Thesis (M.S.)--University of Wyoming, 2008. / Title from PDF title page (viewed on Mar. 11, 2010). Includes bibliographical references (p. 42-45).

M. Tech. Department of Chemical, Metallurgical and Materials Engineering. / Discusses how to synthesize a titanium-manganese oxide composite electrode with improved supercapacitive properties. The research aim was achieved through the following objectives: 1. the mechanisms of the synergistic incorporation of manganese oxide for improving the supercapacitive properties of titanium oxide electrodes. 2. Investigate possible metallurgical interactions and phenomenon during the sintering of the composite. 3. Investigate the electrochemical characteristics of titanium-manganese composite electrodes.

Thesis (Ph.D.(Physics)) -- University of Limpopo, 2020 / We employ molecular dynamics simulations, using DL_POLY code, to study the structural behaviour of β-MnO2 cathode material during discharging through lithium-ion intercalation into the bulk, nanoparticle, nanorod, nanosheet, and nanoporous β-MnO2. It is shown that lithium-ions have an average coordination number of about 5.70 and prefer surface sites with high oxygen coordination. The average lattice parameter values at intercalation of 0.85 Li/Mn are found to be under 4% relative to the experimental values obtained at 0.92 Li/Mn. Moreover, all the lithiated β-MnO2 structures did not collapse at 0.85 Li/Mn as observed in the β-MnO2 mesoporous in experimental work. As lithium is limited, sodium is a good alternative charge carrier in lithium-ion batteries. As a result, we have also performed studies on sodium intercalation into bulk, nanoparticle, nanorod, nanosheet and nanoporous β-MnO2. The microstructures and radial distribution functions show that the β-MnO2 structures could be intercalated up to 0.24 Na/Mn without any obvious structural degradation. Beyond this sodium concentration, the microstructure collapses and become amorphous thus predicting a potentially lower capacity for Na-MnO2-β batteries. Also, as the voltage is an important factor in the energy density of lithium-ion batteries, we have studied the trends in the average intercalation potentials in relation to the various nano architectures. The trend, in increasing value of average intercalation potentials, were found to be bulk structure, nanorod, nanosheet, nanoporous and nanoparticle. This suggests that nanostructuring can enhance cell voltage. Mechanical properties studies on the pure and lithiated bulk and nanorod β-MnO2 were also performed through uniaxial compressive and tensile strain application. The results show that under compressive strain the bulk structure and nanorod mitigate stress through the contraction and collapse of the inherent tunnel structures, known to cause electrochemical inactivity, and also through the shifting of the MnO6 octahedral planes. The collapsing of tunnels was found to occur more on the bulk structure and less on the nanorod, while the MnO6 octahedral plane shifts were found to occur more on the nanorod and less on the bulk structure. Unoccupied 1x2 or conjoined 1x2 were found to result in structural collapse irrespective of the host nanoarchitecture. The X-ray diffraction pattern (v) plots suggest that lithium intercalation and compressive stress application have a similar impact on the underlying structure of the various nanostructures. The microstructure analysis for bulk β-MnO2 under tensile strain reveals that fracture occurred in the brookite region and along the dislocation/stacking fault. The nanorod β-MnO2 mitigated stress through a rutile-to-brookite phase transition which occurred in the unstrained Li0.73MnO2-β and under tensile strain in LixMnO2-β for x = 0.00, 0.03, 0.12, and 0.24. In both the bulk and nanorod β-MnO2 the brookite phase was succeeded by structural breakdown leading to fracture and served as an indicator for imminent structural failure upon more tensile strain application. / National Research Foundation (NRF)

Thesis (MSc. (Physics)) -- University of Limpopo, 2016 / Nanostructured materials are attractive candidates for efficient electrochemical energy storage devices because of their unique physicochemical properties. Introducing nanotube systems as electrode materials represents one of the most attractive strategies that could dramatically enhance the battery performance. Nanostructured manganese based oxides are considered as ideal electrode materials for energy storage devices such as high energy and high power lithium-ion batteries. In this study, computer simulation strategies were used to generate various structures of MnO2 and spinel LiMn2O4 nanotubes; where Miller index, diameter and symmetry are considered as variables. The effect of these variables on nanotube generation was investigated. MnO2 and spinel LiMn2O4 nanotubes were generated using MedeA® software. Lower Miller indices, namely; {001}, {100}, {110} and {111} with diameter ranging from 5Å30Å were investigated for both systems. There are two ways that a nanotube structures could be wrapped along different directions, i.e., a_around_b or b_around_a. It was observed that wrapping direction has an effect on the geometrical structure of the nanotube. MnO2 nanotube generated from {110} revealed that nanotube wrapped along b_around_a gave a close-packed structure compared to its counterpart nanotube wrapped a_around_b. Diameter represents an important structural parameter of nanotubes; however, precise control of nanotube diameter over a wide range of materials is yet to be demonstrated. In this study, it was found that as the diameter of the nanotube is changed, parameters such as cross-sectional area and bond length change as well. The average bond distance of the nanotubes is less than that of MnO2 and LiMn2O4 bulk structure. Molecular dynamics simulation is further used to investigate the structure of MnO2 and LiMn2O4 nanotubes and the effect of temperature on the generated systems. Molecular graphical images used for the atomic positions for the nanotubes were investigated. The nanotube structures are described using radial distribution functions and XRD patterns. The calculated XRD patterns are in good agreement with the experiments, thus validating the generated structural models for the nanotubes. The resulting models conform to pyrolusite polymorph of MnO2 and LiMn2O4, featuring octahedrally coordinated manganese atoms. It was established that the variables have a direct control on nanotube morphology and the stability of generated nanotube model depends on surface morphology and termination. / National Research Foundation (NRF) and Centre for High Performance Computing (CHPC) of CSIR