It is unclear whether all intercalation techniques truly lead to the insertion of atoms between the graphite layers, or also lead to other effects which contribute to expansion. The objective of this project is to better understand the effects caused by different intercalation methods. Three intercalation methods were explored to determine the method which incurs the least damage to the surface and microstructure of the graphite intercalated compounds, yet achieves the best intercalation and therefore expansion. All the main findings are summarised below:  The gas phase sample had virtually no mass loss at the point where expansion took place. Therefore the intercalation was very efficient, producing large expansion without significant mass loss.  The mass loss that only occurs at the sublimation of iron chloride (320 ºC) indicates the excessive "un-intercalated" or residual iron chloride.  After oxidation, before purification, the gas phase sample has 25 % residual mass; this also proves the presence of impurities and residual iron chloride in the exfoliated sample. For the Hummers and electrochemical samples, expansion and mass loss occur over a wide temperature range, this indicates that graphite oxide was formed rather than the theoretically expected "insertion of atoms between the sheets".  The mass losses before 200 ˚C of the samples of the Hummers and electrochemical methods are more evidence that graphite oxide and graphite surface complexes with oxygen were produced.  The Hummers and electrochemical intercalation methods show similar expansion and mass loss curves, therefore it can be concluded that the reaction mechanism for both these methods is alike.  The gas phase method yields the best expansion of 250 % using the TMA, whereas both the other methods deliver approximately 220 %.  Using microwave expansion the electrochemical intercalation method yields the best bulk volume expansion of 1500 %, with the gas phase sample delivering a volume expansion of 1450 %. The Hummers samples are extremely damaged. This is clear from the several and deep oxidation pits visible throughout the basal plane of these samples. The basal plane and the edges are even eroded before purification and oxidation. This intercalation technique employs oxidisers in the preparation method which additionally oxidises the samples. This explains why the Hummers method renders the most damage. The residual material on the gas phase sample acts as catalysts making the sample very reactive and consequently damaging the surface during oxidation. The partially oxidised purified gas phase sample visibly shows the pits and roughened edges. There are two “types” of intercalation. The first intercalation “type” is the actual insertion of atoms or molecules between the graphite layers, whereas the other “type” of intercalation is the production of graphite oxide. The compound comprises carbon, oxygen and hydrogen, obtained by treating graphite with strong oxidisers. The functional groups usually found in graphite oxide are carbonyl (C=O), hydroxyl (-OH), phenol amongst others and also some impurities of sulphur when sulphuric acid is used. Both these intercalation types lead to expansion. It is recommended that a more efficient method for removal of residual material in the gas phase samples be explored. It is also recommended that more research be done to determine the reaction mechanisms during the three different intercalation methods. The graphite surface complexes of the intercalated compounds and the evolved gases during expansion should be analysed. / Dissertation (MEng)--University of Pretoria, 2015. / tm2015 / Chemical Engineering / MEng / Unrestricted

Ce travail présente les études électrochimiques et structurales menées sur deux systèmes : P2/O3-NaxMn1-yFeyO2 et O’3-NaxNiO2 utilisés en tant que matériaux d’électrode positive pour batteries Na-ion.Concernant le système P2/O3-NaxMn1-yFeyO2, l’étude par diffraction des rayons X menée in situ pendantla charge de batteries a montré de nombreuses transitions structurales. Que leur structure soit de type P2ou O3, les matériaux présentent une phase distordue pour les taux d’intercalation (x) les plus élevés etune phase très peu ordonnée pour les taux d’intercalation les moins élevés. Entre ces deux étatsd’intercalation, les phases de type P2 présentent moins de transitions que les phases de type O3. Celaentraine de meilleures propriétés électrochimiques pour les phases de type P2 (meilleure capacité endécharge, meilleure rétention de capacité…). Les spectroscopies d’absorption des rayons X et Mössbauerdu 57Fe ont montré que les couples redox Mn4+/Mn3+ et Fe4+/Fe3+ sont impliqués lors du cyclage, à bas ethaut potentiel, respectivement.Concernant O’3-NaNiO2, la diffraction des rayons-X menée in situ pendant la charge de batteriesO’3-NaNiO2//Na a montré de nombreuses transitions structurales O’3 ↔ P’3 résultant du glissement desfeuillets MO2. Ces transitions s’accompagnent de mises en ordre Na+ - lacunes dans le matériau. La tailledes grains a montré avoir un intérêt majeur puisqu’elle influe sur le nombre de phases présentessimultanément dans le matériau. Lorsque la batterie est déchargée, la phase limitante Na≈0.8NiO2 estobservée et empêche le retour à O’3-NaNiO2 / This work concerns the electrochemical and structural studies carried out on two systems used aspositive electrode materials for Na-ion batteries: P2/O3-NaxMn1-yFeyO2 and O’3-NaxNiO2. Concerning theP2/O3-NaxMn1-yFeyO2 systems, in situ X-ray diffraction carried out during the charge of the batteriesshowed that both materials undergo several structural transitions. Both the P2 and O3 phases show adistorted phase for the higher intercalation rates (x) and a poorly ordered phase for the lower ones.Between these two states, P2-based materials exhibit less structural transitions than the O3-based ones.This is correlated to the better electrochemical properties the P2-based materials exhibit (better dischargecapacity, better capacity retention…). X-ray absorption and 57Fe Mössbauer spectroscopies showed thatthe Mn4+/Mn3+ and Fe4+/Fe3+ redox couples are active upon cycling, respectively at low and high voltage.Concerning O’3-NaNiO2, in situ X-ray diffraction carried out during the charge of O’3-NaNiO2//Nabatteries showed several structural transition between O’3 and P’3 structures, resulting from slab glidings.These transitions are accompanied by Na+ - vacancies ordering within the “NaO6” slabs. Upon discharge,the material does not come back to its initial state and, instead, the Na≈0.8NiO2 phase represents themaximum intercalated state. The occurrence of this limiting phase prevents O’3-NaNiO2 to be consideredas an interesting material for real Na-ion applications.

Les chalcogénures à clusters de Molybdène sont des structures minérales reconnues pour développer des propriétés d’accueil de cations. En effet, la remarquable mobilité des cations dans les matrices minérales construites sur les unités Mo6X8 (avec X= S, Se) des phases de Chevrel définit des systèmes redox réversibles Mo6X8 + xMn+ + xn e- <-> MxMo6X8. A côté de leurs potentialités physiques et chimiques connues, deux applications de ces réactions d’intercalation ont vu jour récemment au sein de nos équipes, en lien avec les préoccupations environnementales et de valorisation matière. Il s’agit du développement de jonction électrochimique de transfert pour la récupération sélective de cations et la mise au point de capteurs, applications validées parfaitement sur des solutions synthétiques mono-cationiques. Or pour leur développement et pour se rapprocher des cas réels, il s’avère indispensable de réaliser des études dans des solutions pluri-cationiques. Ainsi l’objectif de notre travail de thèse a été d’étudier les mécanismes d’intercalation de 4 cations métalliques (Ni2+, Cd2+, Zn2+, Co2+), représentatifs de différents effluents industriels. Une première partie s’est attachée à développer la connaissance du positionnement des processus d’intercalation mixtes de cations, contenus dans des mélanges équimolaires 0,1 M bi et tri cationiques, en mode ex situ afin d’étudier les réponses électrochimiques caractéristiques à chaque électrolyte. Cette connaissance confrontée à des contrôles structuraux et stœchiométriques permet de mieux interpréter et d’identifier les systèmes préférentiels d’insertion ainsi que le mode d’intercalation. Une seconde partie a été réalisée in situ sur grands instruments à l’ESRF afin de suivre la formation des phases au cours de l’intercalation électrochimique. Grâce à ce suivi, nous avons pu déterminer la nature des phases en présence, leur proportion (dans le cas d’un mélange), leur structure ainsi que la position exacte du cation dans la structure, notamment pour l’ion cadmium dans les phases soufrée et séléniée qui a fait l’objet d’un traitement expérimental approfondi. L’originalité de ce travail repose d’une part sur l’étude d’intercalation dans des électrolytes inédits bi et tri cationiques, d’autre part sur la détermination de structures des phases de Chevrel pas encore référencées dans la littérature (Cd2Mo6Se8). Ces résultats constituent un ensemble de données particulièrement utiles pour l’optimisation des performances des capteurs ou celle des protocoles de transferts sélectifs / Molybdenum cluster chalcogenides are mineral structures known for their cation-receiving properties. Indeed, the remarkable cations mobility in the mineral matrices built on the Mo6X8 units (with X = S, Se) of the Chevrel phases defines reversible redox systems Mo6X8 + xMn+ + xn e- <-> MxMo6X8. Regarding their physical and chemical potential, two applications using the intercalation reactions have been recently developed in our teams, in connection with environmental concerns and material recovery. More precisely the development of an electrochemical transfer junction for the selective recovery of cations and the development of sensors, which were perfectly validated on synthetic mono-cationic solutions. But for their development and to get closer to real cases, it is essential to carry out studies in multi-cationic solutions. Thus the objective of our thesis work was to study the mechanisms of intercalation of 4 metal cations (Ni2+, Cd2+, Zn2+, Co2+), representative of different industrial effluents. A first part focused on developing the knowledge of the positioning of the mixed cation intercalation processes, contained in equimolar 0.1 M bi and tri cationic mixtures, in ex situ mode in order to study the electrochemical responses characteristic of each electrolyte. This knowledge, confronted with structural and stoichiometric controls, makes it possible to better interpret and identify the preferred insertion systems as well as the intercalation mode. A second part was performed in situ on great facilities at the ESRF in order to follow phase formation during electrochemical intercalation. Thanks to this monitoring, we were able to determine the phases nature involved, their proportion (in the case of a mixture), their structure as well as the exact position of the cation in the structure, in particular for the cadmium ion in the phases sulfur and selenium which has been the subject of a thorough experimental treatment. The originality of this work rests on the one hand on the intercalation study in new electrolytes bi and tri cationic, on the other hand on the determination of structures of Chevrel phases not yet referenced in the literature (Cd2Mo6Se8). These results constitute a set of data particularly useful for the optimization of sensor performance or selective transfer protocols