AbstractMaterials used for energy conversion applications normally have a complex microstructure and the resulting interfaces within materials profoundly affect their properties. In this project the aim was to develop a tool for investigating such interfaces at the atomic level. The research carried out focuses primarily on four materials; strontium titanate, cerium dioxide, lithium lanthanum titanate and lithium lanthanum zirconate. Current research into energy applications has focussed on the improvement of solid oxide fuel cells (SOFCs) and lithium ion batteries (LIBs) in a variety of different ways, an example of such is the lowering of the operating temperature. Research into the structural and transport properties of bulk materials have been extensive; however, the study of interfaces in these materials has been less prevalent. A model has developed which allows the investigation of grain boundary structures and their effect on the transport properties at the interface.
The focus of this research was initially to develop a method with which we could accurately obtain the structure of grain boundary models for homogenous crystal lattices where a number of different interface models were obtained, some of which have been observed experimentally. The approach to achieving this was through energy minimisation techniques combined with a number of different interatomic potential models where the stability of a boundary could be quantified along with an accurate structural representation. This was further expanded on through molecular dynamics simulations, to investigate the role of the interface and how the local structure influences anionic and cationic diffusion.
Chapter 1 is an overarching review of the relevant literature related to each material, covering predominantly SOFCs and LIBs, with other supporting information. Chapter 2 summarises the computational methodologies used and how energy minimisation and molecular dynamics can be applied to generate structural models. Chapter 3 describes the investigation into the chosen rigid ion interatomic potential models and justifies the case for using each particular model with comparisons to a number of experimentally observed structural, elastic and dynamic properties. Chapters 4 and 5 focus on the interfaces of ceria and strontium titanate, respectively. A work flow was developed and further used to accurately model grain boundaries which are consistent with those observed experimentally In each case the oxygen transport is suppressed by the presence of grain boundaries. Chapter 6 will expand on the previous two by considering heterointerfaces of STO||CeO2 including the incorporation of defects to determine the effect this has on both diffusion and stability. Chapters 7 and 8 will describe work on modelling lithium diffusion in bulk and selected grain boundary dynamics of LLTO and LLZO. Each chapter investigates the bulk and how intrinsic and extrinsic dopants can be incorporated in order to enhance lithium ion diffusion and further grain boundaries of the LIB electrolyte materials. Chapter 9 will conclude this research and will discuss the major findings, the consequences of these findings and how they may be used to drive future research in the field.
|Date of Award||22 Feb 2023|
|Supervisor||Steve Parker (Supervisor), Marco Molinari (Supervisor) & Saiful Islam (Supervisor)|