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Title: Computational study of lithium-ion mobility in stoichiometric solid-state systems
Author: Mulliner, Alexander D.
ISNI:       0000 0004 6346 4830
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2015
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Lithium-ion mobility in solid-state systems is an important field with significance for a range of technological interests, especially in the field of energy materials. The binary lithium compounds Li2O and Li3N have been studied using modern computational techniques to elucidate their diffusion properties. The macroscopic diffusion coefficients have been calculated and the underlying microscopic diffusion processes elucidated using molecular-dynamics-based techniques. In both systems studied the diffusion has been modelled over a wider temperature range than previously attempted. The agreement between the calculated diffusion coefficients and the best available experimental data surpasses previous computational studies. The level of agreement achieved is due to the use of force-field non-equilibrium molecular dynamics (FF-NEMD). This method, which has only recently been applied to solid-state diffusion, allows rates of diffusion that are typically too low to be studied with equilibrium molecular dynamics to be modelled. NEMD uses a fictitious external field to increase the frequency of the rare diffusion events in a predictable manner without changing the nature of the diffusion mechanisms. In the case of Li2O the transition from the non-superionic to the superionic regime has been modelled. The dominant mechanism in each regime has been determined: in the non-superionic regime a concerted mechanism dominates and in the superionic regime an interstitial-mediated mechanism dominates. For Li3N the anisotropic nature of the lithium diffusion coefficient has been modelled. The diffusion mechanisms dominant for different directions have also been elucidated. The rapid diffusion in the (001) plane of the hexagonal structure has been explained by the presence of vacancies on the in-plane Li sites, both in this thesis and in previous works. Previous studies have explained this by the presence of contaminants leading to non-stoichiometric Li3N. However, in this thesis the under occupation of these sites has been explained by the formation of Li2 dumbbells, or site sharing, largely about the interlayer Li sites, but also about the in-plane Li sites. The existence of these dumbbells explains the experimental diffusion coefficients of stoichiometric Li3N.
Supervisor: Battle, Peter ; David, William Sponsor: Science and Technolcogy Facilities Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available