different dispersion corrections (D3 and D-TS) and an on-site Coulomb repulsion term (GGA+U) on the calculated structural and electronic properties is tested. All calculations are based on the Perdew-Burke-Ernzerhof (PBE) functional. An effective U value of 3.5 eV is used for titanium. The deviation of the calculated lattice parameter c for TiS2 from experiment is reduced from 14 % with standard PBE to −2 % with PBE+U and Grimme’s D3 dispersion correction. 1T-TiS2 has a metallic ground state at PBE level whereas PBE+U predicts an indirect gap of 0.19 eV in agreement with experiment. The 7Li chemical shift and quadrupole coupling constants are in reasonable agreement with the experimental data only for PBE+U-D3. An activation energy of 0.4 eV is calculated with PBE+U-D3 for lithium migration via a tetrahedral interstitial site. This result is closer to experimental values than the migration barriers previously obtained at LDA level. The proposed method PBE+U-D3 gives a reasonable description of structural and electronic properties of 1T-LixTiS2 in the whole range 0≤x≤1. © 2017 Walter de Gruyter GmbH, Berlin/Boston 2017

EMSL Geochemistry, Biogeochemistry and Subsurface Science-Science Theme Advisory Panel Meeting

This report covers the topics of discussion and the recommendations of the panel members. On December 8 and 9, 2010, the Geochemistry, Biogeochemistry, and Subsurface Science (GBSS) Science Theme Advisory Panel (STAP) convened for a more in-depth exploration of the five Science Theme focus areas developed at a similar meeting held in 2009. The goal for the fiscal year (FY) 2011 meeting was to identify potential topical areas for science campaigns, necessary experimental development needs, and scientific members for potential research teams. After a review of the current science in each of the five focus areas, the 2010 STAP discussions successfully led to the identification of one well focused campaign idea in pore-scale modeling and five longer-term potential research campaign ideas that would likely require additional workshops to identify specific research thrusts. These five campaign areas can be grouped into two categories: (1) the application of advanced high-resolution, high mass accuracy experimental techniques to elucidate the interplay between geochemistry and microbial communities in terrestrial ecosystems and (2) coupled computation/experimental investigations of the electron transfer reactions either between mineral surfaces and outer membranes of microbial cells or between the outer and inner membranes of microbial cells

Solid-State NMR Spectroscopy Study of Cation Dynamics in Layered Na2Ti3O7 and Li2Ti3O7

Ti-based materials exhibit suitable properties for usage in secondary Li- and Na-ion batteries and were in the focus of several electrochemical and ion conductivity studies. A material of such interest is layer-structured, monoclinic Na2Ti3O7. Additionally, the sodium in Na2Ti3O7 can be replaced completely with lithium to achieve monoclinic Li2Ti3O7, whose electrochemical properties were already investigated as well. Both materials exhibit interesting properties such as zero-strain behavior upon intercalation and high cycling stability. However, there is still a lack of fundamental understanding of the ion diffusivity of both Na and Li in the corresponding host structure. Solid-state nuclear magnetic resonance (NMR) spectroscopy is used here for the first time to reveal the cation dynamics in layered Na2Ti3O7 and Li2Ti3O7. This includes activation energies for the ionic motion and jump rates on the microscopic scale from NMR spin-lattice relaxation (SLR), spin-alignment echo (SAE), and 2D NMR exchange techniques. Moreover, the dimensionality of the ionic motion is investigated by frequency-dependent NMR SLR. Structural details are studied using magic-angle spinning (MAS) NMR spectroscopy. Results for the electric field gradient at the Na and Li site, respectively, are compared with those from theoretical calculations performed within this study. The dynamics are similar for both cations, and the frequency-dependence of the 7Li NMR SLR rate indicates Li motion confined to two dimensions. Thus, these two materials may be regarded a model system for low-dimensional diffusion of two different cations.DFG, 119336273, FOR 1277: Mobilität von Lithiumionen in Festkörpern (molife