Novel and Scalable Synthesis of BnHxz- Boron Clusters as Solid-State Electrolytes for Sodium Batteries and their Detailed Study for Hydrogen Storage Applications

Thesis

Reference

Novel and Scalable Synthesis of BnHxz- Boron Clusters as Solid-State Electrolytes for Sodium Batteries and their Detailed Study

for Hydrogen Storage Applications

GIGANTE, Angelina

Abstract

All-solid-state batteries (ASSBs) promise higher power and energy density compared to batteries based on liquid electrolytes. Recently, a stable 3V ASSB based on the super ionic conductor (1mS cm-1 near room temperature) Na4(B12H12)(B10H10) has demonstrated excellent cycling stability. This electrolyte Na4(B12H12)(B10H10) can be obtained directly using the 5 steps, scalable and solution-based synthesis shown in scheme 1 (TEA = Et4N+).

The use of the wet chemistry in the final step allows the solution processing of the solid electrolyte and to improve the contacts with the cathode material during assembly of the battery. This new synthesis is a cost efficient synthesis for the precursors Na2B10H10 and Na2B12H12 which are commercially very expensive. Two key parameters to tune the kinetics and selectivity of this solvolthermal synthesis of closo-hydroborates were identified: the choice of the counter cation, tetraethylammonium ((C2H5)4N+, TEA+) vs tetrabutylammonium (TBA+), and the solvent.

GIGANTE, Angelina. Novel and Scalable Synthesis of BnHxz- Boron Clusters as

Solid-State Electrolytes for Sodium Batteries and their Detailed Study for Hydrogen Storage Applications. Thèse de doctorat : Univ. Genève, 2020, no. Sc. 5454

DOI : 10.13097/archive-ouverte/unige:140154 URN : urn:nbn:ch:unige-1401548

Available at:

http://archive-ouverte.unige.ch/unige:140154

Disclaimer: layout of this document may differ from the published version.

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To Ezio.

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‘La connaissance, c’est comme un verre d’eau, il faut le remplir tous les jours’

Daniel Sautot

‘Curiosity was the main driving force for me’

Prof. Dr. Akira Yoshino Nobel Prize for Lithium-ion batteries, 2019

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Acknowledgements

First of all, I thank Prof. Dr. Torben Jensen and Dr. Alexandre Fürstenberg to have accepted to be in the jury for my PhD thesis defense. I would like to thank my supervisor Prof. Dr. Hans Hagemann who allowed me to join his research group for my PhD. I thank him to have given to me the possibility to work on different projects, which allowed me to face different chemical problems. I also thank him for letting me worked on the synthesis projects of compounds based on B3H8-, B10H102- and B12H122-

ions for hydrogen storage and solid-state electrolyte for sodium batteries. The synthesis projects have been very important for me because I was looking for a PhD in which I would have been able to synthetize my products by developing an eco-friendly synthesis in order to avoid toxic chemicals, which are dangerous for the environment. Therefore, this PhD was what I was looking for. I thank my supervisor for the scientific discussions, for having always supported me and for having been always available even in the stressful situations where we always found a solution. I thank him to have given to me the possibility to go to USA to Pacific Northwest National Laboratory (PNNL) to work in the HyMARC consortium for the development of hydrogen storage materials. I thank Dr. Romain Moury who had worked with me for three years in all my PhD projects. I thank Romain

because he showed me all techniques for making synthesis. Before starting my PhD, I was a spectroscopist and I had never touched a flask. He always gave me suggestions and he was always available when I needed help. Romain supported me a lot to go to USA and he translated in French the documents that I needed to apply for a scholarship. He has been a great co-worker and I have been so used to work with him for three years that it was not easy to start alone when he left Geneva.

I thank Romain to have always understood the up and down moments, which can happen during the PhD. I thank all people who I have met at the University, everybody gave to me a life lesson to learn and improve each day and they improved my daily life both at University and in Geneva. I thank the people of the research group: Max, Manish, Daniel, Elia, Jacob, Teresa, Julien, and Jafar to have shared good and bad moments. Each shared moment has been an opportunity to reflect and to improve myself. I thank Catherine who always helped me for all documents, especially at beginning when I could not understand the French language. I thank Dominique, Eric, Nahid, Laurent and Patrick who got my job more comfortable in the laboratories. I could have never worked without their precious technical jobs. I thank all people who worked with me in the Synergia projects: Tatsiana, Aristea, Léo, Marion, Arash, Elsa, Dr. Remhof, Dr. Embs, Prof. Dr. Lodziana and Prof. Dr. Černý, their work, availability and collaboration have been an additional opportunity to learn and increase my scientific background. I thank all people who worked with me for the solvent free Mg(B3H8)2

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project in USA: Katharzyna, Ba, Andy, Jesse, Eric, Mark, Noemi, Madison, Prof. Dr. Tom Gennett, Prof. Dr. Craig Jensen, Dr. John Linehan and Dr. Tom Autry. I loved the scientific discussions and the passion for which all of these people work each day and each week-end in order to find a solution to a chemical problem. The American experience has been a great possibility, which does not happen often during a PhD. In each meeting, there was passion, enthusiasm, and determination, which I will always bring with me. I thank Dr. Tom Autrey to have shared with me his passion for the chemistry,

knowledge about the chemistry of borohydrides and the mechanism of reactions. I miss all of you.

I consider my PhD to have been a journey in which a lot of people gave a contribution to get it possible. I thank all Italian people and friends in Geneva and Salerno. Particularly, I thank my brother,

who has always supported my choices and he convinced me to start my research experience in Geneva. Thanks to Ezio. I thank Fabio because he has always supported me for the scientific career.

Fabio is always ready when I need to discuss some chemical problems and when I need to laugh in order to get the things less serious. I thank Grazia and Federica who are always my supporters and Federica for letting me laugh since we were child. My PhD journey is finished but it is just the beginning of my scientific career. In this context, let me thank Daniel Sautot not only for having tried to improve my French, which is still very bad, but also to have given to me the possibility to discuss my research outside the scientific community. Although Daniel does not have a scientific degree, his outstanding interests for the science and environment allow him to think as a scientist. Daniel has always made interesting and stimulating questions, which lead to me to think a lot. I believe that the role of a researcher is to make a contribution in the world. As researcher, I have tried to make my contribution in the field of environment by developing eco-friendly syntheses. Anyhow, I feel that improvements can be done with further ideas and projects. I feel that I can make further contributions to help the environment. We live in a global particular situation where the role of the scientist is very important for the society. Daniel Sautot showed me that no scientific people are waiting for solutions from scientists and this is the driving force, which should motivate the scientific research in trying to face the problem of the global warming. Thanks to Dany for your interest and enthusiasm in science and to have showed to me how no scientific people appreciate the scientific efforts, which are done each day with passion in the laboratories. Without the support of no scientific people as Daniel Sautot, my scientific research would not have a reason because my research is done for improving the life of the people in order to get their life better.

Thanks to everybody to have helped me in my scientific projects.

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Résumé

Les composés octahydrotriborates alcalins ou alcalino-terreux M(B3H8)x et dodécahydro-closo-dodécaborates MxB12H12 (M= Li, Na, Mg ou Ca avec x = 1 or 2) ont récemment attiré beaucoup d’intérêt en vue d’applications de stockage d’hydrogène et comme conducteurs ioniques solides. Cependent leur synthèse est à ce jour toujours un obstacle pour des applications à grande échelle.

Dans cette thèse, une approche nouvelle et sûre pour leur synthèse a été démontrée, partant du borohydrure le meilleur marché, à savoir NaBH4. Ce processus implique d’abord la synthèse solvothermale du tétrabutylammonium octahydrotriborate (C4H9)4NB3H8 (TBAB3H8), qui est à la base des synthèses des autres boranes. TBAB₃H₈ est ensuite converti en sodium octahydrotriborate NaB₃H₈ par une reaction de métathèse avec le sodium tétraphénylborate NaBPh4. Finalement, Na2B12H12 est obtenu avec succès par la décomposition solvothermale de NaB₃H₈. Cette approche s’est avérée quantitative et reproductible, ce qui peut mener au développement de ces boranes pour des applications réelles.

Les batteries entièrement solides (ASSB= « all solid state battery ») promettent des densités de puissance et enrgie supérieures comparés aux batteries basées sur des electrolytes liquides. Récemment, une batterie solide de 3V basée sur le conducteur superionique Na₄(B₁₂H₁₂)(B₁₀H₁₀) (1 mS cm^-1 proche de la température ambiante) a démontré une excellente stabilité de cyclage.

Cet électrolyte Na4(B12H12)(B10H10) peut être directement obtenu par une synthèse en 5 étapes qui peut étendue à des volumes plus grands, comme illustré dans le schéma 1 (TEA = Et₄N⁺).

Schéma 1. Résumé des 5 étapes de reaction partant de NaBH₄ pour preparer d’un côté le nouvel électrolyte Na4(B12H12)(B10H10) et de l’autre Na2B12H12 and Na2B10H10 séparément.

L’usage de la chimie en solution pour la dernière étape permet le traitement en solution de l’électrolyte solide pour améliorer les contacts avec le matériau de cathode lors de l’assemblage de la batterie. Cette nouvelle synthèse est une synthèse plus économique pour les précurseurs Na2B10H10 et Na2B12H12 qui commercialement sont très chers. Deux paramètres clé pour accorder la cinétique et la sélectivité de cette synthèse solvothermale

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ont été identifiés : le choix du contre-cation tétra-éthylammonium ((C₂H₅)₄N⁺, TEA⁺) vs tétra-butylammonium (TBA⁺), et le solvant.

Cette synthèse avec un excellent rendement nous a permis de préparer Na4(B12H12)(B10H10) marqué avec l’isotope ¹¹B à partir de Na¹¹BH4 en vue d’expériences de diffusion de neutrons quasi-élastiques (QENS). De nouvelles expériences QENS avec ces échantillons marqués ont permis d’obtenir des résultats améliorés par rapport à l’étude initiale. Des mesures en fonction de la température par spectroscopie infrarouge et Raman ont également été effectuées. Ces expériences montrent que Na₄(¹¹B₁₂H₁₂)(¹¹B₁₀H₁₀) est stable jusqu’à 300°C et que les anions B12H122-et B10H102- ne sont pas rigides, mais sont sujets à des mouvements de réorientation dynamiques à température ambiante.

Les composés tétra-éthylammonium closododécahydrodécaborate ((C₂H₅)₄N)₂B₁₂H₁₂ (TEA₂B₁₂H₁₂) et tétra-n-butylammonium octahydrotriborate (C4H9)4NB3H8 (TBAB3H8) ont été utilisés comme précurseurs pour la synthèse en solution des composés alcalins et alcalino-terreux avec le closo dodécahydrododécaborate et l’octahydrotriborate. Nous présentons la caractérisation de TEA2B12H12 et TBAB3H8 par la diffraction aux rayons X, l’analyse thermique et la spectroscopie vibrationnelle, qui montrent que ces composés sont stables en dessous de 300 °C. Les sels de TBA⁺ sont beaucoup moins stables. Les propriétés structurales des sels de TEA⁺ and TBA⁺ impliquent aussi un désordre conformationnel qui est systématiquement observe pour les sels de TBA⁺ avec les borohydrures. Des calculs théoriques ont été effectués pour assister les caractérisations structurales et comparaisons avec les spectres vibrationnels expérimentaux.

Les composés octahydrotriborate des ions alkalino-terreux M(B3H8)2 attirent beaucoup d’intérêt ces dernières années pour le stockage d’hydrogène, vu leur teneur gravimétrique en hydrogène. L’étude détaillée de leurs réactions d’hydrogénation-déhydrogénation permet une comprehension améliorée des mécanismes qui interviennent dans le stockage d’hydrogène dans les borohydrures métalliques. Cet aspect est adressé dans le chapitre 7 de cette thèse. M(B₃H₈)₂ (M=Mg ou Ca) sans solvant a été préparé partant de NaBH₄, basé sur les résultats montrés dans les chapitres 2 et 3, avec une réaction de métathèse finale par « ball milling » des solides :

2NaB3H8(s) + MBr2(s) → M(B3H8)2(s) + 2NaBr(s)

Les mesures vibrationnelles à température ambiante montrent que le Mg(B₃H₈)₂ n’est pas un solide moléculaire, mais plutôt un solide ionique. En chauffant, on observe une perte de masse d’environ 30% pour les composés avec M=Mg et M=Ca, qui peut être décrite par la réaction :

M (B3X8)2(s) à “MB4X8” (s) + B2X6(g) + X2(g)

Pour M = Mg, les spectres vibrationnels spectra suggèrent que “MB4X8” est en fait un mélange de Mg(BH4)2

et MgB12H12 et/ou MgB10H10. Pour M=Ca, Ca(BH4)2 n’a pas pu être identifié comme produit de réaction.

Jusqu’à 200°C, on n’observe pas de réaction de décomposition supplémentaire pour les deux composés.

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Le Magnésium octahydrotriborane Mg(B₃H₈)₂ est le premier intermédiaire stable obtenu par la déhydrogenation du borohydrure de magnésium Mg(BH4)2. Ce composé intermédiaire, préparé sans solvant, a été sujet à des réactions de déhydrogénation, et aussi à des réactions de réhydrogénation. Le progrès de ces réactions a été suivi par une variété de techniques expérimentales. L’hydrogénation de Mg(B3H8)2 sans solvant en dessous de 200°C avec 6 bars d’hydrogène en présence d’un excès d’hydrure de magnésium MgH2 montre une conversion presque quantitative en Mg(BH4)2 par des mésures RMN solide « in situ » et en solution « ex situ ». Des mesures calorimétriques sous des conditions expérimentales comparables ont permis de mesurer la chaleur d’hydrogénation et d’estimer la différence de stabilité thermodynamique entre Mg(BH4)2 et Mg(B₃H₈)₂. En l’absence de MgH₂ et H₂, la décomposition thermique de Mg(B₃H₈)₂ sans solvant résulte en la formation d’un mélange complexe de boranes neutres et anioniques. La NMR solide du ¹¹B permet d’identifier les espèces non-volatils et montre la formation d’un mélange de borohydrures comme BH4-, B10H102- et B12H122. TPD-MS de Mg(B₃H₈)₂ dans l’absence de MgH₂ montre la formation des gaz diborane (B₂H₆) et pentaborane (B5H9). En présence d’un excès de MgH2, la formation de B2H6 et B5H9 est fortement réduite.

Les borohydrures des terres rares ont attiré l’intérêt dans la communauté scientifique pour des applications de stockage d’hydrogène et de luminescence. Les spectroscopies infrarouge et Raman peuvent être un outil important pour étudier la structure locale de l’ion BH₄^- dans le cristal. Dans le dernier chapitre, nous présentons une étude vibrationnelle de Pr(¹¹BH4)3. Des mesures in situ par spectroscopie infrarouge en fonction de la température et ex situ par spectroscopie Raman montrent que Pr(¹¹BH₄)₃ est sujet à une transition de phase irréversible autour de 200 °C.

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Abstract

Alkaline or alkaline earth octahydrotriborate M(B3H8)x and dodecahydro-closo-dodecaborate MxB12H12 (M= Li, Na, Mg or Ca with x = 1 or 2) compounds have recently attracted a lot of interest for hydrogen storage and solid electrolyte applications. Nevertheless, their syntheses are still a roadblock for large scale applications. In this thesis, a novel and safe approach for their syntheses starting from the cheapest borohydride, namely NaBH4 has been demonstrated. The process involves first the solvothermal synthesis of tetrabutylammonium octahydrotriborate (C4H9)4NB3H8 (TBAB3H8) being the basis for the syntheses of the others boranes. TBAB3H8 is then converted into sodium octahydrotriborate NaB3H8 by a salt metathesis reaction with sodium tetraphenylborate NaBPh4. Finally, Na2B12H12 is successfully obtained by the solvothermal decomposition of NaB3H8. This approach has shown to be quantitative and reproducible, which could lead to the development of these boranes for real life applications.

All-solid-state batteries (ASSBs) promise higher power and energy density compared to batteries based on liquid electrolytes. Recently, a stable 3V ASSB based on the super ionic conductor (1mS cm^-1 near room temperature) Na4(B12H12)(B10H10) has demonstrated excellent cycling stability.

This electrolyte Na₄(B₁₂H₁₂)(B₁₀H₁₀) can be obtained directly using the 5 steps, scalable and solution-based synthesis shown in scheme 1 (TEA = Et4N⁺).

Scheme 1. Summary of the 5 steps process starting from NaBH₄ to 1^st: the highly conductive electrolyte Na4(B12H12)(B10H10) and 2^nd Na2B12H12 and Na2B10H10 separately.

The use of the wet chemistry in the final step allows the solution processing of the solid electrolyte and to

improve the contacts with the cathode material during assembly of the battery. This new synthesis is a cost efficient synthesis for the precursors Na2B10H10 and Na2B12H12 which are commercially very expensive.

Two key parameters to tune the kinetics and selectivity of this solvolthermal synthesis of closo-hydroborates were identified: the choice of the counter cation, tetraethylammonium ((C2H5)4N⁺, TEA⁺) vs tetrabutylammonium (TBA⁺), and the solvent.

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This high yield synthesis allowed us also to prepare ¹¹B labelled Na₄(B₁₂H₁₂)(B₁₀H₁₀) for quasi elastic neutron scattering (QENS) experiments starting from Na¹¹BH4. New QENS experiments with these labelled samples show improved results with respect to the first investigation. Temperature dependent Infrared and Raman experiments of these samples have also been performed. These experiments show that Na4(¹¹B12H12)(¹¹B10H10) is stable up to 300°C and B12H122-and B10H102- anions are not rigid but they undergo already dynamic reorientational motion at room temperature.

Tetra-n-ethylammonium closododecahydrododecaborate ((C₂H₅)₄N)₂B₁₂H₁₂ (TEA₂B₁₂H₁₂) and tetra-n-butylammonium octahydrotriborate (C4H9)4NB3H8 (TBAB3H8) have been used as organic precursor for

the wet synthesis of alkaline and alkaline earth closo-dodecahydrododecaborate and octahydrotriborate compounds. We present the structural characterization of TEA2B12H12 and TBAB3H8 by the use of XRD, thermal analysis, and vibrational spectroscopy, which reveal a thermal stability below 300°C. The TBA⁺ salts are thermally much less stable. Structural properties of TEA⁺ and TBA⁺ salts imply also conformational disorder which is systematically observed for the TBA⁺ salts with borohydrides. Theoretical calculations have been performed to assist the structural characterizations and comparisons with experimental vibrational spectra.

Alkaline earth octahydrotriborate M(B₃H₈)₂ compounds have attracted a lot of interest in the last years as potential materials for hydrogen storage due to their gravimetric hydrogen content. The detailed study of their hydrogenation-dehydrogenation reactions allows an improved understanding of the mechanism involved for hydrogen storage in metal borohydrides. This aspect is addressed in the chapter 7 of the thesis. Solvent free M(B₃H₈)₂ (M=Mg or Ca) was prepared starting from NaBH₄, based on our results shown in chapters 2 and 3, with a final metathesis reaction performed by ball milling of the solid:

2NaB3H8(s) + MBr2(s) → M(B3H8)2(s) + 2NaBr(s)

Room temperature vibrational spectra show that solid Mg(B3H8)2 is not a molecular crystal with discrete Mg(B3H8)2 molecules, but rather an ionic solid. Upon heating, a mass loss of about 30% is observed around 100 °C for both Mg and Ca compounds, which can be described by the reaction:

M (B3X8)2(s) à “MB4X8” (s) + B2X6(g) + X2(g)

For M = Mg, vibrational spectra suggest that “MB4X8” is in fact a mixture of Mg(BH4)2 and MgB12H12 and/or MgB10H10. For M=Ca, Ca(BH4)2 could not be identified as a product. Up to 200 °C, no further dehydrogenation is observed for both compounds.

Magnesium octahydrotriborane Mg(B3H8)2 is the first stable intermediate formed from the dehydrogenation of magnesium borohydride Mg(BH4)2. This intermediate compound, prepared without solvent, was subjected on one side to dehydrogenation reactions, and on the other side to rehydrogenation reactions. The progress of these reactions was monitored with a variety of experimental techniques. Hydrogenation of solvent free

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Mg(B₃H₈)₂ below 200°C with 6 bar of hydrogen pressure, in the presence of an excess of magnesium hydride MgH2, followed by in-situ solid-state NMR, ex-situ solution NMR show near quantitative conversion to Mg(BH₄)₂. Calorimetry, under comparable reaction conditions, provides a measure of the heat of the hydrogenation to yield an experimental measure of the difference in thermodynamic stability between Mg(BH4)2 and Mg(B3H8)2. In the absence of MgH2 and H2, the thermal decomposition of solvent free Mg(B3H8)2 results in a complex mixture of anionic and neutral boranes. Solid-state ¹¹B NMR provides insights into the non-volatile species and it shows the formation of a mixture of anionic boranes such as BH₄^-, B₁₀H₁₀^2- and B12H122-. TPD-MS of neat solvent free Mg(B3H8)2 in the absence of MgH2 shows formation of volatile diborane (B₂H₆) and pentaborane (B₅H₉). In the presence of excess MgH₂, the formation of B₂H₆ and B₅H₉ is significantly reduced.

Rare-earth (RE) metal borohydride have attracted the interest of the scientific community for hydrogen storage and luminescence applications. Infrared and Raman spectroscopy can be useful tools to study the local

structure of the BH4- ion in the crystal. In the last chapter, we present a vibrational study of Pr(¹¹BH4)3. In-situ variable temperature Infrared and ex-situ Raman experiments reveal that the Pr(¹¹BH₄)₃ undergoes an

irreversible phase transition around 200°C.

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Table of Contents

General Introduction________________________________________________________ 20 1. Context___________________________________________________________________ 20 2. Energy demand: Can we sustain it?_____________________________________________ 21 3. Renewable Energy: are they competitive?________________________________________ 23 4. Energy storage_____________________________________________________________ 24 4.1. Hydrogen storage_________________________________________________________ 25 4.2. Rechargeable batteries_____________________________________________________ 27 5. References________________________________________________________________ 30 Chapter I: Introduction_______________________________________________________ 32 1. Hydrogen storage___________________________________________________________ 32 1.2. Metal Borohydrides: MBH4_________________________________________________ 33 1.2.1. Sodium borohydride NaBH4_______________________________________________ 34 1.2.1.1. Synthesis and structure__________________________________________________ 34 1.2.1.2. Hydrogen storage properties______________________________________________ 35 1.2.2. Lithium borohydride LiBH4________________________________________________ 36 1.2.2.1. Synthesis and structure__________________________________________________ 36 1.2.2.2. Hydrogen storage properties______________________________________________ 37 1.2.3. Calcium borohydride Ca(BH₄)₂_____________________________________________ 38 1.2.3.1. Synthesis and structure__________________________________________________ 38 1.2.3.2. Hydrogen storage properties______________________________________________ 39 1.2.4. Magnesium borohydride Mg(BH4)2__________________________________________ 39 1.2.4.1. Synthesis and structure__________________________________________________ 39 1.2.4.2. Hydrogen storage properties______________________________________________ 41 1.3. Metal Octahydrotriborates: MB3H8____________________________________________ 42 1.3.1. Sodium octahydrotriborate NaB3H8__________________________________________ 42 1.3.1.1. Synthesis and structure__________________________________________________ 42 1.3.1.2. Hydrogen storage properties______________________________________________ 43 1.3.2. Lithium octahydrotriborate LiB3H8__________________________________________ 44 1.3.2.1. Synthesis and structure__________________________________________________ 44

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1.3.2.2. Hydrogen storage properties______________________________________________ 44 1.3.3. Potassium octahydrotriborate KB3H8_________________________________________ 44 1.3.3.1. Synthesis and structure__________________________________________________ 44 1.3.3.2. Hydrogen storage properties______________________________________________ 45 1.3.4. Magnesium octahydrotriborate Mg(B3H8)2____________________________________ 46 1.3.4.1. Synthesis and structure__________________________________________________ 46 1.3.4.2. Hydrogen storage properties______________________________________________ 47 1.3.5. Berillium octahydrotriborate Be(B3H8)2______________________________________ 47 1.3.5.1. Synthesis and structure__________________________________________________ 47 1.3.5.2. Hydrogen storage properties______________________________________________ 48 1.4. References_______________________________________________________________ 49 1.5. Lithium and sodium solid-state batteries_______________________________________ 55 1.5.1. Hydroborates as candidate solid electrolyte for all-solid-state batteries______________ 57 1.5.2. Electrochemistry of all-solid state batteries based on hydroborates_________________ 64 1.5.3. Synthesis of closo-hydroborates____________________________________________ 65 1.6. Solid-state and liquid-state ¹¹B NMR as tools to study the evolution

of dehydrogenation-hydrogenation MB3H8 compounds________________________________

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1.7. Conclusions______________________________________________________________ 73 1.8. References_______________________________________________________________ 75 Chapter 2: An alternative approach to the synthesis of NaB₃H₈

and Na₂B₁₂H₁₂ for solid electrolyte______________________________________________

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Abstract____________________________________________________________________ 79 2.1. Introduction______________________________________________________________ 80 2.2. Experimental part_________________________________________________________ 82 2.2.1. Materials_______________________________________________________________ 82 2.2.2. Apparatuses____________________________________________________________ 82 2.2.3. TBABH₄ synthesis_______________________________________________________ 83 2.2.4. TBAB3H8 synthesis______________________________________________________ 83 2.2.5. NaB3H8 synthesis________________________________________________________ 84 2.2.6. Na2B12H12 synthesis 1____________________________________________________ 84 2.2.7. Na2B12H12 synthesis 2____________________________________________________ 84

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2.3. Results__________________________________________________________________ 85 2.3.1. Synthesis of TBAB3H8____________________________________________________ 85 2.3.1.1. Ex- situ Liquid ¹¹B NMR of TBAB3H8______________________________________ 87 2.3.2. Synthesis of NaB3H8_____________________________________________________ 90 2.3.3. Synthesis of Na2B12H12___________________________________________________ 90 2.4. Conclusions______________________________________________________________ 94 Acknowledgments____________________________________________________________ 94 Authors’s contribution_________________________________________________________ 95 2.5. References_______________________________________________________________ 96 Chapter 3: Direct solution-based synthesis of Na₄(B₁₂H₁₂)(B₁₀H₁₀) solid electrolyte_______ 98 Abstract____________________________________________________________________ 98 3.1. Introduction______________________________________________________________ 99 3.2. Experimental part_________________________________________________________ 101 3.2.1. Materials_______________________________________________________________ 101 3.2.2. Apparatuses____________________________________________________________ 101 3.2.3. TEABH4 synthesis_______________________________________________________ 103 3.2.4. TEAB3H8 synthesis______________________________________________________ 103 3.2.5. TEABD4 synthesis_______________________________________________________ 104 3.2.6. TEAB₃D₈ synthesis______________________________________________________ 104 3.2.7. TEA2B12H12 and TEA2B10H10 synthesis_______________________________________ 105 3.2.8. Na2B12H12 and Na2B10H10 synthesis__________________________________________ 105 3.2.9. Na4B12H12B10H10 synthesis_________________________________________________ 105 3.3. Results__________________________________________________________________ 106 3.3.1. Synthesis process________________________________________________________ 106 3.3.2. Mechanism_____________________________________________________________ 112 3.3.2.1. TEAB3H8_____________________________________________________________ 112 3.3.2.2. TEA2B12H12 and TEA2B10H10_____________________________________________ 114 3.4. Conclusions______________________________________________________________ 117 Acknowledgments____________________________________________________________ 117 Authors’s contribution_________________________________________________________ 118 3.5. References_______________________________________________________________ 119

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Chapter 4: QENS and Vibrational analysis of the solid-state electrolyte Na4B12H12B10H10

containing the ¹¹B isotope______________________________________________________

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Abstract____________________________________________________________________ 123 4.1. Introduction______________________________________________________________ 124 4.2. Experimental part_________________________________________________________ 126 4.2.1. Materials_______________________________________________________________ 126 4.2.2. Apparatuses____________________________________________________________ 126 4.2.3. TEA¹¹BH4 synthesis______________________________________________________ 128 4.2.4. TEA¹¹B3H8 synthesis_____________________________________________________ 128 4.2.5. TEA211B12H12 and TEA211B10H10 synthesis____________________________________ 128 4.2.6. Na411B12H1211B10H10 synthesis______________________________________________ 128 4.3. Results__________________________________________________________________ 129 4.3.1. Synthesis process of compounds containing ¹¹B isotope__________________________ 129 4.3.3. Dynamic of Na411B12H1211B10H10____________________________________________ 130 4.3.4. In-situ variable temperature Infrared of Na411B12H1211B10H10______________________ 136 4.3.5. In-situ variable temperature Raman of Na411B12H1211B10H10_______________________ 138 4.4. Conclusions______________________________________________________________ 140 Acknowledgements___________________________________________________________ 140 Authors’s contribution_________________________________________________________ 141 4.5. References_______________________________________________________________ 142 Chapter 5: Structural characterization of TBAX (X=Br^-, BH4-, B3H8-) and TEA2B12H12, materials for hydrogen storage_________________________________________________

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Abstract____________________________________________________________________ 144 5.1. Introduction______________________________________________________________ 145 5.2. Experimental part_________________________________________________________ 147 5.2.1. Materials_______________________________________________________________ 147 5.2.2. Apparatuses____________________________________________________________ 147 5.2.3. Computational details on TBA⁺ cation_______________________________________ 148 5.3. Results__________________________________________________________________ 149 5.3.1. Structural investigation of TEA₂B₁₂H₁₂_______________________________________ 149 5.3.1.1. Crystal structure of TEA₂B₁₂H₁₂___________________________________________ 149

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5.3.1.2. Infrared and Raman spectra of TEA2B12H12__________________________________ 150 5.3.1.3. Thermal analysis of TEA2B12H12__________________________________________ 152 5.3.2. Experimental and theoretical study of the tetrabutylammonium cation TBA⁺_________ 154 5.3.2.1. Crystal structure of TBAB3H8_____________________________________________ 154 5.3.2.2. Thermal analysis of TBAX (X=Br^-, BH4-, and B3H8-)__________________________ 156 5.3.2.3. Theoretical investigations of TBAX structures_______________________________ 158 5.3.2.4. Comparison between experimental and theoretical Infrared spectra of TBAX compounds__________________________________________________________________

159 5.3.2.5. Comparison between experimental and theoretical Raman spectra of TBAX compounds__________________________________________________________________

161 5.3.2.6. In-situ variable temperature experimental Infrared and Raman spectra of TBAX compounds__________________________________________________________________

163 5.4. Conclusions______________________________________________________________ 170 Acknowledgements___________________________________________________________ 170 Authors’s contribution_________________________________________________________ 171 5.5. References_______________________________________________________________ 172 Chapter 6: Synthesis and Characterization of solvent free Mg(B₃H₈)₂, Ca(B₃H₈)₂ and

their deuterated derivatives: Materials for hydrogen storage________________________

175

Abstract____________________________________________________________________ 175 6.1. Introduction______________________________________________________________ 176 6.2. Experimental part_________________________________________________________ 180 6.2.1. Materials_______________________________________________________________ 180 6.2.2. Apparatuses____________________________________________________________ 180 6.2.3. TBAB₃H₈ synthesis______________________________________________________ 181 6.2.4. NaB3H8 synthesis________________________________________________________ 182 6.2.5. TBAB3D8 synthesis______________________________________________________ 182 6.2.6. NaB3D8 synthesis________________________________________________________ 183 6.2.7. Mg(B₃H₈)₂, Ca(B₃H₈)₂, Mg(B₃D₈)₂ and Ca(B₃D₈)₂ synthesis______________________ 183 6.3. Results__________________________________________________________________ 184 6.3.1. Synthesis of TBAB3D8____________________________________________________ 184 6.3.1.1. Ex- situ Liquid ¹¹B NMR of TBAB3D8______________________________________ 185 6.3.2. Synthesis of solvent free Mg(B₃H₈)₂, Ca(B₃H₈)₂, Mg(B₃D₈)₂ and Ca(B₃D₈)₂__________ 186

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6.3.3. Thermal analysis________________________________________________________ 189 6.3.4. In-situ variable temperature Infrared of Mg(B3H8)2 and Mg(B3D8)2_________________ 193 6.3.5. In-situ variable temperature Raman of Mg(B3D8)2______________________________ 196 6.3.6. In-situ variable temperature Infrared of Ca(B3H8)2 and Ca(B3D8)2__________________ 198 6.4. Conclusions______________________________________________________________ 201 Acknowledgments____________________________________________________________ 201 Authors’s contribution_________________________________________________________ 202 6.5. References_______________________________________________________________ 203 Chapter 7: Controlling the reaction pathways in the thermal decomposition of solvent free Mg(B3H8)2__________________________________________________________________

208

Abstract____________________________________________________________________ 208 7.1. Introduction______________________________________________________________ 209 7.2. Experimental part_________________________________________________________ 211 7.2.1. Materials_______________________________________________________________ 211 7.2.2. Apparatuses____________________________________________________________ 211 7.2.3. Solvent free Mg(B3H8)2 and Mg(B3D8)2 synthesis_______________________________ 214 7.2.4. Solvent free Mg(B₃H₈)₂-4MgH₂ and Mg(B₃D₈)₂-4MgH₂ synthesis_________________ 215 7.2.5. Analysis of samples before in-situ experiments________________________________ 215 7.3. Results__________________________________________________________________ 216 7.3.1. Dehydrogenation of solvent free NaB3H8_____________________________________ 216 7.3.1.1. Ex-situ solution ¹¹B NMR of solvent free NaB3H8_____________________________ 216 7.3.1.2. In-situ solid-state ¹¹B NMR of solvent free NaB3H8____________________________ 219 7.3.2. Dehydrogenation of solvent free Mg(B₃H₈)₂___________________________________ 220 7.3.2.1. Ex-situ solution ¹¹B NMR of solvent free Mg(B3H8)2__________________________ 220 7.3.2.2. In-situ variable temperature ¹¹B NMR of solvent free Mg(B3H8)2_________________ 224 7.3.2.3. DSC experiments for Mg(B3H8)2__________________________________________ 225 7.3.3. Experiments with solvent free Mg(B3H8)2-4MgH2______________________________ 226 7.3.3.1. Ex-situ solution ¹¹B NMR of solvent free Mg(B3H8)2-4MgH2____________________ 226 7.3.3.2. In-situ variable temperature ¹¹B NMR of solvent free Mg(B3H8)2-4MgH2__________ 230 7.3.3.3. DSC of Mg(B3H8)2-4MgH2_______________________________________________ 232 7.3.4. Thermal programmed desorption (TPD) and Mass spectrometry (MS) experiments_____ 233

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7.3.5. Hydrogenation of solvent free Mg(B3H8)2-4MgH2______________________________ 242 7.3.5.1. Ex-situ solution ¹¹B NMR of solvent free Mg(B3H8)2-4MgH2____________________ 242 7.3.5.2. In-situ variable temperature ¹¹B NMR of solvent free Mg(B₃H₈)₂-4MgH₂ with 6 bar of hydrogen____________________________________________________________________

243 7.3.5.3. DSC of Mg(B₃H₈)₂-4MgH₂ with 6 bar of hydrogen____________________________ 244 7.4. Conclusions______________________________________________________________ 246 Acknowledgments____________________________________________________________ 247 Authors’s contribution_________________________________________________________ 248 7.5. References_______________________________________________________________ 249 Chapter 8: Vibrational characterization of Pr(¹¹BH4)3_____________________________ 252 Abstract____________________________________________________________________ 252 8.1. Introduction______________________________________________________________ 253 8.2. Experimental part_________________________________________________________ 254 8.2.1. Apparatuses____________________________________________________________ 254 8.3. Results__________________________________________________________________ 254 8.3.1. Room temperature Infrared and Raman of Pr(¹¹BH4)3____________________________ 254 8.3.2. In-situ variable temperature Infrared of Pr(¹¹BH₄)₃______________________________ 256 8.3.3. Ex-situ Raman of Pr(¹¹BH₄)₃_______________________________________________ 259 8.4. Conclusions______________________________________________________________ 261 Acknowledgments____________________________________________________________ 261 Authors’s contribution_________________________________________________________ 262 8.5. References_______________________________________________________________ 263 General Conclusions and Final Remarks________________________________________ 265