Matar, M.K.H.A.; (2011) A computational study of the structure and properties of titanates and carbon nitride. Doctoral thesis , UCL (University College London).

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Abstract

This thesis presents results of a computational investigation focusing on two classes of materials: • Those of interest as anodes in rechargeable Li ion batteries, particularly TiO2 polymorphs and a novel Ti oxysulfide; and • A graphitic carbon nitride, where the high pressure structural and electronic transformations, and intercalation properties are studied. The first part of the work considers the Li intercalation in TiO2 Brookite. Calculations identify stable sites and low energy configurations during the intercalation process up to full occupancy. It is demonstrated that Li intercalation is homogeneous but consists of three domains corresponding to different ordering patterns. It is shown that Li intercalation is strongly limited by the diffusion of Li ions, especially at high Li content, resulting in the absence of intercalation at normal conditions, and intercalation to high concentrations at elevated temperature or in the nanophase. Later, Li intercalation in the TiO2-B structure is investigated and the most favourable energy sites for Li ions in the structure have been identified including the energy favourable migration pathways. These have been studied by calculating the diffusion barriers in all possible directions within the substrate material. Calculations indicate that the TiO2-B structure has a lower density than other titanates, and the calculated cell voltage is in the range of 0.8 to 1.45 V for low Li content, with the Z direction showing the highest mobility for Li ions; the TiO2-B structure therefore yields a high Li mobility. Calculations show that brookite is suitable to host Li ions, and yields relatively constant voltage values which are suitable for use in actual Li batteries (Brookite anode value calculated as 1.7 V); TiO2-B generates a voltage value which is lower than the brookite voltage which means it is a promising anode in rechargeable Li batteries. In the second part of the thesis a graphitic carbon nitride of composition C6N9H3.HCl is studied. In the first part of the work, the interlayer bonding mechanism that takes place between consecutive layers at high pressure is analysed. Calculations performed also allowed identification of symmetrical hydrogen bonding, a feature which is rarely observed, and the formation of carbon-chlorine bonds (both observations at elevated pressures). In the next stage of the study on Carbon Nitride, intercalation behaviour within the graphitic structure is examined (hence connecting this part of the work to the work performed on the TiO2 substrates). Results indicate that the ability of the examined carbon nitride to act as an anode for rechargeable Li batteries is low because the intercalation energy is not constant as a function of the Li content, in addition to strong structural deformations occurring during intercalation. The combination of factors affecting the cyclabilty of intercalation/deintercalation is expected to make the substrate a poor rechargeable battery. The final section of this thesis is dedicated to the study of the Li intercalation properties of a layered oxysulfide structure of composition Y4Ti4O10S4. Calculations indicate that the material appears to be unsuitable for use as an electrode for Li ion rechargeable batteries due to unsuitable Li intercalation energies, and large structural changes occurring within the substrate during the cycling process.

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