Détermination et calcul premiers principes, par la méthode PAW/GIPAW, de paramètres RMN de fluorures inorganiques

Cette thèse porte sur la détermination et la modélisation, par la méthode PAW/GIPAW (Gauge Including Projector Augmented Waves), de paramètres RMN de fluorures inorganiques. Dans la première partie, une corrélation entre valeurs expérimentales de déplacements chimiques isotropes (diso) de 19F et de constantes d'écran isotropes (iso) de F calculées de fluorures binaires, dont les attributions sont triviales, est établie. Elle permet de prédire les spectres RMN de 19F avec une bonne précision. Les paramètres quadripolaires de ces fluorures sont aussi déterminés et calculés. Dans la seconde partie, la linéarité entre valeurs expérimentales de diso de 19F et valeurs calculées de iso de 19F permet une attribution non ambigüe des raies RMN de 19F de NbF5 et TaF5. Par contre, pour trois des quatre composés MF4 étudiés (b-ZrF4, HfF4, CeF4, ThF4), caractérisés par des gammes de valeurs de diso de 19F plus petites, les corrélations médiocres entre valeurs de diso et de siso de 19F ne le permettent pas. Enfin, NaAsF6 et KPF6 qui présentent des valeurs élevées de couplage 1J 19F-X et des transitions de phase à des températures proches de l'ambiante sont étudiés par DTA ou DSC et diffraction des rayons X sur poudre et RMN du solide multinucléaire à température variable. Les structures de a- et b-NaAsF6 sont déterminées. KPF6 adopte une structure de haute symétrie désordonnée à température ambiante mais les tentatives de détermination des positions atomiques des deux premières phases basse température sont restées vaines. Ce travail souligne les potentialités et quelques limites de cette méthode ainsi que l'attention qui doit être prêtée aux effets des optimisations.This thesis focuses on the determination and the modeling, by the PAW/GIPAW (Gauge Including Projector Augmented Waves) method, of NMR parameters in inorganic fluorides. In the first part, a correlation between experimental 19F isotropic chemical shift (diso) and calculated 19F isotropic shieldings (siso) of binary fluorides with obvious assignments is established that allows to predict 19F NMR spectra with a good accuracy. The quadrupolar parameters of these fluorides are also determined and calculated. In the second part, a complete and unambiguous assignment of the 19F NMR lines of NbF5 and TaF5 is obtained, ensured by the linearity between experimental 19F diso values and calculated 19F siso values. On the other hand, for the studied MF4 (b-ZrF4, HfF4, CeF4, ThF4) compounds, characterized by smaller 19F diso ranges, except for ThF4, the poor correlations between experimental 19F diso and calculated 19F siso values prevent us to propose an assignment of the 19F NMR lines. In the last part, NaAsF6 and KPF6, exhibiting large 19F-X 1J-coupling and phase transitions at temperatures close to room temperature (RT) are investigated by DTA or DSC and variable temperature X-ray powder diffraction and multinuclear solid-state NMR. The structures of a- and b-NaAsF6 are determined. KPF6 adopts a disordered high symmetry structure at RT. Unfortunately, attempts to determine the atomic positions of the two first low temperature phases remain unsuccessful. This work highlights the potentialities and some limitations of this method as well as the care that must be taken when dealing with optimized structures.LE MANS-BU Sciences (721812109) / SudocSudocFranceF

Exploiting Cationic Vacancies for Increased Energy Densities in Dual-Ion Batteries

© 2019 Elsevier B.V. Dual-ion Li–Mg batteries offer a potential route to cells that combine desirable properties of both single-ion species. To maximize the energy density of a dual-ion battery, we propose a strategy for achieving simultaneous intercalation of both ionic species, by chemically modifying the intercalation host material to produce a second, complementary, class of insertion sites. We show that donor-doping of anatase TiO2 to form large numbers of cationic vacancies allows the complementary insertion of Li+ and Mg2+ in a dual-ion cell with a net increase in cell energy density, due to a combination of an increased reversible capacity, an increased operating voltage, and a reduced polarization. By tuning the lithium concentration in the electrolyte, we achieve full utilization of the Ti4+/Ti3+ redox couple with excellent cyclability and rate capability. We conclude that native interstitial sites preferentially accommodate Li+ ions, while Mg2+ ions occupy single-vacancy sites. We also predict a narrow range of electrochemical conditions where adjacent vacancy pairs preferentially accommodate one ion of each species, i.e., a [LiTi ​+ ​MgTi] configuration. These results demonstrate the implementation of additional host sites such as cationic sites as an effective approach to increase the energy density in dual-ion batteries

Correlated Anion Disorder in Heteroanionic Cubic TiOF 2

Resolving anion configurations in heteroanionic materials is crucial for understanding and controlling their properties. For anion-disordered oxyfluorides, conventional Bragg diffraction cannot fully resolve the anionic structure, necessitating alternative structure determination methods. We have investigated the anionic structure of anion-disordered cubic (ReO3-type) TiOF2 using X-ray pair distribution function (PDF), 19F MAS NMR analysis, density functional theory (DFT), cluster expansion modeling, and genetic-algorithm structure prediction. Our computational data predict short-range anion ordering in TiOF2, characterized by predominant cis-[O2F4] titanium coordination, resulting in correlated anion disorder at longer ranges. To validate our predictions, we generated partially disordered supercells using genetic-algorithm structure prediction and computed simulated X-ray PDF data and 19F MAS NMR spectra, which we compared directly to experimental data. To construct our simulated 19F NMR spectra, we derived new transformation functions for mapping calculated magnetic shieldings to predicted magnetic chemical shifts in titanium (oxy)­fluorides, obtained by fitting DFT-calculated magnetic shieldings to previously published experimental chemical shift data for TiF4. We find good agreement between our simulated and experimental data, which supports our computationally predicted structural model and demonstrates the effectiveness of complementary experimental and computational techniques in resolving anionic structure in anion-disordered oxyfluorides. From additional DFT calculations, we predict that increasing anion disorder makes lithium intercalation more favorable by, on average, up to 2 eV, highlighting the significant effect of variations in short-range order on the intercalation properties of anion-disordered materials

Controlled hydroxy-Fluorination Reaction of Anatase to Promote Mg²⁺ Mobility in Rechargeable Magnesium Batteries

International audienceIn anatase TiO2, substituting oxide anions with singly charged (F,OH) anions allows the controlled formation of cation vacancies, which act as reversible intercalation sites for Mg2+. We show that ion-transport (diffusion coefficients) and intercalation (reversible capacity) properties are controlled by two critical parameters: the vacancy concentration and the local anionic environment. Our results emphasise the complexity of this behaviour, and highlight the potential benefits of chemically controlling cationic-defects in electrode materials for rechargeable multivalent-ion batteries

Atomic Insights into Aluminium-Ion Insertion in Defective Anatase for Batteries

International audienceAluminium batteries constitute a safe and sustainable high‐energy‐density electrochemical energy‐storage solution. Viable Al‐ion batteries require suitable electrode materials that can readily intercalate high‐charge Al3+ ions. Here, we investigate the Al3+ intercalation chemistry of anatase TiO2 and how chemical modifications influence the accommodation of Al3+ ions. We use fluoride‐ and hydroxide‐doping to generate high concentrations of titanium vacancies. The coexistence of these hetero‐anions and titanium vacancies leads to a complex insertion mechanism, attributed to three distinct types of host sites: native interstitial sites, single vacancy sites, and paired vacancy sites. We demonstrate that Al3+ induces a strong local distortion within the modified TiO2 structure, which affects the insertion properties of the neighbouring host sites. Overall, specific structural features induced by the intercalation of highly polarising Al3+ ions should be considered when designing new electrode materials for polyvalent batteries

NMR parameters in alkali, alkaline earth and rare earth fluorides from first principle calculations

19F isotropic chemical shifts for alkali, alkaline earth and rare earth of column 3 basic fluorides are measured and the corresponding isotropic chemical shieldings are calculated using the GIPAW method. When using PBE exchange correlation functional for the treatment of the cationic localized empty orbitals of Ca2+, Sc3+ (3d) and La3+ (4f), a correction is needed to accurately calculate 19F chemical shieldings. We show that the correlation between experimental isotropic chemical shifts and calculated isotropic chemical shieldings established for the studied compounds allows to predict 19F NMR spectra of crystalline compounds with a relatively good accuracy. In addition, we experimentally determine the quadrupolar parameters of 25Mg in MgF2 and calculate the electric field gradient of 25Mg in MgF2 and 139La in LaF3 using both PAW and LAPW methods. The orientation of the EFG components in the crystallographic frame, provided by DFT calculations, is analysed in term of electron densities. It is shown that consideration of the quadrupolar charge deformation is essential for the analysis of slightly distorted environments or highly irregular polyhedra.Comment: 18 pages, 8 figures, 4 tables and ES

Lithium Intercalation in Anatase Titanium Vacancies and the Role of Local Anionic Environment

The structure of bulk and nondefective compounds is generally described with crystal models built from well mastered techniques such the analysis of an X-ray diffractogram. The presence of defects, such as cationic vacancies, locally disrupt the long-range order, with the appearance of local structures with order extending only a few nanometers. To probe and describe the electrochemical properties of cation-deficient anatase, we investigated a series of materials having different concentrations of vacancies, i.e., Ti_{1–<i>x</i>–<i>y</i>}□_{<i>x</i>+<i>y</i>}O_{2–4(<i>x</i>+<i>y</i>)}F_4<i>x</i>(OH)_4<i>y</i>, and compared their properties with respect to defect-free stoichiometric anatase TiO₂. At first, we characterized the series of materials Ti_{1–<i>x</i>–<i>y</i>}□_{<i>x</i>+<i>y</i>}O_{2–4(<i>x</i>+<i>y</i>)}F_4<i>x</i>(OH)_4<i>y</i> by means of pair distribution function (PDF), ¹⁹F nuclear magnetic resonance (NMR), Raman and X-ray photoelectron spectroscopies, to probe the compositional and structural features. Second, we characterized the insertion electrochemical properties vs metallic lithium where we emphasized the beneficial role of the vacancies on the cyclability of the electrode under high C-rate, with performances scaling with the concentration of vacancies. The improved properties were explained by the change of the lithium insertion mechanism due to the presence of the vacancies, which act as host sites and suppress the phase transition typically observed in pure TiO₂, and further favor diffusive transport of lithium within the structure. NMR spectroscopy performed on lithiated samples provides evidence for the insertion of lithium in vacancies. By combining electrochemistry and DFT-calculations, we characterized the electrochemical signatures of the lithium insertion in the vacancies. Importantly, we found that the insertion voltage largely depends on the local anionic environment of the vacancy with a fluoride and hydroxide-rich environments, yielding high and low insertion voltages, respectively. This work further supports the beneficial use of defects engineering in electrodes for batteries and provides new fundamental knowledge in the insertion chemistry of cationic vacancies as host sites

Reversible magnesium and aluminium ions insertion in cation-deficient anatase TiO₂

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Détermination et modélisation des paramètres de résonance magnétique nucléaire de (19)F et (27)Al dans les fluoroaluminates (relation avec la structure à courte et moyenne distance)

Cette thèse comporte deux volets. Le premier, expérimental, a consisté en la synthèse d'une série de fluoroaluminates cristallisés et vitreux (binaires CaF2-AlF3 et BaF2-AlF3 et ternaire BaF2- CaF2-AlF3) sous forme de poudre et en leur étude par Résonance Magnétique Nucléaire du Solide haute résolution des noyaux 19F et 27Al. Les expériences RMN les plus récentes (MAS, SATRAS, MQ-MAS) ont été mises en œuvre. Elles ont permis l'obtention de spectres correspondant à environ quatre-vingts sites d'ions fluorure F- et une quinzaine de sites d'ions aluminium Al3+ pour les fluoroaluminates cristallisés. Les paramètres spectroscopiques pertinents (déplacements chimiques isotropes pour 19F et 27Al, paramètres d'interaction quadripolaire pour 27Al) ont été déterminés par reconstruction des spectres des échantillons cristallisés et vitreux. Le second volet du travail a porté sur la modélisation de ces paramètres : 1/ déplacement chimique isotrope de 19F par une méthode semi-empirique (" modèle de superposition " développé au Mans) dans les fluoroaluminates cristallisés et des calculs ab-initio (code GAUSSIAN) sur un très grand nombre de fluorures inorganiques cristallisés; 2/ paramètres quadripolaires de 27Al par des calculs ab-initio (code WIEN2k) dans des fluoroaluminates cristallisés. La comparaison entre les paramètres calculés et expérimentaux est en général tout à fait satisfaisante et permet de discuter les valeurs des paramètres RMN en terme de liaison chimique, de connectivité des entités constituant le réseau et d'ordre à courte et moyenne distances. Elle permet en outre de caractériser le réseau des fluoroaluminates vitreux.This work is divided into two parts. The first one, experimental, is related to the synthesis of crystallized and glassy fluoroaluminates (CaF2-AlF3 and BaF2-AlF3 binaries, and BaF2-CaF2-AlF3 ternary) as powder samples, and their study using 19F and 27Al high resolution solid state Nuclear Magnetic Resonance. The most recent NMR experiments (MAS, SATRAS, MQ-MAS) have been worked out. Experimental spectra have been recorded for about eighty sites of F- fluorine ion and fifteen sites of Al3+ aluminium ion in the crystallized fluoroaluminates. The relevant spectroscopic parameters (isotropic chemical shift for 19F and 27Al, quadrupolar parameters for 27Al) have been determined by reconstruction of the crystallized and glassy samples spectra. The second part of this work is devoted to the modeling of these NMR parameters: 1/ 19F isotropic chemical shift using a semi-empirical method ( superposition model developed in Le Mans) in the crystallized fluoroaluminates and using ab initio calculations (GAUSSIAN code) on a large number of crystallized inorganic fluorides; 2/ 27Al quadrupolar parameters using ab initio calculations (WIEN2k code) in crystallized fluoroaluminates. The comparison between the calculated and experimental parameters was generally satisfactory and allows us to discuss the NMR parameter values in term of chemical bond, connectivity of the network entities, and short and mean range order. Moreover, the network of fluoroaluminate glasses was also characterized.LE MANS-BU Sciences (721812109) / SudocSudocFranceF

Controlled hydroxy-Fluorination Reaction of Anatase to Promote Mg²⁺ Mobility in Rechargeable Magnesium Batteries

In anatase TiO 2, substituting oxide anions with singly charged (F,OH) anions allows the controlled formation of cation vacancies, which act as reversible intercalation sites for Mg 2+. We show that ion-transport (diffusion coefficients) and intercalation (reversible capacity) properties are controlled by two critical parameters: the vacancy concentration and the local anionic environment. Our results emphasise the complexity of this behaviour, and highlight the potential benefits of chemically controlling cationic-defects in electrode materials for rechargeable multivalent-ion batteries. </p